Seminar

Topology and Brain Science

May 16 (Fri) 14:00 - 15:30, 2025

Shiu Gary (Professor, Department of Physics, University of Wisconsin-Madison, USA)

Venue: via Zoom

Event Official Language: English

The index of lattice Dirac operators and K-theory

May 15 (Thu) 13:30 - 15:00, 2025

Hidenori Fukaya (Assistant Professor, Department of Physics, Osaka University)

We show that the Wilson Dirac operator in lattice gauge theory can be identified as a mathematical object in K-theory and that its associated spectral flow is equal to the index. In comparison to the standard lattice Dirac operator index, our formulation does not require the Ginsparg-Wilson relation and has broader applicability to systems with boundaries and to the mod-two version of the indices in general dimensions. We numerically verify that the K and KO group formulas reproduce the known index theorems in continuum theory. We examine the Atiyah-Singer index on a flat two-dimensional torus and, for the first time, demonstrate that the Atiyah-Patodi-Singer index with nontrivial curved boundaries, as well as the mod-two versions, can be computed on a lattice (This seminar is co-organized with FQSP).

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

iTHEMS Biology welcomes 2 new members!

May 15 (Thu) 13:00 - 14:15, 2025

Isaac Planas Sitja (Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
Antoine Diez (Research Scientist, Mathematical Application Research Team, Division of Applied Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

This meeting will be used to welcome 2 new members to the iTHEMS Biology Study Group; Postdoc Isaac Planas-Sitjà and Senior Researcher Antoine Diez. They will each give us a 15-20 min talk to introduce their research. If time permits, let's also use this time to catch up on each other's current research. I hope that many people will join us to welcome these new members and come meet them and hear about their research.

Venue: Hybrid format (4th floor public space & Zoom), Main Research Building

Event Official Language: English

2d Cardy-Rabinovici model with the modified Villain lattice formulation

May 9 (Fri) 14:00 - 15:00, 2025

Nagare Katayama (Ph.D. Student, Yukawa Institute for Theoretical Physics, Kyoto University)

One of the most famous scenarios of the quark confinement problem is the dual superconductor picture. In this picture, the quark confinement is induced by monopole condensation, but in the theory with a θ term, we expect that not only monopole but also dyon condensation is induced, as suggested by Cardy and Rabinovici through their intuitive arguments. In this study, the Witten effect of the theory of two-dimensional compact bosons with the θ term is examined using a modified Villain-type lattice theory that can treat the θ term and dion in a rigorous manner. In addition, we construct the 2d Cardy-Rabinovici model and analyze the phase diagram through the scaling dimension argument and the anomaly matching constraint.

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English

The role of the visual information of fish schooling via selective decision-making

May 8 (Thu) 16:00 - 17:00, 2025

Susumu Ito (Ph.D. Student, Department of Physics, Graduate School of Science, Tohoku University)

Visual cues play crucial roles in the collective motion of animals, birds, fish, and insects. Recently, experiments have revealed that organisms such as fish selectively utilize a portion, rather than the entirety, of visual information. This method of the visual interaction avoids heavy load for small brain of the organisms. However, the previous models using visual interaction implicitly assume that an agent interacts with all visible neighbors. Therefore, we study the effect of the selective decision-making on the collective motion via the agent-based model and the coarse grained continuous model. In the former study, we have constructed a visual model which takes into account the motion of visual attention of agents induced by the visual stimuli, and our model can simultaneously show the spontaneous appearance of various collective patterns and the bifurcation process of the tracking of a neighbor. The later study, the agents corresponds to the density field by the coarse graining, and the visual occlusion is treated in a self-consistent manner via a coarse-grained density field, which renders the interaction effectively pairwise. The model exhibits a discontinuous transition as in the conventional models by the local collision, and but the discontinuity is weakened by the non-locality of visual interaction. Our studies clarify the comprehensive coincidence with experimental results via selective decision-making and the essential role of non-locality in the visual interactions.

Venue: via Zoom

Event Official Language: English

Supernova axion emissivity with Δ(1232) resonance in heavy baryon chiral perturbation theory

May 1 (Thu) 16:00 - 17:30, 2025

Shu-Yu Ho (Postdoctoral Researcher, Institute of Physics, Academia Sinica, Taiwan)

Abstract: In this talk, we evaluate the energy loss rate of supernovae induced by the axion emission process π− + p → n + a with the Δ(1232) resonance in the heavy baryon chiral perturbation theory for the first time. Given the axion-nucleon-∆ interactions, we include the previously ignored Δ-mediated graphs to the π− + p → n + a process. In particular, the Δ_0-mediated diagram can give a resonance contribution to the supernova axion emission rate when the center-of-mass energy of the pion and proton approaches the Δ(1232) mass. With these new contributions, we find that for the typical supernova temperatures, compared with the earlier work with the axion-nucleon (and axion-pion-nucleon contact) interactions, the supernova axion emissivity can be enhanced by a factor of ∼ 4(2) in the Kim-Shifman-Vainshtein-Zakharov model and up to a factor of ∼ 5(2) in the Dine-Fischler-Srednicki-Zhitnitsky model with small tanβ values. Remarkably, we notice that the Δ(1232) resonance gives a destructive contribution to the supernova axion emission rate at high supernova temperatures, which is a nontrivial result in this study.

Venue: #345-347, 3F, Main Research Building (Main Venue) / via Zoom

Event Official Language: English

ComSHeL Launch Meeting

May 1 (Thu) 14:00 - 15:00, 2025

This is the very first meeting of the new Computationally-drive Solutions for Healthier Lives (ComSHeL) Study Group. The study group brings together members from iTHEMS' Fundamental Division together with the ECL Mathematical Genomics Unit and Teams from iTHEMS Applied Math Division (Medical Science Data-driven Math Team and Medical Science Deep Learning Team). The goal of this first meeting will be to discuss and decide on the format for this monthly study group, and to get to know each other (each member introducing their research briefly). I hope you can take the time to join us.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Insights on Issues in the Cold Dark Matter Hypothesis

April 25 (Fri) 14:00 - 15:15, 2025

Yuka Kaneda (Ph.D. Student, University of Tsukuba)

Dark matter accounts for 85% of the matter component of our universe, but its true nature is still unclear. The Lambda-Cold Dark Matter (CDM) model, which thought to be the standard model, reproduces well the statistical properties of the large-scale structure of our universe. However, at the scale of galaxies and dwarf galaxies, serious discrepancies between the predictions of the CDM model and observations have been pointed out. In this study, we tackle on the “cusp-core problem” and the “missing satellite problem,” which are typical examples of such discrepancies, using N-body simulations. In the talk, the physical trigger of cusp-to-core transition and the novel method to find missed satellites are presented.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Asymptotic Waves in Stars

April 23 (Wed) 14:00 - 15:30, 2025

Jim Fuller (Professor, Division of Physics, Mathematics, and Astronomy (TAPIR), California Institute of Technology (Caltech), USA)

Waves propagating through stars often have very short wavelengths in the radial direction, enabling WKB approximations that facilitate understanding. The main types of waves that propagate in stars are acoustic waves (restored by pressure forces) and gravity waves (restored by buoyancy forces). I will also discuss how the properties of these waves are changed by rotation (adding Coriolis and centrifugal forces) and magnetic fields (adding Lorentz forces). Finally, I will discuss how these waves carry energy and angular momentum through stars, and discuss some potential consequences for stellar evolution.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

On IR/UV divergence of inflationary decoherence

April 22 (Tue) 14:00 - 15:30, 2025

Fumiya Sano (Ph.D. Student, Institute of Science Tokyo)

Supported by observational evidence indicating that cosmological scalar perturbations were nearly Gaussian at the beginning of the universe, it is anticipated that the origin of these perturbations is quantum fluctuations. Consequently, cosmic inflation provides a valuable laboratory for testing the quantum nature with/of gravity. Evaluation of the quantumness of the primordial perturbations is an inevitable step for the purpose. However, quantum states of the perturbations are suffered from IR/UV divergence, resulting in fully classical states. In this presentation, I will first review the evaluation of the quantum coherence in de Sitter spacetime as a measure of quantumness, and then show how to regularize the divergence.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Artificial Intelligence and Neuroscience

April 11 (Fri) 14:00 - 15:30, 2025

Junichi Chikazoe (Professor, Center for Brain,Mind and KANSEI Sciences Research, Hiroshima University)

Recent advancements in artificial intelligence have led to various discoveries in the field of neuroscience. For example, it has been demonstrated that the information on orientation columns in the visual cortex and the basic taste information in the gustatory cortex can be extracted by applying machine learning to relatively low-resolution functional MRI data. Additionally, intriguing findings have emerged, such as the information processing structures of artificial neural circuits—designed independently of the brain—showing similarities to those of biological neural networks. In this talk, I will discuss the applications of artificial intelligence in neuroscience and explore future directions in this field.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

iTHEMS Biology Study Group April Launch Meeting (Part 2)

April 10 (Thu) 13:00 - 14:00, 2025

Let's launch our Biology Study Group activities for the new year (Part 2 of 2). This meeting will be used to (1) say welcome to new member (SPDR Kenji Okubo, and Postdoc Lucas Sort); (2) discuss Biology seminar management in light of the new iTHEMS Centre; and (3) catch up on each other's current research. Since this will probably take us 2h, this will be Part 2 of 2 (Part 1 was on 4/3). On 4/10 (Part 2) we will get a 15 min introduction talk by Postdoc Lucas Sort. This meeting is open to all RIKEN and guests. You do not need to be a member of the iTHEMS Biology Study Group.

Venue: via Zoom / Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English

A Strategy for Proving the Strong Eigenstate Thermalization Hypothesis: Chaotic Systems and Holography

April 3 (Thu) 16:00 - 17:00, 2025

Taishi Kawamoto (Ph.D. Student / JSPS Research Fellow DC, Yukawa Institute for Theoretical Physics, Kyoto University)

The strong eigenstate thermalization hypothesis (ETH) provides a sufficient condition for thermalization and equilibration. Although it is expected to hold in a wide class of highly chaotic theories, there are only a few analytic examples demonstrating the strong ETH in special cases, often through methods related to integrability. In this talk, I will explore sufficient conditions for the strong ETH that may apply to a broad range of chaotic theories. These conditions are expressed as inequalities involving the long-time averages of real-time thermal correlators. Specifically, I will discuss bottom-up holographic models that satisfy these conditions under certain assumptions, which are expected to hold in such models. This talk is based on the preprint 2411.09746 [hep-th].

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Gauge subtleties and the finiteness of loop corrections beyond slow roll

April 3 (Thu) 14:00 - 15:30, 2025

Danilo Artigas (JSPS Postdoctoral Research Fellow, Department of Physics Ⅱ, Division of Physics and Astronomy, Graduate School of Science, Kyoto University)

The early universe undergoes a phase of exponential expansion called inflation, under which quantum fluctuations are amplified and later seed cosmological structures. A long-standing question is whether interactions of these quantum fields may significantly affect the n-point statistics of cosmological observables. These corrections are known as loop corrections. Recently, Kristiano and Yokoyama claimed that, in scenarios beyond slow-roll inflation, the one-loop correction of super-Hubble fluctuations could become non-negligible and violate cosmological-perturbation theory. This result is highly debated, and in this talk we will use a non-linear approach known as delta N formalism to evaluate these loop corrections. We find the existence of loop corrections for modes close to the Hubble scale, however, these corrections are quickly suppressed for long-wavelength modes. We also show how the result of Kristiano and Yokoyama may arise when truncating the perturbative expansion, and how this result depends on the chosen gauge.

Venue: Seminar Room #359

Event Official Language: English

iTHEMS Biology Study Group April Launch Meeting (Part 1)

April 3 (Thu) 14:00 - 15:00, 2025

Let's launch our Biology Study Group activities for the new year (Part 1 of 2). This meeting will be used to (1) say welcome to new member (SPDR Kenji Okubo, and Postdoc Lucas Sort); (2) discuss Biology seminar management in light of the new iTHEMS Centre; and (3) catch up on each other's current research. Since this will probably take us 2h, this will be Part 1 of 2 (Part 2 on 4/10). On 4/3 (Part 1) we will get a 15 min introduction talk by SPDR Kenji Okubo. This meeting is open to all RIKEN and guests. You do not need to be a member of the iTHEMS Biology Study Group.

Venue: via Zoom / 4th floor public space, Main Research Building

Event Official Language: English

Omega Meson from Lattice QCD

April 2 (Wed) 15:00 - 16:00, 2025

Haobo Yan (Ph.D. Student, School of Physics, Peking University, China)

The three-body problem, renowned for its unsolvable nature in celestial mechanics and homonymous science fiction, is not only solvable in the quantum realm regarding spectra but also offers profound insights into QCD. In this talk, I will present the first-ever lattice calculation of the resonance parameters for the lightest hadron decaying into three particles, the -meson. By mapping finite-volume energy levels to infinite-volume scattering amplitude, a pole position trajectory is obtained that, when extrapolated to the physical point, shows good agreement with the experiment.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

The rarer-sex effect

March 27 (Thu) 16:00 - 17:00, 2025

Andy Gardner (Professor, School of Biology, University of St Andrews, UK)

The study of sex allocation—that is, the investment of resources into male versus female reproductive effort—yields among the best quantitative evidence for Darwinian adaptation, and has long enjoyed a tight and productive interplay of theoretical and empirical research. The fitness consequences of an individual's sex allocation decisions depend crucially upon the sex allocation behaviour of others and, accordingly, sex allocation is readily conceptualized in terms of an evolutionary game. I will discuss the historical development of understanding of a fundamental driver of the evolution of sex allocation—the rarer-sex effect—from its inception in the writing of Charles Darwin in 1871 through to its explicit framing in terms of consanguinity and reproductive value by William D. Hamilton in 1972. I will show that step-wise development of theory proceeded through refinements in the conceptualization of the strategy set, the payoff function and the unbeatable strategy.

Venue: #445-447, 4F (Hybrid), Main Research Building / via Zoom

Event Official Language: English

Stability of nonsingular black holes

March 27 (Thu) 15:00 - 16:30, 2025

Shinji Tsujikawa (Professor, Graduate School of Advanced Science and Engineering, Faculty of Science and Engineering, Waseda University)

We show that nonsingular black holes (BHs) realized in nonlinear electrodynamics are always prone to Laplacian instability around the center because of a negative squared sound speed in the angular direction. This is the case for both electric and magnetic BHs, where the instability of one of the vector-field perturbations leads to enhancing a dynamical gravitational perturbation in the even-parity sector. Thus, the background regular metric is no longer maintained in a steady state. We also generalize our analysis to the case in which a scalar field is present besides the U(1) gauge field and find no explicit examples of linearly stable nonsingular BHs. Our results suggest that the construction of regular BHs without instabilities is generally challenging within the scheme of classical field theories.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

A Century of Quantum Mechanics

March 24 (Mon) 14:00 - 15:30, 2025

Gordon Baym (Professor Emeritus, University of Illinois, USA)

This is a RIKEN iTHEMS - The Univ. of Tokyo, Phys. Dept. Joint Seminar. This year, 2025, the "International Year of Quantum Science and Technology (IYQ)," is the 100th anniversary of the "formal" start of quantum mechanics, the description of the microscopic world. 1925 is the year in which Werner Heisenberg and others formulated "matrix mechanics," and physicists began to understand how to accurately predict microscopic phenomena. In this talk I will describe how quantum mechanics came about, starting with physicists in the late nineteenth century trying to understand the colors of hot metals and other hot objects, noting crucial advances leading to the fully developed wave and matrix quantum mechanics in the mid 1920's, to steps towards understanding real materials, culminating with spectacular applications such as smartphones, scarcely a century later.

Venue: The Univ. of Tokyo, Faculty of Science Building #4, room 1220 (Main Venue) / via Zoom

Event Official Language: English