# Seminar

## Fallback Accretion in Binary Neutron Star Mergers

July 9 at 16:00 - 17:30, 2021

Dr. Wataru Ishizaki (Postdoctoral Fellows, Yukawa Institute for Theoretical Physics, Kyoto University)

The gravitational wave event GW170817 with a kilonova shows that a merger of two neutron stars ejects matter with radioactivity including r-process nucleosynthesis. A part of the ejecta inevitably falls back to the central object, possibly powering long-lasting activities of a short gamma-ray burst (sGRB), such as extended and plateau emissions. We investigate the fallback accretion with the r-process heating by performing one-dimensional hydrodynamic simulations and developing a semi-analytical model. We show that the usual fallback rate dM/dt \propto t^{-5/3} is halted by the heating. The characteristic halting timescale is $\sim 10^4$--$10^8$ sec for the GW170817-like r-process heating, which is long enough to continue the long-lasting emission of sGRBs. Furthermore, we propose a new interpretation of the recently reported re-brightening in the annual-scale X-ray light curve of GW170817. We model the fallback of the merger ejecta and construct a simple light curve model from the accreting ejecta. We find that the X-ray flux excess can be well explained by the fallback of the post-merger ejecta such as the disk wind from the accretion disk of the merger remnant rather than by the fallback of the dynamical ejecta. The duration of the constant luminosity phase conveys the initial fallback timescale t_0 in the past. Future observations in the next decades will probe the timescale of t_0 \sim 10--10^4 sec, around the time of extended emission in short gamma-ray bursts.

Venue: via Zoom

Event Official Language: English

## Non-Unitary TQFTs from 3d N=4 Rank-0 SCFTs

July 5 at 13:00 - 14:30, 2021

Dr. Myungbo Shim (Kyung Hee University, Republic of Korea)

We propose a novel procedure of assigning a pair of non-unitary topological quantum field theories (TQFTs), TFT_\pm[T_0], to a (2+1)D interacting N=4 superconformal field theory (SCFT) T_0 of rank 0, i.e. having no Coulomb and Higgs branches. The topological theories arise from particular degenerate limits of the SCFT. Modular data of the non-unitary TQFTs are extracted from the supersymmetric partition functions in the degenerate limits. As a non-trivial dictionary, we propose that F = max{ -log |S^{(+)}_{0\alpha}| } = max{ -log |S^{(-)}_{0\alpha}| }, where F is the round three-sphere free energy of T_0 and S^{(\pm)}_{0\alpha} is the first column in the modular S-matrix of TFT_\pm. From the dictionary, we derive the lower bound on F, F > -log(\sqrt{(5-\sqrt{5})/10}) \simeq 0.642965, which holds for any rank 0 SCFT. The bound is saturated by the minimal N=4 SCFT proposed by Gang-Yamazaki, whose associated topological theories are both the Lee-Yang TQFT. We explicitly work out the (rank 0 SCFT)/(non-unitary TQFTs) correspondence for infinitely many examples. Before going to the technical part, we provide some background materials including some peculiar features in 3d gauge theories, some supersymmetries, anyons, and some modular data of MTC in this talk.

Venue: via Zoom

Event Official Language: English

## An introduction to modular functions, conformal field theories, and moonshine phenomena

July 2 at 16:00 - 18:10, 2021

Mr. Mizuki Oikawa (Junior Research Associate, iTHEMS / Student Trainee, iTHEMS / Ph.D. Student, Graduate School of Mathematical Sciences, The University of Tokyo)

Moonshine phenomena are certain mysterious connections between modular functions and finite groups. The first example is the celebrated monstrous moonshine, which connects the J-invariant and the Monster group. Surprisingly, this relationship can be well understood in terms of chiral conformal field theory. In this talk, I would like to explain what is chiral conformal field theory and how it gives moonshine phenomena. In the first part of the talk, the notion of modular function will be introduced and the precise statement of the monstrous moonshine will be given. Then the monstrous moonshine will be explained in terms of vertex operator algebra, a mathematical model of chiral conformal field theory. In the second part of the talk, we focus on the question: what is chiral conformal field theory mathematically? In addition to vertex operator algebras, other mathematical models of chiral conformal field theory, namely conformal nets and Segal conformal field theories, will be introduced. Recent progress on the relationship among these three models, including the Carpi--Kawahigashi--Longo--Weiner correspondence and the geometric realization of conformal nets will also be reviewed.

Venue: via Zoom

Event Official Language: English

## How nucleus size affects chromatin motion? - Experimental measurements and a polymer physics theory

July 1 at 10:00 - 11:00, 2021

Prof. Takahiro Sakaue (Associate Professor, College of Science and Engineering, Department of Physics and Mathematics, Aoyama Gakuin University)

Chromatin moves dynamically inside the cell nucleus, and its motion is often correlated with gene functions such as DNA recombination and transcription. A recent study has shown that during early embryogenesis of the nematode, Caenorhabditis elegans, the chromatin motion markedly decreases with the cell stage. However, the underlying mechanism for this transition has yet to be elucidated. Here we systematically investigate the impact of nuclear size to demonstrate that it is indeed a decisive factor in chromatin mobility. We show that a simple theoretical description, which takes into account the length and time scales of chromatin polymer solution, can quantitatively describe the relationship between the nucleus size and the chromatin motion in vivo. Our results emphasize a regulatory role of nuclear size in restricting chromatin motion, and a generic polymer physics model plays a guiding role in capturing this essential feature. *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

## Toward QCD-based description of dense baryonic matter

June 29 at 13:00 - 14:30, 2021

Dr. Yuki Fujimoto (The University of Tokyo)

The equation of state (EoS) of dense baryonic/quark matter is the crucial ingredient for understanding neutron stars. I briefly review the current state of the high-density matter EoS based on the QCD perspectives. In this talk, I particularly focus on the perturbative QCD (pQCD) EoS, which was previously thought to be useless at realistic density because it is plagued by the large uncertainty. I introduce our recent analysis of the EoS calculated within the pQCD framework with the resummation [Fujimoto & Fukushima, 2011.10891]. I discuss our scheme for the Hard Dense Loop resummation, which turns out to reduce the uncertainty compared with the conventional pQCD estimate without resummation. Our result apparently extends the applicability of the QCD-based EoS down to densities realized inside neutron stars and infers a smooth matching with the baryonic EoS.

Venue: via Zoom

Event Official Language: English

## Theory of Core-Collapse Supernovae

June 25 at 16:00 - 17:00, 2021

Dr. Akira Harada (Special Postdoctoral Researcher, iTHEMS)

Venue: via Zoom

Event Official Language: English

## Precise WIMP Dark Matter Abundance and Standard Model Thermodynamics

June 24 at 16:30 - 17:30, 2021

Dr. Satoshi Shirai (Project Assistant Professor, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)

We are now living in the era of precision cosmology. The relic abundance of dark matter (DM) is now observationally well-determined, and its error is smaller than O(1)%. This means that the same or much higher precision is required when we make theoretical predictions. Weakly Interacting Massive Particle (WIMP) has long been the leading candidate for DM because of its beautiful mechanism to predict the observed relic abundance. WIMP is in the same thermal bath as the Standard Model particles in the beginning. At a certain point when the temperature of the Universe is smaller than the DM mass, it decouples to fix its number density. The yield of the DM is determined by its annihilation cross-section to the Standard Model sector. It seems that there is no ambiguity in the calculation of this process at first: the cross-section is purely theoretical and all the remainings are described in the Standard Model physics. However, the source of the uncertainty does remain in the Standard Model sector. The dilution of the number density of DM particle depends on the expansion rate of the Universe, which is determined by the Standard Model particles. The effective degree of freedom (d.o.f) of the relativistic species controls this factor. We have to deal with the non-equilibrium dynamics to precisely describe the time-evolution of the d.o.f, in which we need numerical approaches. In this talk, he introduced his work to update these calculations. By implementing the latest findings in the non-equilibrium dynamics in i) the neutrino decoupling, ii) the QCD phase transition, iii) the electroweak phase transition, and iv) the perturbative calculations, they found that the final d.o.f is smaller than the previous estimate in more than 1%. This is larger than the level of precision in observations. It is also important that the uncertainty is quantified by them. Another good news is that he makes the calculated d.o.f with its error publically available. With these updates, we now correctly know the points to probe DM!

Venue: via Zoom

Event Official Language: English

## Period variability can provide valuable information in oscillatory systems

June 24 at 10:00 - 11:00, 2021

Prof. Fumito Mori (Assistant Professor, Department of Human Science, Faculty of Design, Kyushu University)

Biological clocks generate temporally precise oscillations although they are subjected to various types of noise. In other words, oscillations with only a small variability in the period are observed under action of noise. In this talk, I focus on period variability in coupled phase oscillators and complex oscillatory dynamical systems, and present the following topics:(i) A phase oscillator subjected to noise can become to generate more precise oscillations not only when it is synchronized with periodic signal but also when it is mutually synchronized with another phase oscillator with stronger noise. (ii) In complex oscillatory dynamical systems, period variability is sensitive to the choice of an output variable and output checkpoint; it can be reduced by an appropriate selection of them. (iii) Noise intensity and coupling strength in synchronized phase oscillators can be inferred from data about period variability. This talk is based on joint researches with Dr. Hiroshi Kori (Tokyo Univ) and Dr. Alexander Mikhailov (Kanazawa Univ). *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

## Introduction to the replica method

June 23 at 13:30 - 15:40, 2021

Prof. Yoshiyuki Kabashima (Professor, Graduate School of Science, The University of Tokyo)

The replica method is a mathematical technique for evaluating the "quenched" average of logarithm (or a real number power) of the partition function with respect to predetermined random variables that condition the objective system. The technique has a long history, dating back at least to a book by Hardy et al in 1930s, but has become well known only since its application to the physics of spin glasses in 1970s. More recently, its application range is spreading rapidly to various fields in information science, including information theory, communication theory, signal processing, computational complexity theory, machine learning, etc. In this talk, we introduce the basic idea of the replica method and its mathematical fault illustrating a few examples. *Detailed information about the seminar refer to the email.

Venue: via Zoom

Event Official Language: English

## Black Hole Information Paradox and Wormholes

June 21 at 13:00 - 14:30, 2021

Dr. Kanato Goto (Special Postdoctoral Researcher, iTHEMS)

In this talk, I will explain about the recent progress in the black hole information paradox that I am involved with. The information paradox arises when a black hole evaporates by emitting Hawking radiation due to the quantum effects. Time dependence of the entropy of Hawking radiation is diagnosis of information loss caused by the black hole evaporation. If information is not lost, the entropy of Hawking radiation should obey the so-called Page curve. In recent research developments, it was found that “the quantum extremal islands” reproduce the unitary Page curve in an evaporating black hole. I will argue about how the quantum extremal islands are derived from the computation of the entropy of Hawking radiation using the gravitational path-integral.

Venue: via Zoom

Event Official Language: English

## Stable eigenvalues of compact anti-de Sitter 3-manifolds

June 18 at 16:00 - 18:10, 2021

Dr. Kazuki Kannaka (Special Postdoctoral Researcher, iTHEMS)

Geometric objects that have been investigated in detail so far, such as closed Riemann surfaces, are sometimes locally homogeneous. Loosely speaking, their infinitesimal behavior is the same at each point. In this talk, I would like to explain the idea of investigating such objects using the Lie group theory.In the first part of the talk, I will recall the notions of Lie group actions and their quotient spaces with examples, and then explain the definitions of locally homogeneous spaces and their deformations (Teichmüller spaces). In the second part of the talk, I will consider anti-de Sitter manifolds as a special case, i.e., Lorentzian manifolds of negative constant curvature. As in the Riemannian case, a differential operator called the Laplacian (or the Klein-Gordon operator) is defined on Lorentzian manifolds. Unlike the Riemannian case, it is no longer an elliptic differential operator but a hyperbolic differential operator. In its spectral analysis, new phenomena different from those in the Riemannian case have been discovered in recent years, following pioneering works by Toshiyuki Kobayashi and Fanny Kassel. I would like to explain stable eigenvalues of the hyperbolic Laplacian of anti-de Sitter 3-manifolds with recent progress.

Venue: via Zoom

Event Official Language: English

## The origin and dispersal of buckwheat

June 10 at 10:00 - 11:00, 2021

Dr. Jeffrey Fawcett (Senior Research Scientist, iTHEMS)

Buckwheat, which soba noodles is made from, originated from a wild species that is distributed in southwest China, around Yunnan, Sichuan, and Tibet. We are trying to understand when, where, and how it originated and then spread across the world and came to Japan. To do so, we are using genomic data of wild samples from China and cultivated samples from various parts of the world. I will give a brief introduction about buckwheat, explain what we already know about its origin and dispersal, and show a bit of our results. I will also explain the significance of studying "domestication", that is, the process that plants/animals that humans currently use originated from their ancestral wild species. *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

## iTHEMS-phys Intro Meeting on June 7, 2021

June 7 at 13:00 - 14:30, 2021

Dr. Tomoya Naito
(Student Trainee, Quantum Hadron Physics Laboratory, RIKEN Nishina Center for Accelerator-Based Science (RNC) / Ph.D. Student, Department of Physics, Graduate School of Science, The University of Tokyo)

Dr. Takeru Yokota
(Special Postdoctoral Researcher, iTHEMS)

Dr. Naomi Tsuji
(Postdoctoral Researcher, iTHEMS)

13:00-13:20 [JST] Tomoya Naito 13:20-13:40 [JST] Takeru Yokota 13:40-14:00 [JST] Naomi Tsuji 14:00- Free discussion

Venue: via Zoom

Event Official Language: English

## An overview of genome-wide epistasis and co-selection analysis

June 3 at 10:00 - 11:00, 2021

Dr. Yingying Xu (Special Postdoctoral Researcher, iTHEMS)

This talk is a summary of research that have done by me and my team during 2016~2019. I was a postdoc researcher in Aalto university/Helsinki university in Finland. In the team, a worldwide active collaboration has happened between many fields including statistical physics, biology, computer science and statistics. The target is to analyze ultra-high dimensional large population genomic datasets of two major human pathogens, Streptococcus pneumoniae and Neisseria meningitidis, without phenotypic data. Interacting networks of resistance, virulence and core machinery genes are identified. Many different approaches have been invented and they can be generally applied to other datasets with similar mathematical setting. I will explain methods based on statistical model [1,2], mutual information [3], and theoretical performance analysis for statistical model [4]. In the end, I will briefly introduce a new phenomenon of random matrix which is discovered during the research process for statistical significance filtering [5]. *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

## iTHEMS-phys Intro Meeting on June 1, 2021

June 1 at 13:00 - 15:00, 2021

Dr. Kengo Kikuchi
(Special Postdoctoral Researcher, iTHEMS)

Dr. Enrico Rinaldi
(Research Fellow, Physics Department, University of Michigan, USA)

Dr. Hiroshi Yokota
(Postdoctoral Researcher, iTHEMS)

13:00-13:20 [JST] Kengo Kikuchi 13:20-13:40 [JST] Enrico Rinaldi 13:40-14:00 [JST] Hiroshi Yokota 14:00- Free discussion

Venue: via Zoom

Event Official Language: English

## iTHEMS-phys Intro Meeting on May 31, 2021

May 31 at 13:00 - 15:00, 2021

Dr. Ryo Namba
(Senior Research Scientist, iTHEMS)

Dr. Naritaka Oshita
(Special Postdoctoral Researcher, iTHEMS)

Dr. Yuki Yokokura
(Senior Research Scientist, iTHEMS)

Dr. Shigehiro Nagataki
(Deputy Program Director, iTHEMS)

13:00-13:20 [JST] Ryo Namba 13:20-13:40 [JST] Naritaka Oshita (10 mins break) 13:50-14:10 [JST] Yuki Yokokura 14:10-14:30 [JST] Shigehiro Nagataki 14:30- Free discussion

Venue: via Zoom

Event Official Language: English

## Magnetorotational Instability: Current Understanding and Perspective

May 28 at 16:00 - 17:00, 2021

Dr. Takashi Minoshima (Researcher, Japan Agency for Marine-Earth Science and Technology (JAMSTEC))

The differentially rotating flow can be destabilized in the presence of a weak magnetic field through the magnetorotational instability (MRI). The MRI is considered as a possible mechanism for outward angular momentum transport and subsequent mass accretion in accretion disks. Numerous studies have been devoted to understand its nature and judge whether it can supply the power sufficient for observed transport efficiency. For example, the MHD simulation studies have attempted to reveal the scaling of the MRI on numerical (e.g., resolution and domain size) as well as physical parameters (e.g., magnetic field intensity and configuration). In this talk, I would like to discuss current understanding and perspective of the MRI through theoretical and numerical studies. I will especially focus on the impact of transport coefficients (viscosity, resistivity, and their ratio) on the evolution of the MRI and disk.

Venue: via Zoom

Event Official Language: English

## A Mathematical Model for Stem Cell Competition to Maintain a Cell Pool Injured by Radiation Exposure

May 27 at 10:00 - 11:00, 2021

Dr. Kouki Uchinomiya (Central Research Institute of Electric Power Industry)

Cancer risk of low-dose-rate ionizing radiation exposure is one of the most important issues in radiation protection. Tissue stem cells have been considered one of the targets of radiation-induced carcinogenesis. There has been a hypothesis that the carcinogenic effects of radiation can be reduced if damaged stem cells are eliminated via stem cell competition between damaged and intact stem cells. This would be particularly effective under very low-dose-rate conditions, where only a few stem cells in a stem cell pool are affected by radiation. In this presentation, I will introduce a simple mathematical model to discuss the influence of stem cell competition on the accumulation of radiation damage and show that the character of damaged cells and the size of the stem cell pool may affect the accumulation of radiation damage. *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

## Theory of Anomalous Floquet Higher-Order Topology

May 26 at 22:00 - 23:15, 2021

Dr. Rui-Xing Zhang (University of Maryland, College Park, USA)

Periodically-driven or Floquet systems can realize anomalous topological phenomena that do not exist in any equilibrium states of matter, whose classification and characterization require new theoretical ideas that are beyond the well-established paradigm of static topological phases. In this work, we provide a general framework to understand anomalous Floquet higher-order topological insulators (AFHOTIs), the classification of which has remained a challenging open question. In two dimensions (2D), such AFHOTIs are defined by their robust, symmetry-protected corner modes pinned at special quasienergies, even though all their Floquet bands feature trivial band topology. The corner-mode physics of an AFHOTI is found to be generically indicated by 3D Dirac/Weyl-like topological singularities living in the phase spectrum of the bulk time-evolution operator. Physically, such a phase-band singularity is essentially a "footprint" of the topological quantum criticality, which separates an AFHOTI from a trivial phase adiabatically connected to a static limit. Strikingly, these singularities feature unconventional dispersion relations that cannot be achieved on any static lattice in 3D, which, nevertheless, resemble the surface physics of 4D topological crystalline insulators. We establish the above higher-order bulk-boundary correspondence through a dimensional reduction technique, which also allows for a systematic classification of 2D AFHOTIs protected by point group symmetries. We demonstrate applications of our theory to two concrete, experimentally feasible models of AFHOTIs protected by C2 and D4 symmetries, respectively. Our work paves the way for a unified theory for classifying and characterizing Floquet topological matters. *Detailed information about the seminar refer to the email.

Venue: via Zoom

Event Official Language: English

## iTHEMS-phys Intro Meeting on May 25, 2021

May 25 at 13:00 - 15:00, 2021

Dr. Takuya Sugiura
(Postdoctoral Researcher, iTHEMS)

Dr. Takumi Doi
(Senior Research Scientist, Quantum Hadron Physics Laboratory, RIKEN Nishina Center for Accelerator-Based Science (RNC))

Dr. Shoichiro Tsutsui
(Special Postdoctoral Researcher, Quantum Hadron Physics Laboratory, RIKEN Nishina Center for Accelerator-Based Science (RNC))

Dr. Kanato Goto
(Special Postdoctoral Researcher, iTHEMS)

13:00-13:20 [JST] Takuya Sugiura 13:20-13:40 [JST] Takumi Doi (10 mins break) 13:50-14:10 [JST] Schoichiro Tsutsui 14:10-14:30 [JST] Kanato Goto 14:30- Free discussion

Venue: via Zoom

Event Official Language: English

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