Universality class for driven interfaces and... integrable spin hydrodynamics?

May 19 (Mon) at 15:00 - 17:00, 2025

Kazumasa A. Takeuchi (Associate Professor, Graduate School of Science, The University of Tokyo)

The Kardar-Parisi-Zhang (KPZ) universality class, originally formulated to describe driven systems such as growing interfaces, has undergone several paradigm shifts [1]. One major breakthrough was the discovery of exact solutions for one-dimensional models within the KPZ class — remarkable given their non-equilibrium and non-linear nature — enabled by underlying integrability. These exact results revealed nontrivial fluctuation properties, some closely linked to random matrix theory, which were subsequently observed in real experiments on driven interfaces. But more recently, the KPZ framework appears to be entering a new phase, extending unexpectedly to integrable spin chains at thermal equilibrium [2,3]. Although this connection was nearly dismissed when clear discrepancy in full counting statistics was reported, the speaker and collaborators numerically found that various two-point quantities agree precisely with KPZ exact solutions, so the KPZ class indeed governs integrable spin chains, yet only their two-point quantities [4]. I will also discuss a recent hydrodynamic theory aiming to bridge spin chains and KPZ, which, currently, falls short of fully explaining the numerical observations and calls for further refinement [2,3].

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

Event Official Language: English

Stable homotopy theory of invertible quantum spin systems

May 16 (Fri) at 16:00 - 18:00, 2025

Yosuke Kubota (Associate Professor, Graduate School of Science, Kyoto University)

In the past decade, A. Kitaev proposed that the set of invertible gapped quantum spin systems would form an \Omega-spectrum. This conjecture is considered to have potentially significant application to the study of SPT phases. Recently, we give a mathematically rigorous realization of this proposal with the language of functional analysis and operator algebra. This gives a unified proof of a series of existing researches. The proof also suggests to understand Kitaev's proposal from the viewpoint of coarse geometry of metric spaces. This association leads us to the concept of localization flow.

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

Event Official Language: English

Topology and Brain Science

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

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

Venue: via Zoom

Event Official Language: English

Mini-Workshop on Neuro Science

May 16 (Fri) at 9:00 - 12:00, 2025

Satoshi Iso (Director, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
Takuya Isomura (Unit Leader, Brain Intelligence Theory Unit, RIKEN Center for Brain Science (CBS))
Taro Toyoizumi (Team Director, Laboratory for Neural Computation and Adaptation, RIKEN Center for Brain Science (CBS))
Kazuo Okanoya (Professor, Advanced Comprehensive Research Organization (ACRO), Teikyo University)
Atsushi Iriki (Specially Appointed Professor, Advanced Comprehensive Research Organization, Teikyo University)
Gary Shiu (Professor, Department of Physics, University of Wisconsin-Madison, USA)

[Program] 9:00am – 9:05am Satoshi Iso (iTHEMS RIKEN) “Opening Remarks” 9:05am – 9:35am Takuya Isomura (CBS RIKEN) “Triple equivalence integrates neural network dynamics, statistical inference, and computation” 9:45m – 10:15am Taro Toyoizumi (CBS RIKEN) “Chaotic neural dynamics facilitate probabilistic computations through sampling” 10:25am – 10:55am Kazuo Okanoya (Teikyo University) “Brain topography and auditory processing in birds and rats” 11:05am – 11:30am Atsushi Iriki (Teikyo University, iTHEMS RIKEN) “From Classical to Quantum: Rethinking Science, Consciousness, and Civilization” 11:30am – Gary Shiu (University of Wisconsin) “Summary: Mathematical Science and Brain” Followed by a seminar 2:00pm – 3:30pm Prof. Gary Shiu (University of Wisconsin) Topology and Brain Science Room 359, 3rd floor, Main Research Building, RIKEN

Venue: #345-347, Main Research Building, RIKEN Wako Campus (Main Venue) / via Zoom

Event Official Language: English

The index of lattice Dirac operators and K-theory

May 15 (Thu) at 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) at 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) at 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) at 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

3rd Mathematical Application Research Team Meeting

May 2 (Fri) at 13:30 - 15:00, 2025

Antoine Diez (Research Scientist, Mathematical Application Research Team, Division of Applied Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

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

Event Official Language: English

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

May 1 (Thu) at 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) at 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

The 28th MACS Colloquium

April 25 (Fri) at 14:45 - 18:30, 2025

Shizuo Kaji (Professor, Institute of Mathematics for Industry, Kyushu University / Professor, Center for Science Adventure and Collaborative Research Advancement (SACRA), Graduate School of Science, Kyoto University)

14:45-15:00 Teatime discussion 15:00-16:00 Talk by Prof. Shizuo Kaji (Professor, Institute of Mathematics for Industry, Kyushu University / Professor, Center for Science Adventure and Collaborative Research Advancement (SACRA), Graduate School of Science, Kyoto University) 16:15-17:20 2024 Study Group introduction session 17:30-18:30 Discussion

Venue: Science Seminar House (Map 9), Kyoto University

Event Official Language: Japanese

Insights on Issues in the Cold Dark Matter Hypothesis

April 25 (Fri) at 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) at 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

From Quarks to Neutron Stars: Insights from kHz gravitational waves

April 23 (Wed) - 24 (Thu), 2025

Astronomical observations of binary neutron star mergers and supernovae provide unique opportunities to explore physics spanning from quarks to neutron stars. The groundbreaking multi-messenger observation of GW170817, combining electromagnetic signals and gravitational waves, vividly demonstrated this potential. Furthermore, if a core-collapse supernova were to occur in our galaxy, it would likely become the first event where electromagnetic signals, neutrinos, and gravitational waves are detected together. The interior of neutron stars is not yet well understood due to their extreme density, where exotic states of matter may emerge. Gravitational waves, particularly in the kilohertz band, play a crucial role in extracting information about such ultra-high densities. In recent years, several concepts for kilohertz-band gravitational wave detectors, such as Australia's NEMO, have been proposed to advance multi-messenger studies of high density astrophysical objects. KAGRA in Japan is also exploring upgrades aimed at enhancing high-frequency sensitivity, which could significantly improve sky localization accuracy and enable the detection of post-merger signals. This workshop aims to explore the transformative potential of high-frequency gravitational wave observations for multi-messenger astrophysics, with a focus on their implications for nuclear and neutron star physics. The workshop aims to spark dynamic discussions and cultivate new collaborations among participants through invited talks by leading experts and engaging poster presentations.

Venue: via Zoom

Event Official Language: English

On IR/UV divergence of inflationary decoherence

April 22 (Tue) at 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

iTHEMS x academist Online Event "World of Mathematical Sciences 2025"

April 19 (Sat) at 10:00 - 15:30, 2025

Yuuka Kanakubo (Postdoctoral Researcher, RIKEN-Berkeley Center, Division of Global Collaborations and Research Talent Development, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
Kan Kitamura (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
Leo Speidel (RIKEN ECL Research Unit Leader, Mathematical Genomics RIKEN ECL Research Unit, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
Yuki Yokokura (Senior Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Venue: via Zoom

Event Official Language: Japanese

Artificial Intelligence and Neuroscience

April 11 (Fri) at 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