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2017

Oct 6

Room/TimeRoom 907 / 16:00-17:30
SpeakerToshiaki Fujimori (Keio University)
TitleExact resurgent trans-series and all-order multi-bion contributions in CP^N quantum mechanics
AbstractI 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

Room/TimeRoom 907 / 16:00-17:30
SpeakerShun Furusawa (RIKEN)
TitleNuclear Equation of State for Core-Collapse Supernovae
AbstractThe 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

Room/TimeRoom 907 / 16:00-17:30
SpeakerNoriyuki Sogabe (Keio University)
TitleNew dynamic critical phenomena in nuclear and quark superfluids
AbstractWe 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

Room/TimeRoom 907 / 16:00-17:30
SpeakerDaisuke Satow (Goethe Universität)
TitleTransport coefficients of QGP in strong magnetic fields
AbstractWe 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

Room/TimeRoom 907 / 16:00-17:30
SpeakerTakuya Kanazawa (RIKEN)
TitlePhases of circle-compactified QCD with adjoint fermions at finite density
AbstractWe 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

Room/TimeRoom 907 / 16:00-17:30
SpeakerTomoya Hayata (Chuo University)
TitleChiral magnetic effect in multi-Weyl semimatals
AbstractNew 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. [1] Tomoya Hayata, Yuta Kikuchi, Yuya Tanizaki, arXiv:1703.02040 [cond-mat.mes-hall]

March 16

Room/TimeRoom 907 / 14:00-15:30
SpeakerYork Schroder (UBB Chillan)
TitleFive-loop Beta function and anomalous dimensions in QCD
AbstractI provide an update on a long-term project that aims at evaluating massive vacuum Feynman integrals at the fi ve-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

Dec. 2

Room/TimeRoom 907 / 16:00-17:30
SpeakerRyutaro Matsudo (Chiba University)
TitleGauge-invariant definition of magnetic monopoles in Yang-Mills theory
AbstractMagnetic 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.

Nov. 25

Room/TimeRoom 907 / 16:00-17:30
SpeakerShi Pu (Univ. of Tokyo)
TitleBjorken flow in relativistic magnetohydrodynamics
AbstractIn 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.

Nov. 18

Room/TimeRoom 201b / 15:00-16:00
SpeakerAtsushi Hosaka
TitleExotic hadrons - what we have learned and do not -
AbstractStarting 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.

Oct. 14

Room/TimeRoom 907 / 16:00-17:30
SpeakerNaoki Yamamoto (Keio University)
TitleChiral turbulence in supernovae
AbstractChirality 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.

Sep. 29

Room/TimeRoom 201a / 10:30-12:00
SpeakerJan Pawlowski (Heidelberg University)
TitleAsymptotic Safety in Quantum Gravity
AbstractIn 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

SpeakerTetsuya Yoshida (Tokyo Institute of Technology)
TitleSpectroscopy of mesons and baryons in a magnetic field by cylindrical Gaussian expansion method
AbstractIn 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).

June 17

SpeakerYuya Tanizaki (RIKEN Nishina Center, RIKEN BNL Research Center)
TitleLefschetz-thimble path integral for studying the sign problem and review of the current situation
AbstractRecently, 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.

Mar. 25

SpeakerHiroyuki Fujita (Univ. of Tokyo)
TitleChiral magnetic effect in Weyl semimetals: the role of electromagnetic field analysis
AbstractFor 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

Nov. 20

SpeakerKazuhiko Kamikado (RIKEN)
TitlePhonons and pions on the spatially modulated chiral condensate
AbstractWe 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.

Nov. 6

SpeakerSho Ozaki (KEK)
TitleKondo effect in QCD
AbstractIn 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.

Oct. 26

SpeakerKoichi Murase (Univ. of Tokyo)
TitleHydrodynamic fluctuations in high-energy nuclear collisions
AbstractHydrodynamic 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 \( v_n \) 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

SpeakerGo Mishima (Univ. of Tokyo)
TitleDiquark bound states at far beyond ladder truncation
AbstractThe 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

SpeakerZhang Sen (Okayama Inst. for Quantum Physics)
TitleOn the Cancellation of Radiation from the Unruh detector
AbstractA 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.

Feb. 19

SpeakerYoshimasa Hidaka (RIKEN)
TitleGeneralization of the Nambu-Goldstone theorem
AbstractSymmetry 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, \( Q_a \), minus half the rank of the expectation value of \( [iQ_a,Q_b] \).

Jan. 23

SpeakerNaoki Yamamoto (Keio Univ.)
TitleTopological transport phenomena: from condensed matter physics to astrophysics
AbstractWe 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.

Jan. 9

SpeakerYusuke Tsunoda (Univ. of Tokyo)
TitleStructure of medium-mass nuclei studied by Monte Carlo Shell-Model calculations
AbstractWe study nuclear structure of Ni and Cu isotopes, especially neutron-rich ones in the \( N\sim 40 \) region by Monte Carlo shell model (MCSM) calculations in \( pfg_9d_5 \) model space (\( 0f_{7/2} \), \( 1p_{3/2} \), \( 0f_{5/2} \), \( 1p_{1/2} \), \( 0g_{9/2} \), \( 1d_{5/2} \)). 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 \( pfg_9d_5 \) 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 \( ^{68} \)Ni, 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

Dec. 5

SpeakerSanjin Benic (Univ. of Tokyo / Zagreb U.)
TitleA self-consistent bag model
AbstractThe 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.

Oct. 17

SpeakerHana Saito (DESY Zeuthen)
TitleTemperature dependence of the chiral condensate in the Schwinger model with Matrix Product State
AbstractIt 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.

Sep. 26

SpeakerAntonino Flachi (Universidade de Lisboa)
TitleTesting semi-classical quantum field theory in curved space with graphene-like materials
AbstractOwing 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

SpeakerShuji Sasagawa (Rikkyo Univ.)
TitleEffect of Curvature on Chiral and Deconfinement Phase Transitions
AbstractQCD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

SpeakerYoji Ohashi (Keio Univ.)
TitleSpin-gap phenomena and pseudogap effects in the BCS-BEC crossover regime of an ultracold Fermi gas
AbstractWe 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 \( \chi \) exhibits non-monotonic temperature dependence, and is suppressed near the superfluid phase transition temperature \( T_c \), 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 \( \chi \) takes a maximal value, to determine the spin-gap regime (\( T_c \le T \le T_s \)) 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 \( \chi \) 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-\( T_c \) cuprates.

June 6

SpeakerShigehiro Yasui (KEK)
Titleエキゾチックな重いハドロンと原子核
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。