2023

Thu, Dec. 7th,

Hadron scatterings at nonzero chemical potential in the hadronic phase

Kotaro Murakami  (Tokyo Insititute of Technology)
Room 907 (17:00-18:00)

Abstract: We formulate a calculation method for hadron scatterings at $T=0$ and $\mu \neq 0$ in the hadronic phase. Although it is believed that the physical quantities are not changed by a small $\mu$ at zero temperature, the correlation functions depend on $\mu$. First, we analytically derive the modification of the dispersion relation for a single hadron by $\mu$ from the Euclidean-time dependence of the corresponding correlation functions. Next, we extend the HAL QCD method at $\mu= 0$ to the case of $\mu \neq 0$, where we have found that the interaction potential can depend on $\mu$ only through the effective mass while the scattering phase shifts are independent of $\mu$. We also numerically analyze the hadron scatterings in QC$_{2}$D at nonzero quark chemical potential. Finally, we propose a technique to reduce the systematic error for the scatterings amplitudes caused by the inelastic contributions by utilizing the asymmetric property of the correlation functions at nonzero $\mu$. 

Thu, November 30th,

Real time non-perturbative dynamics in Schwinger model - jet production, charge transport, and more

Shuzhe Shi (Tsinghua University)
Online via Zoom (17:00-18:00)

Abstract: Addressing QCD scattering processes theoretically requires a real-time, nonperturbative method. It is well known that the Schwinger model [QED in (1+1) dimensions] shares many common properties with QCD, including confinement, chiral symmetry breaking, and the existence of vacuum fermion condensate. As a step in developing such an approach, we report here on fully quantum simulations, using classical devices, of a massive Schwinger model. We study the chiral condensate and entanglement entropy caused by jet propagation[PhysRevLett.131.021902]. We also explore the propagation of vector and axial charge, i.e., CMW, and observe different oscillation frequencies[PhysRevD.108.074001]. The phase structure of the Schwinger model at finite temperature and chemical potential will also be discussed[2305.00996].

Thu, November 9th,

Topological charge fractionality and higher-group of lattice higher-form gauge fields

Okuto Morikawa (Osaka University)
Room 907 (17:00-18:00)

Abstract: Recently, the notion of symmetry is generalized so that we can get further information on low-energy dynamics in strongly coupled field theories. This is described in terms of topological operators and also category. It would be crucial that topology of gauge fields is nontrivial in a fully regularized framework; e.g., continuity appears to be lost under lattice regularization. Luescher addressed this issue for SU(N) gauge fields and defined the topological charge on a lattice explicitly. We apply Luescher's construction to generalized symmetries. We consider lattice SU(N) gauge theories coupled with Z_N 2-form gauge fields, and show the fractionality of the topological charge on SU(N)/Z_N principal bundle. Also the mixed 't Hooft anomaly and higher-group structure are realized on the lattice. We become interested on the lattice in monopole physics as a topological phenomenon.

Thu, November 2nd,

Hyperon puzzle of neutron stars probing with hypernuclear data and correlation function of heavy-ion collision 

Asanosuke Jinno (Kyoto University)
Room 907 (17:00-18:00)

Abstract: The hyperon puzzle in neutron stars refers to the problem that most of the equations of state with hyperons (e.g. Lambda) are not sufficiently stiff to support the observed massive neutron stars. Although various solutions to the puzzle have been proposed, the presence or absence of hyperons in neutron stars remains uncertain. To deepen the quantitative discussion, it is crucial to constrain both the Lambda potential in nuclear matter and the Lambda-nucleon interaction based on the experimental data. However, the current data could not impose enough constraints at high densities. 

In this talk, we discuss the possibility to constrain the Lambda potential at high densities[1] using the future high-resolution hypernuclear data that will be measured at J-PARC. Furthermore, we propose Lambda-alpha (Helium-4) correlation function of heavy-ion collision experiments to determine the extent of the repulsive core of the Lambda-nucleon interaction.

[1] A. Jinno, K. Murase, Y. Nara, and A. Ohnishi, arXiv:2306.17452 [nucl-th].

Thu, October 26th,

Chiral magnetic waves in quark matter inside neutron stars and gravitational waves 

Sota  Hanai (Keio University)
Room 907 (17:00-18:00)

Abstract: It is important to unravel the internal structure of neutron stars in astrophysics. One effective way to study the interior of neutron stars is analyzing their seismic oscillations. Recently, the chiral magnetic wave (CMW), which is a density wave propagating along magnetic fields due to the chirality of fermions, has been studied in the context of the heavy ion collision experiments.

In this talk, we show that the CMW can appear as a seismic oscillation in quark matter, such as the two-flavor color superconductivity, inside neutron stars. We also discuss the frequency and amplitude of a new type of gravitational wave radiated by the seismic oscillation. This gravitational wave could be a new probe of the magnetic field and quark matter in neutron stars.

Wed, October 18th,

Three ways of calculating composite-particle spectra of gauge theories in the Hamiltonian formalism 

Akira Matsumoto (YITP)
Room 907 (17:00-18:00)

Abstract: Determination of the mass spectrum of composite particles (hadrons) is one of the key issues in QCD, which has been precisely calculated by the Monte Carlo simulation based on the Lagrangian formalism. We newly consider three distinct methods to compute the mass spectrum in the Hamiltonian formalism, where we can apply quantum computation and tensor network methods.
The first one, the correlation-function scheme, is similar to the conventional Euclidean method in the Monte Carlo study. The second one, the one-point-function scheme, uses the boundary effect to compute the mass spectrum efficiently. The third one, the dispersion-relation scheme, generates the excited states to obtain the dispersion relation and quantum numbers. These methods are demonstrated using the density-matrix renormalization group (DMRG) in the 2-flavor Schwinger model,
which shares important properties with QCD. We show that their results are consistent with each other, and discuss their potential applications.
(preprint: https://arxiv.org/abs/2307.16655)

August 14th,

Chiral kinetic theory involving nonlinear quantum corrections

Kazuya Mameda (Tokyo University of Science)
Room 907 (16:00-17:00)

Abstract: The chiral kinetic theory (CKT) is a great theoretical framework for the transport phenomena of massless degrees of freedom. In spite of various developments, the usual CKT includes only the linear quantum correction. In this talk, I explain how to formulate the CKT with the nonlinear corrections, not only under electromagnetic fields but also under gravitational fields. From the generalized CKT, I also show several intriguing implications, including the consistency with QED, nondissipative transport under gravity, potential issues on the CKT etc.

July 3

Generalized chiral instabilities, linking numbers, and non-invertible symmetries

Ryo Yokokura (Keio University)

Room 907

Abstract:  We demonstrate a universal mechanism of a class of instabilities in infrared regions for massless Abelian p-form gauge theories with topological interactions, which we call generalized chiral instabilities. Such instabilities occur in the presence of initial electric fields for the p-form gauge fields. We show that the dynamically generated magnetic fields tend to decrease the initial electric fields and result in configurations with linking numbers, which can be characterized by non-invertible global symmetries. The so-called chiral plasma instability and instabilities of the axion electrodynamics and (4+1)-dimensional Maxwell-Chern-Simons theory in electric fields can be described by the generalized chiral instabilities in a unified manner. We also illustrate this mechanism in the (2+1)-dimensional Goldstone-Maxwell model in electric field.

June 21

Rigidly-rotating scalar fields: between real divergence and imaginary fractalization

Maxim Chernodub (Institut Denis Poisson, CNRS, France)

Room 907

Abstract:  The thermodynamics of rigidly rotating systems experience divergences when the system dimensions transverse to the rotation axis exceed the critical size imposed by the causality constraint. The rotation with imaginary angular frequency, suitable for numerical lattice simulations in Euclidean imaginary-time formalism, experiences fractalization of thermodynamics in the thermodynamic limit when the system's pressure becomes a fractal function of the rotation frequency. Our work connects two phenomena by studying how thermodynamics fractalizes as the system size grows. We examine an analytically-accessible system of rotating massless scalar matter on a one-dimensional ring and the numerically treatable case of rotation in the cylindrical geometry and show how the ninionic deformation of statistics emerges in these systems. We discuss a no-go theorem on analytical continuation between real- and imaginary-rotating theories. Finally, we compute the moment of inertia and shape deformation coefficients caused by the rotation of the relativistic bosonic gas and make a comparison with the high-temperature limit of rotating gluon gas which was computed numerically from the first principles. Recent results on rotational gluon plasma instability will also be briefly mentioned. 

(following ArXiv:2303.03147 and ArXiv:2304.05998)

May 17

Yang-Lee singularity, semicircle theorem and nonunitary criticality in BCS superconductivity 

Hongchao Li  (TokyoU)

Room 907

Abstract:  Yang-Lee theory investigates phase transitions in terms of zeros of partition functions. We find that Yang-Lee zeros of a BCS superconductor are distributed on a semicircle on the complex plane of the interaction strength due to the Fermi-surface instability. A renormalization-group analysis shows that the semicircle theorem holds for a generic quantum many-body fermionic system with a marginal coupling, in sharp contrast with the Lee-Yang circle theorem for the Ising spin system. Furthermore, we unveil the nonunitary criticality in BCS superconductivity with complex-valued interaction strength which shows singularity at each Yang-Lee zero. These singularities are caused by exceptional points and constitute a universality class distinct from that of the conventional Yang-Lee edge singularity.

April  28

Momentum shell and rapid stiffening in quarkyonic matter from an explicit duality:

Exactly solvable model analysis

Yuki Fujimoto  (Institue for Nuclear Theory, US)

Room 907

Abstract:  We present a dual description of quarkyonic matter in terms of quarks and baryons. We construct an exactly solvable quantum-mechanical model that gives free energy in terms of the quark and baryon distribution in the momentum space and we postulate the dual transformation between these distributions. By solving this model, we discuss two natural consequences of the quarkyonic duality: First, the shell structure in the baryon distribution emerges in the momentum space as the density increases. Second, we observe rapid stiffening in the equation of state around a few times the normal nuclear density.

April  21

Higgs-confinement continuity in light of particle-vortex statistics

Yui Hayashi  (YITP, Kyoto University)

Room 907, 16:00 - 17:00

Abstract: The Higgs-confinement continuity is of crucial importance to the quark-hadron continuity conjecture, which proposes a smooth crossover between nuclear superfluid and color superconducting phases. Some gauge theories with superfluidity, including dense QCD, have nontrivial Aharonov-Bohm (AB) phases around vortices in the Higgs regime. It has been recently discussed whether this nontrivial AB phase implies a Higgs-confinement transition. In this talk, we discuss this issue and support the Higgs-confinement continuity. By performing explicit calculations in relevant lattice models, we illustrate how the AB phase shows a smooth connection between the Higgs and confining regimes. This talk is based on arXiv:2303.02129 [hep-th].

February 14th

Perturbative-QCD and the neutron stars

Oleg Komoltsev ( the University of Stavanger )

in-person, Zoom, 16:00 - 17:00

Abstract: 

     Rapid advancement in neutron-star observations allows unprecedented empirical access to cold, ultra-dense QCD matter. The combination of these observations with theoretical calculations reveals previously inaccessible features of the equation of state (EOS) and the phase diagram of QCD. In this talk, I demonstrate how perturbative-QCD calculations at asymptotically high densities based solely on causality and stability provides robust constraints on the equation of state at neutron-star densities. By comparing the calculations to neutron-star observations using a Bayesian framework, I show that perturbative-QCD calculations offer significant information beyond current observations. The results support the hypothesis of quark matter cores in most massive neutron stars and the QCD input softening the equation of state at high densities. In addition, I discuss the modern approach to treat the perturbative uncertainties and its impact on the EOS.

2022

November 18th

Tensor renormalization group approach to quantum fields on a lattice

Shinichiro Akiyama ( U. Tokyo )

in-person, Zoom, 16:00 - 17:00

Abstract: 

     Tensor renormalization group (TRG) approach is a variant of the real-space renormalization group to evaluate the path integral defined on the thermodynamic lattice, without resorting to any probabilistic interpretation for the given Boltzmann weight. Moreover, since the TRG can directly deal with the Grassmann variables, this approach can be formulated in the same manner for the systems with bosons, fermions, or both. These advantages of the TRG approach have been confirmed by the earlier studies of various lattice theories, which suggest that the TRG enable us to investigate the parameter regimes where it is difficult to access with the standard stochastic numerical methods, such as the Monte Carlo simulation. In this talk, explaining our recent applications of the TRG approach to several lattice models, we demonstrate the efficiency of the TRG as a tool to investigate lattice theories, particularly in higher dimensions.

November 11th

Continuous transformation from hadrons to quarks in medium by means of a quark model

Daiki Suenaga (RIKEN)

Zoom, 16:00 - 17:00

Abstract: 

     How do hadrons change into quarks in medium? This is one of the most important and challenging questions of QCD, which is relevant to, e.g., understanding of the internal structure of neutron stars. At high density, discussions based on the symmetry structures suggest that the transition from hadron matter to quark matter is continuous. In this talk, I explain our intuitive picture describing such a continuous transition focusing on the quark occupation probability, by means of a quark model. In particular, as an instructive application, I present our latest investigation of the occupation probability in a meson gas at temperature.

October 28th

Velocity of Sound beyond the High-Density Relativistic Limit from Lattice Simulation of Dense Two-Color QCD

Etsuko Itou (RIKEN)

in-person, Zoom, 16:00 - 17:00

Abstract: 

     We obtain the equation of state (EoS) for two-color QCD at low temperature and high density from the lattice Monte Carlo simulation. We find that the velocity of sound exceeds the relativistic limit (cs2/c2=1/3) after BEC-BCS crossover in the superfluid phase. Such an excess of the sound velocity is previously unknown from any lattice calculations for QCD-like theories. This finding might have possible relevance to the EoS of neutron star matter revealed by recent measurements of neutron star masses and radii. This talk is based on arXiv:2207.01253.

October 21st

Exploring strongly interacting matter with deep learning

Lingxiao Wang (FIAS)

Zoom, 16:00 - 17:00

Abstract: 

     Deep learning can help us to explore strongly interacting matter from three perspectives, heavy-ion collisions(HIC), Lattice QCD calculations, and neutron star(NS) observations. I will first briefly introduce data- and physics-driven deep learning, and further their applications in HICs[1] and Lattice QCD[2] respectively. Then I will focus on using physics-driven deep learning to infer equation of states(EoSs) from NS observables(particularly mass-radius)[3]. Combined with a deep neural network-parameterized EoS(NNEoS), we have designed an automatic differentiation framework to utilize the Tolman–Oppenheimer–Volkoff equation for optimizing the parameters of NNEoS. In chi-square fitting, a reduced EoS was inferred from the latest 18 NSs. The corresponding rebuilt mass-radius curve and tidal deformability are consistent with the current facts. In our ongoing works, an improved fully-physical framework is manifested it can identify EoSs with(out) first-order phase transition from mock tests. In the end, I will discuss the potential advantages of physics-driven deep learning and our future work.

[1] arXiv:2105.13761, Phys. Rev. D 103, 116023;

[2] Phys. Rev. D 106, L051502; Comput. Phys. Commun. 282, 108547 (2023);

[3] arXiv:2209.08883; JCAP08 (2022) 071.

July 15th

Coleman-Weinberg Abrikosov-Nielsen-Olesen strings

Yu Hamada (KEK)

Zoom, 16:00 - 17:00

Abstract: 

     Vortex strings are topological solitons in field theories and appear in various systems, such as superconductor, neutron star, early universe and so on. We clarify the nature of vortex strings in the Abelian-Higgs-like model with a Coleman-Weinberg type Higgs potential, which is inspired by the argument of the naturalness problem in particle physics. This model has the gauge U(1) symmetry and the classical scale invariance. The model has no vortex string solution at the classical level, while quantum corrections spontaneously break the U(1) symmetry and stable vortex strings exist. The interaction between the two vortex strings is found to be very different from that of the Abelian-Higgs model with the conventional second- and fourth-order type potentials. Implications for particle physics and cosmology are also discussed.

June 24th

Inverse renormalization group for quantum field theories

Dimitrios Bachtis (Swansea University)

Zoom, 17:00 - 18:00

Abstract: 

     The inverse renormalization group method enables the generation of configurations for systems of increasing lattice size in absence of the critical slowing down effect. In this talk, I will discuss the construction of inverse renormalization group transformations with the use of machine learning for quantum field theories and for systems with continuous degrees of freedom. The method will then be applied on configurations of the two-dimensional \phi^{4} theory to iteratively increase the lattice volume of the system. I will then discuss the emergence of inverse renormalization group flows in parameter space and present accurate calculations of multiple critical exponents for the phase transition of the \phi^{4} theory.

June 3rd

Noether's 1st theorem with local symmetries

Shinya Aoki (Kyoto University)

Room 233, 13:00 - 14:00

Abstract: 

 We propose a general method to derive a conserved current associated with a global symmetry which is a part of local symmetries. While a current derived from local symmetries of a  matter sector with a covariant background gauge field  is not conserved in general, we show that the current associated with a special type of global transformations is on-shell conserved. We also point out that the conserved current can be modified by adding a current  trivially conserved as a constraint by the Noether's 2nd theorem. We apply this derivation to an $U(1)$ gauge theory, general relativity and a non-abelian gauge theory. In general relativity, the associated conserved charge agrees with the one recently proposed from a different argument. 

May 27th

Lattice QCD with neural network 

Akio Tomiya (IPUT in Osaka)

16:00 - 17:00

Abstract: 

The neural network is the most sophisticated framework to approximate functions under the name of the universal approximation theorem. In this talk, we introduce a gauge covariant neural network for lattice QCD, which is nothing but trainable smearing. As an example, for application, we show results for dynamical QCD simulations. We show some preliminary results for the flow-based sampling algorithm. We do not assume the audience has deep knowledge of machine learning.

May 18

Associated quarkonium production and double parton scatterings in high energy accelerator experiments

Nodoka Yamanaka (KMI)

Zoom, 16:00 - 17:00

Abstract: 

 Quarkonium production in proton-proton and proton-antiproton collisions provides interesting means to study the parton content and their correlations in the proton. Recent experimental LHC and Tevatron data of J/psi + Z, J/psi + W and J/psi + J/psi production suggest the relevance of double parton scatterings (DPSs) as opposed to single parton scatterings (SPSs). In this talk, we review the corresponding SPS contributions and discuss upper limits set up by quark-hadron duality. We show that the DPS currently presents a puzzle as regards to the associated quarkonium production.

2021

December 24

Kramers-Wannier-like duality defects in (3 + 1)d gauge theories 

Kantaro Ohmori (U. Tokyo)

Zoom, 16:00 - 17:00

Abstract: 

The 1+1d Majorana fermion has a chiral Z2 symmetry which is "broken" after gauging the non-chiral fermion parity due to their mixed anomaly. However it is better to think that the symmetry is preserved even after gauging in the form of topological defect and the defect implements the Kramers-Wannier self-duality of the Ising CFT. This talk will be about an analogue of this story for some examples of (3+1)d continuum QFTs, e.g. N=1 SO(3) SYM. In particular I will explicitly construct topological defects associated to self-dualities under gauging a one-form (aka center) symmetry of QFTs.

On the course, I will also touch about the 1+1 dimensional SU(3) adjoint QCD which possesses similar KW-like “triality” topological defect and its interesting consequence.

references: https://arxiv.org/abs/2111.01141, https://arxiv.org/abs/2008.07567

December 17

Confinement and Chiral Symmetry Breaking in Near-SUSY Theories 

Ofri Telem (UC. Berkeley)

Zoom, 11:00 - 12:00

Abstract: 

The study of non-supersymmetric, strongly coupled gauge theories is notoriously hard, and exact results are scarce. In this talk, I present a new way to study the vacua of near-supersymmetric theories by perturbing their supersymmetric versions with Anomaly Mediated Supersymmetry Breaking (AMSB). Thanks to the UV-insensitivity of AMSB, SUSY results can be perturbed with no ambiguities, even when applied to composite fields. Using our method, we analyze the phases of $SO(N)$ gauge theory with flavors. The abelian Coulomb and free magnetic phases do not survive supersymmetry breaking and collapse to a confining phase, providing one of the first demonstrations of true confinement with chiral symmetry breaking in a non-supersymmetric vacuum. Our results are exact in the small SUSY breaking limit, and we briefly discuss the prospects of extrapolating them to large SUSY breaking.

December 3

Higher form symmetry and its application to spontaneous symmetry breaking and axion electrodynamics 

Yoshimasa Hidaka (KEK)

Zoom, 16:00 - 17:00

Abstract: 

Ordinary symmetries are symmetries acting on point particles. The concept can be generalized to symmetries acting on extended objects, such as vortices and domain walls. Such symmetries are called higher form symmetries. In this talk, I introduce the concept of higher form symmetries and the current status of generalized global symmetries including higher groups and non-invertible symmetries. As an example of the application, I discuss spontaneous breaking of higher form symmetries in a nonrelativistic system and the axion electrodynamics which exhibits a higher group structure.

November 26 

Diffractive dissociation in high-energy electron-nucleus scattering 

Anh Dung Le (CPHT, Ecole Polytechnique)

Zoom, 17:00 - 18:00

Abstract:

Diffractive dissociation in electron-nucleus scattering is the phenomenon in which the virtual photon mediating the interaction fluctuates into a set of partons part of which goes into the final state while the nucleus is kept intact, leaving a rapidity gap between the nucleus and the produced particles. It turns out that diffractive events are due to specific scattering partonic configurations, and the rapidity gap distribution is related to the statistics of those configurations. In this talk, I will present our recent results on diffractive electron-nucleus scattering in both analytical and numerical aspects. On the analytical side, we investigated the partonic mechanism of diffraction, which enabled us to derive a complete asymptotic expression for the rapidity gap distribution. We also made some predictions for diffraction at future electron-ion colliders, based on the numerical solutions to the QCD high-energy evolution equations.

November  19

Anomaly inflow and the η invariant 

Kazuya Yonekura (Tohoku University)

Zoom, 16:00 - 17:00

Abstract: 

I will explain a modern understanding of anomalies, which can describe nonperturbative anomalies. In the case of fermions, I sketch the derivation of the formula for anomalies in terms of the so-called Atiyah-Patodi-Singer η invariant which generalizes Chern-Simons invariant. A higher dimensional bulk theory and the anomaly inflow mechanism plays the crucial role. The talk is based on https://arxiv.org/abs/1909.08775

November 12 

Double Holography and Its Recent Progress 

Zixia Wei (YITP)

Zoom, 16:00 - 17:00

Abstract: 

Double holography is a holographic correspondence which has three equivalent descriptions: 1) a d-dimensional BCFT (boundary CFT) without gravity; 2) a d-dimensional AdS gravity coupled to a d-dimensional CFT through an interface; 3) a d+1-dimensional AdS gravity with an end-of-the-world brane. Double holography has been playing a crucial role in recent studies of Hawking radiation and information paradox, since it relates description 2), in which a black hole in AdS can evaporate by emitting radiation to a heat bath, to two much better-understood descriptions. In this talk, I will firstly review basic concepts and results in double holography. Then I will introduce some recent progresses of doubly holographic models in which I was involved [1-4].

[1] I. Akal, Y. Kusuki, T. Takayanagi and Z. Wei, “Codimension two holography for wedges”, Phys. Rev. D 102 (2020) 126007 [2007.06800].

[2] I. Akal, Y. Kusuki, N. Shiba, T. Takayanagi and Z. Wei, “Entanglement Entropy in a Holographic Moving Mirror and the Page Curve”, Phys. Rev. Lett. 126 (2021) 061604 [2011.12005].

[3] I. Akal, Y. Kusuki, N. Shiba, T. Takayanagi and Z. Wei, “Holographic Moving Mirrors”, Class. Quant. Grav. 38 (2021) 22, 224001 [2106.11179]

[4] H. Omiya and Z. Wei, “Causal Structures and Nonlocality in Double Holography”, [2107.01219]

November  5

Hadrons in nuclear matter from QCD and experimental data

Philipp Gubler (JAEA)

Zoom, 16:00 - 17:00

Abstract: 

Studying the behavior of hadrons in nuclear matter from the first principles of QCD is a longstanding and still not fully resolved problem, caused in part by the difficulty of applying lattice QCD simulations to systems with a non-zero chemical potential. Analogously, it has been difficult to extract unambiguous conclusions from experimental studies of hadrons in matter because of the scarcity of clear experimental signals and problems in the correct interpretation of the available experimental data.

In this talk, I will give an overview of recent developments in this field, especially focusing on the study of the phi meson in nuclear matter and my own theoretical work based on QCD sum rules [1,2]. I will also give an introduction to the ongoing experiments at J-PARC [3], using pA (proton-nucleus) reactions to generate vector mesons in nuclei and how such reactions can be simulated using a transport approach [4,5].   

[1] PG, K. Ohtani, Phys. Rev. D 90, 094002 (2014).

[2] H.J. Kim, PG, Phys. Lett. B 805, 135412 (2020).

[3] S. Ashikaga et al., (J-PARC E16 Collaboration), JPS Conf. Proc. 26, 024005 (2019).

[4] PG, E. Bratkovskaya, in progress.  

[5] W. Cassing and E.L. Bratkovskaya, Nucl. Phys. A 831, 215 (2009).

October 29 

Revisiting Wess-Zumino-Witten terms

Yasunori Lee (IPMU, the Univeristy of Tokyo)

Zoom, 16:00 - 17:00

Abstract: 

Wess-Zumino-Witten (WZW) terms are topological terms in the non-linear sigma models which are believed to be the low-energy effective descriptions of QCDs in 3+1d. It turns out that there are various subtle issues concerning these WZW terms, some of which cannot be fully understood from a conventional cohomology or homotopy point of view. In this talk, I will revisit such issues from a modern bordism point of view, and explain for example that the definition of WZW terms actually requires the spin structure on the spacetime manifold.

October 15

Field theoretical analysis of SU(3) antiferromagnetic triangular lattice 

Itsuki Takahashi (Kyoto University)

Zoom, 16:00 - 17:00

Abstract:

SU(3) antiferromagnet is a generalization of usual SU(2) spins system which is well known as Heisenberg model. Antiferromagnetic spins on two spatial dimensional lattice is expected to support quantum phase transition between Néel and VBS phases, and has been investigated for several decades.

We have recently studied SU(3) antiferromagnetic triangular lattice based on analytical calculation from the Néel order side. Various field theoretical methods, including gauge theory, skyrmion, and anomaly matching, are used there. Especially, topological properties play an important role in the lattice theory. It was revealed that skyrmion tunneling events drastically affect its phase, depending on the spin representation. In this talk, I will show how to formulate the low-energy effective theory and discuss its possible phases.

July 2

Chiral gravitational effect in cosmology

Jun'ya Kume (the University of Tokyo)

Zoom, 16:00 - 17:00

Abstract:

In this talk, I present "chiral gravitational effect" (CGE) which originates from the chiral gravitational anomaly, and can be understood as a gravitational counterpart of the chiral magnetic effect (CME). Since the anomaly connects the left-right asymmetry of the fermions with the helicity of the gravitational waves, there is a non-trivial interplay between the gravitational waves and the chiral plasma through this effect. While the static fermion background was assumed in the pioneer work of this effect, non-trivial time dependence of the background needs to be taken into account for the application to cosmology. We show that such an extension can be done by the effective theoretic description. While comparing it with CME, I present the general features of CGE in time dependent backgrounds. 

June 25

Quantum phase transition and Resurgence: Lessons from 3d N=4 SQED

Takuya Yoda (Kyoto University)

Zoom, 16:00 - 17:00

Abstract:

We study a resurgence structure of a quantum field theory with a phase transition to uncover relations between resurgence and phase transitions. In particular, we focus on three-dimensional N=4 supersymmetric quantum electrodynamics (SQED) with multiple hypermultiplets, where a second-order quantum phase transition has been recently proposed in the large-flavor limit. We provide interpretations of the phase transition from the viewpoints of Lefschetz thimbles and resurgence. For this purpose, we study the Lefschetz thimble structure and properties of the large-flavor expansion for the partition function obtained by the supersymmetric localization. We show that the second-order phase transition is understood as a phenomenon where a Stokes and anti-Stokes phenomenon occurs simultaneously. The order of the phase transition is determined by how saddles collide at the critical point. In addition, the phase transition accompanies an infinite number of Stokes phenomena due to the supersymmetry. These features are appropriately mapped to the Borel plane structures as the resurgence theory expects. Given the lessons from the SQED, we provide a more general discussion on the relationship between the resurgence and phase transitions. In particular, we show how the information on the phase transition is decoded from the Borel resummation technique.

June 18

Femtoscopic study on the hadron-hadron interaction

Yuki Kamiya (Institute of Theoretical Physics, CAS)

Zoom, 16:00 - 17:00

Abstract:

Recently, the femtoscopic technique using the momentum correlation function has getting much attention as a new method to study the hadron-hadron interaction. The momentum correlation function is well described by the convolution of the emitting source function and the relative wave function in the pair rest frame and is suitable to investigate the low-energy hadron-hadron interaction of short-lived hadron pairs. In this seminar, we discuss how we can extract the information on the hadron interaction from correlation functions in detail. The theoretical and experimental situation of the various hadron pairs will be reviewed. Finally, we discuss the feature prospects and the required extension for the further study. 

June 11

Tensor networks -- Lagrangian approach

Shinji Takeda (Kanazawa University)

Zoom, 16:00 - 17:00

Abstract:

Tensor networks are powerful method to study quantum many-body system and their striking feature is free of the sign problem which is a serious drawback of Monte Carlo method. In my talk, first we review tensor networks focusing on Lagrangian (path integral) approach which is useful to study quantum field theory in high energy physics. As an example of the approach, we talk about study of two dimensional complex scalar field theory at finite density. Finally we address remaining issues and discuss future prospect of tensor networks.

May 14

Resumming non-global logarithms in hadron collisions for Nc = 3

Takahiro Ueda (Seikei University)

Zoom, 16:00 - 17:00

Abstract:

Soft gluon radiation causes non-global logarithms (NGLs) when measurements are limited to a part of the full phase space. As usual logarithmically enhanced terms, if there is a large hierarchy between the hard and soft scales, then NGLs can break down a perturbative expansion and hence all-order resummation is needed. The resummation of NGLs beyond the large-Nc limit is a nontrivial task, even at the leading-logarithmic level. Some years ago, we developed a framework to resum NGLs at the finite Nc in the leading-logarithmic approximation, which can be implemented as a numerical code and has been applied to observables in electron-positron annihilation. In this talk, based on our recent work, I will discuss applications of the framework to the Higgs decay and Higgs-plus-dijet production in proton-proton collisions, where the results involve higher "color multipoles".

2020

November 30

Wigner functions and quantum kinetic theory of polarized photons

Di-Lun Yang (Keio university)

Zoom, 16:00 - 17:00

Abstract:

Quantum transport of circularly polarized photons is a fundamental issue in various areas of physics. Particularly, it is essential to develop a transport theory to delineate the spin transport of polarized photons. We derive the Wigner functions of polarized photons in the Coulomb gauge with the hbar expansion applied to quantum field theory, and identify side-jump effects for massless photons. We also discuss the photonic chiral vortical effect for the Chern-Simons current and zilch vortical effect for the zilch current in local thermal equilibrium. Moreover, using the real-time formalism, we construct the quantum kinetic theory (QKT) for polarized photons. By further adopting a specific power counting scheme for the distribution functions, we provide a more succinct form of an effective QKT. This photonic QKT involves quantum corrections associated with self-energy gradients in the collision term, which are analogous to the side-jump corrections pertinent to spin-orbit interactions in the chiral kinetic theory for massless fermions. The same theoretical framework can also be directly applied to weakly coupled gluons in the absence of background color fields.  

November 24

Chiral kinetic theory of anomalous transport induced by torsion

Lan-lan Gao (Fudan university, Stony Brook University)

Zoom, 10:00 - 11:00

Abstract:

For the case of Dirac/Weyl particles in the electromagnetic field, the previous explanation of chiral anomaly is related to the Berry curvature and the field curvature. However when we deformed a Weyl semimetal, the Berry connections are mixed in coordinate space and momentum space. To solve it, I will introduce a phase space description of Berry curvature and show the anomaly is due to "monopole charge function" in phase space. 

In condensed matter system, torsion is also a kind of a deformation. Deformation will change the shape of Weyl cone and induce a synthetic electromagnetic field by shift the weyl point. I will show the topologically protected chiral anomaly and anomalous transport are independent of the shape. And I will also show the synthetic electromagnetic field induced by torsion and compression can lead to a chiral chemical potential. So, the anomalous current finally show up in this case with a magnetic field.

October 19

Complex Langevin simulations of finite density QCD

Shoichiro Tsutsui (RIKEN)

Zoom, 16:00 - 17:00

Abstract:

Complex Langevin method (CLM) is one of a promising way to evade sign problems, and it have been applied to condensed matters, cold atoms and string theories. In this talk, I present recent progress of complex Langevin simulations of finite density QCD.

First, I clarify the applicability and limitation of the CLM. On the basis of a criterion proposed in Ref. [1], the CLM is not guaranteed if the probability distribution of the drift term of the Lagevin equation shows the power-law decay. This issue occurs, for instance, when the Dirac operator has near-zero eigenvalues. In the thermodynamic and chiral limits, these eigenvalues are directly related to chiral condensates due to the generalized Banks-Casher relation [2]. Therefore, in these limits, the CLM will be applicable only in the chiral symmetry restored phases. I demonstrate this scenario by numerical simulations on a 24^3×12 lattice with mπ > 520 Mev [3].

Second, I propose several lattice setups where conventional methods are not applicable due to the severe sign problem. A candidate is a lattice with small aspect ratios and fine lattice spacings, in which the Dirac spectrum would have a gap. I show numerical results performed on 8^3×16 and 16^3×32 lattices for μ/T=1.6-9.6.

In particular, I show the expectation value of the quark number have a plateau with respect to μ with the height of 24 (=3 (color) × 4 (flavor) × 2 (spin)) for both lattices [4]. I also discuss strategies to find chiral symmetry restoration at finite density and color superconductivity.

[1] J. Nishimura and S. Shimasaki, Phys. Rev. D92, 011501 (2015),  K. Nagata, J. Nishimura, and S. Shimasaki, Phys. Rev. D94, 114515 (2016).

[2] K. Splittorff, Phys. Rev. D91, 034507 (2015),  K. Nagata, J. Nishimura, and S. Shimasaki, JHEP 07, 073 (2016).

[3] ST, Y. Ito, H. Matsufuru, J. Nishimura, S. Shimasaki, and A. Tsuchiya, PoS LATTICE2018, 144 (2018). 

[4] Y. Ito, H. Matsufuru, J. Nishimura, S. Shimasaki, A. Tsuchiya and ST, arXiv:2007.08778 (to be published in JHEP).

July 20

 Zilch and chiral kinetic theory of photon

Xu-Guang Huang (Fudan University)

Webex, 15:00- 16:00

Abstract:

For sourceless Maxwell equations, there exist a tower of parity-odd conserved currents called Zilch. I will review some general properties of Zilch and its experimental significance in optics. In a photon gas, the rotation can induce a Zilch counterpart of the chiral vortical effect. I will discuss how such Zilch vortical effect is understood from the kinetic point of view.

July 14

 Chaos of interquark force via holography

Tetsuya Akutagawa (Osaka University)

Webex, 14:00- 15:00

Abstract:

As the low energy of QCD, spectra of hadrons exhibit quantum chaos. However, we do not know what is the origin of the chaos. Because we cannot evaluate sensitivity to the initial conditions in the quantum system, it is difficult to quantitatively analyze the chaos. One solution is a measure of the classical chaos via holography. We investigate the chaos of the interquark force in the large N_c QCD by using holography. We find that the interquark force is less chaotic for larger interquark distance and this result is well approximated by a hypothesis that the chaos originates in the endpoints of the QCD string.

July 7

 Low-dimensional fluctuations in strongly interacting Fermi gases with a population imbalance

Hiroyuki Tajima (Kochi University)

Webex, 16:00- 17:00

Abstract:

Ultracold atoms provide us with an ideal platform to investigate strongly correlated many-body systems due to the tunability of various physical parameters such as interactions and dimensionality. One of the most important problems in this field is a gas of strongly interacting two-component fermions with population imbalance [1]. While this system is expected to exhibit a variety of non-trivial phases such as Fulde-Ferrell-Larkin-Ovchinnikov state, the overall phase diagram is still unclear. From a theoretical viewpoint, a main difficulty originates from the so-called sign problem which occurs in the presence of the population imbalance [2]. In this talk, we present our theoretical study of an attractively interacting nonrelativistic one-dimensional system based on a diagrammatic approach at finite temperature [3]. Our results of thermodynamic quantities show an excellent agreement with those of a quantum Monte Carlo simulation in the balanced case and a complex Langevin method in the imbalanced case. Examining single-particle excitation spectra, we predict the remarkable pseudogap effect induced by strong pairing fluctuations. We also report the current progress of our complex Langevin study in this system. 

[1] D. E. Sheehy and L. Radzihovsky, Ann. Phys. 322, 1790 (2007).

[2] J. E. Drut, J. Phys. Conf. Ser. 1041, 012005 (2018).

[3] H. Tajima, S. Tsutsui, and T. M. Doi, arXiv:2005.12124

June 29

 Hadronic Paschen-Back effect under strong magnetic field

Sachio Iwasaki (Tokyo Institute of Technology)

Webex, 16:00- 17:00

Abstract:

QCD dynamics under a strong magnetic field is of great interest to the field of relativistic heavy-ion collisions and magnetars. In this talk, I will discuss a new effect we recently found in Ref.[1], 'Hadronic Paschen-Back effect (HPBE)', which is analogous to the Paschen-Back effect observed in atomic physics. This effect is induced by the interplay between a strong magnetic field and finite orbital angular momenta in hadronic systems. It allows the wave functions to drastically deform and leads to anisotropic decays. Such a decay gives a possibility to measure the strength of the magnetic field in heavy-ion collision at LHC, RHIC and SPS, which has not experimentally been measured. As an example of HPBE, I will report our results [1] of the mass spectra, wave functions, and mixing ratios of P-wave charmonia in a wide range of magnetic fields by using the potential model and a numerical few-body technique.

Furthermore, I will talk about a systematic study for the radiative decays of P-wave quarkonia by HPBE based on potential non-relativistic QCD in Ref.[2].

[1] S. Iwasaki, M. Oka, K. Suzuki, T. Yoshida, “Hadronic Paschen-Back effect,” arXiv:1802.04971 

[2] S. Iwasaki, K. Suzuki, “Quarkonium radiative decays from the Hadronic Paschen-Back effect,” Phys. Rev. D98, 054017 (2018)

May 25

Quantum criticality of magnetic catalysis in (2+1) dimensions

Yasuhiro Tada (University of Tokyo)

Webex, 14:00-15:00

Abstract:

Interacting Dirac fermions exhibit chiral symmetry breaking in presence of a magnetic field, which is known as "magnetic catalysis". The magnetic catalysis has been studied not only in hadron physics, but also in condensed matter physics where there are various kinds of Dirac semimetals such as graphene. Although it has been extensively studied in both research fields, the magnetic field dependence near a chiral phase transition point is rather poorly explored. In this study, we investigate quantum criticality of magnetic catalysis in a simple lattice model  (staggered fermions with a four-fermion interaction) by a non-perturbative numerical calculation with density matrix renormalization group. We show that the order parameter at the quantum critical point of Z2 symmetry breaking is well characterized by the (2+1)-dimensional chiral Ising universality class. We also establish a phase diagram in the interaction-magnetic field plane. 

Ref: YT, arXiv:2005.01990.

May 18

Study of a method of measure of free energy in quantum field theories

Matteo Zoccolan (University of Tokyo)

Webex, 16:00-17:00

Abstract:

The aim is to study the  efficiency of a new method to calculate free energy differences in a physical system. Here the method is applied to the case of pure gauge QCD, or SU(3) Yang-Mills theory, on a lattice. Usually to calculate the free energy difference between two systems of actions S1 and S2 one takes ratios of partition functions of a system varying with small steps of a parameter λ between the two actions. Partition functions becomes more peaked on larger volumes, thus needing smaller steps and leading to a lost of efficiency of the calculation. The new method does not suffer from the above problem since it does not depend on the volume of the system. Instead I take the derivative of the free energy in the varying parameter λ and then I integrate this function in λ, this integral is the free energy difference between the two systems. The integral is performed with the gaussian quadrature method that reduces the number of values that has to be evaluated. I wrote a code to perform numerical simulations of the SU(3) Yang-Mills theory on a lattice. The code has been executed on Milano Bicocca University's Wilson cluster using parallel computing commands to speed up the gathering of data. Finally I checked if my results agreed with those available in the literature.

Mar 2

Dark Matter Heating vs. Rotochemical Heating in Old Neutron Stars

Keisuke Yanagi (University of Tokyo)

Room 907, 15:00-16:00

Abstract:

WIMP dark matters (DMs) in the Universe accumulate in neutron stars (NSs) through their interactions with nucleons. It has been known that their annihilation inside the NS core causes late-time heating, with which the surface temperature is kept at T_s ≃ (2−3)×10^3 K for the NS age t>10^6−10^7 years. Without the DM heating, such old NSs become much colder than 10^3 K. Thus the measurement of NS surface temperature can be used as a new probe of WIMP DMs. Because of the strong gravity of NSs, DM heating has several advantages compared to the direct detection experiments on the earth.

Despite such advantages, potential heating sources other than the DM heating is rarely discussed in the particle physics community. In particular, the rotochemical heating, internal heating caused by the out-of-equilibrium beta reactions in a NS, should be considered because it does not assume any exotic physics. In fact, if the rotochemical heating operates in a NS, it can conceal the DM heating effects.

In our work, we reevaluate the significance of the DM heating in NSs, including the effect of the rotochemical heating. We first show that the rotochemical heating actually explains observed old warm NSs. Then we compare DM heating to rotochemical heating, and show that the signature of DM heating can still be detected in old ordinary pulsars.

2019

November 29

Topological order in the color-flavor locked phase of (3+1)-dimensional U(N) gauge-Higgs system

Ryo Yokokura (KEK)

Room 907, 16:00-17:00

Abstract:

We study a (3+1)-dimensional U(N) gauge theory with N-flavor fundamental scalar fields, whose color-flavor locked (CFL) phase has topologically stable non-Abelian vortices. The U(1) charge of the scalar fields must be Nk+1 for some integer k in order for them to be in the representation of U(N) gauge group. This theory has a Z_{Nk+1} one-form symmetry, and it is spontaneously broken in the CFL phase, i.e., the CFL phase is topologically ordered if k is not 0. We also find that the world sheet of topologically stable vortices in CFL phase can generate this one-form symmetry.

July 23

Tidal deformability, and new relativistic models IOPB-I and G3

Bharat Kumar (Tsukuba University)

Room 907, 16:00-17:00

Abstract:

In the first part of my talk, I will discuss the tidal deformability for neutron and hyperon stars using relativistic mean field equations of state. The second part of my talk , we will discuss two new parameter sets for the energy density functional such as G3 and IOPB-I for finite nuclei, and infinite nuclear matter system within the effective field theory motivated relativistic mean field (ERMF) formalism. The isovector part of the ERMF model employed in the present study includes the coupling of nucleons to the δ and ρ mesons and the cross-coupling of ρ mesons to the σ and ω mesons. The results for the finite and infinite nuclear systems obtained using our parameter sets are in harmony with those data extracted from various experiments. In particular, the neutron-skin thickness of 208Pb nucleus and canonical radius of the neutron star are compatible with the GW170817. The low-density behavior of the equation of state for pure neutron matter is in good agreement with other microscopic models. Also, we calculate the maximum mass, and tidal deformability which are in quite well with the GW170817 as well as with the pulsar data.

June 28

In-medium quarkonium as open quantum system: from potential to real-time dynamics

Alexander Rothkopf (University of Stavanger)

Room 907, 16:00-17:00

June 21

Axial anomaly and hadronic properties in a nuclear medium

Gergely Fejős (Keio University)

Room 907, 16:00-17:00

Abstract:

I will show recent results on meson and nucleon dynamics at finite baryon density and temperature, by coupling the nucleon field and the omega meson to the three-flavor linear sigma model. Using the functional renormalization group (FRG) method, I will show how to calculate hadronic properties at the nuclear liquid-gas transition and argue that mesonic fluctuations increase the strength of the coefficient of the U_A(1) breaking determinant operator. Density dependence of the meson masses and partial restoration of chiral symmetry will also be discussed.

May 10

Anomalous effects of dense matter under rotation

Kentaro Nishimura (Keio University)

Room 907, 16:00-17:00

Abstract:

We study the anomaly induced effects of dense baryonic matter under rotation. We derive the anomalous terms that account for the chiral vortical effect in the low-energy effective theory for light Nambu-Goldstone modes. The anomalous terms lead to new physical consequences, such as the anomalous Hall energy current and spontaneous generation of angular momentum in a magnetic field (or spontaneous magnetization by rotation). In particular, we show that, due to the presence of such anomalous terms, the ground state of the quantum chromodynamics (QCD) under sufficiently fast rotation becomes the “chiral soliton lattice” of neutral pions that has lower energy than the QCD vacuum and nuclear matter. We briefly discuss the possible realization of the chiral soliton lattice induced by a fast rotation in noncentral heavy ion collisions.

April 19

Sign problem in canonical approach -introduction to activities of FEFU group-

Hideaki Iida (University of Tokyo)

Room 907, 16:00-17:00

Abstract:

In this talk, I will introduce the recent activities of the group of particle physics at Far Eastern Federal University in Russia. One of the main subjects of their researches is the sign problem in the canonical approach. I will briefly mention what is the canonical approach and its history. After that, I will talk about a recent study of Lee-Yang zeros from lattice QCD at finite density in the canonical approach, by which we can know the information of phase transition.

February 8

Soliton-induced non-thermal fixed points in a one-dimensional antiferromagnetic spin-1 Bose gas

Kazuya Fujimoto (University of Tokyo)

Room 907, 16:00-17:00

Abstract:

Ultracold atomic gases have been excellent testbeds for studying universal non-equilibrium phenomena. Actually, thanks to the high controllability and feasibility, several experiments have successfully observed a variety of universal dynamics such as the Kibble-Zurek mechanism, phase ordering dynamics, and turbulent cascades. Recently, a non-thermal fixed point (NTFP), which is a universal thermalization scenario originally proposed in quark-gluon plasmas, has attracted great interest in the field of ultracold atomic gases because signatures of NTPFs are experimentally observed in one-dimensional (1D) scalar and spinor Bose gases [1,2]. However, the mechanisms for the emergence of the 1D NTFPs have been open. In this seminar, we show our recent results for relaxation dynamics in a 1D antiferromagnetic spinor Bose gas [3]. By using the truncated Wigner approximation, we find scale-invariant relaxation dynamics consistent with the NTFP scenario. Furthermore, we uncover that magnetic soliton excitations play a crucial role in the emergence of universal relaxation dynamics.

2018

November 16

Inhomogeneous chiral condensate and vortex state in rotating frame

Lingxiao Wang (University of Tokyo)

Room 907, 16:00-17:30

Abstract:

In this work, we try to calculate the inhomogeneous chiral condensate in a finite space self-consistently. For that purpose, we begin with U(1) symmetry Nambu--Jona-Lasinio (NJL) model at chiral limit, and derive the Bogoliubov-de Gennes (BdG) theory [1] in finite rotating system. In 2+1 dimensions system [2], the renormalized NJL model gives the influence of the boundary on the condensate. The inhomogeneous scalar condensate fluctuates near the boundary [3-4], which influences inside slightly. The angular velocity plays the similar role as the baryon chemical potential and suppresses the chiral condensate in the temperature-angular velocity phase diagram [4-6]. One found is that the system with vortex state dominates under finite rotation [4, 7], which is an exotic topological state in strong interaction system.

October 26

Topological classification of non-Hermitian insulators and superconductors

Kohei Kawabata (University of Tokyo)

Room 907, 16:00-17:30

Abstract:

Topological phases of matter have been widely explored in equilibrium closed systems [1], but richer properties appear in nonequilibrium open systems that are effectively described by non-Hermitian Hamiltonians [2]. While several unique properties were uncovered, no research has established a comprehensive theoretical framework for non-Hermitian topological systems. In this seminar, we discuss the topological classification of non-Hermitian insulators and superconductors [3-5], as a generalization of the tenfold classification of Hermitian systems [6-8]. After clarifying symmetry and energy gaps for non-Hermitian Hamiltonians, we provide the periodic table that classifies all the non-Hermitian topological systems in a general manner.

June 8

On thermal effective potential of pure gauge theory at large N

Hiromichi Nishimura (RBRC)

Room 907, 15:00-16:30

Abstract:

In this talk we will discuss the effective potential in terms of the Polyakov loop in the limit of large number of colors N.  We show that it is analytically more tractable at infinite N and argue that the effective potential becomes a simple function of the Polyakov loop.  We will demonstrate this by showing the known perturbative two-loop calculation as well as some partial results of the three-loop contributions. 

May 24

How curved geometry emerges in hydrodynamics

Masaru Hongo (RIKEN)

Room 907, 11:00-12:30

Abstract:

Finite temperature quantum systems can be described by the imaginary-time formalism, where the path integral of the Euclidean action appears. However, it is only applicable to globally thermalized systems, and we have to generalize the imaginary-time formalism to describe locally thermalized systems, e.g. QGP created in high-energy heavy-ion collisions. In this talk, I will explain how we can describe the locally thermalized quantum systems with the help of curved geometry, and show a modern derivation of nondissipative transport including anomaly-induced transport such as the chiral magnetic effect.

April 27

Fundamental physics for quantum many-body systems investigated by cold atom experiments

Munekazu Horikoshi (University of Tokyo)

Room 907, 16:00-17:30

Abstract:

In a low-energy quantum system, particles behave as matter wave with quantum statistics, and the influence of the inter-particle interactions appears as the symmetry and the phase shift of the scattering wave. While the scattering length is a parameter describing the phase shift, it is an important physical quantity giving the coupling constant between two particles. Since the scattering length is controllable in cold atomic systems by using Feshbach resonances, we can study various interacting quantum many-body systems experimentally in a wide range of coupling constants. Especially, the unitary regime, where the scattering length diverges, are interested in various quantum fields from quark matter to neutron matter, since they have similar low-energy scattering conditions. In this seminar, we deal with the spin-1/2 Fermi particle system interacting with tunable s-wave scattering length. Firstly, we confirm the physical meaning of the scattering length and various universal physical laws described by s-wave scattering length. Secondly, we will introduce an experiment that investigated the equation of state (EOS) of fermions from the BCS regime to the unitary regime using ultracold 6Li atomic gases [1]. Finally, we show application of the EOS to dilute neutron matter.

2017

December 15

Out-of-time-order correlators in quantum mechanics

Keiju Murata (Keio University)

Room 907, 16:00-17:30

Abstract:

The out-of-time-order correlator (OTOC) is considered as a measure of quantum chaos. We formulate how to calculate the OTOC for quantum mechanics with a general Hamiltonian. We demonstrate explicit calculations of OTOCs for a harmonic oscillator, a particle in a one-dimensional box, a circle billiard and stadium billiards. For the first two cases, OTOCs are periodic in time because of their commensurable energy spectra. For the circle and stadium billiards, they are not recursive but saturate to constant values which are linear in temperature. Although the stadium billiard is a typical example of the classical chaos, an expected exponential growth of the OTOC is not found. We also discuss the classical limit of the OTOC. Analysis of a time evolution of a wavepacket in a box shows that the OTOC can deviate from its classical value at a time much earlier than the Ehrenfest time.

December 5

Learning Disordered Topological Phases by Statistical Recovery of Symmetry

Nobuyuki Yoshioka (University of Tokyo)

Room 907, 16:00-17:30

Abstract:

In machine learning, computational algorithms are constructed and executed to optimize the quantified objective of the problem to be solved. The surging development of the state-of-the-art techniques has led condensed matter physicists to realize the effectiveness of the tools in their own research field, such as phase classification [1,2], solving quantum many-body problem [3,4] and speeding up the Monte Carlo simulation [5]. 

Firstly we start with basic information to understand the classification by the artificial neural network (ANN), which maps out in our work the quantum phase diagram of disordered topological superconductor in class DIII. Given the disorder that keeps the discrete symmetries of the ensemble as a whole, translational symmetry which is broken in the quasiparticle distribution individually is recovered statistically by taking an ensemble average. By using this, we classify the phases by the ANN that learned the quasiparticle distribution in the clean limit and show that the result is totally consistent with the calculation by another independent approach.

If all three phases, namely the Z2, trivial, and the thermal metal phases appear in the clean limit, the machine can classify them with high confidence over the entire phase diagram. If only the former two phases are present, we find that the machine remains confused in the certain region, leading us to conclude the detection of the unknown phase which is eventually identified as the thermal metal phase.

November 28

Filling and Symmetry-Based Indicator of Many-Body Chern Number

Haruki Watanabe (University of Tokyo)

Room 907, 16:00-17:30

Abstract:

The topology of quantum many-body systems are deeply related to the average number of particles of the system and/or the symmetry representation of the ground state. One of the most established example of this relation is the so-called (Hasings-Oshikawa-) Lieb-Schultz-Mattis theorem, a theorem that applies to a very wide class of Hamiltonian regardless of the interaction strength or the spatial dimension of the system.  In this talk, I will first overview the HOLSM theorem and then discuss recent advances concerning the many-body Chern number.

The talk will be based on

A. Matsugatani, Y. Ishiguro, K. Shiozaki, and H. Watanabe, arXiv:1710.07012.

November 24

Electrodynamics of Chiral Matter

Zebin Qiu (University of Tokyo)

Room 907, 16:00-17:30

Abstract:

A many-body system with chiral fermions can exhibit novel transport phenomena that violate parity and time-reversal symmetries, such as the chiral magnetic effect, the anomalous Hall effect, and the anomalous electric charge. We study the electromagnetic and optical properties of such systems by examining the electromagnetic sector of the Chern-Simons theory. Based on the Maxwell-Chern-Simons equations, we derive modified laws for the generation of static electromagnetic fields, and the propagation of electromagnetic waves.

October 20

Condensed Matter Application of Lattice QCD 

Arata Yamamoto (University of Tokyo)

Room 907, 16:00-17:30

October 6

Exact resurgent trans-series and all-order multi-bion contributions in CP^N quantum mechanics

Toshiaki Fujimori (Keio University)

Room 907, 16:00-17:30

Abstract:

I will discuss resurgence structures in CP^N quantum mechanics derived from a supersymmetric model with a small SUSY breaking deformation parameter. By using the standard Rayleigh-Schrodinger perturbation theory, we exactly determine how the ground state energy respond to the deformation. The full resurgent trans-series expressions for the expansion coefficients of the ground state energy are found exactly in CP^1 quantum mechanics. We then discuss the semi-classical bion contributions in the complexified path integral formalism and compare them with the exact results. We determine all exact saddle point solutions corresponding to multi-bion configurations in the complexified theory. By evaluating the complexified quasi-moduli integral, we obtain all order multi-bion contributions which are consistent with the exact results.

June 23

Nuclear Equation of State for Core-Collapse Supernovae

Shun Furusawa (RIKEN)

Room 907, 16:00-17:30

Abstract:

The core-collapse supernovae are one of the most fascinating phenomena in astrophysics. The mechanism of these events is not clearly understood yet because of their intricacies. One of the underlying problems is uncertainties in the equations of state of hot and dense matter. In this talk, I will give an overview about the nuclear equation of state and its roles in the numerical simulations of supernovae. If time permits, I also introduce a recent study about transitions of hadronic matter to three-flavor quark matter, which may be realized during or after core-collapse supernovae.

June 9

New dynamic critical phenomena in nuclear and quark superfluids

Noriyuki Sogabe (Keio University)

Room 907, 16:00-17:30

Abstract:

We study the static and dynamic critical phenomena near the possible high-density QCD critical point in the superfluid phase of nuclear and quark matter. In particular, we find that its dynamic universality class is different from those studied in QCD and condensed matter systems so far. We argue that this novelty stems from the interplay between the chiral criticality and the presence of the superfluid phonon---a feature specific for high-density QCD critical point.

May 8

Transport coefficients of QGP in strong magnetic fields

Daisuke Satow (Goethe Universität)

Room 907, 16:00-17:30

Abstract:

We compute the transport coefficients in magnetohydrodynamics at finite temperature, in strong magnetic fields (B), which is expected to be generated in heavy ion collision. We use the lowest Landau level approximation, in which the 1-to-2 scattering process is kinematically allowed in contrast to the B=0 case. We find that this effect of the magnetic field is significant in the electrical conductivity and the bulk viscosity: These quantities become sensitive to the current quark mass, and we discuss its physical origin in terms of the chirality conservation and the conformal invariance. This presentation is partially based on Phys. Rev. D 94, 114032 (2016) and arXiv:1610.06839 [hep-ph].

April 7

Phases of circle-compactified QCD with adjoint fermions at finite density

Takuya Kanazawa (RIKEN)

Room 907, 16:00-17:30

Abstract:

We study chemical-potential dependence of confinement and mass gap in QCD with adjoint fermions in spacetime with one spatial compact direction. By calculating the one-loop effective potential for the Wilson line in the presence of chemical potential, we show that a center-symmetric phase and a center-broken phase alternate when the chemical potential in unit of the compactification scale is increased. In the center-symmetric phase we use semiclassical methods to show that photons in the magnetic bion plasma acquire a mass gap that grows with the chemical potential as a result of anisotropic interactions between monopole-instantons. For the neutral fermionic sector which remains gapless perturbatively, there are two possibilities at non-perturbative level. Either to remain gapless (unbroken global symmetry), or to undergo a novel superfluid transition through a four-fermion interaction (broken global symmetry). If the latter is the case, there exists a new type of BEC-BCS crossover of the diquark pairing across 3 and 4 dimensions.

March 27

Chiral magnetic effect in multi-Weyl semimatals

Tomoya Hayata (Chuo University)

Room 907, 16:00-17:30

Abstract:

New topological phase of matters named Weyl semimetal has attracted growing attention in recent years. In a Weyl semimetal, Weyl fermions are realized as low-energy excitations near band touching points called Weyl points. The Weyl points act as monopoles in momentum space, and lead to topological transport phenomena such as the chiral magnetic effect. Since condensed matter system does not have Lorentz and rotational symmetries, there appear Weyl excitations with nonrelativistic dispersion relations, which results in the exotic chiral magnetic effect. In this talk, our recent work on such an exotic chiral magnetic effect in multi-Weyl semimetals is presented [1]. Multi-Weyl semimetal is a Weyl semimetal with multiple-monopole charge. We show that in static magnetic fields, the CME is manifestly topological, and given by the straightforward extension of that for conventional WSMs with unit monopole charge to multiple case. On the other hand, the dynamic CME with time-dependent magnetic field is not manifestly topological, but we found the topological feature hidden there. We propose an experimental setup to measure the multiple monopole charge via the topological nature hidden in the dynamic CME.

March 16

Five-loop Beta function and anomalous dimensions in QCD

York Schroder (UBB Chillan)

Room 907, 14:00-15:30

Abstract:

I provide an update on a long-term project that aims at evaluating massive vacuum Feynman integrals at the five-loop frontier, with high precision and in various spacetime dimensions. A number of applications are sketched, mainly concerning the determination of anomalous dimensions, for quantum field theories in four, three and two dimensions.

2016

December 2

Gauge-invariant definition of magnetic monopoles in Yang-Mills theory

Ryutaro Matsudo (Chiba University)

Room 907, 16:00-17:30

Abstract:

Magnetic monopoles play the important role in the dual-superconductor picture, which is one of the most promising scenarios for quark confinement. However, in the Yang-Mills theory, it is not trivial to define magnetic monopoles. A well-known method to define monopoles is the Abelian projection, but in this method, we have to fix the gauge. In contrast to this, by using the field decomposition, we can define monopoles in the gauge-invariant way. Recently, we found that this method, which was obtained for the gauge group SU(N), can be applicable to any semisimple compact gauge groups. In this talk, firstly, I will discuss this method for the case of the gauge group SU(2). Secondly, I will explain how to extend this method to the case of the gauge group SU(N). Finally, I will explain how to extend this method to the case of any other semisimple compact gauge groups.

November 25

Bjorken flow in relativistic magnetohydrodynamics

Shi Pu (University of Tokyo)

Room 907, 16:00-17:30

Abstract:

In the initial stage of relativistic heavy-ion collisions, strong magnetic fields appear due to the large velocity of the colliding charges. The evolution of these fields appears as a novel and intriguing feature in the fluid-dynamical description of heavy-ion collisions. In this work, we study analytically the one-dimensional, longitudinally boost-invariant motion of an ideal fluid in the presence of a transverse magnetic field. Interestingly, we find that, in the limit of ideal magnetohydrodynamics, i.e., for infinite conductivity, and irrespective of the strength of the initial magnetization, the decay of the fluid energy density e with proper time τ is the same as for the time-honoured “Bjorken flow” without magnetic field. Then, we also extend our discussion with the non-zero magnetization effect. We find that the temperature and energy density decay more slowly because of the non-vanishing magnetization. For values of the magnetic field typical for heavy-ion collisions, this effect is, however, rather small. These solutions can serve both to gain insight on the dynamics of heavy-ion collisions in the presence of strong magnetic fields and as testbeds for numerical codes.

November 18

Exotic hadrons - what we have learned and do not -

Atsushi Hosaka (Osaka University)

Room 201b, 15:00-16:00

Abstract:

Starting from an overview of the current status of the XYZ and pentaquark exotic hadrons, several theoretical attempts are discussed to understand unexpected phenomena in the hadron spectroscopy. One of plausible explanations is a hadronic molecule picture where hadron-like correlations are enhanced near the threshold region, which has been successfully applied to several X,Y,Z states. Yet the other important ingredients are the quark dynamics such as diquarks and compact multiquark structures. We discuss the recent attempt to explore these problems in the current and future experimental studies, and in phenomenological analyses on heavy hadron productions and decays.

October 14

Chiral turbulence in supernovae

Naoki Yamamoto (Keio University)

Room 907, 16:00-17:30

Abstract:

Chirality of neutrinos modifies the conventional hydrodynamic behavior at the macroscopic scale and leads to anomalous transport phenomena in neutrino matter. We argue that such chiral transport of neutrinos should play important roles in the evolution of core-collapse supernovae, and, in particular, lead to the possible inverse energy cascade from small to large scales, which may be relevant to the origin of the supernova explosion.

September 29

Asymptotic Safety in Quantum Gravity

Jan Pawlowski (Heidelberg University)

Room 201a, 10:30-12:00

Abstract:

In the first part of the talk I briefly review the asymptotic safety scenario in quantum gravity. In the second part the stability as well both in pure quantum gravity and in coupled matter-gravity systems is discussed. Applications range from potential restrictions on UV-extensions of the Standard Model to dilaton-induced slow-roll inflation.

The talk is based on original material from arXiv:1605.01858, arXiv:1604.02041, arXiv:1601.04597, arXiv:1510.07018, arXiv:1506.07016.

July 22

Spectroscopy of mesons and baryons in a magnetic field by cylindrical Gaussian expansion method

Tetsuya Yoshida (Tokyo Institute of Technology)

Abstract:

In this talk, I will discuss the spectroscopy of neutral mesons and baryons in a magnetic field. In vacuum, spectroscopy of mesons and baryons can be analyzed by using a potential model with constituent quarks. Gaussian expansion method (GEM) is a powerful and useful tool to preciously calculate hadron masses from such models. The conventional GEM is, however, applied only for the system having the spherical symmetry. In order to obtain hadron masses and wave functions in a magnetic field from the potential model, we newly introduce cylindrical Gaussian expansion method (CGEM), where the basis of a special wave function is expanded by a superposition of two Gaussian bases for transverse and longitudinal directions in the cylindrical coordinate. First, I will give the review of constituent quark model and GEM. Then, I will show our results of the mass shifts and deformation of the wave functions of heavy mesons (quarkonia, heavy-light mesons) and light baryons (neutron, delta) by magnetic effects. Moreover, we will also discuss magnetic behaviors of excited states (2S and 3S).

July 17

Lefschetz-thimble path integral for studying the sign problem and review of the current situation

Yuya Tanizaki (RIKEN Nishina Center, RIKEN BNL Research Center)

Abstract:

Recently, Picard-Lefschetz theory gets much attention in the context of the sign problem, because it enables us to study the system with the complex classical action nonperturbatively by employing the semiclassical analysis. In this seminar, after its brief introduction, I will apply it to the one-site Hubbard model. This model has a severe sign problem, which looks quite similar to that of the finite-density QCD at low temperatures. By solving this model using the Lefschetz-thimble path integral, we are trying to understand the structure of the sign problem of finite-density QCD. Especially, I give a qualitative picture (or speculation) about the early-onset problem of the baryon number density, called the baryon Silver Blaze problem. I would also like to review the latest development of this field if time allows.

March 25

Chiral magnetic effect in Weyl semimetals: the role of electromagnetic field analysis

Hiroyuki Fujita (University of Tokyo)

Abstract:

For relativistic Weyl fermions in 3+1 dimensions, an electric current proportional to the external magnetic field is predicted. This phenomenon is called Chiral Magnetic Effect (CME) and recent studies on "Weyl semimetals” in condensed matter physics renewed the interest on the subject as a realistic problem which can be investigated experimentally. 

In this talk, I will start from brief introduction of developments in the field of “topological phase of matter” and Weyl semimetals. After some comments on CME in lattice models especially focusing on its qualitative difference from that in the relativistic field theory, I will move onto the main content discussed in our recent paper [1]. I will show that for the understanding of CME-dominated transport properties of Weyl semimetals, it is fundamentally important to do the electromagnetic field analysis inside the sample.

[1] Hiroyuki Fujita and Masaki Oshikawa, arXiv: 1602.00687

2015

November 20

Phonons and pions on the spatially modulated chiral condensate

Kazuhiko Kamikado (RIKEN)

Abstract:

We discuss properties of low energy excitations on the spatially modulated chiral condensate. On the spatially modulated chiral condensation, as well as the chiral symmetry, translational symmetry is spontaneously broken. Therefor phonon and pion appear as a low energy excitation. We derive energy eigenvalue equations of the phonon and pion on the spatially modulated chiral condensate and solve them. We show, due to thermal fluctuation of the phonon, the spatially modulated chiral condensation becomes unstable and, instead of the spatially modulated chiral condensate, the quasi-long-range order of the order parameter emerges. We evaluate the exponents which characterise the power low of the correlation function of the order parameter.

November 6

Kondo effect in QCD

Sho Ozaki (KEK)

Abstract:

In condensed matter physics, Kondo effect is known as an enhancement of electrical resistance of impure metals with decreasing temperature/energy. This phenomenon is the first known example of asymptotic freedom in physics, which is found well before the discovery of that of QCD. Kondo effect is caused by the combination of the following ingredients: (0) heavy impurity, (i) existence of Fermi surface, (ii) quantum fluctuations (loop effects), (iii) non-Abelian nature of interaction (e.g. spin-flip interaction in the case of condensed matter physics). In this talk, we will discuss Kondo effect realized in QCD. We found the characteristic behavior of Kondo effect in quark matter with heavy quark impurity. There, the color exchange interaction mediated by gluons plays the role of the third condition (iii) for the appearance of Kondo effect. Furthermore, we found a novel type of Kondo effect induced by strong magnetic fields, which is possible even without Fermi surface (chemical potential). In addition to the fact that the magnetic field dose not affect the color degrees of freedom, dimensional reduction to 1+1 dimensions as well as finiteness of the density of state of LLL at E=0 are essential for the magnetically induced QCD Kondo effect.

October 26

Hydrodynamic fluctuations in high-energy nuclear collisions

Koichi Murase (University of Tokyo)

Abstract:

Hydrodynamic fluctuations are thermal fluctuations of hydrodynamics which arise in the dissipative currents such as the shear stress tensor, the bulk pressure, and the diffusion currents. The hydrodynamic fluctuations become important when the scale separation of the macroscopic and microscopic dynamics of a system is not large enough. In high-energy nuclear collision experiments, two nuclei accelerated to almost the speed of light collide with each other to create a new state of the nuclear matter, quark-gluon plasma. The dynamics of the created matter is described by relativistic hydrodynamic models. To investigate the effects of the hydrodynamic fluctuations in the high-energy nuclear collisions, we first consider the properties of the fluctuations in relativistic systems. Then we implement them into our dynamical model and perform event-by-event numerical simulations to investigate the effects on observables. It turned out that the observables such as the flow coefficients vn are substantially changed by the hydrodynamic fluctuations, which affects the quantitative determination of the transport coefficients of the created matter such as shear viscosity, etc.

June 12

Diquark bound states at far beyond ladder truncation

Go Mishima (University of Tokyo)

Abstract:

The Bethe-Salpeter equation in the diquark channel is investigated by employing the Dyson-Schwinger method together with the Munczek-Nemirovsky model. The novelty of our study is a resummation of completely-crossed ladder diagrams in the Bethe-Salpeter kernel. These diagrams are enhanced due to their color factors in the diquark channel, but not in the meson channel. In our analysis, diquark bound-state solutions exist in the Bethe-Salpeter equation.

June 5

On the Cancellation of Radiation from the Unruh detector

Zhang Sen (Okayama Institute for Quantum Physics)

Abstract:

A uniformly accelerated detector (Unruh detector) in Minkowski vacuum is excited as if it is exposed to the thermal bath with temperature proportional to its acceleration. In the inertial frame, both of an excitation and a deexcitation of the detector are accompanied by emission of radiation into the Minkowski vacuum, so one may suspect the Unruh detector emits radiation like the Larmor radiation from an accelerated charged particle. However, it is found the energy momentum tensor of the radiation is miraculously cancelled by a quantum interference effect. We find that the cancellation occurs if the Green function satisfies a relation similar to the Kubo-Martin-Schwinger (KMS) condition. We then study two examples, Unruh detectors in the 3+1 dimensional Minkowski spacetime and in the de Sitter spacetime.

February 19

Generalization of the Nambu-Goldstone theorem

Yoshimasa Hidaka (RIKEN)

Abstract:

Symmetry and its spontaneous breaking are of basic importance for understanding the low energy physics in many-body systems. When a continuum symmetry is spontaneously broken, there appears a zero mode called Nambu-Goldstone (NG) mode, which is well developed in Lorentz invariant systems. In contrast, in non-Lorentz invariant systems, the NG theorem has not been well developed. In this talk, we discuss the recent progress in generalization of NG theorem, and discuss the counting rule for NG modes using the Langevin equation derived from Mori's projection operator method. We give the detailed derivation of the counting rule, i.e., the number of NG modes is equal to the number of broken generators, Qa, minus half the rank of the expectation value of [iQa,Qb].

January 23

Topological transport phenomena: from condensed matter physics to astrophysics

Naoki Yamamoto (Keio University)

Abstract:

We discuss novel transport phenomena for chiral fermions and their relevance in condensed matter physics, nuclear physics, and astrophysics. In particular, we give a simple argument that chiral plasmas are dynamically unstable and tend to reduce the chirality imbalance. This "chiral plasma instability" may provide a possible new mechanism for the gigantic and stable magnetic field of magnetars.

January 9

Structure of medium-mass nuclei studied by Monte Carlo Shell-Model calculations

Yusuke Tsunoda (University of Tokyo)

Abstract:

We study nuclear structure of Ni and Cu isotopes, especially neutron-rich ones in the N∼40 region by Monte Carlo shell model (MCSM) calculations in pfg9d5 model space (0f7/2, 1p3/2, 0f5/2, 1p1/2, 0g9/2, 1d5/2). Effects of excitation across N=40 and other gaps are important to describe properties such as deformation, and we include this effects by using the pfg9d5 model space. We can calculate in this large model space without any truncation, as an advantage of MCSM. In the MCSM, a wave function is represented as a linear combination of angular-momentum- and parity-projected deformed Slater determinants. We can study intrinsic shapes of nuclei by using quadrupole deformations of MCSM basis states before projection. In doubly-magic 68Ni, there are oblate and prolate deformed bands as well as the spherical ground state from the calculation. Such shape coexistence can be explained by introducing the mechanism called Type II shell evolution, driven by changes of configurations within the same nucleus mainly due to the tensor force. The properties of other nuclei including intrinsic shapes are also discussed.

2014

December 5

A self-consistent bag model

Sanjin Benic (University of Tokyo, Zagreb University)

Abstract:

The QCD phase diagram consists roughly of a hadronic and a quark-gluon phase. While hadrons are abundant at low temperatures and/or densities, they are suppressed in the high temperature and/or high density phase where quarks and gluons dominate. We propose a simple framework that considers this switching between the relevant degrees of freedom. We introduce an auxiliary field to control the relative abundance of the hadronic and the quark-gluon sector. This simple model is then applied to the QCD phase transition at extreme densities where we need to switch from nucleons to quarks.

October 17

Temperature dependence of the chiral condensate in the Schwinger model with Matrix Product State

Hana Saito (DESY Zeuthen)

Abstract:

It is known that the lattice QCD at finite chemical potential doesn't work efficiently since a notorious sign problem spoils the Monte Carlo sampling in this case. In our study, to search for another possibility of numerical method for QCD at finite density, we employ Hamiltonian formulation with Tensor Network (TN) approach. TN methods form a family of efficient techniques of approximating quantum states. We employed one of the tensor network techniques, called Matrix Product States, to investigate the Schwinger model, which has similar properties to QCD. As a first step of our approach, we consider a case of zero chemical potential. In this talk, we show our results for the temperature dependence of the chiral condensate in the massless model and we compare to the analytical formula derived by Sachs and Wipf.

September 26

Testing semi-classical quantum field theory in curved space with graphene-like materials

Antonino Flachi (Universidade de Lisboa)

Abstract:

Owing to its geometrical versatility and emergent relativistic behaviour, graphene-like materials offers an ideal system for testing the semi-classical nature of quantum field theory (QFT) in curved space. This is important for, at least, two distinct reasons. At the level of applications, the interplay between quantum and geometrical effects may suggest novel ways to induce a mass gap in graphene. At a fundamental level, it may offer an opportunity to test the validity at low energies of the semi-classical approximation in-built in QFT in curved space. In this talk, we discuss an interesting example where these effects show up in the staggered magnetization in graphene as a result of the presence of a localized curvature and of the spontaneous break-down of the discrete symmetry related to the bipartite nature of the honeycomb lattice.

July 4

Effect of Curvature on Chiral and Deconfinement Phase Transitions

Shuji Sasagawa (Rikkyo University)

Abstract:

QCDはカイラル対称性の破れとカラーの閉じ込めという2つの性質を持っている。 これらによる相は温度や密度効果によって相転移を起こすことが知られている。 しかし、この2つの相転移の関係性についてはほとんど分かっていない。 これに対して、2つの相転移は区別すべきなのかを知るために重力(曲率)の寄与を加えて、 臨界点に差が出るのかを調べた。 その結果、臨界点の差は曲率の強さによって広がることが確認できた。 今回は、Nambu-Jona-lasinioモデルに重力の寄与を加えた他の話題も合わせて紹介する。

June 27

Spin-gap phenomena and pseudogap effects in the BCS-BEC crossover regime of an ultracold Fermi gas

Yoji Ohashi (Keio University)

Abstract:

We theoretically discuss magnetic properties of an ultracold Fermi gas in the BCS-BEC crossover region. Within the framework of an extended T-matrix approximation, we show that the uniform spin susceptibility χ exhibits non-monotonic temperature dependence, and is suppressed near the superfluid phase transition temperature Tc, because of the formation of preformed singlet pairs. To characterize this spin-gap phenomenon, we conveniently introduce the spin-gap temperature Ts as the temperature at which χ takes a maximal value, to determine the spin-gap regime (Tc≤T≤Ts) over the entire BCS-BEC crossover region. In the weak-coupling BCS regime, the spin-gap temperature Ts is found to agree well with the so-called pseudogap temperature T∗ below which pairing fluctuations lead to the decrease of the normal-state density of states around the Fermi level. The agreement is, however, absent in the BEC regime, where Ts is much higher than T∗. In this strong-coupling regime, we show that the spin-gap behavior of χ can be simply understood by treating the system as a gas mixture of two-body bound molecules and thermally dissociated free Fermi atoms. Our results indicate that an ultracold Fermi gas is a useful system to assess the preformed pair scenario which has been discussed as a possible mechanism of the pseudogap and spin-gap phenomena in high-Tc cuprates.

June 6

エキゾチックな重いハドロンと原子核

Shigehiro Yasui (KEK)

Abstract:

最近X, Y, Zと 呼ばれるチャームやボトムを含むエキゾチックなハドロンの観測が相次いで報告されている。このような状態では重いクォークに関するスピン対称性が重要な役割を果たしていると考えられている。今回の発表では、スピン対称性を出発 点として様々なエキゾチックな重いハドロンの構造を議論するとともに、核媒質中の重いハドロンについても考察する。具体的な例として反D中間子あるいはB中間子と核子の 系を考えて、重いハドロンの有効理論に基づいて得られる様々な状態の質量スペクトラムとスピン対称性の関係を議論する。

April  18

Non-Invertible Kennedy-Tasaki Transformation and Applications to Gapless-SPT

Linhao Li (The University of Tokyo)

Room 907, 15:00 - 16:00

Abstract: Kennedy-Tasaki (KT) transformation was originally explored as a highly non-local unitary transformation that maps between a $\Z_2\times \Z_2$ spontaneously symmetry breaking phase and a symmetry protected ordered phase on an open chain. In this talk, we propose a way to define it on a closed chain, by sacrificing unitarity. The operator realizing such a non-unitary transformation satisfies the non-invertible fusion rule and implements a generalized gauging of the $\Z_2\times \Z_2$ global symmetry. We further use the KT transformation to study topological signatures of different gapless symmetry protected topological (gSPT) phases in a unified framework, and the phase diagram under symmetric perturbations. This talk is based on the recent work with Yunqin Zheng and Masaki Oshikawa.