# セミナーレポート

2022-01-25

## NEW WG Seminar by Mr. Tomohiro Tanogami on January 20, 2022

Tomohiro Tanogami from Kyoto University gave a talk titled "A simple XY model for cascade transfer." Cascade transfer is a ubiquitous phenomenon that appears in various physical systems. During a cascade transfer an inviscid conserved quantity such as energy is transferred conservatively from large/small to small/large scales, which results in a universal scaling law in the spectrum (e.g., Kolmogorov energy spectrum). In the talk, Tomohiro proposed a simple XY model which shows inverse energy cascade with a non-Kolmogorov energy spectrum and pointed out that the model may characterize a novel "universality class" of cascade transfer. Tomohiro also explained possible relations between his model and, e.g., spin and atmospheric turbulences. About 20 physicists have joined the seminar and enjoyed discussions from various viewpoints. Reported by Hidetoshi Taya

2022-01-20

## iTHEMS Biology Seminar by Dr. Hiroyasu Miyazaki on January 20, 2022

In the iTHEMS Biology Seminar on 20th January, Dr. Hiroyasu Miyazaki (RIKEN iTHEMS) gave a talk on the topological aspect of chemical reaction network. His talk is composed of two parts: motivation and result part and method part. In the first part, he mentioned the reduction of the chemical reaction network in E. Coli and its result which shows the same steady state as that before the reduction. In the second part, he explained how the network should be reduced. The topological information of the network can be described by using the matrix and the vector. He explained that the quotient network is defined to have a sequence of linear maps called “exact sequence”, which describes the relation among the original network, the subnetwork and the quotient network. We enjoyed his beautiful talk. From the audiences, the lively discussion arose during and after the talk. Thank you very much, Hiroyasu! Reported by Hiroshi Yokota

2022-01-18

## DMWG Seinar by Dr. Kanji Mori on January 17, 2022

In this era of multi-messenger astronomy, axion-like particle (ALP) searches using astrophysical transients are now excitingly discussed. ALPs are an extended class of particles from QCD axion, which could solve the strong CP problem of the Standard Model, and dark matter (DM) candidates of various connections to the physics beyond the Standard Model. ALPs could interact with our sector through their coupling to the photon, electron, and/or nucleons. The coupling of ALP to the photon is previously investigated using observations of SN 1987A, the most famous supernova explosion. MeV gamma-ray signals are expected when considering the conversion of ALP outside the stellar envelope, hence no observations of the associated gamma-ray emission to the supernova lead to the constraint on the coupling strength, for example. Also, the existence of such coupling should modify the dynamics of supernovae. When a star explodes, the difference between the masses of the progenitor star and the resulting compact object sources the radiation, neutrino emission, and kinetic energy of the ejecta material. ALP could be an additional channel for the energy release while its absorption heats the supernova. The balance between heating and cooling controls the supernova dynamics and shock restoration, which directly relates to the electromagnetic observables. Dr. Mori has shown that the heating rate is very sensitive to the coupling strength and enhancement of the electromagnetic emission is expected in the pre-supernova stage. The signatures could also be visible in the neutrino sector after the explosion. The results are fantastic, and there are a lot of things to do. For example, the inclusion of the ALP-nucleon coupling effects is an interesting direction to pursue. Updates in the understanding of stellar models are important as well. We should see the signature of ALP in the near future in this field! Reported by Nagisa Hiroshima

2022-01-17

## iTHEMS Biology Seminar by Dr. Wataru Nishima on January 13, 2022

Dr. Wataru Nishima (New Mexico Consortium, USA) gave a very interesting presentation about his published paper on the spike protein of the SARS-CoV-2, the virus which causes COVID-19 [1]. He described the conformational change which the SARS-CoV-2 spike protein undergoes when it fuses with the host cell to initiate infection. His work offers and interesting alternative view of the process wherein parts of the spike protein ends up inside the host cell. This differs from the usual view where the spike protein would be left on the outside of the cell after virus fusion with the host. This alternative hypothesis could have important implication for better understanding disease severity, e.g. based on the possible toxic effect of spike protein components shed as part of the fusion process. Dr. Nishima's expertise is primarily in Molecular Dynamics and Bioinformatics and he is very interested to establish new collaborations with iTHEMS members. Reported by Catherine Beauchemin

2022-01-12

## iTHEMS Biology Seminar by Prof. Tetsuya Yamamoto on January 6, 2022

On January 6th, Prof. Tetsuya Yamamoto gave us a comprehensive talk about the physical aspects of nuclear bodies. He first introduced the liquid-liquid phase separation and other self-assembly mechanisms in soft-matter physics, such as micelle formation, with motivations from the experimental side. In the main part, he told us about three recent topics regarding how RNAs' dynamics and sequence properties lead to different types of assembly of nuclear bodies. The mathematical formulation, schematic figures, and comparisons with experiments were clear to understand. We are very grateful to Prof. Yamamoto for the exciting seminar. Reported by Kyosuke Adachi

2021-12-28

## iTHEMS Math Seminar by Prof. Yosuke Morita on December 3, 2021

On December 3, Yosuke Morita from Kyoto university gave a talk titled “The Conley index of topological dynamical systems” at the iTHEMS math seminar. He reviewed the fundamental concepts of study of topological dynamical systems and introduced the notion of index neighborhood which is used to define his refined Conley index. His talk contained many instructive examples which enable us to understand his talk easily. Finally, his refined Conley index is defined as a functor into a category of (equivariant) condensed sets. This gives new conceptual understanding of Conley index in terms of condensed set introduced by Clausen and Scholze recently. His talk was very interesting and stimulated many questions and discussions. I believe it was a very worthwhile time for many participants. Reported by Masaki Taniguchi

2021-12-28

## DMWG Seinar by Prof. Hidetoshi Otono on December 15, 2021

Among kinds of approaches for dark matter (DM), collider experiments are advantageous in their high precision. The FASER (ForwArd Search ExpeRiment at the LHC) is equipment to probe long-lived particles created at the interaction point of the LHC at the distance of about 400m. Particles pass through the absorber between the FASER detector and the interaction point and have enough lifetime to reach the FASER detector should leave signatures there. Dark photon and axion-like particles are good candidates for such species, as well as DM. The equipment is set at the front of the LHC beam direction. The position is really good at reducing the background particles and making use of already existing but not used space. Multiple layers of scintillators, emulsion for neutrino detection, silicon trackers, the decay volume for new particles with dipole magnets, and calorimeter at the endpoint consists of the facility. In combination with the works of these pieces, new long-lived particle signatures as rare-decay signatures of neutral pion decay can be searched. It is already working well with LHC Run 3 and neutrino properties are now being probed. Furthermore, a new piece of the facility is decided to be installed to enhance the accessibility to axion-like particle signature in the channel of 2 photon decay. The installation does not disturb the current measurement and will be completed by the end of 2023. The FASER experiments should become a key facility to probe new particles at m~O(1) GeV scale. In addition, our understanding of the Standard Model sector is also significantly improved with its measurements. The collaboration is now intensively working. The idea for physics with FASER is highly welcomed. Stay tuned and work hard! Reported by Nagisa Hiroshima

2021-12-22

## NEW WG Seminar by Dr. Tomoki Ozawa on December 20, 2021

Tomoki Ozawa (Tohoku/AIMR) gave a talk on "Quantum metric of topological and non-topological insulators in AMO and other systems." Quantum metric, or more generally quantum geometry, is attracting great interest recently in condensed-matter physics. Tomoki gave a pedagogical introduction to the concept of quantum metric as well as recent experimental progresses including the first detection of the quantum metric using a diamond NV center. Tomoki then explained his recent results on this topic, in particular, on relations among quantum metric, topology, and the Kähler geometry. More than 30 people have joined the seminar and enjoyed fruitful discussions. Reported by Hidetoshi Taya

2021-12-22

## NEW WG Seminar by Dr. Yusuke Yamada on December 15, 2021

Yusuke Yamada (RESCEU/Tokyo) gave a talk on "Cosmological particle production as Stokes phenomena." Particle production from the vacuum takes place in the presence of time-dependent backgrounds, and is of interest in various fields of physics. Yusuke reconsidered the particle production from a viewpoint of the Stokes phenomenon, a mathematical concept in theory of differential equations, and discussed its application to the early Universe. Yusuke pointed out that the Stokes-phenomenon viewpoint gives a systematic and powerful framework to investigate the realtime dynamics of the particle production. About 20 people have joined the seminar, and enjoyed active discussions during and even after the seminar. Reported by Hidetoshi Taya

2021-12-17

## iTHEMS Biology Seminar by Prof. Hidetoshi Shimodaira on December 16, 2021

On December 16, Dr. Motomu Matsui (University of Tokyo) gave a lecture titled "Revisiting Standard Methods for Phylogenetic Tree Inference" at the iTHEMS Biology Seminar. He described the Graph Partitioning (GS) method, a new molecular phylogenetic analysis method based on graph theory and clustering, and explained its usefulness when used with real data. The GS method can reveal protein family relationships that cannot be clarified by conventional molecular phylogenetic analysis using maximum likelihood or distance methods. It was a very meaningful seminar for many participants. Thank you very much! Reported by Yuki Yazaki

2021-12-17

## ABBL/iTHEMS Astro Seminar by Prof. Jin Matsumoto on December 10, 2021

Prof. Jin Matsumoto (Assistant Professor, Keio Institute of Pure and Applied Sciences (KiPAS), Graduate School of Science and Technology, Keio University) gave a nice presentation on "Magnetic field dependence of neutrino-driven core-collapse supernova models". In the presentation, Prof. Matsumoto introduced his neutrino-radiation-hydrodynamics supernova code (3DnSNe, Takiwaki et al. 2016) to include magnetohydrodynamics (MHD). Using this code, he performed three-dimensional MHD simulations for the evolution of non-rotating stellar cores focusing on the difference in the magnetic field of the progenitors. He found that the neutrino-driven explosion occurs in both the weak and strong magnetic field models. It was concluded that the neutrino heating is the main driver for the explosion in his models, whereas the strong magnetic field slightly supports the explosion. Reported by Shigehiro Nagataki

2021-12-10

## iTHEMS Math Seminar by Dr. Ryosuke Iritani on December 8, 2021

On December 8, Dr. Ryosuke Iritani (iTHEMS) gave a talk in Math Seminar. He talked about expectation value of certain random variables. He and his collaborators obtained a result on its explicit formula together with an algorithm of its fast calculation. This was the last Math Seminar talk of the year 2021. After the seminar there was an online Christmas party. Reported by Michiya Mori

2021-12-10

## iTHEMS Biology Seminar by Prof. Hidetoshi Shimodaira on December 9, 2021

In this week’s biology seminar, the invited speaker Prof. Hidetoshi Shimodaira introduced his recent work about selective inference. Selective inference is an important statistical problem described also by another word “the file-drawer effect”. For example, Journals are much more likely to publish studies with low P values, and the readers never hear about the great number of studies that showed no effect and were filed way. This makes it difficult to assess the strength of a reported P value. The challenge of correcting for the effects of selection is a complex one. Prof. Hidetoshi Shimodaira explained a method to improve the previously proposed approximately unbiased test by adjusting the selection bias. This method is applied to predict trees and edges in phylogeny. In the seminar, he started by an interesting introduction of Darwin's theory of evolution and the tree of life. Based on this theory, modern phylogenetic inference is developed by analyzing DNA sequences of species as system relationships like trees. He also showed us, different statistical tests can give different P values of trees and edges in the trees. Therefore, we should be careful doing the tests and aware of the bias types in the problem. We thank Prof. Hidetoshi Shimodaira for his great talk and precious time with us! Reported by Yingying Xu

2021-12-07

## iTHEMS Biology Seminar by Dr. Yuka Suzuki on November 18, 2021

On November 18, Yuka Suzuki from Okinawa Institute of Science and Technology (OIST) gave a talk titled "Spatial structure in ecology: the effects of dispersal network structure on biodiversity pattern and stability in metacommunities" at the iTHEMS Biology Seminar. She introduced the basic concepts of spatial structure in ecology and explained how computational tools and network theoretical concepts are used to investigate spatial structures in ecology using her own study. Her talk was easy to understand and also very interesting and stimulated many questions and discussions. I believe it was a very worthwhile time for many participants. Thanks Yuka! Reported by Jeffrey Fawcett

2021-12-07

## iTHEMS Math Seminar by Prof. Koichi Taira on November 26, 2021

In November 26, there was a seminar by professor Kouichi Taira. He explained the relations between self-adjointness and the completeness of the classical dynamics. He especially explained a conjecture that self-adjointness and completeness of classical dynamics are equivalent. He then gave some examples on this conjectures. Reported by Keita Mikami

2021-12-07

## iTHEMS Math Seminar by Dr. Shinichiro Seki on November 19, 2021

On November 19, we invited Professor Shin-ichiro Seki from Aoyama Gakuin University to give a talk in the Math Seminar. He explained about the graph removal lemma, which is one of the key ingredients of the proof of his recent joint work with Kai, Mimura, Munemasa and Yoshino on a generalization of the celebrated Green-Tao theorem. In the first half of the talk, he gave a survey of Szemerédi's regularity lemma and the graph removal lemma, and explained how to extend the removal lemma to the case of (weighted) hypergraphs. In the second half of the talk, he presented Fox's result on a quantitative version of the graph removal, and discussed the prospects for future research. After the seminar, we had an online drinking party, and enjoyed a lot of discussion with the speaker. Reported by Hiroyasu Miyazaki

2021-12-07

## NEW WG Seminar by Prof. Ryosuke Oketani on November 25, 2021

Ryosuke Oketani from Kyushu University gave a talk on "Imaging Theory of Optical Microscopy: Basic to Super Resolution." Optical microscopy is a powerful tool to observe microscopic objects such as living micro-organisms. Recently, several super-resolution techniques have been developed, which enabled us to overcome the limit in spatial resolution caused by the wave nature of light. Ryosuke explained the basics and recent theoretical developments of optical microscopy and the super-resolution techniques. We also had fruitful discussions on the theory of optical microscopy from an interdiciplinary point of view. Reported by Hidetoshi Taya

2021-12-01

## ABBL/iTHEMS Astro Seminar by Dr. Hirotaka Yoshino on November 12, 2021

Dr. Yoshino gave us a talk on his recent work on axions around rotating black holes. He showed results of his numerical simulations of so-called “superradiant instability” in which an axion field around a rotating black hole extracts the energy of the black hole. These results suggest that every astrophysical black hole is expected to wear a cloud of the axion. Reported by Akira Mizuta (ABBL, RIKEN)

2021-11-30

## Quantum Matter SG seminar by Prof. Harshman Nathan on November 17, 2021

From American University, Prof. Nathan Harshman gave a seminar talk about topological exchange statistics in one spatial dimension on Nov. 17th, 2021. To introduce exchange statistics as done in the literature, he started with statistics for bosons and fermions and extended fractional and non-abelian exchange statistics. Then, the speaker raised questions about particle exchange in 1D and the collisions of the particles. Can a collision and an exchange at all be distinguished in one dimension? Is it reasonable to include points of coincidence in the configuration space? The way to resolve these problems is to introduce orbifolds. After pedagogically explaining the concepts of the orbifolds, he began to use the orbifold topological approach to study exchange statistics for any dimension. This approach offers great chances for novel abelian and nonabelian anyons in effectively 1D cold atom and condensed matter systems. Reported by Thore Posske (University of Hamburg, Germany) and Ching-Kai Chiu

2021-11-30

## iTHEMS Biology Seminar by Prof. Catherine Beauchemin on October 14, 2021

Defective interfering virus particles (DIPs) are viruses that are defective in a very specific way that allows them to out-compete standard, non-defective virus. It is difficult to count DIPs because they can look too similar to standard virus. So instead, people are counting them based on their effect on suppressing the standard virus population. In this talk, C. Beauchemin explained the basic biology of virus replication, what are DIPs, and how they compete with standard virus. She presented her group's mathematical model (ordinary differential equation) that describes co-infection competition with DIPs and standard virus. She also showed applications of the mathematical model to show how experiments to count DIPs can give incorrect results, and proposed some solutions.

2021-11-30

## Information Theory SG Seminar by Dr. Enrico Rinaldi on November 29, 2021

In the first part, he introduced the concept of simulation-based inference (SBI), which is inference methods driven by machine learning, and show some examples. In the later part, he showed some preliminary results of SBI on a biological neural circuit. We had a great discussion during Enrico’s talk. Thanks again for the fantastic talk, Enrico! We look forward to working with you. Reported by Akinori Tanaka

2021-11-24

## iTHEMS Math Seminar by Prof. Song Sun on October 22, 2021

In this seminar, Professor Song Sun started with explaining the most basic concepts like, Riemannian metric, curvature in differential geometry. He then introduced one of the most important equations differential geometry, the so-called Einstein equation. After defining the holonomy group of a Riemannian manifold, he explained Berger's classification of holonomy groups of Riemannian manifolds which are not locally symmetric. He then pointed out the importance of studying Riemannian manifolds with special/exceptional holonomy groups, e.g. Calabi-Yau manifolds, hyperkahler manifolds, G2 and Spin(7) manifolds. He concentrated on hyperkahler 4-manifolds and explained their geometry and topology in more details. In the end, he introduced his recent research result on studying Gromov-Hausdorff limit of hyperkahler 4-manifolds. Reported by Yalong Cao

2021-11-24

## iTHEMS Math Seminar by Prof. Siqi He on October 15, 2021

In this seminar, Professor Siqi He started with mean curvatures and minimal submanifolds in differential geometry. Then he introduced the notion of calibrated geometry of Harvey-Lawson. As important examples of calibrated submanifolds, special Lagrangian submanifolds in Calabi-Yau manifolds are introduced. Then he explained a branch cover problem raised by Simon Donaldson and sketched a proof of it. Reported by Yalong Cao

2021-11-24

## iTHEMS Math Seminar by Prof. Chen Jiang on October 8, 2021

In this seminar, Professor Chen Jiang started with several basic notions like geometric genus and canonical volume in birtaional geometry and introduced the question of geography problem. On algebraic surfaces, the boundary of geography problem is given by Miyaoka-Yau inequality and Noether inequality. Then he explained generalizations of the story on 3-folds, including his joint work with M. Chen and J.K.Chen. Finally he gave an outline for the proof of his main theorem. Reported by Yalong Cao

2021-11-24

## iTHEMS Math Seminar by Prof. Yukinobu Toda on October 1, 2021

In this seminar, Professor Yukinobu Toda started with the example of counting rational curves on hypersurfaces in complex projective spaces to illustrate ideas in enumerative geometry. Then he introduced several advanced topics including Donaldson-Thomas theory and mirror symmetry. Finally he explained his recent work on wall-crossing formulae for categorified DT invariants on resolved conifold. Reported by Yalong Cao

2021-11-18

## NEW WG Seminar by Prof. Naoto Nagaosa on November 15, 2021

Naoto Nagaosa (Tokyo/RIKEN) gave a talk on "Geometry in optical responses of quantum materials." Geometry and topology provide new insight into optical responses of solids and are of interdisciplinary interest in physics. After an excellent review on this topic made over the past decade, Naoto mainly discussed (i) shift current in noncentrosymmetric quantum materials driven by Berry phases, and (ii) Riemannian geometry in nonlinear optical responses. About 50 physicists from various fields have joined the seminar and enjoyed fruitful discussions during and after the seminar. Reported by Hidetoshi Taya

2021-11-17

## iTHEMS Biology Seminar by Dr. Takaharu Mori on November 4, 2021

On Nov 4th 2021, Dr. Takaharu Mori from Molecular Science Lab (Sugita Lab), RIKEN gave an online talk, entitled, "Protein structure modeling from cryo-electron microscopy data". He talked about the theoretical method to determine three-dimensional structures of biomolecules at near-atomic resolution, using a molecular dynamics (MD) simulation and also microscopy data. In addition, he showed their combined approach of coarse-grained model and all-atom model. According to him, Bayesian inference and machine learning are useful for the determination of biomolecule's 3D structure which can be the seeds of future collaborations between us. During the seminar, there were many discussions with iTHEMS members especially about the theoretical method to use recent microscopy data. We enjoyed his talk very much. Thanks, Mori-san! Reported by Gen Kurosawa

2021-11-17

## iTHEMS Biology Seminar by Dr. Jae Kyoung Kim on November 11, 2021

On Nov 11th 2021, Prof. Jae Kyoung Kim from KAIST, South Korea gave an online talk at iTHEMS. The title of the talk is “Toward mathematical medicine: development of a new drug and digital medicine for sleep disorders”. In the seminar, Dr. Kim talked about mathematical model (e.g. IBM) of our daily rhythms when environment within cells is crowded by lipids, and its implication for obesity. In addition, he showed their successful collaboration with Pfizer and Samsung medical center. The talk thrilled us very much. There were many intensive discussions with iTHEMS members and also experimental biologists from outside. Thanks, Jae! Reported by Gen Kurosawa

2021-11-15

## iTHEMS Math Seminar by Prof. Todor Milanov on November 12, 2021

Professor Todor Milanov gave a talk on his recent research on K-theoretic GW theory. He first recalled the definition of Gromov-Witten invariants and its K-theoretic generalization. Then he mentioned how to use genus 0 GW invariants to define quantum product on the usual cohomology group of a symplectic manifold. Such formulation has deep connection to integrable systems. He then introduced his research results, including a proof of the fact that small J-function in quantum cohomology of a Fano manifold can be obtained as a limit q -->1 of the small J-function in quantum K-theory. Reported by Yalong Cao

2021-11-15

## iTHEMS Math Seminar by Prof. Naichung Conan Leung on November 5, 2021

Professor Conan Leung gave an introductory talk to mirror symmetry. He first started with the discussion on the sum of inner angles of a triangle to illustrate the difference between our familiar Euclidean spaces and spaces with nontrivial curvature. He then introduced the notion of complex numbers and how it is related to solving polynomial equations in algebra. Recalling that gravity theory and quantum theory having very different nature of behaviour, he introduced super string theory as a potential unification of these two theories and its great impact to the study of modern mathematics, e.g. enumerative geometry, geometry of special holonomy. Finally he introduced mirror symmetry and how to understand it from the perspective of Fourier transform following a proposal of Strominger, Yau and Zaslow. Reported by Yalong Cao

2021-11-11

## Information Theory SG Seminar by Dr. Michiya Mori on November 4, 2021

On November 4, Dr. Michiya Mori gave a talk entitled “Boolean algebras and operator algebras” at the Information Theory SG Seminar. He explained some of the basic concepts of Boolean algebra. Then he talked about some interesting issues and his contributions in the field. It was a very informative and stimulating talk. Reported by Yukimi Goto

2021-11-11

## Astro-AI WG/Information Theory SG Joint Seminar by Dr. Takehiko Saito on November 8, 2021

The seminar was held online plus 4 people on-site. Dr.Saito gave a talk about “hunting hypernuclei by machine learning in nuclear emulsions”. Recent experimental studies with heavy ion beams have revealed that the nature of the hypertriton is unclear, especially on its biding energy and lifetime. The group led by Dr. Saito conducted measurements of them using nuclear emulsion, which requires a huge human load on visual image analyses. Therefore, they have developed machine learning models to detect events associated with production and decay of hypertriton in nuclear emulsions data, and successfully have discovered hypertriton events. Reported by Naomi Tsuji

2021-11-10

## iTHEMS Theoretical Physics Seminar by Prof. Katsuki Aoki on November 2, 2021

On November 2, 2021, at a seminar as part of the iTHEMS-phys seminar series, Dr. Katsuki Aoki (YITP, Kyoto University) delivered a talk on the topic of consistency conditions on the Standard Model of particle physics imposed from the requirements of a more fundamental ultraviolet (UV) theory. Underlying assumptions on UV completion can impose constraints on its low-energy effective field theories (EFTs). The swampland program aims to clarify consistent and inconsistent EFTs with quantum gravity and aims to understand quantum gravity from low-energy physics and vice versa. One of the most well-established constraints is called positivity bounds, provided that general assumptions such as Poincare invariance and unitarity are satisfied at all scales. Dr. Aoki first explained how these consistency conditions arise especially in the presence of gravity. He then showed that the positivity bound is violated if the Standard Model coupled to General Relativity is extrapolated up to 10^16 GeV, requiring new physics there or below. The precise value of the cutoff is determined by hadronic physic while it is insensitive from non-gravitational physics beyond the Standard Model. This is a signal from established physics for the necessity of quantum gravity below 10^16 GeV. Reported by Ryo Namba

2021-11-02

## Report of iTHEMS colloquium - High-Energy Neutrino Astrophysics in the Multimessenger Era

Our Universe is filled with mysterious high-energy emissions: Cosmic-rays (CR), neutrinos, and gamma-ray photons. They could be generated at the same source populations although we have not yet confirmed. Among these messenger particles, the neutrino is a special one to probe their origins. Cosmic rays are deflected by the magnetic field, while high-energy photons are attenuated during its propagation. Only the neutrino could preserve the information about the production sites. Also, the neutrino should be a messenger to physics beyond the Standard Model. The fact that the neutrinos have finite masses itself requires the extension of the Standard Model. Its origin could be related to dark matter, new kinds of interactions, and so on. On-going and planned experiments should enable us to access these problems by combining spectrum, timing, and flavor information. In this colloquium, Prof. Kohta Murase reviewed the above contents focusing on the latest results from neutrino observations and the development of the source modeling in the multi-messenger approach. About 100 people have enjoyed his talk. Lots of questions and some deep discussions related to the topic continued after the main part of the talk. We would like to thank him again and express our great appreciation for your attendance. Reported by Nagisa Hiroshima

2021-10-29

## iTHEMS Biology Seminar by Prof. Ikkyu Aihara on October 28, 2021

In the Biology Seminar on 28th October, Ikkyu Aihara (University of Tsukuba) gave a talk on frog chorus and its synchronization phenomena using experimental, fieldwork, and mathematical models. An earthquake that had occurred just before the seminar made me a bit nervous about whether we can hold the seminar, but the talk thankfully went very well. As I'm a big fan of Kuramoto-model as well as Ikkyu's work, I really enjoyed the talk on the whole. I saw one of the most interesting experiments of his ongoing work, and so I'm really looking forward to seeing the work being published. I also enjoyed a private conversation between Ikkyu and Kazuyuki Aihara before and during the talk! Thank you again for the great talk, Ikkyu-san! Reported by Ryosuke Iritani

2021-10-27

## Quantum Matter SG seminar by Dr. Robert Peters on October 20, 2021

The Quantum Matter Study Group invited Dr. Robert Peters from Kyoto University to talk about nonlinear responses in strongly correlated systems. In the beginning, the speaker introduced nonlinear responses by reviewing several related experimental data, including a nonlinear photo-induced current. Then, going beyond linear response theory, he explained the approach to compute nonlinear responses in strongly correlated systems. Here, the essential step of his work was to neglect vertex corrections, which enables the analysis of higher-order correlation functions within established frameworks of, e.g., dynamical mean field theory. Equipped with this tool, he showed nonreciprocal conductivity in a ferromagnetic non-centrosymmetric heavy Fermion compound and giant nonlinear Hall effect in a Weyl-Kondo-Semimetal. Reported by Thore Posske (University of Hamburg, Germany) and Ching-Kai Chiu

2021-10-27

## DMWG Seinar by Prof. Takashi Toma on October 20, 2021

Among the varieties of dark matter (DM) candidates, the so-called WIMP (an abbreviation of the weakly interacting massive particle) is famous for its beautiful mechanism to achieve the current DM density. In the early Universe, WIMP was in the thermal bath of the Standard Model (SM) particles. In this stage, (i) DM+DM->SM+SM, (ii)SM+SM->DM+DM interaction as well as (iii)DM+SM-> DM+SM interaction occur frequently enough. The strategy searching for each process corresponds to indirect, collider, and direct detection experiments. The cross-section (i.e., the interaction rate) of the above processes correlates with each other. The dropping off of the interaction rate of the process (i) below to the Hubble expansion rate of the Universe fixes the number density of DM particles. We need only two parameters in this WIMP freeze-out scenario: mass and the interaction cross-section. Nowadays the constraints from direct detection experiments are so severe that WIMP in the low mass range of m<O(10)GeV are almost excluded. However, such constraints could be evaded by considering a different mechanism for fixing the particle number density. If DM interacts as (i)' DM+DM->DM+SM, which is a process referred to as co-annihilation, the cross-section of the process (iii) could be suppressed in a velocity-dependent way. A concrete example introduced in this talk is the one considering the SM particle in the process (i)' as the neutrino. From the momentum conservation, DM accumulated in the Sun annihilates to produce boosted DM and neutrino, hence we expect a double-peak spectrum in large-volume neutrino experiments for this case. The solution to the core-cusp problems of usual WIMP and the origin of the neutrino mass, which is another important problem in the Standard Model, are also within the focus of this story. The world of DM is not closed on its own. It should be a key to understanding nature and obtaining a picture of our Universe! Reported by Nagisa Hiroshima

2021-10-27

## NEW WG Seminar by Mr. Akihiro Yamada on October 20, 2021

Akihiro Yamada (Keio U.) gaive a talk on "Floquet vacuum engineering: laser-driven chiral soliton lattice in the QCD vacuum" based on his recently published paper [1]. After briefly introducing the Floquet theory and the chiral perturbation theory, Akihiro showed emergence of chiral soliton lattice structure in the QCD vacuum under a very strong laser field with large frequency. About 20 people have joined the seminar, and we had fruitful discussions during and after the seminar. Reported by Hidetoshi Taya

2021-10-27

## iTHEMS Biology Seminar by Dr. Hiroshi Yokota on October 21, 2021

In the iTHEMS Biology seminar on October 21, I talked about the driving force and the mechanism of the chromosome formation. The chromosome is constructed by chromatin fiber condensed into the rod-like shape. The rod-like shape comes from the consecutive chromatin loops. The driving force of the chromosome formation is one of the controversial issues. Two hypotheses on the driving force are considered: energy gain from ATP hydrolysis and thermal fluctuation energy. In this talk, I discussed the driving force derived from the free energy of the chromatin loop. Moreover, the mechanism associated with each driving force was also discussed by using the dynamical model based on the free energy. The result implies that these mechanisms dynamically switch. This study is collaborated with Masashi Tachikawa in Kyoto University who is also visiting scientist in iTHEMS. As there were various questions and discussions from the audiences, I was so happy. Thank you very much!

2021-10-19

## Quantum Matter SG seminar by Mr. Seishiro Ono on October 12, 2021

Quantum Matter Study Group invited Seishiro Ono from the University of Tokyo to give a talk about symmetry indicators for topological (nodal) superconductors. First, he gave a review of symmetry indicators for topological insulators. In condensed matter physics, using topological invariants to search for topological materials is difficult; alternatively, symmetry indicators serve a simpler calculation to identify topological states of matter since this method considers the properties located at symmetry invariant momenta. After the introduction, the speaker extended the indicator approach to topological (nodal) superconductors. Although the different types of superconducting pairings lead to complications in the symmetry-based classification, the speaker showed that their work classifies all the (magnetic) space groups with possible superconducting pairings. Furthermore, he demonstrated their own database server listing all the classification of the topological superconductors by using the approach of symmetry indicators. The work serves as an important guide for hunting topological superconductors. Reported by Ching-Kai Chiu

2021-10-18

## ABBL/iTHEMS Astro Seminar by Dr. Nobuya Nishimura on October 15, 2021

At the seminar, there were lots of questions and answers for r-process nucleosynthesis and kilonova events. The speaker presented lots of his excellent works on them, and the audience enjoyed his presentation with curiosity. Reported by Shigehiro Nagataki

2021-10-12

## Quantum Matter SG seminar by Prof. Bohm-Jung Yang on October 7, 2021

The Quantum Matter study group had the honor to invite Prof. Bohm-Jung Yang to give a talk on October 7th. He talked about wave function geometry and anomalous Landau levels of flat bands. He started with a heuristic introduction by explaining the geometry matters in a flat band, including the notion of the (Hilbert-Schmidt) quantum distance and the quantum metric. Particularly, he used a Kagome lattice as an example of the singular touching point of the flat band and quadratic band locked by the geometry. Furthermore, in the presence of a magnetic field, the singular flat band evolves into very dense anomalous Landau levels. Remarkably, the relation between the Landau level spreading and the maximum of the quantum distance can be described by a universal formula, independent of material parameters. In consequence, deducing the Landau level spreading in (spectroscopic) experiments would allow for the measurement of the quantum distance, thus probing the quantum geometry of the wave functions. The study was extended to the (spin-orbit-coupled) Lieb lattice with one flat band sandwiched by two dispersive bands. In this case, the Landau level spreading of the isolated flat band is determined by the fidelity tensor. This talk showed us the interesting and subtle interplay between the band flatness and the Landau levels. Reported by Ching-Kai Chiu and Chen-Hsuan Hsu (CEMS, RIKEN)

2021-10-11

## iTHEMS Theoretical Physics Seminar by Prof. Keisuke Izumi on September 13, 2021

At the seminar organized by the iTHEMS-physics study group, Prof. Keisuke Izumi discussed the S-matrix unitarity toward UV completion. Einstein gravity is not renormalizable and does not hold perturbative unitarity at high energy. This is the main reason why the construction of quantum gravity is difficult. A conjecture was proposed by Llewellyn Smith, "renormalizablility and tree-unitarity at high energy give the same conditions". This conjecture would be important because it shows that, if a theory is constructed such that unitarity is satisfied, renormalizablility holds automatically, and vice versa. Unfortunately, a counterexample was pointed out. If a theory involves higher derivatives, there exists a theory which is renormalizable but does not satisfy tree-unitarity. A candidate of quantum gravity, the quadratic gravity (R_{\mu\nu}^2 gravity), is one of the examples. Therefore, Llewellyn Smith's conjecture would not be useful for the discussion of quantum gravity. Then, Prof. Izumi and his collaborators introduced a new conjecture, "renormalizablility and S-matrix unitarity (or often called pseudo-unitarity) at high energy give the same conditions". In his talk, Prof. Izumi explained Llewellyn Smith's conjecture and his contribution to it. Then, he introduced his new conjecture. Finally, he showed that his conjecture works well even in theories with higher derivatives. Reported by Ryo Namba

2021-09-29

## Report of iTHEMS colloquium - Finding Gravitational Waves from the Early Universe

Prof. Eiichiro Komatsu, a director at the Max Planck Institute for Astrophysics (Germany), shared his enthusiasm for observational aspects of cosmology and in particular the primordial gravitational waves (GWs) generated during the earliest stage of our Universe, called cosmic inflation. Such GWs distort the space while propagating and leave footprints as the polarizations of the cosmic microwave background (CMB) and as a more direct signal at the GW interferometers. As a world-leading expert on all areas of cosmology, Prof. Komatsu explained the fundamental aspects of CMB and GW observations and their implications in understanding the physics of the primordial Universe (from his remark, "We all came from quantum fluctuations"). He made his talk visually accessible to non-experts while providing fresh interpretations of the data in cosmological observations. He also clarified the importance and excitement of the upcoming LiteBIRD project, the first CMB mission led by Japan, which expects its launch toward the end of 2020's. After the official part of his talk, he remained to be available for both scientific and non-scientific discussions with the audience in an informal setting and exchanged more technically involved thoughts as well as his everyday-life experiences. Reported by Ryo Namba

2021-09-17

## NEW WG Seminar by Prof. Yuta Murakami on September 15, 2021

Yuta Murakami from Tokyo Institute of Technology told us about "High-harmonic generation in strongly correlated systems." High-harmonic generation (HHG) by strong AC electric fields is one of the hottest topics in cond-mat physics. After a brief overview of HHG in various materials such as gasses and semi-conductors and also recent experimental results in Mott insulators, Yuta explained his recent numerical studies on HHG in strongly correlated systems and clarified unique features of HHG arising from the strongly-correlated nature. About 30 people joined the seminar and we enjoyed fruitful discussions during and after the seminar. Reported by Hidetoshi Taya

2021-09-17

## NEW WG Seminar by Prof. Matteo Baggioli on September 10, 2021

Matteo Baggioli (Jiao-Tong U. Shanghai) gave a talk on "Towards a description of amorphous solids and viscoelastic materials using effective field theory and holographic methods." Amorphous and viscoelastic materials appear in many places, not only in scientific problems but also in our daily life. Despite its ubiquity, there are many intriguing phenomena that are not yet understood, in particular, in the non-linear regime. In the talk, Matteo explained his recent attempts to formulate non-linear responses in amorphous and viscoelastic materials based on holographic and effective-field-theory techniques. Matteo also discussed open problems and future directions that we can pursue in the future. There were about 30 participants from various fields of physics, and we had fruitful discussions from a broad point of view during and even after the talk. Reported by Hidetoshi Taya

2021-09-09

## iTHEMS Biology Seminar by Dr. Euki Yazaki on September 9, 2021

Molecular phylogenetic analysis is one of the most important analysis methods in biology. If used well, it can help us understand how organisms diversified and how genes evolved. The methods of molecular phylogenetic analysis are subject to debate, so we are familiar with the mathematical background. However, we are not so familiar with how the data sets for phylogenetic analysis are made up. In this Biology Seminar, the main alignment algorithms for making data sets were explained and discussed. Scoring is important in alignment algorithms, and there was a discussion on scoring for amino acid and nucleic acid mutations and scoring for gap penalties. The discussion on alignment distance was also heated. I hope the audience all enjoyed it. I would like to have another discussion on phylogenetic analysis and mathematical background next time. Thank you very much!

2021-09-09

## ABBL-iTHEMS Joint Seminar by Dr. Yosuke Mizuno on September 3, 2021

Prof. Yosuke Mizuno (Tsung-Dao Lee Institute, Shanghai JiaoTong University) gave an excellent talk on "The Polarised ring ofthe Supermassive Black Hole in M87: EHT observations andtheoretical modeling". He introduced that the Event HorizonTelescope (EHT) had mapped the central compact radio source of theelliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. Recently EHT provided new images of the polarised emissionaround the central black hole in M87 on event-horizon scale. Thispolarised synchrotron emission probes the structure of magnetic fieldsand the plasma properties near the black hole. He and hiscollaborators found that the net azimuthal linear polarisationpattern may result from organised, poloidal magnetic fields in the emission region. In a quantitative comparison with a large simulated polarimetric image library, he found that magnetically arrested accretion disks are favoured to explain polarimetric EHT observations. In this talk, he also discussed about a new modelling study of M87jets in the collimation and acceleration region. Reported by Shigehiro Nagataki

2021-09-03

## Quantum Matter SG seminar by Dr. Levente Rózsa on September 1, 2021

Dr. Levente Rózsa gave a talk about Yu-Shiba-Rusinov (YSR) states and other localized states appearing in superconductors for Quantum Matter Study Group on September 1st, 2021. There are three distinct types of localized in-gap states in superconductors —- Majorana bound states, YSR states, and Caroli-de Genne-Matricon states. Dr. Rózsa first introduced Majorana bound states emerging in topological superconductors. Next, he talked about the anisotropic spacial distribution of the YSR states on the La surface (0001). The YSR state is a localized in-gap state induced by magnetic impurity in a superconductor. He showed that this anisotropy stems from the anisotropic Fermi surface. Furthermore, he discussed the localized states appearing in vortex cores are Caroli-de Genne-Matricon states, instead of Majorana bound states. He went through the experimental details and compared them with his theoretical simulation. The talk was well-organized and clear. We thank Dr. Rózsa for sharing his interesting research works. Reported by Thore Posske (University of Hamburg, Germany) and Ching-Kai Chiu

2021-09-03

## iTHEMS Biology Seminar by Dr. Yingying Xu on September 2, 2021

In today's Biology Seminar, Yingying Xu (iTHEMS) introduced one paper arguing the quantitative explanation for immune-escape of virus by means of quasispecies dynamics models with Gillespie algorithm. She firstly explained the background information about the immune systems. She then taught us how the authors simulated the dynamics and made a comparison with experimental data. We finally discussed potential future directions to further development of quantitative modeling of chronic or acute infectious virus, highlighting a potential issue in estimating the mutation rate of virus. We learned a lot from this journal club, thank you so much for the great talk, Yingying! Reported by Ryosuke Iritani