Seminar

Social evolution in viruses

April 22 (Wed) at 10:00 - 10:45, 2020

Asher Leeks (International Program Associate, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / Ph.D. Student, Department of Zoology, University of Oxford, UK)

Viruses are normally thought of as solitary organisms. However, in reality viruses often interact with one another, and these interactions can have important consequences for how viruses evolve and cause disease. In this talk, I will show how simple models of virus-virus interactions can help us to understand some puzzling aspects of virus biology. At the end, I will also talk about a new modelling project on Covid-19 joint with Ryosuke, which considers how human actions such as social distancing could influence the evolution of virulence.

Venue: via Zoom

Event Official Language: English

Holography from field theories: a realization of AdS/CFT correspondence and beyond

March 18 (Wed) at 13:30 - 14:30, 2020

Sinya Aoki (Professor, Yukawa Institute for Theoretical Physics, Kyoto University)

*Seminar room changed from 160 to 433. We argue that the Anti-de-Sitter (AdS) geometry in $d+1$ dimensions naturally emerges from an arbitrary conformal field theory in $d$ dimensions using the free flow equation. We first show that an induced metric defined from the flowed field generally corresponds to the quantum information metric, called the Bures or Helstrom metric, if the flowed field is normalized appropriately. We next verify that the induced metric computed explicitly with the free flow equation always becomes the AdS metric when the theory is conformal. We also show that the conformal symmetry in $d$ dimensions converts to the AdS isometry in $d+1$ dimensions after $d$ dimensional quantum averaging. This guarantees the emergence of AdS geometry without explicit calculation. We next apply this method to non-relativistic systems with anisotropic scaling symmetries, such as Lifshitz field theories and Schr\”odinger invariant theories. In consequence we obtain a new hybrid geometry of Lifshitz and Schr\”odinger spacetimes as a general holographic geometry. We also show that the bulk hybrid geometry is realized by an Einstein-Maxwell-Higgs system plus a gauge fixing term for diffeomorphism, which may be interpreted as a holographic dual of a general non-relativistic system at the boundary.

Venue: #433, Main Research Building

Event Official Language: Japanese

Index of the Wilson-Dirac operator revisited: a discrete version of Dirac operator on a finite lattice

February 25 (Tue) at 16:00 - 18:10, 2020

Mikio Furuta (Professor, The University of Tokyo)

The Wilson-Dirac operator is a discrete version of Dirac operator defined on regular lattices. When the discrete version is a fine approximation of the Dirac operator on a Z/2-graded Clifford module on a torus, it is known that (1) an integer-valued index is defined for the Wilson-Dirac operator, and (2) the index is equal to the Atiyah-Singer index of the Dirac operator on the torus. These have been well established up to around 2000. The strategy of all the previous works is to make use of the discrete version of the heat kernel for Neuberger's overlap Dirac operator. Therefore the strategy cannot be generalized to mod 2 index nor family version of index. In this talk I would like to explain a new approach to the index of Wilson-Dirac operator which can be immediately generalized to these various cases. Joint work with H. Fukaya, S. Matsuo, T. Onogi, S. Yamaguchi and M. Yamashita.

Venue: Seminar Room #160

Event Official Language: English

Mining for Dark Matter substructure: Learning from lenses without a likelihood

February 17 (Mon) at 14:00 - 15:30, 2020

Johann Brehmer (Postdoctoral Researcher, New York University, USA)

Dr. Brehmer gives us a talk about a method to deduce DM small structures. Please join us! The subtle imprint of dark matter substructure on extended arcs in strong lensing systems contains a wealth of information about the small-scale distribution of dark matter and, consequently, about the underlying particle physics. However, teasing out this effect is challenging since the likelihood function for realistic simulations of population-level parameters is intractable. Structurally similar problems appear in many other scientific fields ranging from particle physics to neuroscience to epidemiology, which has prompted the development of powerful simulation-based inference techniques based on machine learning. We give a broad overview over these methods, and then apply them to the problem of substructure inference in galaxy-galaxy strong lenses. In this proof-of-principle application to simulated data, we show that these methods can provide an efficient and principled way to simultaneously analyze an ensemble of strong lenses, and can be used to mine the large sample of lensing images deliverable by near-future surveys for signatures of dark matter substructure.

Venue: #424-426, Main Research Building

Event Official Language: English

Solved and open problems regarding the neighborhood grid data structure

February 7 (Fri) at 16:00 - 18:10, 2020

Martin Skrodzki (Visiting Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / Fellow, German Academic Scholarship Foundation, Germany)

February 7 at 16:00-17:00 17:10-18:10, 2020 In 2009, Joselli et al. introduced the neighborhood grid data structure for fast computation of neighborhood estimates for point clouds in arbitrary dimensions. Even though the data structure has been used in several applications and was shown to be practically relevant, it is theoretically not yet well understood even in the two-dimensional case. The purpose of this talk is to present the data structure, give a time-optimal building algorithm, and motivate several associated questions from enumerative combinatorics as well as low-dimensional (probabilistic) geometry. In case of questions that have been solved in the past, corresponding proofs will be provided. For the open question, the talk will list them as an outlook to possible future collaboration.

Venue: Seminar Room #160

Event Official Language: English

Biology Talk

February 5 (Wed) at 16:00 - 16:30, 2020

Yasuo Yasui (Assistant Professor, Laboratory of Crop Evolution, Kyoto University)

Dr. Yasuo Yasui will give a 20-30 min introduction of the buckwheat project. Please feel free to join !

Venue: Seminar Room #160

Event Official Language: Japanese

ABBL/iTHEMS seminar - talk on ultra-high energy cosmic rays

January 31 (Fri) at 14:00 - 15:00, 2020

Eiji Kido (Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Venue: Seminar Room #132

Event Official Language: English

Nucleon Structure from Quantum Chromodynamics

January 30 (Thu) at 11:00 - 12:00, 2020

Jason Chang (Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / LBNL/UCB, USA)

Talk of this seminar is a colloquium style, so that non-expert can enjoy. Please feel free to join. The origin of matter is one of the longest standing mysteries that have captured the human imagination. The modern description of particle and nuclear physics hypothesizes that our matter filled universe must have resulted in the underlying physical processes favoring the preservation of matter over antimatter during the initial formation of our universe. This mechanism is attributed to the fundamental breaking of particle and antiparticle symmetry in physics beyond the Standard Model. One source of asymmetry is hypothesized to reside in the neutrino sector, and intense international efforts are being pursued to observe this phenomena in neutrino scattering experiments. Precise interpretation of experimental observations benefits from a Standard Model prediction of how nuclear matter interacts with neutrinos. The modern theory governing matter and their properties is the theory of the strong interaction, quantum chromodynamics (QCD). In this talk I will discuss a QCD calculation of the nucleon form factor at zero momentum transfer, which is related to how a neutrino at rest interacts with a single proton or neutron, followed by current progress on the calculation of the proton radius which is related to the slope of the form factor. Together the calculations paves a novel way forward towards a precise determination of the nucleon form factor up to momentum transfer relevant for neutrino scattering. I will end the talk by discussing future milestones and challenges as we work towards calculations for nuclear physics starting from QCD.

Venue: #433, Main Research Building

Event Official Language: English

ABBL/iTHEMS seminar - talk on neutron stars

January 24 (Fri) at 14:00 - 15:00, 2020

Hajime Sotani (Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / Research Scientist, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Venue: Seminar Room #132

Event Official Language: English

Semiclassical methods in mathematical quantum mechanics

January 23 (Thu) at 16:00 - 18:10, 2020

Shu Nakamura (Professor, Gakushuin University)

Plan of the seminar: we separate each talk into two. In the first 60 minutes the speaker gives an introductory talk for non-mathematicians. After a short break, the second 60 minutes is spent for a bit more detailed talk for mathematicians (working in other areas). We welcome you joining both parts of the seminar or only the first/second half. Talk 1: Semiclassical analysis, microlocal analysis and scattering theory. I plan to talk about overview on the semiclassical analysis and related topics, especially its intrinsic relationship with microlocal analysis and (microlocal) scattering theory. Roughly speaking, the microlocal analysis is an application of semiclassical idea to the analysis of singularities, and its analogue in momentum space is the microlocal scattering theory. We discuss basic notions of these, and mention several recent results. Talk 2: Microlocal structure of the scattering matrix with long-range perturbations. As an example of topics discussed in Talk 1, we discuss recent results on the scattering matrix with long-range perturbations. In particular, we show that the scattering matrix is expressed as a Fourier integral operator, and in some cases we can decide its spectral properties. Our approach is fairly geometric and abstract, and thus applies not only to usual Schrödinger operators but also to higher order operators and discrete Schrödinger operators.

Venue: #435-437, Main Research Building

Event Official Language: English

Tipping point detection of nonlinear dynamics by dynamic network biomarkers, and short-term time-series data prediction by randomly distributed embedding

January 15 (Wed) at 14:00 - 16:30, 2020

Luonan Chen (Professor, Excutive director, Chinese Academy of Sciences, China)

He will talk about theoretical study on complex network data, including clinical data. His talk consists of two parts: 14:00-15:00: Detecting the tipping points of dynamic processes by dynamic network biomarkers 15:00-15:30: Coffee break 15:30-16:30: Predicting future dynamics of short-term time-series data by randomly distributed embedding First part of his talk is for general audience. In second part of his talk, he will explain mathematical basis of his study in detail.

Venue: Large Meeting Room, 2F Welfare and Conference Building (Cafeteria)

Event Official Language: English

Particle acceleration in young supernova remnants with X-ray and gamma-ray observations

January 9 (Thu) at 10:00 - 11:00, 2020

Naomi Tsuji

A supernova remnant (SNR) is a prominent candidate as an accelerator of galactic cosmic rays. In a widely accepted theory of diffusive shock acceleration, a particle is accelerated by diffusing back and forth across shock waves. The spectral cutoff shape of accelerated electrons is determined by diffusion and cooling and appears in the X-ray and TeV gamma-ray energy ranges, respectively, through synchrotron radiation and inverse Compton scattering. To measure the cutoff shape precisely and explore the corresponding diffusion coefficient and acceleration efficiency, we systematically analyze young SNRs that emit nonthermal X-rays and TeV gamma-rays. We found a variety of particle acceleration depending on individual SNRs: the cutoff energy is well-reproduced by the theoretical prediction in Kepler’s SNR and Tycho’s SNR, while it might be affected by the surroundings in Cassiopeia A and SN 1006. Putting all the eleven SNRs analyzed in this study together, the acceleration tends to be more efficient as the SNR evolves and becomes older, which implies a connection with turbulent production.

Venue: Seminar Room #160

Event Official Language: English

The 32nd QCD Club

December 26 (Thu) at 15:00 - 17:00, 2019

Event Official Language: Japanese

Constraining superheavy dark matter with the multi-messenger observations of accompanying radiation

December 20 (Fri) at 14:00 - 15:00, 2019

Yana Zhezher (Institute for Cosmic Ray Research (ICRR), The University of Tokyo)

One of the main alternatives for the weakly interacting massive particles (WIMP) dark matter scenario are the super-heavy X particles with masses larger than the weak scale by orders of magnitude. We assume the experimentally more plausible scenario of decaying superheavy dark matter (SHDM), which leads to the production secondary particles: electrons, positrons, gamma rays and neutrinos. The hypothetical X-particle has two main parameters: it’s mass MX and lifetime ?, which can be indirectly constrained by comparisons of predicted flux of secondary particles with the astrophysical observations. We present the limits on the SHDM parameters derived with the multi-messenger data from the Fermi-LAT, IceCube and other experiments.

Venue: Seminar Room #132

Event Official Language: English

Scalable Majorana vortex modes in iron-based superconductors

December 18 (Wed) at 13:30 - 15:00, 2019

Ching-Kai Chiu (Senior Research Associate, Kavli Institute for Theoretical Sciences, University of Chinese Academy of Sciences, China)

The iron-based superconductor FeTexSe1−x is one of the material candidates hosting Majorana vortex modes residing in the vortex cores. It has been observed by recent scanning tunneling spectroscopy measurement that the fraction of vortex cores possessing zero-bias peaks decreases with increasing magnetic field on the surface of FeTexSe1−x. The hybridization of two Majorana vortex modes cannot simply explain this phenomenon. We construct a three-dimensional tight-binding model simulating the physics of over a hundred Majorana vortex modes in FeTexSe1−x. Our simulation shows that the Majorana hybridization and disordered vortex distribution can explain the decreasing fraction of the zero-bias peaks observed in the experiment; the statistics of the energy peaks off zero energy in our Majorana simulation are in agreement with the experiment.

Venue: #435-437, Main Research Building

Event Official Language: English

Multiple Zeta Values: Interrelation of Series and Integrals

December 17 (Tue) at 16:00 - 18:10, 2019

Syuji Yamamoto (Associate Professor, Keio University)

Plan of the seminar: we separate each talk into two. In the first 60 minutes the speaker gives an introductory talk for non-mathematicians. After a short break, the second 60 minutes is spent for a bit more detailed talk for mathematicians (working in other areas). We welcome you joining both parts of the seminar or only the first/second half. Abstract: This is an introduction to multiple zeta values (MZVs). Although the study of MZVs is related to various areas of mathematics, we will concentrate on the algebraic structures of MZVs themselves. The key point is that MZVs have two kinds of representations: nested series and iterated integrals. We present how these two representations yield rich algebraic relations among MZVs.

Venue: Seminar Room #160

Event Official Language: English

Weak lensing cosmology by Subaru HSC survey

December 12 (Thu) at 10:30 - 12:00, 2019

Chiaki Hikage (Project Associate Professor, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)

Place: IPMU seminar room C

Venue: Kavli IPMU Building

Event Official Language: English

Seminar talk on GRB190114C

December 11 (Wed) at 13:45 - 15:30, 2019

Susumu Inoue (Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

Detection of very high energy gamma-rays (~TeV) from GRB190114C by MAGIC telescope is reported in the latest nature issue. Dr. Susumu Inoue (iThems) who is one of MAGIC team members will give a seminar talk on this exciting event.

Venue: Seminar Room #132

Event Official Language: English

Noncommutative crepant resolutions and some higher dimensional flops

December 4 (Wed) at 16:00 - 18:10, 2019

Wahei Hara (JSPS Research Fellow, Faculty of Science and Engineering, Waseda University)

Plan of the seminar: we separate each talk into two. In the first 60 minutes the speaker gives an introductory talk for non-mathematicians. After a short break, the second 60 minutes is spent for a bit more detailed talk for mathematicians (working in other areas). We welcome you joining both parts of the seminar or only the first/second half. Abstract: We will talk about the theory noncommutative resolution of singularities. Noncommutative resolution is a noncommutative analog of usual (geometric) resolution of singularities, and allows us to generalise the idea of McKay correspondence to a large class of singularities. In the first part of the talk, we discuss the classical McKay correspondence, the definition of noncommutative crepant resolution, and some known results in lower dimensions. In the second half, we will discuss some concrete examples of noncommutative crepant resolutions in higher dimensions.

Venue: Seminar Room #160

Event Official Language: English

A multiscale study of turbulent heating in hot accretion flows

November 18 (Mon) at 14:00 - 15:00, 2019

Yohei Kawazura (Assistant Professor, Tohoku University)

Recently, the Event Horizon Telescope (ETH) collaboration revealed the stunning picture of radiation from the vicinity of the black hole. For accurate interpretation of the observation, it is crucial to understand the nature of plasma in the accretion disk. The disks that EHT is observing are called radiatively inefficient accretion flows, in which the plasma is hot and dilute, and consequently collisionless. In collisionless plasma, ions and electrons can have different temperatures as they do not thermally relax through Coulomb interaction. The ion-to-electron temperature ratio is the key to interpreting the observation because we can measure only the electrons' energy via radiation. To study ion and electron heating, kinetic treatment, rather than hydrodynamic treatment, is necessary. However, kinetic plasma turbulence is an extremely challenging subject. Therefore, we utilized gyrokinetics that is widely used in magnetic confinement fusion research. Our new multiscale approach treats a "large scale" where turbulence is driven by magnetorotational instability via MHD, and a "small scale" where turbulence is dissipated via gyrokinetics. Using this approach, we formulated a prescription of ion-to-electron heating ratio. In my talk, I will also present basic knowledge that is necessary to study collisionless turbulent heating.

Venue: Seminar Room #132

Event Official Language: English