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

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

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

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

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 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

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

Ecology and Evolution of Mammal-Microbe Interactions

May 29 (Thu) at 16:00 - 17:00, 2025

Taichi A Suzuki (Assitant Professor, Biodesign Center for Health Through Microbiomes, Arizona State University, USA)

A critical open question in microbiome research is identifying key host-microbial interactions that influence host fitness. While the disruption of coevolved host-microbial interactions is known to affect host fitness in simpler systems (e.g., insects and their symbionts), understanding the extent and consequences of host-microbial coevolution in more complex systems (e.g., mammals and their gut microbiota) remains a major challenge. My research has identified multiple species of gut microbes in adults and children that share a parallel evolutionary history with humans by analyzing paired human genotypes and bacterial strain genotypes. In another line of work, I applied a selection experiment demonstrating that selection and transmission of the microbiome and its metabolites can alter mouse locomotion behavior within four rounds of microbiome transfer, without any changes to the mouse genome. Finally, I will briefly outline my future plans to study the effects of disrupting evolutionary stable host-microbial associations on the phenotypes of deer mice (Peromyscus spp.) in the Madrean Sky Islands and genetically diverse human populations in Arizona. Biosketch: Assistant Professor at Arizona State University since 2023. MS at University of Arizona, PhD at University of California Berkeley, and Postdoc at Max Planck Institute for Biology. My group integrates evolutionary genomics, microbial ecology, and biomedical research to study host-microbial interactions using wild rodents and humans.

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

Event Official Language: English

From Galaxies to Cosmological Structures: The Multi-Scale Influence of Cosmic Rays

June 13 (Fri) at 14:00 - 15:15, 2025

Ellis Owen (Special Postdoctoral Researcher, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Cosmic rays interact with astrophysical systems over a broad range of scales. They go hand-in-hand with violent, energetic astrophysical environments, and are an active agent able to regulate the evolution and physical conditions of galactic and circum-galactic ecosystems. Depending on their energy, cosmic rays can also escape from their galactic environments of origin, and propagate into larger-scale cosmological structures. In this talk, I will discuss the impacts of cosmic rays retained in galaxies. I will show they can deposit energy and momentum to alter the initial conditions of star-formation, modify the circulation of baryons around galaxies, and have the potential to regulate long-term galaxy evolution. I will highlight some of the astrophysical consequences of contained hadronic and leptonic cosmic rays in and around galaxies, and how their influence can be probed using signatures including X-rays, gamma-rays and neutrinos. I will also discuss what happens to the cosmic rays that escape from galaxies, including their interactions with the magnetized large-scale structures of our Universe, and the fate of distant high-energy cosmic rays that do not reach us on Earth.

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

Event Official Language: English

iTHEMS-TheoryCenter(KEK) Scientific Writing and DEI Workshop

June 24 (Tue) - 25 (Wed), 2025

Ashleigh Griffin (Professor, Department of Biology, University of Oxford, UK)
Stuart West (Professor, Department of Biology, University of Oxford, UK)
Ryosuke Iritani (Senior Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

This is a two-day KEK-iTHEMS workshop on scientific writing and diversity, equity, and inclusion. For more details, please visit the workshop website via the relevant link.

Venue: RIKEN Wako Campus, Head Quarter Build., 2F Large Conference Room

Event Official Language: English

Recent Developments and Challenges in Tensor Networks: Algorithms, Applications to science, and Rigorous theories

July 28 (Mon) - August 8 (Fri), 2025

Venue: Panasonic Hall, Yukawa Institute for Theoretical Physics, Kyoto University

Event Official Language: English

iTHEMS-NCTS Workshop

August 18 (Mon) - 21 (Thu), 2025

This workshop aims to strengthen collaboration between researchers at RIKEN iTHEMS and the National Center for Theoretical Sciences in Taiwan. It will be a four-day event, with the first two days dedicated to interdisciplinary topics. The last two days will focus on specialized areas, with one day devoted to condensed matter physics and the other to high-energy physics, including quantum gravity.

Venue: via Zoom / RIKEN Wako Campus

Event Official Language: English