Luca V. Delacretaz, University of Chicago

[QCD theory Seminar] Nonlinear bosonization of Fermi liquids

##### Landau’s theory of Fermi liquids is a cornerstone of theoretical physics. I will show how to formulate Fermi liquid theory as an effective field theory of bosonic degrees of freedom, using the formalism of coadjoint orbits. While at the linear level, this theory reduces to existing multidimensional bosonization approaches, it necessarily features nonlinear corrections that are fixed by the geometry of the Fermi surface. These are crucial to reproduce nonlinear response, such as higher-point functions of currents or densities. The effective field theory framework furthermore systematically parametrizes corrections to Fermi liquid behavior, and provides a new angle to approach non-Fermi liquids. (Based on https://arxiv.org/abs/2203.05004).

Daiki Ueda, Peking University

Entropy constraints on effective field theory

##### Relative entropy is a non-negative quantity and depicts a difference between two probability distributions. In this talk, I explain constraints on perturbative corrections to the Euclidean effective action by the relative entropy. I show that the relative entropy can yield the constraints on some effective field theories generated by a class of UV theories with a specific sign of interactions between heavy and light degrees of freedom. For such a class of theories, I show that the interactions increase thermodynamic entropy at fixed energy and charge, which is intimately connected with the extremality relations of black holes exhibiting Weak-Gravity-Conjecture.

Sigtryggur Hauksson, IPHT, CEA-Saclay

[QCD theory Seminar] Jet broadening in a non-equilibrium QCD medium

##### Heavy-ion collisions produce far-from-equilibrium QCD matter which rapidly becomes a hydrodynamic medium. In this talk, I discuss how jets are shaped by the initial stage of such collisions. Most importantly, I show that anisotropic momentum broadening of jets at early times leads to net spin in emitted gluons. This means that there is net spin polarization of jet partons which is constant at all energy scales. I furthermore discuss jet broadening in the later hydrodynamic stage. Using tools of non-equilibrium field theory, I show that momentum broadening is substantially reduced at lower transverse momenta compared with a medium in thermal equilibrium.

Kentaroh Yoshida, Kyoto University

Yang-Baxter sigma models from 4D Chern-Simons theory

##### Recently, a 4D Cherns-Simons (CS) theory has been proposed by Costello and Yamazaki as a unified theory of 2D classically integrable field theories. In this talk, I will explain how to derive Yang-Baxter sigma models from this 4D CS theory.

Luca Visinelli, Tsung-Dao Lee Institute

[JpDe Joint Seminar] Axion Miniclusters in the Milky Way

##### Axion miniclusters (AMCs) are relatively dense, gravitationally bound clumps of dark matter (DM) QCD axions. AMCs have intriguing observational consequences for Earth-based axion detectors, for DM substructure searches with microlensing, and for radio signatures from AMC encounters with neutron stars (NSs). However, the properties of AMCs in the Milky Way may be drastically altered by tidal interactions with ordinary stars. We present Monte Carlo simulations following the evolution of AMCs orbiting in the Milky Way, which can be used to estimate the properties of AMCs throughout the Galaxy today and can be easily recast. We use this information as a key ingredient in estimating the rate, duration, flux, and sky locations of radio signals from axion-photon conversion due to NS encounters with AMCs. The resulting radio transients are within reach of current and future radio telescopes, opening a new avenue for detecting QCD axion DM.

Koji Hashimoto, Kyoto University

A bound on energy dependence of chaos

##### We conjecture a chaos energy bound, an upper bound on the energy dependence of the Lyapunov exponent for any classical/quantum Hamiltonian mechanics and field theories. The conjecture states that the Lyapunov exponent λ(E) grows no faster than linearly in the total energy E in the high energy limit. In other words, the exponent c in λ(E) ∝ E^c

(E→∞) satisfies c ? 1. This chaos energy bound stems from thermodynamic consistency of out-of-time-order correlators (OTOC’s) and applies to any classical/quantum system with finite N / large N (N is the number of degrees of freedom) under plausible physical conditions on the Hamiltonians. To the best of our knowledge the chaos energy bound is satisfied by any classically chaotic Hamiltonian system known, and is consistent with the cerebrated chaos bound by Maldacena, Shenker and Stanford which is for quantum cases at large N. We provide arguments supporting the conjecture for generic classically chaotic billiards and multi-particle systems. The existence of the chaos energy bound may put a fundamental constraint on physical systems and the universe.

Yuki Yokokura, RIKEN, iTHEMS

Interior Metric of Slowly Formed Black Holes in a Heat Bath

##### We study a spherical black hole formed slowly in a heat bath in the context of ordinary field theory, which we expect to have the typical properties of black holes.

We assume that the matter field is conformal and that the metric satisfies the semi-classical Einstein equation. Then, as a necessary condition, its trace part must be satisfied, which is determined only by the metric through the 4-dimensional Weyl anomaly independently of the quantum state. With some physically reasonable assumptions, this equation restricts the interior metric to a certain class.

Such metrics are approximately warped products of AdS_2 and S^2 with almost Planckian curvature.

Among them, we find one that is consistent with Hawking radiation and is smoothly connected to the exterior Schwarzschild metric slightly outside the Schwarzschild radius.

This leads to a picture that the black hole is a dense object with a surface (not a horizon), which evaporates due to Hawking-like radiation when taken out of the bath.

[arXiv: 2108.02242]

大森寛太郎, 東京大

Intensive lecture on Quantum Anomalies as Projective Phase (in Japanese)

##### この講義では0+1次元と1+1次元における場の量子論とその対称性の量子異常(量子アノマリー、’t Hooft anomaly)を扱う。特に、対称性の状態空間(ヒルベルト空間)への作用と量子異常の関係に焦点を当てたい。

講義の前半では0+1次元場の理論、つまり量子力学について、まず対称性を取り扱うための基本となるWignerの定理を紹介する。

Wignerの定理においては一般に量子力学の状態空間は対称性群の元で射影表現をなすが、この射影因子が量子力学系のダイナミクスを拘束する例を見る。また、量子力学における対称性の射影因子と1+1次元の対称性保護トポロジカル相の関係をみる。

後半では1+1次元の場の理論において、射影表現の類似物が場の量子論を線分上で境界条件付きで量子化することにより得られることを見て、U(1)の場合にこれが量子異常のよりconventionalな定義と一致することを見る。

時間が許せば1+1次元の$Z_2$対称性の量子異常の繰り込み群流れへの応用(CP1模型)を紹介したい。

講義ノートは https://kantohm11.github.io/symmetry_review/ において公開される。(現在準備中で、順次更新する。)

Masaru Hongo, Niigata Univ.

[QCD Theory Seminar] Effective field theoretical approach to weakly bound Borromean nuclei

##### We construct an effective field theory of a two-neutron halo nucleus in the limit where the two-neutron separation energy and the neutron-neutron two-body virtual energy are smaller than any other energy scale in the problem, but the scattering between the core and a single neutron is not fine-tuned, and the Efimov effect does not operate. The theory has one dimensionless coupling, which formally runs to a Landau pole in the ultraviolet. I will demonstrate some universal properties of the system, such as the ratio of the mean-square matter radius and charge radius and the shape of the E1 dipole strength function. I will also discuss the application of our EFT to 22C nucleus, where higher-order corrections to our theory are estimated as of order 20% or less if the two-neutron separation energy is less than 100 keV and the s-wave scattering length between a neutron and a 20C nucleus is less than 2.8 fm.

##### [Ref]

– M. Hongo, D. T. Son, arXiv:2201.09912 [nucl-th]

Ryosuke Sato, Osaka University

Chiral symmetry breaking in large N QCD