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Seminar 2022 / セミナー情報2022

格子ゲージ理論における熱化と量子カオス

2023年2月21日（月） 13:00 @H526
Speaker: 早田 智也 (慶應大)

Abstract:
量子多体系の複雑なダイナミクスを理解することは幅広い分野にまたがる重要な問題の一つである。特に孤立量子系の熱化と量子カオスは非常に精力的に研究されている問題である。本講演では、格子上の非可換ゲージ理論について、ハミルトニアン形式に基づく手法を用いて熱化と量子カオスを解析した研究について紹介する。また、ハミルトニアン形式に基づく格子ゲージ理論は悪名高い符号問題を回避し、実時間や有限密度系を解析する有力な手法として近年注目を集めている。これらの問題を解析するための手法の開発とその応用についても紹介したい。

QCD相転移と初期宇宙におけるバリオン数生成

2023年2月15日（月） 13:00 @H526
Speaker: 村　勇志 (阪大)

強い場の物理と重イオン衝突の初期過程

2022年12月19日（月） 13:30 @H526
Speaker: 田屋 英俊 (RIKEN iTHEMS)

Abstract:
強い場にさらされた物理系を考える。こうした物理系では、強い場は結合定数の小ささを補うことで、普通の物理系ではありえない興味深い非摂動現象が起こる。例えば、強い場の下で真空が粒子対を作って自発的に崩壊してゆく現象(Schwinger機構)など。Schwinger機構をはじめとする強い場の物理は、最近の高強度レーザー実験で検証が期待されているのみならず、重イオン衝突の初期過程等の極限状況の物理を理解する上でも重要な役割を果たす。本セミナーでは、(1)強い場の物理、特に講演者が主に研究を行ってきたSchwinger機構の研究、に関してレビューを行い、そして、(2)Schwinger機構の観点から、重イオン衝突初期の実時間発展ダイナミクスがどのように理解できるのか、ということを議論する。特に、Bjorken膨張するような古典的なカラー電磁場を初期条件とし、弱結合・平均場近似の範囲でQCDを数値的に解くことで、O(1fm/c)程度の早い時間スケールで粒子生成が完了することを見る。

QCDの臨界点探索に対する熱力学的アプローチ

2022年10月28日（金） 14:00 @H526
Speaker: 北尾　紫洸

Journal Ref.: M. Stephanov, K. Rajagopal, and E. Shuryak, "Event-by-event fluctuations in heavy ion collisions and the QCD critical point"
Phys. Rev. D60 (1999) 114028

格子上のゲージ理論とクォークの閉じ込め

2022年10月28日（金） 15:30 @H526
Speaker: 平岩　義寛

Journal Ref.: Kenneth G. Wilson, "Confinement of quarks"
Phys. Rev. D10 (1974) 2445

Shear viscosity of an ultracold Fermi gas in the BCS-BEC crossover region

2022年7月15日（金） 13:30 @H526
Speaker: 鏡原 大地 (近畿大学)

Abstract:
Ultracold Fermi gases provide us the unique opportunity to systematically study various physical properties of a quantum many-body system from the weak-coupling limit to the strong-coupling limit [1], by using the advantage that a pairing interaction of this system is tunable by adjusting the threshold energy of a Feshbach resonance [2]. Recent experimental developments enable us to discuss the non-equilibrium transport properties of this system [3]. Among transport properties, the shear viscosity has attracted much interest from the viewpoint of the so-called Kovtun-Son-Starinets (KSS) conjecture, which states that the ratio of the shear viscosity η to the entropy density s, η/s, has a lower bound [4]. The measured η/s of a unitary Fermi gas is very close to the KSS lower bound [5]. It is also reported that the shear viscosity takes a minimum in the slightly stronger coupling region than the unitarity limit [6]. This fact makes us expect that we would obtain a smaller value of η/s than that of the unitarity limit when we systematically investigate the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region. Motivated by these situations, we theoretically investigate the shear viscosity as well as the entropy density of an ultracold Fermi gas in the BCS-BEC crossover region [7,8]. Including pairing fluctuations within the framework of the self-consistent T-matrix approximation [9], we examine these quantities in the normal state above the superfluid transition temperature. We mainly focus on the shear viscosity η and discuss how interaction and quantum effects influence the behavior of η. We also discuss η/s and point out that two quantum phenomena, namely the Pauli exclusion principle and bound-state formation, are crucial keys to obtaining the lower bound of η/s in this system [8].

[1] I. Bloch, J. Dalibard, and W. Zwerger, Rev. Mod. Phys. 80, 885 (2008); S. Giorgini, L. P. Pitaevskii, and S. Stringari, Rev. Mod. Phys. 80, 1215 (2008); Q. Chen, J. Stajic, S. Tan, K. Levin, Phys. Rep. 412, 1 (2005).
[2] C. Chin, R. Grimm, P. Julienne, and E. Tiesinga, Rev. Mod. Phys. 82, 1225 (2010).
[3] C. Cao, E. Elliott, J. Joseph, H. Wu, J. Petricka, T. Schäffer, and J. E. Thomas, Science 331, 58 (2010).
[4] P. K. Kovtun, D. T. Son, and A. O. Starinets, Phys. Rev. Lett. 94, 111601 (2005).
[5] J. A. Joseph, E. Elliott, and J. E. Thomas, Phys. Rev. Lett. 115, 020401 (2015).
[6]E. Elliott, J. A. Joseph, and J. E. Thomas, Phys. Rev. Lett. 113, 020406 (2014).
[7] D. Kagamihara, D. Inotani, and Y. Ohashi, J. Phys. Soc. Jpn. 88, 114001 (2019).
[8]D. Kagamihara and Y. Ohashi, J. Phys. Soc. Jpn. 89, 044005 (2020).
[9]R. Haussmann, Phys. Rev. B 49, 12975 (1994); T. Enss, R. Haussmann, W. Zwerger, Ann. Phys. 326, 770 (2011).

Exploring the criticality of QCD with effective field theory for fluctuating hydrodynamics

2022年6月24日（金） 13:30 @H526
Speaker: 曽我部　紀之 (IMP)

Abstract:
A non-equilibrium effective field theory framework has recently been formulated for fluctuating hydrodynamics [1]. In this talk, we present an example of applying this novel formalism to study the critical properties of QCD. In the view that non-Gaussian fluctuations of baryon density are important for the QCD critical point search, we derive evolution equations for the critical non-Gaussian fluctuations of a conserved density and obtain closed-form solutions based on field theory techniques [2]. Those results can be readily implemented for simulations in realistic situations of heavy-ion collisions. In addition, we find that nonlinear interactions among noise fields, which are missing in traditional stochastic hydrodynamics, could potentially contribute to the quartic (fourth-order) fluctuations in the scaling regime in off-equilibrium situations.

[1] Michael Crossley, Paolo Glorioso, and Hong Liu, “Effective field theory of dissipative fluids,” JHEP 09 (2017) 095.
[2] Noriyuki Sogabe and Yi Yin, “Off-equilibrium non-Gaussian fluctuations near the QCD critical point: an effective field theory perspective,” JHEP 03 (2022) 124.

Dynamical properties for Roper resonance from holographic QCD approach.

2022年6月3日（金） 13:30 @H526
Speaker: 藤井 大輔 (RCNP)

Abstract:
The Roper resonance, the first excited state of the nucleon, is one of the best established baryon resonances. Yet, its properties have not been consistently explained by effective models of QCD, such as the non-relativistic quark model. In this talk, we propose an alternative approach in the Sakai-Sugimoto model that is one of the holographic models of QCD. In particular, we analyze the helicity amplitude of the electromagnetic transitions at the leading of ’t Hooft coupling 1/λ. We demonstrate that the recently observed data by CLAS are explained in the present approach. Furthermore, we obtain the decay width of the one pion emission for the Roper resonance, and have successfully reproduced its value within the uncertainty of the experimental data. They are features that has not been given by the conventional non-relativistic quark model, and it may be concluded that the Sakai-Sugimoto model captures the nature of the Roper resonance well. We attribute this success to the baryon picture of the Sakai-Sugimoto model. In this model, it is known that baryons are interpreted as instantons on the D8 brane in terms of string theory. The dynamics of the baryon are given as collective motions of instantons / solitons, which is quite different from the quark model based on a single particle picture of quarks. Therefore, in this talk, we would like to explain the method of collective coordinate quantization of gauge theories in detail.

A fully implicit numerical solver for relativistic hydrodynamics

2022年5月20日（金） 13:30 @H526
Speaker: Nathan Touroux 氏(D2)

Abstract:
In the context of heavy-ion collisions, and especially the study of the quark and gluon plasma (QGP), taking into account dynamical flow is very important (i.e. presence of elliptic flow). On the experimental side, the QGP can only be studied using heavy ion collisions, which are out of equilibrium phenomena. Consequently we use relativistic viscous hydrodynamic to model the dynamical flow in the QGP. More precisely, we are designing a new implicit solver for relativistic hydrodynamics. This implicit property allows in particular better numerical stability that would be required to introduce fluctuations in the model. First, we present a brief history of relativistic hydrodynamics in the context of heavy ion collisions. Then we review the existing hydrodynamic codes and their limitation. Finally we introduce the new implicit numerical solver methodology and show its successful benchmark. As an outlook we briefly discuss what extension to the current field of numerical hydrodynamics could be done with an implicit solver.