Topological Image Analysis

December 25 (Thu) 12:00 - 13:00, 2025

Shizuo Kaji (Professor, Institute of Mathematics for Industry, Kyushu University)

Topological Data Analysis (TDA) applies algebraic topology to the study of data such as point clouds. When applied to image and volumetric data, TDA provides a way to capture the topological features that characterise shapes and spatial structures. In this talk, I will outline the strengths and limitations of TDA for image analysis, and compare its capabilities with those of deep neural networks. I will also present hands-on examples using our open-source software Cubical Ripser. Finally, I will highlight a new direction in the use of TDA for image processing.

Venue: Room C501, West Zone 1 Building D, Ito Campus, Kyushu University, SUURI-COOL (Kyushu) (Main Venue) / via Zoom

Event Official Language: English

Cosmological correlators beyond the de-Sitter invariance

December 24 (Wed) 15:00 - 17:00, 2025

Zhu Yuhang (Postdoctoral Researcher, Particle Theory and Cosmology Group, Center for Theoretical Physics of the Universe (CTPU), Institute for Basic Science (IBS), Republic of Korea)

Cosmological correlators serve as powerful probes of the physics that governed the Universe in its earliest moments. Yet analytic calculations of correlators involving massive spinning fields are highly challenging. Recent progress in the cosmological bootstrap program has greatly deepened our understanding of these correlators. In this talk, we will show how to extend the bootstrap program beyond exact de Sitter invariance by studying two types of symmetry breaking: explicit scale-invariance breaking and boost breaking. We will present the boundary differential equations that characterise correlators in these settings and highlight the rich phenomenology that emerges. Finally, we will also show recent developments in approximation methods, based on the combination of exact WKB and saddle-point method, which provide a precise, efficient, and physically transparent way to capture and classify the non-analytic features of correlators.

Venue: via Zoom / Seminar Room #359

Event Official Language: English

Neural Network Quantum States for Quarkonium in Medium: Real-Time Open Quantum-System Dynamics

December 19 (Fri) 16:00 - 17:00, 2025

Tom Magorsch (Ph.D. Student, Department of Physics, Technical University of Munich, Germany)

Many phenomena in high energy physics can not be described by Euclidean-time Monte Carlo estimates alone, but require genuine real-time evolution and a treatment of non-equilibrium effects. However, such simulations are computationally challenging. One such example is the evolution of heavy quarkonium in the quark gluon plasma produced in heavy-ion collisions. In this talk, I will introduce the open quantum system treatment of in-medium quarkonium. I will then give an overview on neural network quantum states as a variational approach to the real-time simulation of open quantum systems. As a controlled benchmark system, I will study the application to the Caldeira-Leggett model and conclude with an outlook on future applications of neural network based simulation of quarkonia in medium.

Venue: via Zoom

Event Official Language: English

Two-component dark matter scenario in a pseudo-Nambu-Goldstone dark matter model - Tomohiro Abe

December 18 (Thu) 14:00 - 15:30, 2025

Tomohiro Abe (Assistant Professor, Kobayashi-Maskawa Institute for the Origin of Particles and the Universe (KMI), Nagoya University)

The WIMP dark matter (or thermal dark matter) is one of the leading candidates for dark matter and is widely studied. On the other hand, recent progress in direct detection experiments places severe constraints on WIMP dark matter models. Pseudo-Nambu-Goldstone boson dark matter models (pNG DM models) can explain the measured value of the dark matter energy density via the freeze-out mechanism and also naturally suppress the dark matter-nucleon scattering. The amplitudes for the processes are proportional to the momentum transfer squared from the dark sector to the visible sector; It is suppressed by t ~ 0 for the dark matter-nucleon scattering process, while it is proportional to s ~ 4 m_DM^2 for annihilation processes. The simplest realization faces the domain wall problem and needs to be extended. Several models have been proposed, but each model brings other issues, a large hierarchy in new energy scales, parameter tunings, and a Landau pole of a new gauge coupling. In this talk, I would like to propose a new pNG DM model based on SU(2)_gauge times SU(2)_global symmetry that overcomes those issues. The model predicts a two-component DM scenario, where one is pNG, and the other is an ordinary WIMP. I will show that the effective spin-independent cross section in the direct detection experiments is smaller than the current upper bound and larger than the prospect of the Darwin project in a wide range of parameter space in the two-component DM scenario.

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

Event Official Language: English

Origin and evolutionary history of an urban underground mosquito

December 18 (Thu) 13:00 - 14:00, 2025

Yuki Haba (Postdoc, Zuckerman Institute, Columbia University, USA)

Urbanization is rapidly reshaping landscapes around the world, which poses questions about whether and how quickly animals and plants can adapt. Culex pipiens form molestus, more commonly known as the "London Underground mosquito," has been held up as a benchmark for the potential speed and complexity of urban adaptation. This intraspecific lineage within Cx. pipiens, a major West Nile virus vector, is purported to have evolved human biting and a suite of other human-adaptive behaviors in the subways and cellars of northern Europe within the past 200 years. Form molestus features prominently in textbooks as well as scholarly reviews of urban adaptation. Yet, the hypothesis of in situ urban evolution has never been rigorously tested. I will talk our recent efforts to understand the contentious origin and evolutionary history of the urban, human-biting mosquito. Our synthesis and meta-analysis of rich yet confusing literature show that its London Underground origin is unlikely (Haba and McBride 2022 Current Biology). Whole genome resequencing and population genomics of 800+ mosquitoes across ~50 countries again debunk the in situ evolution hypothesis and instead support that molestus first adapted to human environments >1000 years ago in the Mediterranean or Middle East, most likely in ancient Egypt or another early agricultural society (Haba et al. 2025 Science). I will outline implications of our results in urban evolutionary biology as well as in public health. Speaker Bio Yuki Haba, Ph.D., is an evolutionary biologist passionate about understanding how and why diverse behaviors evolve in nature. He is currently a Leon Levy Scholar in Neuroscience at Columbia University's Zuckerman Mind Brain Behavior Institute. He aims to take multi-desciplinary approaches, combining genomics, neuroscience, and field-based behavioral ecology to comprehensively understand the evolution of behavior. Yuki completed his PhD at Princeton, MA at Columbia, and undergraduate degree at the University of Tokyo. Personal webpage: https://yukihaba.github.io/

Venue: Seminar Room #359

Event Official Language: English

A bi-fidelity Asymptotic-Preserving Neural Network approach for multiscale kinetic problems

December 17 (Wed) 11:00 - 12:00, 2025

Liu Liu (Assistant Professor, Department of Mathematics, The Institute of Mathematical Sciences, The Chinese University of Hong Kong, Hong Kong)

In this talk, we will introduce a bi-fidelity Asymptotic-Preserving Neural Network (BI-APNNs) framework, designed to efficiently solve forward and inverse problems for the linear Boltzmann equation. Our approach builds upon the previously studied Asymptotic-Preserving Neural Network (APNNs), which employs a micro-macro decomposition to handle the model’s multiscale nature. We specifically address a bottleneck in the original APNNs: the slow convergence of the macroscopic density in the near fluid-dynamic regime. This strategy significantly accelerates the training convergence as well as improves the accuracy of the forward problem solution, particularly in the fluid-dynamic limit. We show several numerical experiments on both linear Boltzmann and the Boltzmann-Poisson system that this new BI-APNN method produces more accurate and robust results for forward and inverse problems compared to the standard APNNs. This is a joint work with Zhenyi Zhu and Xueyu Zhu.

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

Event Official Language: English

Phase transition in parametrized quantum circuits

December 16 (Tue) 10:00 - 12:00, 2025

Xiaoyang Wang (Postdoctoral Researcher, Quantum Mathematical Science Team, Division of Applied Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

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

Event Official Language: English

Is the Fornax galaxy cluster hiding its radio sources?

December 15 (Mon) 14:00 - 15:15, 2025

Alvina Yee Lian On (PD, Physics Division, National Center for Theoretical Sciences, Taiwan)

Recently, the ASKAP POSSUM and MeerKAT surveys revealed an apparent lack of radio source counts in the Fornax galaxy cluster field. The sources in this patch of sky also appeared to be less polarised. These observations are peculiar and could be important signatures of depolarisation on megaparsec scales. In this talk, I will present some of our recent results, where we quantified the effects on polarisation of radio point sources behind a merging galaxy cluster simulation. The merger gives rise to a large-scale intracluster shock, which compresses the gas and aligns the magnetic field along the shock front. The in-falling subcluster also triggers gas sloshing near the cluster center, resulting in the formation of cold fronts. Our ray-tracing calculations revealed that, generally, bright sources do not experience any significant changes in polarisation, whereas faint sources either get severely depolarised or enhanced as their radiation propagates through the intracluster medium. I will highlight the physical conditions under which polarisation may change significantly along the line-of-sight and discuss how these may impact our interpretations of radio observations, particularly with the SKA.

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

Event Official Language: English

RIKEN Quantum hands-on seminar on IBM and QEDMA software for quantum computing beginners

December 15 (Mon) 9:30 - 12:20, 2025

This seminar will be conducted in a hybrid format, both in-person and via Zoom. Since it includes hands-on sessions, we kindly ask you to consider attending in person whenever possible to ensure more effective learning. The overview of the seminar is as follows: Program: 9:30 - 10:50 Yuri Kobayashi (IBM Quantum) "Introduction to IBM Quantum and Qiskit" 10:50 - 11:00 Break 11:00 - 12:20 Ori Alberton (QEDMA) "Introduction to QESEM error mitigation software: Theory,  use-case demonstrations and usage tutorial" Abstract of Yuri Kobayashi's tutorial: This talk introduces HPC researchers to the IBM Quantum platform and the Qiskit SDK, providing a practical orientation to quantum programming without assuming prior quantum-computing knowledge. Attendees will learn how to construct and execute quantum circuits using Qiskit, explore available simulators, and real quantum backends and submit jobs to IBM’s cloud-based quantum processors. The session will also showcase how to map your problem to quantum circuits through specific use-case applications. Abstract of Ori Alberton's tutorial: This talk introduces QESEM, Qedma’s characterization-based software tool for reliable, high-accuracy quasi-probabilistic error mitigation. We will begin by explaining why error mitigation methods are expected to be the first to unlock quantum advantage, and why they will continue to play a central role even as error correction becomes feasible. We will highlight some of the key innovations underlying QESEM’s operation and present recent results on IBM Heron r2 devices demonstrating its capabilities in the largest utility-scale unbiased error mitigation experiment to date. In addition different use-cases and demonstrations ran by QESEM users will be presented. Finally, we will provide guidance on how to start using QESEM to obtain error-mitigated results in experiments on IBM quantum systems.

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

Event Official Language: English

Fracture squares and separable algebras

December 12 (Fri) 16:00 - 17:30, 2025

Luca Pol (Postdoctoral Researcher, Max Planck Institute for Mathematics in Bonn, Germany)

In this talk I will present a way to reconstruct a category from its subcategories of complete and local objects while retaining the symmetric monoidal structure. As an application of this machinery I will discuss how to calculate separable algebras in equivariant homotopy theory.

Venue: via Zoom / 3F 345-347 Seminar Room, Main Research Building

Event Official Language: English

Mathematical Application Research Team Meeting #11

December 12 (Fri) 15:00 - 16:30, 2025

So Matsuura (Professor, Department of Physics Hiyoshi, Keio University)

Mathematical Application Research Team invites Prof. So Matsuura from Keio University for this meeting. You are welcome to join the meeting. Title: Phases and Duality in Fundamental Kazakov-Migdal Model on the Graph Abstract: In this talk, we will explore the fundamental Kazakov-Migdal (FKM) model on a generic graph, where the partition function is represented by the Ihara zeta function weighted by unitary matrices. The effective action of the FKM model is described by a summation of all Wilson loops on the graph, which can be regarded as an extension of the usual Wilson action in lattice gauge theory. We show that the FKM model on regular graphs exhibits an exact strong/weak coupling duality, reflecting the functional equation of the Ihara zeta function. We also discuss the relation between the stability of the FKM model and the pole distribution of the Ihara zeta function. Interestingly, the FKM model universally exhibits the so-called Gross-Witten-Wadia (GWW) phase transitions. We estimate the phase structure of the FKM model in both small and large coupling regions, which is validated through numerical simulations. If time permits, we will examine the phase transition of the FKM model from the Young diagram perspective and discuss its relation with the GWW phase transition indicated by the eigenvalue distribution.

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

Event Official Language: English

Widespread conservation of genetic effect sizes between human groups across traits

December 12 (Fri) 13:30 - 15:00, 2025

Simon Robert Myers (Professor, University of Oxford, UK)

Understanding genetic differences between populations is essential for avoiding confounding in genome-wide association studies and improving polygenic score (PGS) portability. We developed a statistical pipeline to infer fine-scale Ancestry Components and applied it to UK Biobank data. Ancestry Components identify population structure not captured by widely used principal components, improving stratification correction for geographically correlated traits. To estimate the similarity of genetic effect sizes between groups, we developed ANCHOR, which estimates changes in the predictive power of an existing PGS in distinct local ancestry segments. ANCHOR infers highly similar (estimated correlation 0.98 ± 0.07) effect sizes between UK Biobank participants of African and European ancestry for 47 of 53 quantitative phenotypes, suggesting that gene–environment and gene–gene interactions do not play major roles in poor cross-ancestry PGS transferability for these traits in the United Kingdom, and providing optimism that shared causal mutations operate similarly in different populations.

Venue: via Zoom / Seminar Room #359

Event Official Language: English

Kyoto University MACS Program Study Group RIKEN Visit and Research Exchange Meeting —Discussing the Cutting Edge with RIKEN Researchers—

December 12 (Fri) 12:30 - 15:10, 2025

12:00 – 13:30 Participate in Coffee Meeting 13:30 – 14:10 Ryosuke Iritani (Senior Research Scientist, RIKEN iTHEMS) — Research Keywords: Mathematical Biology, Evolutionary Ecology 14:10 – 14:30 Break 14:30 – 15:10 Okuto Morikawa (Special Postdoctoral Researcher, RIKEN iTHEMS) — Research Keywords: Lattice Field Theory, Conformal Field Theory, Quantum Field Theory, Particle Physics

Venue: #345-347, 3F, Main Research Building

Event Official Language: Japanese

From perturbations of operators to noncommutative condensers

December 11 (Thu) 16:00 - 17:00, 2025

Dan Voiculescu (Professor, Department of Mathematics, University of California, Berkeley, USA)

A numerical invariant, the quasicentral modulus underlies the multivariable generalizations of the classical Weyl-von Neumann-Kuroda and Kato-Rosenblum theorems. There are also connections to the Kolmogorov-Sinai dynamical entropy. I will also point out some of the open problems. Recently a noncommutative analogy with condenser capacity in nonlinear potential theory is emerging, that provides a new perspective on the subject.

Venue: via Zoom / #359, 3F, Main Research Building

Event Official Language: English

Self-organized mechano-chemical instabilities drive the emergence of tissue morphogenesis in digit organoids

December 11 (Thu) 13:00 - 14:00, 2025

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

Tissue morphogenesis is an emergent phenomenon: macroscopic structures cannot be predicted from a mere list of genes and cells. We examine here how digits arise from a spherical limb bud and present a framework linking microscopic cellular behavior to morphogenesis. To extract digit morphogenesis in vitro, we created a limb-mesenchyme organoid that breaks symmetry and forms digit-like cartilage. Analyzing cell behavior, iterating between experimental evidence and cellular-based models, shows that microscopic mechanisms like differential adhesion between distal and proximal autopod cells, chemotaxis toward Fgf8b, and biased traction can drive tissue-wide deformations by convergent extension that eventually lead to the formation of digit structures. Taking the continuum limit of these microscopic rules yields a modified Cahn–Hilliard equation, that is well known to describe fluid interfaces and so-called fingering instabilities, but that is shown here to recapitulate well organoid morphogenesis. Taken together, this work suggests that the emergence of “fingers” can be explained in a theoretical framework as a type of fingering instability.

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

Event Official Language: English

Graph polynomials and quantum field theory

December 9 (Tue) 15:00 - 17:00, 2025

Michael McBreen (Assistant Professor, Department of Mathematics, The Institute of Mathematical Sciences, The Chinese University of Hong Kong, Hong Kong)

The Tutte polynomial was introduced in the 1940s as a two-variable generalisation of the chromatic polynomial of a graph. It is the universal matroid invariant satisfying a deletion-contraction relation, and is the subject of much recent work. I will describe a geometric realisation of the Tutte polynomial via the cohomology of a symplectic dual pair of hypertoric varieties. The same construction associates an interesting two-variable polynomial to any pair of symplectically dual spaces, whose one-variable specialisations recover the respective Poincare polynomials. Joint work with Ben Davison.

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

Event Official Language: English

Generative sampling with physics-informed kernels

December 8 (Mon) 14:00 - 15:00, 2025

Renzo Kapust (Ph.D. Student, Institute for Theoretical Physics, University Heidelberg, Germany)

We construct a generative network for Monte-Carlo sampling in lattice field theories and beyond, for which the learning of layerwise propagation is done and optimised independently on each layer. The architecture uses physics-informed renormalisation group flows that provide access to the layerwise propagation step from one layer to the next in terms of a diffusion equation for the respective renormalisation group kernel through a given layer. Thus, it transforms the generative task into that of solving once the set of independent and linear differential equations for the kernels of the transformation. As these equations are analytically known, the kernels can be refined iteratively. This allows us to structurally tackle out-of-domain problems generally encountered in generative models and opens the path to further optimisation. We illustrate the practical feasibility of the architecture within simulations in scalar field theories.

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

Event Official Language: English

Full exceptional collections on Fano threefolds and the braid group action

December 5 (Fri) 16:00 - 17:30, 2025

Anya Nordskova (Postdoctoral researcher, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU))

The bounded derived category D^b(X) of coherent sheaves on an algebraic variety X is a powerful tool that encodes a wealth of information about X. In some cases D^b(X) admits a particularly nice description via so-called full exceptional collections, which allow one to view D^b(X) as being glued from the simplest building blocks, each equivalent to the derived category D^b(pt) of a point. In this situation the set of all full exceptional collections admits an action of the braid group. In 1993, Bondal and Polishchuk conjectured that this braid group action is always transitive. After a short historical overview I will sketch the idea behind the proof of Bondal-Polishchuk's conjecture in the case when X is a Fano threefold of Picard rank 1 (e.g. the projective space P^3). This is the first 3-dimensional case where the transitivity of the braid group action has been verified. The talk is based on joint work with Michel Van den Bergh.

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

Event Official Language: English

About analytic continuation of quantum field theories in non-integer dimensions

December 5 (Fri) 14:00 - 15:30, 2025

Slava Rychkov (Professor, Institut des hautes études scientifiques, France)

Analytic continuation in dimension has been used first as a way to regularize perturbative quantum field theory. But since the work of Wilson and Fisher, quantum field theory in d-dimension has been used more radically, to connect theories living say, in d=4, to theories in d=3 and d=2. Mathematically it's not fully clear what this means. I will give some thoughts about this subject, and I will describe some recent paradoxes which arise when one consider expansion of O(N) models around d=2, based on recent work with Fabiana De Cesare.

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

Event Official Language: English

The Functional Renormalisation Group: From the physics of strongly correlated systems to generative models

December 5 (Fri) 10:30 - 12:00, 2025

Jan Martin Pawlowski (Professor, Institute for Theoretical Physics, University of Heidelberg, Germany)

In the past decades, the functional renormalisation group (fRG) has matured into a comprehensive approach to strongly-correlated (non-perturbative) systems, covering quantitatively both universal and non-universal phenomena. The fRG also constitutes an ideal approach for unravelling structural aspects of quantum field theories. This is not only interesting for studies in mathematical physics, but also guides systematic diagrammatic expansion schemes. It is also used to set up novel statistical (lattice) approaches to non-perturbative phenomena. In the present talk I survey these advances and illustrate the progress with selected examples ranging from ultracold atoms, QCD and quantum gravity to novel generative architectures for lattice simulations and beyond.

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

Event Official Language: English