Seminar 2020 / セミナー情報2020

Heavy quark dynamics in a medium

2021年2月15日（月） 13:30 @zoom
Speaker: Balbeer Singh 氏(Physical Research Laboratory, India)

Abstract:
High energy nuclear collision experiments give us the opportunity to study the properties of strongly interacting QCD matter under extreme conditions such as temperature, baryon density, and magnetic field. Such extreme conditions are supposed to have existed few microseconds after the big bang in the evolution of the universe and possibly exist at the core of ultracompact astrophysical objects like neutron stars. Moreover, one needs well-calibrated probes to investigate the properties of quark-gluon plasma created in these high energy nuclear collision experiments. In this regard, heavy quarks (HQs), produced during the initial hard collisions, have been identified as one of the promising tools. Furthermore, the suppression of quarkonium states has long been proposed as one of the promising probes to study the properties of quark-gluon plasma. In this talk, I would discuss the transport properties of HQs (charm and bottom) in the semi-QGP. In particular, I will talk about the non-perturbative effects arising from the Polyakov loop and constituent quark mass on HQ transport. I would also discuss the effects of the magnetic field on quarkonium potential (both real and imaginary parts) as well as thermal decay width.

Massive Yang-Mills理論に基づいた、有限密度2カラーQCDにおけるグルーオン二点関数の研究

2021年1月14日（木） 15:00 @ H601 & zoom
Speaker: 末永大輝氏 (RCNP)

Abstract:
本研究では「massive Yang-Mills理論」を低エネルギーQCDの有効模型として採用し、摂動論的QCD が適用出来ない中間的な密度領域におけるQCD物質中のグルーオン二点関数を調べた。特に、格子QCD 計算結果が存在する2カラー・2フレーバーQCD物質中のグルーオン二点関数を、1ループレベルで調べた。またその際、格子QCD計算結果が示唆するようにカラー超伝導効果を取り入れた。その結果massive Yang-Mills理論によるグルーオン二点関数は、ゼロ質量Yang-Mills理論に基づいた計算を改良し、より格子QCD計算結果を再現することを示した。また本講演では、カラー超伝導効果を取り入れた際の、ゲージ対称性を破らない正則化の方法についても詳しく議論する。

Replica evolution of classical field in 4+1 dimensional spacetime toward real time dynamics of quantum field

2020年10月12日（月） 13:30 @ H524 & zoom
Speaker: 大西　明氏（京大基研）

Abstract:
Replica evolution of the classical field is proposed as an approximate simulator of real-time quantum field dynamics at finite temperatures. We consider $N$ classical field configurations $(\phi_{\tx},\pi_{\tx} (\tau=0,1,\cdots N-1)$, dubbed as replicas, which interact with each other via the $\tau$-derivative terms and evolve with the classical equation of motion. The $\tau$-derivative terms in the Hamiltonian, $\xi^2 \sum_x (\phi_{x+\hat{\tau}}-\phi_x)^2/2$, correspond to the kinetic part of the Euclidean action in the imaginary time formalism of the finite temperature quantum field theory by regarding the replica index $\tau$ as the imaginary time index.Thus the replica evolution is technically the same as the molecular dynamics part of the hybrid Monte-Carlo sampling. The partition function of replicas at temperature $\xi$ is proven to be proportional to that in quantum field theory at temperature $T=\xi/N$. At the same time, the time dependence of the replica-index average of field variables is described by the classical equation of motion when the fluctuations are small. We examine the statistical and dynamical properties of the $\phi^4$ theory in the 4+1 spacetime dimensions. We note that the Rayleigh-Jeans divergence in the classical field can be removed in replica evolution with $N \geq 2$ by including the mass counterterm. We also find that the thermal mass obtained from the unequal time correlation function at zero momentum grows as a function of the coupling as in the perturbative estimate in the small coupling region. Hence the replica evolution, the classical field theory with improved quantum statistical property, would be a candidate to represent the real-time evolution of the quantum field.
A. Ohnishi, H. Matsuda, T. Kunihiro, T. T. Takahashi, arXiv:2008.09556 [hep-lat]]

冷却原子気体における符号問題およびpolaron energy

2020年7月27日（月） 13:30 @ H524 & zoom
Speaker: 土居孝寛氏 (RCNP)

Abstract:
有限密度における格子QCDでは有効作用が複素数になり、強力な計算手法であるモンテカルロ法の適用が困難になる。これは符号問題と呼ばれる未解決問題で、QCD以外の様々な物理系でも現れる。例えば冷却原子気体でも符号問題は現れ、QCDの場合と同じく理論研究の進展を妨げている。本セミナーでは符号問題の基礎および符号問題の解決策の１つとして考えられている複素ランジュバン法を解説した後、1次元2成分フェルミ原子気体を記述するGaudin-Yang modelに複素ランジュバン法を適用した結果を紹介する。特に、粒子数のインバランスがある系におけるpolaron energyの計算結果が絶対零度における厳密解(Bethe Ansatz)とconsistentな結果を得たので、その詳細について紹介する。