セミナー

Phase Transitions as the Breakdown of Statistical Indistinguishability

2026年6月29日(月) 15:00 - 16:00

宮原 英之 (北海道大学 情報科学研究院 准教授)

We introduce a novel characterization of phase transitions based on hypothesis testing. In our formulation, a phase transition is defined as the breakdown of statistical indistinguishability under vanishing parameter perturbations in the thermodynamic limit. This perspective provides a general, order-parameter-free framework that does not rely on model-specific insights or learning procedures. We show that conventional approaches, such as those based on the Binder parameter, can be reinterpreted as special cases within this framework. As a concrete realization, we employ a distribution-free two-sample run test and demonstrate that the critical point of the two-dimensional Ising model is accurately identified without prior knowledge of the order parameter.

会場: セミナー室 (359号室) 3階 359号室とZoomのハイブリッド開催

イベント公式言語: 英語

Noncritical Conformal Gravity and 4D Liouville Theory

2026年6月12日(金) 15:00 - 16:30

太田 信義 (大阪公立大学 南部陽一郎物理学研究所 客員教授)

We study the quantum aspects of the conformal gravity in four dimensions, specifically addressing a known discrepancy in beta functions between general quadratic curvature theories and conformal gravity, which corresponds to two scalar degrees of freedom. We demonstrate that this mismatch is resolved by carefully introducing gauge-fixing and ghost terms via the BRST symmetry, which effectively adds the two scalar modes. Drawing lessons from two-dimensional quantum gravity and Liouville theory, we proceed to integrate the four-dimensional trace anomaly to derive a consistent Liouville action, which is given by a free-field action for the conformal mode with a consistent conformal anomaly. We give the condition that the BRST transformation is anomaly free. Finally I would like to talk about some application of this theory.

会場: セミナー室 (359号室) 3階 359号室とZoomのハイブリッド開催

イベント公式言語: 英語

Quantum Improved Black Holes in Asymptotically Safe Gravity

2026年6月11日(木) 15:00 - 16:30

Chiang-Mei Chen (Professor, Department of Physics, National Central University, Taiwan)

In this talk, I will explore quantum-improved black hole solutions within the framework of asymptotic safety. In this approach, the Newton coupling becomes scale-dependent, necessitating a meaningful identification between the energy scale and a corresponding physical (length) scale to derive observable consequences for black hole spacetimes. I will argue that the requirement of consistency with the first law of black hole thermodynamics provides a physically motivated criterion for this scale-setting, particularly near the event horizon. Applying this principle, we propose a specific identification scheme that leads to a regularized geometry capable of resolving the ring singularity of Kerr black holes.

会場: セミナー室 (359号室) 3階 359号室とZoomのハイブリッド開催

イベント公式言語: 英語

Disorder and Defects in Critical Systems

2026年6月8日(月) 13:30 - 15:00

Baishali Roy (Postdoctoral Fellow, Indian Institute of Technology Kanpur, India)

Real critical systems are often constrained by boundaries and affected by impurities. In 3d, the effect of disordered impurities on the boundary can be modeled by a random magnetic field on a two-dimensional defect. In this talk, I will discuss how such disorder affects the Wilson-Fisher fixed point in d=4−\epsilon dimensions. By analyzing the one-loop RG flow of the defect couplings using the replica formalism, we find a non-trivial "dirty" fixed point which represents a new boundary universality class, stabilized by the bulk \phi^4 interaction. Disordered systems at critical points are known to exhibit logarithmic behavior — I will also discuss how operator mixing in the replica limit gives rise to a logarithmic defect CFT in our setup.

会場: 研究本館 3階 359号室 (メイン会場) / via Zoom

イベント公式言語: 英語

reflection positivity in de Sitter space

2026年6月5日(金) 10:30 - 11:30

鈴木 優樹 (京都大学 基礎物理学研究所 博士課程（学術振興会特別研究員）)

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 英語

AI and Scientific Discovery

2026年6月3日(水) 14:00 - 15:30

Joseph Ledsam (Google Health Lead, Japan, Google Japan)

Artificial intelligence is having a transformative impact on health and scientific discovery. This presentation will trace the evolution from foundational breakthroughs to the sophisticated capabilities of today's large-scale AI models. It will explore how these advanced systems are creating new possibilities across the healthcare landscape, from accelerating therapeutic development to enhancing diagnostic processes and interpreting complex medical data. The session will also take a deeper look at the future possibilities for AI in health and explore the emerging role of agentic AI in scientific discovery. The core theme is the responsible development of AI to create tools that assist scientists, support healthcare professionals, and empower users. Bio: Dr Joseph Ledsam leads Google Health in Japan, where he works across AI research, digital health and health in Google products. He has led research in medical AI, genomics and drug discovery published in journals including Nature, Nature Medicine and Nature Methods. Before moving to Japan he worked as a medical doctor in the UK, and founded the Health Research and Genomics teams in Google DeepMind. He obtained his medical degree from The University of Leeds, UK, and was a research fellow at University College London during his clinical residency.

会場: 研究本館 435-437号室 (メイン会場) / via Zoom

イベント公式言語: 英語

Bootstrapping Cosmological Correlators

2026年5月28日(木) 16:00 - 18:00

Mang Hei Gordon Lee (Post-Doctoral fellow, Leung Center for Cosmology and Particle Astrophysics, National Taiwan University, Taiwan)

Currently there are hundreds of models describing inflation, a period of accelerated expansion in our universe. Each model lead to different imprints in cosmological observables, and for the purpose of testing the idea of inflation itself, it is essential to understand which predictions are model independent. This lead to the idea of cosmological bootstrap, a set of constraints from physical principles and symmetries alone. In this talk I will give an overview on the cosmological bootstrap program. I will first explain how locality, unitarity and symmetry can constrain the kinematics of cosmological correlators. I will then talk about some recent progress on constructing positivity bounds on cosmology, which places constraints on the interactions of fields in inflation.

会場: 研究本館 3階 359号室とZoomのハイブリッド開催

イベント公式言語: 英語

Introduction to categorification and link homology

2026年5月28日(木) 14:00 - 15:30

Mikhail Khovanov (Professor, Department of Mathematics, Johns Hopkins University, USA)

Quantum link invariants relate topology in 3 dimensions to mathematical physics and representation theory. They admit liftings to 4-dimensional structures, known as link homology. We will explain how the skein relations for quantum invariants turn into homological structures at this higher level and how semisimple representation theory turns into non-semisimple representations and homological algebra upon categorification.

会場: 大河内記念ホール (メイン会場) / via Zoom

イベント公式言語: 英語

Cooperating on networks: inequality and social structure

2026年5月27日(水) 14:00 - 15:00

Manuel Staab (Lecturer, University of Queensland, Australia)

We analyse how inequality in endowments and social structure jointly affect individuals' ability to cooperate. Individuals repeatedly invest in a local public good ("cooperation'') in an environment that is described by a distribution of endowments and a network of beneficiaries. We measure the cooperativeness of an environment by the minimum discount factor needed to sustain (any) cooperation in equilibrium. We characterise the endowment distribution that maximises cooperativeness for any given network and the corresponding minimum discount factor. The latter is shown to be inversely proportional to the maximal index of the graph describing the network. The corresponding dominant eigenvalue of the adjacency matrix characterises the most cooperative income distribution. Moreover, we show that if an environment maximises cooperativeness (over all income distributions and networks of a certain size), then the network is described by a nested split graph. We further show that this is the same class of graphs that maximise welfare for any given discount factor, and yet, the most cooperative graph need not be equal to the most efficient.

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 英語

Harnessing inequality for cooperation

2026年5月26日(火) 14:00 - 15:00

Maria Kleshnina (Senior Lecturer, School of Mathematical Sciences, Queensland University of Technology, Australia)

Inequality in resources is widely thought to undermine cooperation in social dilemmas. Yet cooperation among unequals is ubiquitous: between senior and junior colleagues, firms of different sizes, nations with asymmetric stakes. Here, we offer a resolution to this puzzle and derive a novel prediction: if the returns from cooperation are shared in accordance with the individuals' strategic incentives, inequality enables and strengthens cooperation. We develop a strategic framework to systematically explore cooperation when the returns of a joint project can be shared unevenly. We characterise the optimal sharing rule, which we call resilient sharing, that can sustain cooperation in repeated interactions when no other rule can. Resilient sharing equalises incentives to defect across players, but is neither egalitarian nor proportional. Surprisingly, it typically rewards weaker partners beyond their relative contributions. We show that cooperation can be sustained through direct reciprocity in any environment whenever individual contributions are sufficiently unequal. Evolutionary simulations and a behavioural experiment confirm the central prediction: under resilient sharing, cooperation succeeds among unequal partners where it fails among equals. This suggests that cooperation is more likely to evolve and thrive when individuals can vary contributions and divide returns flexibly, pointing to the role of institutions and norms in harnessing inequality to stabilize cooperation.

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 英語

Singularities of differentiable maps and Thom polynomials

2026年5月22日(金) 15:00 - 17:30

田邊 真郷 (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 基礎科学特別研究員)

Singularities are locations where something is exceptional. In particular, singularities of differentiable maps are mathematical concepts corresponding to stationary points of functions and apparent contours of surfaces under projection onto the retina. These are unavoidable in general, but important to study the shape of spaces and behavior of maps. The theory for them was initiated by R. Thom in 1950's, and have been deeply studied by many researchers.

会場: 理化学研究所 和光キャンパス (メイン会場) / via Zoom

イベント公式言語: 英語

Entanglement entropy and conformal bounds for five-dimensional CFTs

2026年5月21日(木) 15:00 - 16:00

Javier Moreno (京都大学 基礎物理学研究所 基研特任助教)

Abstract: The entanglement entropy of spatial regions in odd-dimensional conformal field theories contains a universal constant contribution that encodes important information about the theory. This quantity can be defined in a robust way using mutual information between slightly deformed versions of a given region. In three-dimensional conformal field theories, it is known that this quantity is always positive and bounded from below by the value corresponding to a spherical region. There is also strong evidence that, for any region, its normalized value is maximized by the free scalar theory. In this work, we show that the situation changes significantly in five dimensions. Although the spherical region remains a local minimum under small shape deformations, more general regions can lead to values that become arbitrarily large in magnitude, with either sign. This implies that, in five-dimensional conformal field theories, the quantity is not bounded from above or below. We also demonstrate that the analogous maximization property observed in three dimensions does not hold in five dimensions when considering general regions. Despite this, we find that existing evidence is consistent with a weaker statement: for small deformations of a spherical region, the normalized quantity remains bounded above by the free scalar result across all five-dimensional conformal field theories. This leads to a new conjectured universal bound relating two key physical quantities—the coefficient governing stress-tensor correlations and the sphere free energy—which appears to hold for all currently known examples.

会場: via Zoom / セミナー室 (359号室)

イベント公式言語: 英語

Stochastic Thermodynamics: noise and energetics of nanoscale systems

2026年5月21日(木) 13:00 - 14:00

Jean-Charles Delvenne (Professor, Applied Mathematics, Université Catholique de Louvain, Belgium)

Stochastic thermodynamics, initiated three decades ago, aims at quantifying the fluctuations of physical observables in relation with thermodynamic quantities (such as heat or entropy production). A typical result is the Thermodynamic Uncertainty Relation, which states that a high entropy production is required to obtain a favorable noise-to-signal ratio for some observables (such as displacement of a molecular motor) in stationary out-of-equilibrium systems. It is especially relevant for nanoscale systems, where the fluctuations cannot be neglected. This includes biological systems (e.g. biological motors such as kinesin along a microtubule), electronic systems (transistor-based memories), chemical reactions, etc. The talk will be both a tutorial on some basic results or applications, and a presentation of some recent results and perspectives.

会場: via Zoom / セミナー室 (359号室)

イベント公式言語: 英語

On Mean-Field Games

2026年5月21日(木) 10:30 - 11:30

アントワーヌ・ディエズ (理化学研究所 数理創造研究センター (iTHEMS) 数理展開部門 数学応用研究チーム 研究員)

Stochastic differential games with a large number of players are notoriously challenging, both theoretically and numerically, typically when it comes to computing Nash equilibria. Yet, when many players interact somehow symmetrically by responding only to the average behavior of the others, the game can surprisingly become more tractable by taking the limit of an infinite number of players. This is in direct analogy with the so-called « mean-field theory » which simplifies the analysis of large systems of interacting particles in statistical physics. Introduced independently about two decades ago by Lasry and Lions (mathematics) and Caines, Huang and Malahamé (engineering), the theory of Mean-Field Games has since been greatly developed with various applications in engineering, economical, social and biological sciences. The goal of this short lecture is to introduce the key concepts, particularly the deep connections between game theory, Partial Differential Equations and stochastic analysis, and to showcase a few striking recent applications.

会場: セミナー室 (359号室) (メイン会場) / via Zoom

イベント公式言語: 英語

Positivity constraints for the gravitational path integral

2026年5月21日(木) 10:00 - 11:50

ガブリエル・ディウバルド (理化学研究所 数理創造研究センター (iTHEMS) 国内外連携・人材育成部門 理研バークレーセンター 特別研究員)

For a quantum theory of gravity to have a well-defined Hilbert space, the inner product between different states of open and closed universes must be positive semi-definite. Positivity however is not manifest in the low-energy effective theory and in fact imposes nontrivial constraints on the theory. Working in the Gravitational Path Integral (GPI) approach, we derive the general set of positivity constraints on the closed and open universe Hilbert spaces. In the case of AdS gravity, open universe positivity in principle follows from CFT unitarity, however the holographic description of closed universes remains unclear. Strikingly, we exhibit positivity of closed universes across many theories and prove that open positivity implies closed positivity, showing that the CFT 'knows' about the closed universe hilbert space. We then analyze positivity constraints on gravitational theories coupled to axions. We present a method to compute off-shell axion wormholes in AdS and flat space which we use to show that positivity is violated if the axion shift symmetry is exact. In low-energy EFTs where these wormholes are perturbatively stable, to restore positivity the wormhole must have a non-perturbative instability due to instantons that breaks the shift symmetry. Positivity then leads to a proof of a sharp version of the Axion Weak Gravity Conjecture A-WGC, including precise numerical constants. For the QCD axion this provides a bound on the axion decay constant which has phenomenological and experimental consequences for axion searches. In string theory, positivity gives a bound on the coupling between the axion and the dilaton in the low energy effective action.

会場: via Zoom

イベント公式言語: 英語