Seminar Report
362 news

20211222
Seminar ReportNEW 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 timedependent 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 Stokesphenomenon 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

20211217
Seminar ReportiTHEMS 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

20211217
Seminar ReportABBL/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 neutrinodriven corecollapse supernova models". In the presentation, Prof. Matsumoto introduced his neutrinoradiationhydrodynamics supernova code (3DnSNe, Takiwaki et al. 2016) to include magnetohydrodynamics (MHD). Using this code, he performed threedimensional MHD simulations for the evolution of nonrotating stellar cores focusing on the difference in the magnetic field of the progenitors. He found that the neutrinodriven 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

20211210
Seminar ReportiTHEMS 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

20211210
Seminar ReportiTHEMS 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 filedrawer 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

20211207
Seminar ReportiTHEMS 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

20211207
Seminar ReportiTHEMS 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 selfadjointness and the completeness of the classical dynamics. He especially explained a conjecture that selfadjointness and completeness of classical dynamics are equivalent. He then gave some examples on this conjectures. Reported by Keita Mikami

20211207
Seminar ReportiTHEMS Math Seminar by Dr. Shinichiro Seki on November 19, 2021
On November 19, we invited Professor Shinichiro 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 GreenTao 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

20211207
Seminar ReportNEW 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 microorganisms. Recently, several superresolution 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 superresolution techniques. We also had fruitful discussions on the theory of optical microscopy from an interdiciplinary point of view. Reported by Hidetoshi Taya

20211201
Seminar ReportABBL/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 socalled “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)

20211130
Seminar ReportQuantum 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 nonabelian 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 ChingKai Chiu

20211130
Seminar ReportiTHEMS 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 outcompete standard, nondefective 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 coinfection 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.

20211130
Seminar ReportInformation Theory SG Seminar by Dr. Enrico Rinaldi on November 29, 2021
In the first part, he introduced the concept of simulationbased 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

20211124
Seminar ReportiTHEMS 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 socalled 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. CalabiYau manifolds, hyperkahler manifolds, G2 and Spin(7) manifolds. He concentrated on hyperkahler 4manifolds and explained their geometry and topology in more details. In the end, he introduced his recent research result on studying GromovHausdorff limit of hyperkahler 4manifolds. Reported by Yalong Cao

20211124
Seminar ReportiTHEMS 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 HarveyLawson. As important examples of calibrated submanifolds, special Lagrangian submanifolds in CalabiYau manifolds are introduced. Then he explained a branch cover problem raised by Simon Donaldson and sketched a proof of it. Reported by Yalong Cao

20211124
Seminar ReportiTHEMS 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 MiyaokaYau inequality and Noether inequality. Then he explained generalizations of the story on 3folds, 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

20211124
Seminar ReportiTHEMS 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 DonaldsonThomas theory and mirror symmetry. Finally he explained his recent work on wallcrossing formulae for categorified DT invariants on resolved conifold. Reported by Yalong Cao

20211118
Seminar ReportNEW 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

20211117
Seminar ReportiTHEMS 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 cryoelectron microscopy data". He talked about the theoretical method to determine threedimensional structures of biomolecules at nearatomic resolution, using a molecular dynamics (MD) simulation and also microscopy data. In addition, he showed their combined approach of coarsegrained model and allatom 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, Morisan! Reported by Gen Kurosawa

20211117
Seminar ReportiTHEMS 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

20211115
Seminar ReportiTHEMS Math Seminar by Prof. Todor Milanov on November 12, 2021
Professor Todor Milanov gave a talk on his recent research on Ktheoretic GW theory. He first recalled the definition of GromovWitten invariants and its Ktheoretic 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 Jfunction in quantum cohomology of a Fano manifold can be obtained as a limit q >1 of the small Jfunction in quantum Ktheory. Reported by Yalong Cao

20211115
Seminar ReportiTHEMS 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

20211111
Seminar ReportInformation 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

20211111
Seminar ReportAstroAI WG/Information Theory SG Joint Seminar by Dr. Takehiko Saito on November 8, 2021
The seminar was held online plus 4 people onsite. 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

20211110
Seminar ReportiTHEMS Theoretical Physics Seminar by Prof. Katsuki Aoki on November 2, 2021
On November 2, 2021, at a seminar as part of the iTHEMSphys 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 lowenergy effective field theories (EFTs). The swampland program aims to clarify consistent and inconsistent EFTs with quantum gravity and aims to understand quantum gravity from lowenergy physics and vice versa. One of the most wellestablished 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 nongravitational 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

20211102
Seminar ReportReport of iTHEMS colloquium  HighEnergy Neutrino Astrophysics in the Multimessenger Era
Our Universe is filled with mysterious highenergy emissions: Cosmicrays (CR), neutrinos, and gammaray 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 highenergy 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. Ongoing 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 multimessenger 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

20211029
Seminar ReportiTHEMS 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 Kuramotomodel 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, Ikkyusan! Reported by Ryosuke Iritani

20211027
Seminar ReportQuantum 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 photoinduced 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 higherorder correlation functions within established frameworks of, e.g., dynamical mean field theory. Equipped with this tool, he showed nonreciprocal conductivity in a ferromagnetic noncentrosymmetric heavy Fermion compound and giant nonlinear Hall effect in a WeylKondoSemimetal. Reported by Thore Posske (University of Hamburg, Germany) and ChingKai Chiu

20211027
Seminar ReportDMWG Seminar by Prof. Takashi Toma on October 20, 2021
Among the varieties of dark matter (DM) candidates, the socalled 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 crosssection (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 freezeout scenario: mass and the interaction crosssection. Nowadays the constraints from direct detection experiments are so severe that WIMP in the low mass range of m
DM+SM, which is a process referred to as coannihilation, the crosssection of the process (iii) could be suppressed in a velocitydependent 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 doublepeak spectrum in largevolume neutrino experiments for this case. The solution to the corecusp 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 
20211027
Seminar ReportNEW WG Seminar by Mr. Akihiro Yamada on October 20, 2021
Akihiro Yamada (Keio U.) gaive a talk on "Floquet vacuum engineering: laserdriven 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

20211027
Seminar ReportiTHEMS 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 rodlike shape. The rodlike 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!

20211019
Seminar ReportQuantum 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 symmetrybased 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 ChingKai Chiu

20211018
Seminar ReportABBL/iTHEMS Astro Seminar by Dr. Nobuya Nishimura on October 15, 2021
At the seminar, there were lots of questions and answers for rprocess 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

20211012
Seminar ReportQuantum Matter SG seminar by Prof. BohmJung Yang on October 7, 2021
The Quantum Matter study group had the honor to invite Prof. BohmJung 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 (HilbertSchmidt) 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 (spinorbitcoupled) 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 ChingKai Chiu and ChenHsuan Hsu (CEMS, RIKEN)

20211011
Seminar ReportiTHEMS Theoretical Physics Seminar by Prof. Keisuke Izumi on September 13, 2021
At the seminar organized by the iTHEMSphysics study group, Prof. Keisuke Izumi discussed the Smatrix 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 treeunitarity 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 treeunitarity. 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 Smatrix unitarity (or often called pseudounitarity) 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

20210929
Seminar ReportReport 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 worldleading 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 nonexperts 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 nonscientific discussions with the audience in an informal setting and exchanged more technically involved thoughts as well as his everydaylife experiences. Reported by Ryo Namba

20210917
Seminar ReportNEW WG Seminar by Prof. Yuta Murakami on September 15, 2021
Yuta Murakami from Tokyo Institute of Technology told us about "Highharmonic generation in strongly correlated systems." Highharmonic generation (HHG) by strong AC electric fields is one of the hottest topics in condmat physics. After a brief overview of HHG in various materials such as gasses and semiconductors 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 stronglycorrelated nature. About 30 people joined the seminar and we enjoyed fruitful discussions during and after the seminar. Reported by Hidetoshi Taya

20210917
Seminar ReportNEW WG Seminar by Prof. Matteo Baggioli on September 10, 2021
Matteo Baggioli (JiaoTong 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 nonlinear regime. In the talk, Matteo explained his recent attempts to formulate nonlinear responses in amorphous and viscoelastic materials based on holographic and effectivefieldtheory 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

20210909
Seminar ReportiTHEMS 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!

20210909
Seminar ReportABBLiTHEMS Joint Seminar by Dr. Yosuke Mizuno on September 3, 2021
Prof. Yosuke Mizuno (TsungDao 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 eventhorizon 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

20210903
Seminar ReportQuantum Matter SG seminar by Dr. Levente Rózsa on September 1, 2021
Dr. Levente Rózsa gave a talk about YuShibaRusinov (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 ingap states in superconductors — Majorana bound states, YSR states, and Carolide GenneMatricon 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 ingap 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 Carolide GenneMatricon states, instead of Majorana bound states. He went through the experimental details and compared them with his theoretical simulation. The talk was wellorganized and clear. We thank Dr. Rózsa for sharing his interesting research works. Reported by Thore Posske (University of Hamburg, Germany) and ChingKai Chiu

20210903
Seminar ReportiTHEMS 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 immuneescape 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

20210830
Seminar ReportiTHEMS Biology Seminar by Dr. Gen Kurosawa on August 26, 2021
During this covid19 crisis, we check our body temperature (Tb) every day since Tb is essential variable of our body. In fact, some organisms like humans try to keep Tb constant, but some other organisms chose completely different strategies, including hibernation (冬眠). On August 26, I talked about hibernation at the iTHEMS Biology Seminar. Hibernation is a strategy for the organisms, including a primate to survive in a severe season with limited food and water availability. Although there have been a lot of studies about hibernation since the era of Aristotle, fundamental problems of hibernation remain unknown. Recently, we started to investigate mathematically body temperature profile of hibernating hamsters. During hibernation, the organisms drastically decrease their basal metabolisms, drop their body temperature (Tb) more than 10 degree, and become immobile. Mysteriously, Tb during hibernation does not remain constant at very low value, but greatly fluctuates with inconstant period of several days. At the seminar, I showed that a simple model can reproduce well and forecast Tb data during hibernation. Thankfully, there were a lot of questions and suggestions about the method of timeseries analysis, hibernating species, and so on which are precious to me. This study is the collaboration with Prof. Yoshifumi Yamaguchi at Institute of Low Temperature Science, Hokkaido Univ and Shingo Gibo at iTHEMS. I’m really enjoying this research. I am happy if the audiences enjoyed the seminar and forgot about the heat. Thanks!

20210825
Seminar ReportiTHEMS Biology Seminar by Dr. Naohiro Kawamoto on August 19, 2021
In iTHEMS Biology Seminar on August 19th, Dr. Naohiro Kawamoto (Osaka University) gave a talk on cyanobacterial circadian oscillations. It is known that three proteins (KaiA, KaiB and KaiC) are sufficient for generating selfsustained circadian oscillations in cyanobacterium. In this talk, Dr. Kawamoto experimentally showed that the lack of kaiA causes damped oscillation. Then, he analyzed the benefit of the damped circadian oscillation by using a simple mathematical model of the interaction between the damped and selfsustained oscillators. This analysis revealed that kaiAless damped oscillator can resonate to another oscillatory process. Finally, he talked about evolution of related to evolution of circadian clock system for future work. After the talk, we enjoyed discussion about mathematical modeling of various types of coupled oscillators. Thank you very much, Naohiro! Reported by Shingo Gibo

20210825
Seminar ReportQuantum Matter SG seminar by Prof. DawWei Wang on August 23, 2021
On August 23rd, 2021, Quantum Matter Study Group invited Prof. DawWei Wang to give a talk on the application of machine learning in condensed matter physics. First, he briefly introduced learning and emphasized using supervised machine learning in condensed matter physics. The practical condensed matter problem he focused on is the physics of manybody systems since the manybody problem is challenging to solve. By randomly choosing small portions of a manybody Hamiltonian, machine learning can almost accurately predict the energy of the manybody system. Prof. Wang used the 1D FermiHubbard model and the 1D Ising model to show the consistency between the learning prediction and the known solutions. The limitation of this approach is that the system size has to be fixed. To resolve this problem, he used the transfer learning approach to extend the prediction to a larger system size by learning from small systems. In the end, he talked about identifying the topological phase transition points by improving the machine learning approach in the literature. The talk is very comprehensive and informative. We thank Prof. Wang for giving a wonderful talk. Reported by ChingKai Chiu

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Seminar ReportiTHEMS Math Seminar by Dr. Yalong Cao on August 6, 2021
The final Math Seminar of this semester was held on August 9th. This time, we invited our new colleague Dr. Yalong Cao as a speaker. First, he gave an overview of the celebrated YangMIlls theory in real dimensions 3 and 4. Next, it was explained that one can complexify the above YangMills theory on CalabiYau 3 and 4folds. The point is that in algebrogeometric setting, moduli spaces of YangMills connections admit nice compactifications. Finally, he explained the connection between the above story and the enumerative geometry, specifically on counting (stable) coherent sheaves and complex ASD connections on CalabiYau 4folds. He and his collaborators proposed a conjectural sheaftheoretic interpretation of KlemmPandharipande’s BPS invariants of CY 4folds defined using GromovWitten theory. Reported by Hiroyasu Miyazaki

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Seminar ReportiTHEMS Theoretical Physics Seminar by Dr. Shunichiro Kinoshita on August 16, 2021
Shunichiro Kinoshita (Chuo U.) gave a talk on "Application of AdS/CFT to nonequilibrium phenomena in external electric fields. "After a brief introduction to the AdS/CFT correspondence, Shunichiro analyzed a holographic QCD system under strong electric fields constructed with D3/D7 branes. Shunichiro considered two kinds of electricfield configurations.The first one was a field with a sudden switching on (quenched setup). Shunichiro discussed how the confinementdeconfinement phase transition occurs and showed the possibility of deconfinement driven by turbulence. The second configuration was rotating electric fields. Shunichiro discussed how the frequency of the electric field affects the phase transition and argued the possibility of a novel state, distinct from the vacuum, for infinitesimally small electric fields with finite frequency. Reported by Hidetoshi Taya

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Seminar ReportInformation Theory SG Seminar by Prof. Masayuki Ohzeki on August 4, 2021
On August 4, Prof. Masayuki Ohzeki (Tohoku University/Tokyo Institute of Technology/Sigmai Co., Ltd.) gave us a talk on the quantum annealing. In the first part, after a general introduction, he demonstrated how to use the DWave machine from our personal computers. It is remarkable that we can easily manipulate a number of quantum spins ourselves to solve an optimization problem. In the second part, he reviewed working principles for quantum annealing: he started from the original proposal in 1998 and explained more recent methods such as the reverse annealing. He also explained carefully the usefulness and the limits of the quantum annealing. After discussing the theoretical perspective and how the machine is implemented using the superconducting qubits, he presented the application of the quantum annealer to the real world. He showed his collaborations with many companies and how the quantum annealing can be used to solve reallife problems, such as the optimization of the evacuation routes or listing the hotel recommendation on the web. A lot of interactive discussions were made during and after the talk. We really thank Prof. Ohzeki for his great talk. Reported by Ryusuke Hamazaki

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Seminar ReportiTHEMS Biology Seminar by Dr. Ai Niitsu on August 5, 2021
In iTHEMS biology seminar on August 5th, Dr. Ai Niitsu (RIKEN Theoretical Molecular Science Lab.) gave a talk about the membrane peptide design and computational modeling. First, she mentioned the recent machine learning approach for the prediction of the protein structures. Then, she pointed out that the prediction of the membrane related structure and stoichiometry of protein complexes is still challenging. Next, she explained the computational design of the coiledcoil peptides in membrane. The amino acid sequence of the peptide is determined so that the interchain interaction is satisfied. As a result, she obtained some stable transmembrane assemblies of the peptide. Along with the modelling of the peptide assemblies and their conductance estimate, she experimentally measured the conductance of the peptide channel in membrane by imposing various voltages. The high voltage induces the multiconductance state, which differs from the low voltage case where a stable single channel was observed. Based on further computational modelling and experiments, that multiconductance state was suggested to come from the change in the peptide structures and stoichiometry of a single pore. Her computational method and experimental results attract the audience interests and induce various discussions. Thank you very much for great talk, Ai! Reported by Hiroshi Yokota

20210802
Seminar ReportInformation Theory SG Seminar by Dr. Qibin Zhao on July 28, 2021
In the Information Theory SG seminar on July 28th, Dr. Qibin Zhao gave us an exciting talk about the basics and applications of tensor networks (TNs) in machine learning. His talk was divided into three parts, (i) tensor methods for data representation, (ii) TNs in deep learning modeling, and (iii) frontiers and future trends. In the first part, he started with a graphical introduction of tensors and dimension reduction methods. He then presented the important question of how the imperfect data represented by tensors can be completed using the lowrank approximation, and introduced several possible approaches with impressive examples. Here he explained the TNs and decomposition methods in detail using diagrammatic notations, which can concisely express the tensor operations such as the contraction. In the second part, he talked about the useful application of TNs to model compression in machine learning. He illustrated the ways to represent the neural network model by tensors and to learn the weights through the contraction. Interestingly, the density matrix renormalization group (DMRG), which was originally proposed in the quantum physics field, may be used as a learning algorithm. In the last part, he showed an overview of recent topics on TNs and machine learning such as TNs for probabilistic modeling and supervised learning with projected entangled pair states (PEPS). Intriguingly, the topological structure of TNs can be optimized for a given image, and the learned topology significantly depends on the input data and is more complex than conventional simple structures such as lines, trees, or cycles. The clearly structured talk took us from the basics to the cuttingedge subjects, and there were many questions and discussions during the talk. We are deeply grateful to Dr. Qibin Zhao for his excellent talk on the fastgrowing interdisciplinary field. Reported by Kyosuke Adachi
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