Seminar

Quantification model of energy of loop structure on biopolymer

June 3 (Wed) at 10:00 - 10:45, 2020

Hiroshi Yokota (Postdoctoral Researcher, iTHEMS)

During cell division, the chromatin fiber condenses into a rod-like shape, which is the so-called chromosome. The chromosome is constructed by consecutive chromatin loop structures whose excluded volume interaction gives chromosome its stiffness. So far, the energy source for the loop growing has remained a controversial issue. In this seminar, we quantify the energy source by calculating the free energy difference before and after a model polymer chain creating a loop structure.

Venue: via Zoom

Event Official Language: English

Localization and universality in non-Hermitian many-body systems

May 29 (Fri) at 15:00 - 16:30, 2020

Ryusuke Hamazaki (Senior Research Scientist, iTHEMS / RIKEN Hakubi Team Leader, Nonequilibrium Quantum Statistical Mechanics RIKEN Hakubi Research Team, RIKEN Cluster for Pioneering Research (CPR))

Recent study on isolated quantum many-body systems have revealed two different phases distinguished by their dynamics and spectral statistics. One is an ergodic phase whose spectral statistics exhibit universality of random matrices, and the other is a many-body localized phase where dynamics is constrained due to strong disorder. In this talk, we show that novel and rich physics concerning such localization and universality appears in non-Hermitian many-body systems, which have been utilized in diverse scientific disciplines from open quantum systems to biology. As a first topic, we analyze non-Hermitian quantum many-body systems in the presence of interaction and disorder [1]. We demonstrate that a novel real-complex transition occurs upon many-body localization of non-Hermitian interacting systems with asymmetric hopping that respect time-reversal symmetry. As a second topic, we show that “Dyson’s threefold way,” a threefold symmetry classification of universal spectral statistics of random matrices, is nontrivially extended to non-Hermitian random matrices [2]. We report our discovery of two distinct universality classes characterized by transposition symmetry, which is distinct from time-reversal symmetry due to non-Hermiticity. We show that the newly found universality classes indeed manifest themselves in dissipative quantum many-body ergodic systems described by Lindblad equations.

Venue: via Zoom

Event Official Language: English

Knotted 2-spheres in the 4-space and Yang-Mills gauge theory

May 27 (Wed) at 16:00 - 18:10, 2020

Masaki Taniguchi (Special Postdoctoral Researcher, iTHEMS)

The classification problem of knots is one of the central topics in a study of topology. In the first part, we review classical knot theory and theory of 2-dimensional knots in the 4-dimensional space. In the second part, we focus on a problem considered in differential topology. In the studies of differential topology, people are interested in the difference between continuous and smooth. As the main result of this talk, we introduce a theorem that tells us the difference between continuous and smooth 2-dimensional knots. The proof uses Yang-Mills gauge theory for 4-manifolds obtained by the surgery of 2-knots.

Venue: via Zoom

Event Official Language: English

Forgetting in Reinforcement Learning Links Sustained Dopamine Signals to Motivation

May 27 (Wed) at 10:00 - 10:45, 2020

Ayaka Kato (RIKEN Center for Brain Science (CBS) / Ph.D. Student, The University of Tokyo)

Dopamine (DA) has been suggested to have two reward-related roles: (1) representing reward-prediction-error (RPE), and (2) providing motivational drive. Role(1) is based on the physiological results that DA responds to unpredicted but not predicted reward, whereas role(2) is supported by the pharmacological results that blockade of DA signaling causes motivational impairments such as slowdown of self-paced behavior. Whereas synaptic/circuit mechanisms for role(1), i.e., how RPE is calculated in the upstream of DA neurons and how RPE-dependent update of learned-values occurs through DA-dependent synaptic plasticity, have now become clarified, mechanisms for role(2) remain unclear. We modeled self-paced behavior by a series of ‘Go’ or ‘No-Go’ selections in the framework of reinforcement-learning assuming DA's role(1), and demonstrated that incorporation of decay/forgetting of learned-values, which is presumably implemented as decay of synaptic strengths storing learned-values, provides a potential unified mechanistic account for the DA's two roles, together with its various temporal patterns.

Venue: via Zoom

Event Official Language: English

Gradient Flow Equation and Its Applications

May 15 (Fri) at 13:30 - 15:00, 2020

Kengo Kikuchi (Special Postdoctoral Researcher, iTHEMS)

Gradient flow is the one of the methods to suppress the ultraviolet divergence in gauge theories. The any correlation functions in terms of the flowed field, which is defined by the gradient flow equation, are finite without additional renormalizations. Because of this surprising property, the methods has been studied widely, especially in the lattice field theory. In this seminar, we introduce what the gradient flow is briefly. And we show our work, “generalized gradient flow equation”, which is the gradient flow equation for field theories with nonlinearly realized symmetry. Applying the formalism to a supersymmetric theory and O(N) non linear sigma model, we obtain the SUSY gradient flow and the Large N gradient flow. We also refer to the current research, the gradient flow of the supersymmetric theory with the non-renormalization theorem and the new formalism to obtain the sphalerons, which is one of the static classical solutions, using gradient flow methods, if time allows.

Venue: via Zoom

Event Official Language: English

How many electrons can atoms bind?

May 13 (Wed) at 16:00 - 18:10, 2020

Yukimi Goto (Special Postdoctoral Researcher, iTHEMS)

In this talk, I will introduce the mathematical studies on the ionization problem. Some experimental & numerical evidences say that any doubly charged atomic ion X^{2-} is not stable. This 'fact' is called the ionization conjecture in mathematical physics literatures. My hope is to illustrates the interplay between mathematical and physical ideas. The talk is directed towards researchers on various aspects of quantum mechanics. In the first part, we will discuss the many-body aspects of quantum mechanics and introduce some basic notions. The second part will deal with the mathematical results in some approximation theories.

Venue: via Zoom

Event Official Language: English

Models to describe how virus spreads in vitro

May 13 (Wed) at 10:00 - 10:45, 2020

Catherine Beauchemin (Senior Visiting Scientist, iTHEMS / Professor, Department of Physics, Ryerson University, Canada)

This is meant as an introductory talk about my research field, virophysics, in which I apply the rigour and methods of physics to study virology. My focus is primarily in vitro (how virus spreads cell-to-cell in a cell culture) but I occasionally do some in vivo work. Recently, my hobby became tracking COVID-19 spread.

Venue: via Zoom

Event Official Language: English

Slip or (k)not: Geometry and mechanical performance of physical knots

May 7 (Thu) at 17:00 - 17:45, 2020

Tomohiko Sano (PD, École polytechnique fédérale de Lausanne, Switzerland)

Knots can impart unique mechanical function to filamentary structures, with examples ranging across length scales, including DNA, polymer-chains, shoelaces, climbing ropes, tennis racket, and surgical sutures. Even though knot theory has a long history of studies in the realm of mathematics, mechanics-based studies of physical knots are much rarer. The fundamental challenge in the understanding of their mechanics under a wide range of loading conditions stems from the fact that their topology, geometry, elasticity, and friction are all tangled ingredients. In this talk, combining experiments, simulations and theory, we present the recent progress on the predictive framework for the knot performance.

Venue: via Zoom

Event Official Language: English

From Eigenvalues to Resonances

May 1 (Fri) at 16:00 - 18:10, 2020

Keita Mikami (Research Scientist, iTHEMS)

Resonance is one of the most studied object in mathematical study of Schrödinger operators. One possible reason is that resonance is appeared in many other fields like arithmetic, physics, and topography. This series of talks target both mathematicians and researchers in other fields. The goal of the talk is to introduce the study of resonances for two body Schrödinger operators. In the first part, we briefly review spectral theory and how we use it in the study of Schrödinger operators. The aim of this part is to introduce the audience some basic notions used in the study of Schrödinger operators. In the second part, we give brief introduction of resonances and its application to both mathematicians and researchers in other fields. We start from mathematical definition of resonances to its applications in the other fields.

Venue: via zoom

Event Official Language: English

Analyses of large-scale sequence data from “PROTIST” can reveal the eukaryotic phylogeny and evolution

April 30 (Thu) at 10:00 - 10:45, 2020

Euki Yazaki (Postdoctoral Researcher, iTHEMS)

Most of the phylogenetic diversity of eukaryotes is made up of unicellular eukaryotic microorganisms called protists, some of which have not known phylogenetic home (called Orphans). Orphans are likely to hold important keys to the evolution of eukaryotes. In this seminar, I will introduce present case studies that reveal phylogenetic home and organelle evolution of orphan protists through sequence analysis based on large-scale sequence data.

Venue: via Zoom

Event Official Language: English

Characterizing the continuous gravitational-wave signal from boson clouds around Galactic isolated black holes

April 27 (Mon) at 16:00 - 17:00, 2020

Sylvia Zhu (Postdoctoral Researcher, Deutsches Elektronen-Synchrotron DESY, Germany)

Bosons such as axions or axion-like particles can form enormous clouds around black holes via the superradiance instability. As the bosons annihilate in the presence of the black hole, they produce a long-lived, slowly-evolving continuous gravitational-wave signal that is potentially detectable using the current generation of gravitational-wave interferometers.A non-detection can disfavor the existence of axions in certain mass ranges, although this is highly dependent on the Galactic black hole population. In this talk, I will discuss the expected annihilation signal from the population of isolated stellar-mass black holes in the Galaxy, and the prospects for detecting the signal using standard searches for continuous gravitational waves.

Venue: via Zoom

Event Official Language: English

Social evolution in viruses

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

Asher Leeks (International Program Associate, 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, 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, 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, iTHEMS / Research Scientist, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Venue: Seminar Room #132

Event Official Language: English