# Coffee Meeting Log

2023-05-19

## Theory of diffusive shock acceleration

Hirotaka Ito (Research Scientist, iTHEMS / Research Scientist, Spin isospin Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Particle acceleration is an ubiquitous physical process in the Universe. Diffusive shock acceleration (DSA) is widely accepted as the predominant mechanism responsible for particle acceleration in various astrophysical phenomena. This theory postulates that the energy distribution of the accelerated particles conforms to a power-law form characterized by E^{-2}. In this 15-minute talk, I will outline the derivation of this power-law index of -2 within the theory of DSA.

2023-04-28

## Look up the sky at multiwavelength

Naomi Tsuji (Visiting Scientist, iTHEMS / Assistant Professor, Faculty of Science, Kanagawa University)

Light (i.e., photons or electromagnetic waves) provides us with important information of sources which emit the light. Measurements of light from the sky/Universe at different wavelengths can open up a different fields of physics. While my major is observations of high-energy (X-ray or gamma-ray) photons from astrophysical sources at the distance, since I moved to Kanagawa University I also work for optical measurements of nearby stars by using a telescope at the university. I will briefly introduce my current works at the coffee meeting.

2023-04-21

## Discrete approximation to the Laplacian

Keita Mikami (Research Scientist, iTHEMS)

In this coffee talk, I will introduce recent results on the approximation of Laplacian by discrete operators. Very modestly, the differential is a limit of difference. This modest picture is not always true if we consider differential operators. However, if one considers the norm resolvent convergence, one can show that we can approximate Laplacian by second-order difference operators. Refferences: Nakamura, S., Tadano, Y.: On a continuum limit of discrete Schrödinger operators on square lattice. J. Spectr. Theory 11 (2021), no. 1, 355-367.

2023-04-14

## When a Cretan says all Cretans are liars

Tsukasa Tada (Coordinator, iTHEMS / Vice Chief Scientist, iTHEMS)

The famous phrase "All Cretans are liars" said by a Cretan philosopher poses a paradox. This millenia-old paradox lies at the core of Goedel's incompleteness theorem. Recently this paradox has been discussed in the physics literature. I will introduce this recent interest and share my own thoughts.

2023-03-24

## The introduce of twisted bilayer graphene

Congcong Le (Postdoctoral Researcher, iTHEMS)

Graphene, a two-dimensional material possessing Dirac points, is recognized as a topological semimetal. Twisted bilayer graphene, a configuration produced by rotating two layers of graphene relative to each other, can host the energy flat bands contributed by Dirac points. These flat bands serve as a fruitful playground for investigating strong correlations and topological phases, and hence the study of twisted bilayer graphene has brought excitement and impacts to the condensed matter physics community. In this coffee meeting, I will present the introduction of twisted bilayer gaphene.

2023-03-10

## Theoretical analysis of supernova neutrino data

Akira Harada (Special Postdoctoral Researcher, iTHEMS)

A supernova explosion is the explosive death of a massive star. Neutrino observation is important to examine the mechanism. However, a theoretical framework to analyze the observed neutrinos has not been established. In order to make a tractable theory, we constructed an analytic neutrino signal model based on the Lane-Emden equation and thermal neutrino emission. Besides, to bridge the theory and observation, I developed a code, "SPECIAL BLEND," to estimate the mass and radius of the central proto-neutron star by fitting the analytic model to the supernova neutrino observation with the Bayesian approach. Using SPECIAL BLEND, I analyzed the neutrinos from the supernova SN1987A and estimated the mass and radius of the central neutron star.

2023-03-03

## Gravitational methods for condensed matter and a curious puzzle for superfluid effective field theory

Matteo Baggioli (Associate Professor, School of Physics and Astronomy, Shanghai Jiao Tong University, China)

At the beginning of my PhD in theoretical gravitational physics, my advisor gave me a copy of Kittel's book "Introduction to solid state theory" to study. I was confused, very confused! After almost ten years of research, I have fully embraced the power of the gauge-gravity duality as a tool to investigate open questions in different fields such as condensed matter or nuclear theory. After briefly reviewing the most salient features and successes of this program, I will share with you an interesting puzzle for the effective field theory of superfluids that I recently became aware of. Maybe, we can find a solution to it together!

2023-02-24

## Physical and local observables in gravitational theory

Christophe Goeller (Humboldt Fellow, Ludwig-Maximilians-Universität München, Germany)

I will discuss a general formalism for the construction of dynamical reference frames and local and physical observables in gauge and gravitational theories and their interactions with the gauge charges and their associated field transformations.

2023-02-17

## Can you hear the shape of a drum?

Christopher Bourne (Visiting Scientist, iTHEMS / Assistant Professor, Advanced Institute for Materials Research (AIMR), Tohoku University)

I will review what it means to 'hear the shape of a drum' and how it offers a glimpse into the interesting ways that analysis and geometry interact.

2023-02-10

## Generative models and stochastic differential equations

Akinori Tanaka (Senior Research Scientist, iTHEMS / Senior Research Scientist, RIKEN Center for Advanced Intelligence Project (AIP))

Generative models, a.k.a statistical models, are widely accepted frameworks for generating “new” samples from given dataset. Recently, a class of generative models, called diffusion models, made great impact on not only researcher’s community but communities on SNS and industries also. I would like to introduce these developments within 15 min. on the blackboard.

2023-02-03

## Quantum decoherence and the Caldeira-Leggett model

Takeru Yokota (Special Postdoctoral Researcher, iTHEMS / Postdoctoral Researcher, The Institute for Solid State Physics (ISSP), The University of Tokyo)

Quantum decoherence is one of the fundamental phenomena of quantum systems caused by coupling to an environment. Such a phenomenon is theoretically analyzed by, for example, the Caldeira-Leggett model, which describes a quantum particle coupled to an environment represented as a collection of harmonic oscillators, and is also investigated by various experiments including cavity QED systems and quantum circuits. In this talk, I will briefly introduce these topics and may refer to our recent related work [Takeru Yokota, Kanta Masuki, Yuto Ashida, arXiv:2208.14107].

2023-01-27

## Quantum many-body physics with cold atoms

Yuta Sekino (Postdoctoral Researcher, iTHEMS / Postdoctoral Researcher, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Physical systems consisting of many quantum particles are ubiquitous in diverse fields of physics. Such systems called quantum many-body systems include solids surrounding us, superconductors used for quantum computations, and neutron matter in neutron stars. Understanding of such quantum many-body systems is, however, often challenging. Among various systems, cold atoms, which are atomic gases below 10^-6 kelvin, are ideal platforms for investigating quantum many-body physics because of their simplicity and high controllability. In this talk, I explain how studies of cold atoms have contributed to deepen our understanding of quantum many-body systems.

2023-01-20

## complexity in non-unitary dynamics of photons

Ken Mochizuki (Hamazaki Hakubi Lab.)

Sampling probability distributions of photons in optical networks can be hard for classical computers. Therefore, the boson sampling problem has been extensively explored from the viewpoint of the quantum supremacy. However, sampling photon distributions is not always hard and physical situations determine the hardness, which suggests that the computational complexity can be used to characterize quantum dynamics and quantum phases. I discuss the distinction between unitary and non-unitary dynamics from such a point of view, where the former and later respectively correspond to isolated and open quantum systems.

2023-01-13

## How can we describe the hierarchical structure of our Universe?

Nagisa Hiroshima (Visiting Scientist, iTHEMS / Assistant Professor, Department of Physics, Faculty of Science, University of Toyama)

Our Universe exhibits a highly hierarchical structure. Halos, which are gravitationally bounded objects of dark matter, are building blocks of such structures. Halos are distributed in more than 20 orders of magnitudes in the mass scale. For example, our Milky Way resides in a halo of which mass is about a trillion solar masses. Indications about the nature of dark matter could be obtained by studying the hierarchical structures of dark matter halos while it needs considerations about the schemes to cover a wide mass range. In this talk, I will introduce a simple framework to describe the structure based on analytical descriptions.

2022-12-23

## Physics Through the Looking Glass

Puttarak Jai-akson (Postdoctoral Researcher, iTHEMS)

The terminology Carrollian has recently shown up in many fields of theoretical physics. In this short talk, I will explain what Carrollian means and why some physicists find it interesting.

2022-12-09

## Supernova signatures of neutrino mass ordering

Maria Manuela Saez (Postdoctoral Researcher, iTHEMS)

Supernovae events are one the most powerful cosmic sources of neutrinos with energies of several MeV. The emission of neutrinos and antineutrinos of all flavors carries away the gravitational binding energy of the compact remnant and drives its evolution from the hot initial to the cold final state. I will briefly describe how to detect these neutrinos from Earth, and how to use these data to address the neutrino mass ordering problem.

2022-12-02

## Quantum many body treatment of collective neutrino flavor oscillations

Ermal Rrapaj (Postdoctoral Researcher, iTHEMS)

Collective flavor oscillations occur in extreme environments like supernovae and binary neutron star mergers where many neutrinos are produced in a short period of time. I will describe the quantum many body treatment and how it effects the evolution.

2022-11-25

## Counting by Drawing

Michel Van Garrel (Assistant Professor, School of Mathematics, University of Birmingham, UK)

How many other circles can you find that meet each of the 3 circles in exactly 1 point? I will talk about fun questions like this one that are solved by drawing, and have been with mathematics all along.

2022-11-18

## Knot theory and its interactions with other fields

Masaki Taniguchi (Special Postdoctoral Researcher, iTHEMS)

Knot theory is one of subjects in the field of topology. I'll exlain what are purposes of knot theory and its interactions with other fields.