Seminar

8th Mathematical Application Research Team meeting

September 5 (Fri) 10:30 - 11:50, 2025

Sonia Mahmoudi (Assistant Professor, Mathematical Science Group, Advanced Institute for Materials Research (AIMR), Tohoku University)

The 8th meeting of the Mathematical Application Research Team invites Sonia Mahmoudi for a talk.

Venue: via Zoom / #359, 3F, RIKEN Wako Campus

Event Official Language: English

Geometry of 2d topological field theories and integrable hierarchies

September 4 (Thu) 15:00 - 17:00, 2025

Zhe Wang (Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

In this talk, I will explain a mathematical formulation of 2d topological field theories making use of integrable hierarchies, which is a framework initiated by B. Dubrovin and developed by many other mathematicians. The talk is divided into two parts. The first 45 minutes is a gentle introduction on how the mathematical structure called Frobenius manifolds naturally appears from topological field theories. The remaining part of the talk is devoted to explaining relationships between Frobenius manifolds and integrable hierarchies via the example of the KdV hierarchy.

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

Event Official Language: English

Synthesizing the evolutionary invasion analysis for high-dimensional population dynamics

September 4 (Thu) 13:00 - 14:00, 2025

Ryosuke Iritani (Senior Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

I will present a linear-algebraic (spectral) method for analyzing nonnegative matrices to study the dynamics of natural selection. This is a joint project with Troy Day (Queen's University, Canada). Within adaptive dynamics theory, evolutionary invasion analysis provides a powerful framework for studying adaptive evolution. It allows us to evaluate (i) whether a new type of individuals (mutants) can successfully invade and replace the resident type, and (ii) whether recurrent substitutions converge to an equilibrium that resists further invasion (an evolutionary Nash equilibrium). A central task is to quantify the reproductive success of mutants, which corresponds to computing the spectral radius (largest eigenvalue) of a nonnegative matrix. However, the high dimensionality of population dynamics often makes the analytical treatment of eigenvalues intractable. To address this problem, we have developed a methodology that applies to any high-dimensional adaptive dynamics system. I will first introduce the principles of adaptive dynamics and the associated eigenvalue problem. I will then present our new method, which translates the high-dimensional eigenvalue problem into another, lower-dimensional eigenvalue problem of arbitrary size, using (i) Perron–Frobenius theory and (ii) graph-theoretic arguments.

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

Event Official Language: English

Ensemble transform Kalman filter (ETKF)

September 3 (Wed) 13:00 - 14:00, 2025

Tatsuro Iwanaka (Research Associate, Data Assimilation Research Team, RIKEN Center for Computational Science (R-CCS))

Venue: R511, Computational Science Research Building (Main Venue) / via Zoom

Event Official Language: English

A Fresh Look at Late-Time Hawking Radiation

September 2 (Tue) 14:00 - 15:30, 2025

Wei-Hsiang Shao (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

There is now a common belief that non-perturbative quantum gravity effects are relevant for resolving the black hole information puzzle. But could such effects also largely alter Hawking radiation itself, the main culprit that led to the puzzle in the first place? There are two main lessons that I would like to convey from this presentation: 1. For large black holes formed by dynamical collapse, the usual description of Hawking radiation in the low-energy effective theory breaks down at an early stage, signaling the need for a UV theory to describe the origin of late-time radiation. 2. In UV models of the radiation field that incorporate a form of nonlocality motivated by string theory, Hawking radiation becomes a transient phenomenon that occurs only for a brief period of time. This behavior suggests a major deviation from the conventional picture of black hole evaporation based on local quantum field theory.

Venue: via Zoom / #359, 3F, RIKEN Wako Campus

Event Official Language: English

Dust formation in the universe: reconstructing of theoretical models

August 29 (Fri) 14:00 - 15:15, 2025

Kyoko Tanaka (Appointed Associate Professor, Tokyo Woman's Christian University)

The formation process of cosmic dust is the starting point of solid matter and is important for understanding the evolution of cosmic material and planet formation processes. The nucleation process at the initial stage of the phase transition is a key to how cosmic dust is formed and evolves. Recent studies of the nucleation have shown possibilities that are very different from the theoretical models that have been considered so far. We have investigated the nucleation process using molecular dynamics simulations, which allow us to observe the nucleation process at the molecular level and obtain new information. We also present our attempt to develop and reconstruct a new theoretical model to elucidate a comprehensive picture of cosmic dust formation in collaboration with experiments.

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

Event Official Language: English

Shadow formation in gravitational collapse: redshift and blueshift by spacetime dynamics

August 28 (Thu) 16:00 - 17:30, 2025

Yasutaka Koga (Assistant Professor, Department of Information and Computer Science, Faculty of Information Science and Technology, Osaka Institute of Technology)

A black hole illuminated by a background light source is observed as a black hole shadow. For a black hole formed by the gravitational collapse of a transmissive object, redshift of light due to the spacetime dynamics is expected to play a crucial role in the shadow formation. In this talk, we investigate the redshift of light caused by the spacetime dynamics. First, we consider a spherical shell model. We see that the collapse of a shell typically leads to the redshift of light, while blueshift can be also observed in some cases. This result suggests that a shadow image is generally formed in the late stage of the gravitational collapse of a transmissive object. Second, we consider a general, dynamical, spherically symmetric spacetime and propose a new covariant formula for the redshift of light. This formula relates the dynamical redshift to the energy-momentum tensor of the background spacetime and provides its intuitive interpretation with Newtonian analogy.

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

Event Official Language: English

The link between ecology and evolution in the speciation process

August 28 (Thu) 13:00 - 14:00, 2025

José Said Gutiérrez-Ortega (Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Both ecological and evolutionary processes shape biodiversity, but these are usually studied separately. Ecologists focus on current dynamics, while evolutionary biologists examine long-term changes. The intersection between the two perspectives lies in understanding speciation: the process of how new species arise. Understanding speciation can clarify how ecological processes build up into the global patterns we see in evolution, and in turn, how evolutionary trends promote ecological processes. Using observational data compiled from macroecological and phylogenetic methods on multiple plant and animal groups, I suggest that the ecological patterns left by the factors promoting speciation in a community correspond to the speciation/extinction dynamics within that community. This ecological-phylogenetic correspondence represents a connection between the ongoing and the long-term dynamics, an idea that may unify the disciplines of ecology and evolution. I expect this talk can promote discussion on the topics of eco-evolutionary dynamics, so that I can get some feedback from you, and that we can create chances for collaboration.

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

Event Official Language: English

Gyromagnetic Angular Momentum Interconversion in Neutron Stars

August 28 (Thu) 10:00 - 12:00, 2025

Hiroshi Funaki (Specially Appointed Assistant Professor, Faculty of Science and Technology, Keio University)

[NOTE] This informal seminar mainly organized by ABBL will be held in Japanese and is a joint event for GWX-EOS Working Group and iTHEMS-ABBL Joint Astro Study Group. Abstract: We propose a novel mechanism for angular momentum (AM) exchange between the crust and core of a neutron star (NS) via the gyromagnetic effect. Using extended hydrodynamics, we model the star by incorporating macroscopic AM and microscopic AM originating from neutron orbital and spin AM. We reveal that macroscopic dynamics in the crust can inform microscopic AM in the core leading to neutron spin polarization, and offer alternative scenario of (anti-)glitches. This work highlights the overlooked multi-scale AM interconversions in NS physics, paving the way for gyromagnetic astrophysics.

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

Event Official Language: Japanese

Analysis of inflationary models in higher-dimensional uniform inflation

August 27 (Wed) 16:00 - 18:00, 2025

Hirose Takuya (Assistant Professor, Kyushu Sangyo University)

In this talk, we consider higher-dimensional uniform inflation, in which the extra dimensions expand at the same rate as three-dimensional non- compact space during inflation. We compute the cosmological perturbation in $D+4$ dimensions and derive the spectral index $n_s$ and the tensor- scalar ratio $r$. We analyze five inflationary models: chaotic inflation, natural inflation, quartic hilltop inflation, inflation with spontaneously broken SUSY, and $R^2$ inflation. By combining the results from these models with the Planck 2018 constraints, we discuss that it is not desirable for the extra-dimensional space to expand at the same rate as the three-dimensional non-compact space, except for the case of one extra dimension. This talk is based on arXiv:2501.13581[hep-ph].

Venue: Seminar Room #359

Event Official Language: English

Observational constraint of non-scalar phantom dark energies

August 26 (Tue) 16:00 - 17:30, 2025

Hsu-Wen Chiang (Postdoc, Department of Physics, Southern University of Science and Technology, China)

Recent observation of DESI strongly disfavors cosmological constant. Given the lack of constraint regarding the fundamental field that constitutes a dynamical dark energy, people traditionally resort on a hypothetical scalar field. We instead consider minimally coupled non-spinless field as alternative, specifically the extended Proca-Nuevo theory (spin-1) and 3-form field (spin-3). Both theories at the background level permit pure phantom (w < -1) and phantom crossing (w < -1 to w > -1) scenarios. Furthermore, with reasonable choice of EFT parameters we can decouple the scalar perturbation of the dark energy from the matter sector. However, the Lorentz constraint within the higher-spin field inevitably modifies the response of the scalar potential to the matter perturbation. This leads to an enhancement of the matter power spectrum most obvious in BAO fullshape analysis. We then perform MCMC analysis and show that the Hubble tension is alleviated, and the non-spin-0 models are preferred marginally over a cosmological constant.

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

Event Official Language: English

A Discussion on Quantum Machine Learning for Medical Data

August 26 (Tue) 14:00 - 15:00, 2025

Satoru Sugimoto (Senior Research Scientist, Medical Science Data-driven Mathematics Team, Division of Applied Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Our team is investigating the applicability of machine learning using quantum computers to medical data. In this talk, we will provide a brief overview of supervised machine learning for medical data as a topic for discussion.

Venue: via Zoom

Event Official Language: English

Tamely Ramified Geometric Langlands Correspondence

August 22 (Fri) 15:00 - 19:00, 2025

Yuki Matsubara (Ph.D. Student, Centre for Quantum Mathematics, University of Southern Denmark, Denmark)

The geometric Langlands correspondence (GLC) is a geometric analogue of the Langlands conjecture in number theory, relating algebraic geometry, representation theory, and many other areas. Since A. Kapustin and E. Witten pointed out the relation between GLC and mirror symmetry, there have been various studies on GLC from a physics perspective as well as a mathematical perspective. First talk: An introduction to Langlands conjecture for everyone This is an entirely accessible overview of the Langlands conjecture. Starting from famous topics, such as the Pythagorean theorem and Fermat’s Last Theorem, I will introduce the statement and motivations behind the Langlands conjecture. No prior background will be assumed, and technical details will often be sketched rather than fully developed, so that anyone with a general mathematical curiosity can follow along. Second talk: On a certain tamely ramified geometric Langlands correspondence In this talk, I will present my research. Arinkin’s 2001 result established the geometric Langlands correspondence for the case G = SL2 on the complex projective line P1 with four fixed regular singularities. When one attempts to extend this to five or more singularities, it turns out to be more natural to decompose the correspondence into a Radon transform-type correspondence and a “GLC‑like” correspondence. I will report on the calculations of cohomology that support the proof of this GLC‑like correspondence in the P1 with five fixed regular singularities case.

Venue: via Zoom / #359, Seminar Room #359

Event Official Language: English

Femtoscopy: Probing Fundamental Matter Properties at the Fermi Scale

August 22 (Fri) 13:30 - 15:00, 2025

Yijie Wang (Postdoctoral Researcher, Tsinghua University, Beijin, China / Visiting Scientist, Radioactive Isotope Physics Laboratory, RIKEN Nishina Center for Accelerator-Based Science (RNC))

Femtoscopy, a cutting-edge technique grounded in intensity interferometry (correlation function analysis), enables in-depth exploration of fundamental properties of matter, including space, time, and interactions, at the Fermi scale. Originating from the Hanbury Brown and Twiss (HBT) correlation function, which was utilized in 1956 to measure the angular radius of Sirius, this method has been extended to the subatomic realm, emerging as a pivotal tool for deciphering space-time structures and particle interactions. This talk focuses on three representative femtoscopy studies: First, by combining femtoscopic interferometry with optical deblurring algorithms, it reveals the non-Gaussian freeze-out spatial distribution of protons and antiprotons in Au+Au relativistic heavy-ion collisions, challenging conventional wisdom [Chinese Physics Letters 42, 031401 (2025)]. Second, in the 30 MeV/u ⁴⁰Ar + ¹⁹⁷Au reaction, femtoscopy is employed to determine the proton emission timescale at approximately 100 fm/c and uncover the kinetic law of preferential emission of neutron-rich particles, making an “ultra-fast” video for heavy-ion collisions [Physics Letters B, 825, 136856 (2022)]. Third, using a high-resolution neutron array, femtoscopy accurately measures the neutron-neutron scattering length and effective range, as well as the space-time size of the neutron emission source, providing crucial data for the study of charge symmetry breaking in nuclear forces and nuclear symmetry energy [Physical Review Letter, 134, 222301 (2025)]. These achievements fully demonstrate the significant value of femtoscopy in advancing the frontiers of nuclear and particle physics, spanning from experimental observations to theoretical modeling. Dr. Yijie Wang is the Post Doc of Tsinghua University and Visiting Scientist of RIKEN. He studied physics at Jilin University, China, and obtained his Ph. D. degree at Tsinghua University in 2021. Then, he continued researches in Tsinghua University as Post Doc up to now. His interests focus on heavy ion collision experiment, nuclear equation of state, advanced detection system development and femtoscopy.

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

Event Official Language: English

Positive neutrino masses with DESI DR2 via matter conversion to dark energy

August 21 (Thu) 14:30 - 16:00, 2025

Kevin Croker (Assistant Research Scientist, Arizona State University, USA)

The Dark Energy Spectroscopic Instrument (DESI) is a massively parallel spectroscopic survey on the Mayall telescope at Kitt Peak, which has released measurements of baryon acoustic oscillations determined from over 14 million extragalactic targets. We combine DESI Data Release 2 with CMB datasets to search for evidence of matter conversion to dark energy (DE), focusing on a scenario mediated by stellar collapse to cosmologically-coupled black holes (CCBH). In this physical model, which has the same number of free parameters as LambdaCDM, DE production is determined by the cosmic star formation rate density (SFRD), allowing for distinct early- and late-time cosmologies. Using two SFRDs to bracket current observations, we find that the CCBH model: accurately recovers the cosmological expansion history, agrees with early-time baryon abundance measured by BBN, reduces tension with the local distance ladder, and relaxes constraints on the summed neutrino mass. For these SFRDs, we find a peaked positive summed mass < 0.149 eV (95% confidence) and summed mass equal to 0.106 +0.050 -0.069 eV respectively, in good agreement with lower limits from neutrino oscillation experiments. A peak in positive summed neutrino mass results from late-time baryon consumption in the CCBH scenario and is expected to be a general feature of any model that converts sufficient matter to dark energy during and after reionization.

Venue: #359, Seminar Room #359

Event Official Language: English

Detecting ghost ancestors in the human lineage

August 21 (Thu) 13:00 - 14:00, 2025

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))

Event Official Language: English

Perturbed Observation

August 20 (Wed) 13:00 - 14:00, 2025

Venue: R511, Computational Science Research Building (Main Venue) / via Zoom

Event Official Language: English

Dynamic Scaling Analysis for Enzymatic Degradation and Network Growth of DNA Liquid Droplets

August 14 (Thu) 13:00 - 14:00, 2025

Michio Tateno (JSPS Overseas Research Fellow, Material Research Laboratory, University of California Santa Barbara, USA)

In this talk, I will introduce two novel pattern formation dynamics exhibited by phase-separated liquid droplets composed of DNA nanoparticles: 1) By enzymatically inactivating the phase-separation ability of the nanoparticles, we observed the process by which droplets gradually disappeared. Notably, the droplet-size distribution density remained unchanged, while only the total number of droplets decreased over time. 2) We also observed the formation of a novel two-dimensional wire-like network pattern, in which two types of droplets are arranged in a one-dimensional, alternating manner. We confirmed that the characteristic size of the network follows power-law growth over nearly two decades, with a universal growth exponent that is independent of droplet viscosity and inter-droplet wetting affinity. We analyze these results within the framework of the dynamic scaling hypothesis and discuss the physical mechanisms underlying these behaviors.

Event Official Language: English

Extended Kalman filter: Lecture 2

August 13 (Wed) 13:00 - 14:00, 2025

Venue: R511, Computational Science Research Building (Main Venue) / via Zoom

Event Official Language: English

Cosmological Collider with Boundary Interactions

August 7 (Thu) 16:00 - 17:30, 2025

Yi Wang (Professor, Department of Physics, Hong Kong University of Science and Technology, Hong Kong)

We first review cosmological collider physics. Namely, how the mass, spin, width and parity of the primordial particles during inflation may be read off from the density correlations in the CMB and the Large Scale Structure of the universe, and how this can be related to particle physics such as the Standard Model. After that, we will introduce relations between bulk and boundary terms in cosmological collider physics, and highlight a class of interactions which were overlooked in the literature.

Venue: via Zoom

Event Official Language: English