# Coffee Meeting Log

2022-06-10

## Data analysis, parameter estimation and error bars

Catherine Beauchemin (Deputy Program Director, iTHEMS / Professor, Department of Physics, Ryerson University, Canada)

Even though my research is focused on virophysics, most of my day-to-day work consists in estimating the values of a model's parameters based on observational data. In this talk, I want to introduce the simple mathematics and computational methods involved in parameter estimation, and some of the common pitfalls. I will use specific example from my past and current work.

2022-05-27

## How do you measure the distance between the Earth and the Sun?

Shigehiro Nagataki (Deputy Program Director, iTHEMS / Chief Scientist, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Accurate measurement of distance is crucial to understanding the universe. In 1998, about 25 years ago, human beings became convinced that the universe is filled with dark energy, which was revealed by accurately determining the distance to supernovae locations. It is the first step in measuring the distance in the universe to determine the distance between the Earth and the Sun. How do you determine the distance between the Earth and the Sun? Of course, you cannot use a ruler from the Earth to the Sun. Once the distance between the Earth and the Sun is determined, the mass of the Sun can be obtained. Once the distance to the Sun is known, the radius of the Sun can also be determined. Then, by using physics, we can understand what is going on inside the Sun even though we cannot see the inside of the Sun by photons. From this understanding, we know that the Sun has a life span of about 5 billion years and that human beings have only about 1 billion years left to live on the Earth. In this 15-minute talk, I would like to present how to determine the distance between the Sun and the Earth.

2022-05-20

## Origami Embeddings of flat tori

Takashi Tsuboi (Deputy Program Director, iTHEMS / Musashino University)

Flat tori appear in many places in mathematics; in complex analysis, in geometry, in algebra, ... It cannot be smoothly isometrically embedded in the 3-dimensional Euclidean space but Nash and Kuiper showed that it is possible in C1 smoothability in 1954-55. It is natural to ask whether we can embed flat tori isometrically as a piecewise-linear object, and Burago-Zalgaller showed it possible in 1996. Their construction is theoretically simple but actually complicated to show it as an object. But Henry Segerman gave a nice simple embedding, a Hinged Flat Torus, and this lead us to find a simple isometric piecewise-linear embedding for the flat torus of any modulus, which I am happy to talk about.

2022-05-13

## A dynamical proposal to resolve the cosmological constant problem

Ryo Namba (Senior Research Scientist, iTHEMS)

Our universe is observed to be expanding at an accelerated rate. The expansion is driven by some unknown object that has a constant energy density, thus called "cosmological constant" (a.k.a. dark energy). Since gravitational effects, which drive the expansion, do not discriminate any forms of matter/energy, this constant is expected to receive various contributions from high-energy physics. However, the observed value of the constant is smaller than the theoretical expectation by many orders of magnitute, the discrepancy called the cosmological constant problem. A mechanism has been proposed to cancel the large contributions by a classical dynamics in the early universe, but it essentially empties the universe altogether, not just the cosmological constant. We propose a concrete scenario that subsequently "reheats" the universe with energetic matter, thus completing the mechanism of the cosmological constant relaxation.

2022-04-22

## Comparison between conventional computers and quantum computers

Ching-Kai Chiu (Senior Research Scientist, iTHEMS)

The transistor invention ushered in the era of conventional computers. However, Moore’s law will fail one day due to the unavoidable quantum limit. Scientists are building quantum computers to continue the computing era. In this talk, I will compare conventional computers and quantum computers. Furthermore, I will briefly introduce that a topological superconductor can be one of the platforms for quantum computing.

2022-04-15

## Generalized generating function for proportion values and its application

Ryosuke Iritani (Research Scientist, iTHEMS)

I will talk about a potentially interesting and useful methodology I partly devised, based on (probability) generating function methods.

2022-03-18

## Weighted and directed XRP network

Abhijit Chakraborty (Kyoto University)

XRP is a well-known crypto currency. In this talk, I will introduce the structure of weighted directed networks from XRP transactions.

2022-03-11

## Borromean nucleus: an extreme state of the atomic nucleus

Masaru Hongo (Visiting Scientist, iTHEMS / Postdoctoral Research Associate, Physics Department, The University of Illinois at Chicago (UIC), USA)

I will introduce the Borromean nucleus, an exotic extreme state of the atomic nucleus.

2022-03-04

## Study on quantum interactions and number theory

Masato Wakayama (Senior Advisor, iTHEMS / Fundamental Mathematics Research Principal, NTT Institute for Fundamental Mathematics)

In this Coffee-meeting, I will discuss the energy spectra of certain models of quantum interactions (between photon and two-level system) and introduce the study in number theory that arise from them including future perspective and conjectures.

2022-02-25

## Derived Categories in Algebraic Geometry

Naoki Koseki (Postdoctoral Research Associate, School of Mathematics, University of Edinburgh, UK)

I will explain how derived categories play roles to connect various areas of mathematics, especially birational and enumerative geometry. The key ideas are "Categorifications" and "Semi-orthogonal decompositions"

2022-02-18

## Stochastic chemical reaction systems

Yuji Hirono (Visiting Scientist, iTHEMS / Junior Research Group Leader/Assistant Professor, Research Division, Asia Pacific Center for Theoretical Physics, Republic of Korea)

"Chemical reactions play important roles in various fields including engineering and biology. In this talk, I plan to give a short introduction to the description of stochastic chemical reaction systems."

2022-02-04

## Universal form of stochastic equations of motion

Masato Itami (Kyoto University)

I will review the universal form of stochastic time evolution for slow variables in equilibrium systems and talk about one attempt to derive a universal form in nonequilibrium systems.

2022-01-28

## Determination of hadronic interactions from lattice QCD

Takuya Sugiura (Postdoctoral Researcher, iTHEMS)

The interactions between hadrons (such as proton and neutron) govern the structure of atomic nucleus. I will briefly discuss how we can define and quantify the hadronic interactions through numerical calculations of lattice QCD.

2022-01-21

## From the thermonuclear supernova to the supernova remnant

Gilles Ferrand (Research Scientist, iTHEMS / Researcher, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Type Ia supernovae (SNe) are believed to be the thermonuclear explosion of a white dwarf star, but their explosion mechanism(s) remain unclear. In this talk we make the connection with the subsequent phase, the supernova remnant (SNR), when the stellar ejecta interact with the interstellar medium. I will outline how simulations and observations can help test theoretical models, with an emphasis on our recent work based on the 3D morphology.

2022-01-14

## Cosmic origin of r-process elements?

Nobuya Nishimura (Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

The astronomical site of r-process nucleosynthesis has been a long-standing mystery in nuclear astrophysics studies. However, in recent years, there has been significant progress. We confirmed the r-process-driven transient "kilonova" associated with binary-neutron-star mergers. In this talk, I will discuss the possibility of the r-process in magneto-rotational supernovae, which are in the extreme class of core-collapse supernovae. Although this scenario is physically challenging and hypothetical, observational evidence appears to grow. Since this scenario is related to supernovae with many observational examples, we expect future direct/indirect observation. I will also discuss the theoretical perspective of observational properties.

2021-12-24

## Quantum chaos, Entanglement and Black holes

Tomoki Nosaka (Research Part-time Worker, iTHEMS)

Quantum chaos is a notion to characterize the mechanism of how the time evolution of a quantum many body system, which should be in principle reversible, end up with the thermal equilibrium where the information of the initial condition is lost. This resembles the black hole information problem, which suggests that one can address various fundamental questions related to black holes through the study of quantum chaos. In this talk I will overview the notions of quantum chaos and black hole geometry, and then introduce an interesting phenomenon called revival where these notions are tightly entangled with each other.

2021-12-17

## Dimensional regularization

Matthias Berwein (Postdoctoral Researcher, iTHEMS)

Dimensional regularization is one of the most important tools for analytic calculations in quantum field theory. I will explain, how the concept of integration over (integer) n variables can be extended to a continuous number D, the dimension, and how this can be used to make sense of the divergent parts of quantum corrections.

2021-12-10

## Dispersal and organization of glioma cells

Gilberto Nakamura (Postdoctoral Researcher, iTHEMS)

Cell migration plays a pivotal role in modeling the invasiveness of gliomas (a type of brain cancer) while the formation of new tumors often relies on cell-cell adhesive forces. I will talk about motility-induced phase separation as an alternative explanation to tackle both cell migration and tumor formation.

2021-12-03

## Machine leaning approaches for weather prediction

Shigenori Otsuka (Research Scientist, iTHEMS / Research Scientist, Data Assimilation Research Team, RIKEN Center for Computational Science (R-CCS))

Recently, many groups are applying deep learning techniques for weather prediction problems. In this talk, I would like to introduce our research activities at R-CCS. We have been developing an algorithm to integrate data-driven approaches and process-driven approaches for precipitation nowcasting.

2021-11-26

## Evolution of core-collapse supernovae to their supernova remnants

Masaomi Ono (Research Scientist, iTHEMS / Research Scientist, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Observations of nearby supernova remnants (SNRs) could potentially provide the clues to elucidate the dynamical evolution of core-collapse supernovae (CCSNe), chemical evolution of elements in the ejecta, explosion mechanisms, properties of neutron stars and progenitor stars. In this talk, our recent three-dimensional hydrodynamical models for the evolution of CCSNe to their SNR are introduced. Additionally, an attempt to model the molecule formation in the ejecta is also mentioned.