Among the numbers of dark matter (DM) scenarios, Weakly Interacting Massive Particle (WIMP) is one of the best-studied particles. It attracts special attention because of its beautiful mechanism to achieve the relic abundance of dark matter which is referred to as the thermal freeze-out. In the early Universe, the annihilation of the WIMP into the standard model (SM) particles and its inverse process are frequent enough to sustain the thermal equilibrium. At a certain point, WIMP decouples from the thermal bath then its number density, which is directly related to the relic abundance, is fixed. The strength of the coupling between WIMP and the SM sector determines the annihilation frequency.

The same coupling is also responsible for the energy transfer between the SM particles and WIMP. The constraints on that scattering cross-section (i.e., the energy transfer between two sectors) have already been severely constrained by direct detection experiments. This motivates us to consider such a model that the coupling is so small and the relic abundance is achieved by a resonant annihilation at a certain mass. In this way, we can satisfy the requirements from the results of direct detection experiments.

The small coupling between the SM sector and WIMP could lead to a non-equilibrium between them around the freeze-out epoch. Such a situation is referred to as the kinetic decoupling. The kinetic decoupling before the freeze-out affects the calculation of the relic density, which is a crucial part of the WIMP scenario. In this talk, Dr. Abe clarified the effect of the kinetic decoupling in terms of the coupling constant taking two examples of fermionic and scalar DM in the Higgs portal scenario. He calculated the freeze-out process in detail and showed that the scattering cross-section, which is measured at direct detection experiments, should also be different from the case where assumes a complete thermal equilibrium.

We could see the signature of such DM in near-future experiments. We should carefully work on the synergy between different types of experiments in this high-precision era of DM search.

The iTHEMS Theoretical Physics Seminar is hold on June 12, 2020. The speaker is Masaru Hongo in University of Illinois at Chicago/RIKEN iTHEMS. The title is ”Field theoretical approach to relativistic hydrodynamics”.

Hydrodynamics is a low-energy effective theory of a conserved charge density, which describes a long-distance and long-time behavior of many-body systems. It is applicable not only to a non-relativistic weakly-interacting dilute gas but also a relativistic strongly-interacting dense liquid like a quark-gluon plasma. The main purpose of this seminar is to explain how we can derive the hydrodynamic equation from the underlying field-theoretical description of systems [1-3]. The derivation is based on the recent development of non-equilibrium statistical mechanics, and they show that the procedure to derive hydrodynamic equations is similar to the so-called renormalized/optimized perturbation theory. Also, to describe transport phenomena in local thermal equilibrium, they give a path-integral formula for a thermodynamic functional, which results in the emergence of thermally induced curved spacetime [2]. These results enable us to derive hydrodynamic equation based on quantum field theories.

On 10th June, Martin Skrodzki talked about Turing models. Historically, Turing's models have been very successful to describe various pattern-formations on a two dimensional plane. He briefly introduced examples and then talked about three dimensional space, showing amazingly diverse patterns the model can cover. His talk generated stimulation discussion among the audience. Thank you so much for the great talk, Martin!
-Ryosuke Iritani

Title: PT-symmetric non-Hermitian quantum many-body system using ultracold atoms in an optical lattice with controlled dissipation
Author: Yosuke Takasu, Tomoya Yagami, Yuto Ashida, Ryusuke Hamazaki, Yoshihito Kuno, Yoshiro Takahashi
arXiv: http://arxiv.org/abs/2004.05734v2