Seminar

Gauge subtleties and the finiteness of loop corrections beyond slow roll

April 3 (Thu) at 14:00 - 15:30, 2025

Danilo Artigas (JSPS Postdoctoral Research Fellow, Department of Physics Ⅱ, Division of Physics and Astronomy, Graduate School of Science, Kyoto University)

The early universe undergoes a phase of exponential expansion called inflation, under which quantum fluctuations are amplified and later seed cosmological structures. A long-standing question is whether interactions of these quantum fields may significantly affect the n-point statistics of cosmological observables. These corrections are known as loop corrections. Recently, Kristiano and Yokoyama claimed that, in scenarios beyond slow-roll inflation, the one-loop correction of super-Hubble fluctuations could become non-negligible and violate cosmological-perturbation theory. This result is highly debated, and in this talk we will use a non-linear approach known as delta N formalism to evaluate these loop corrections. We find the existence of loop corrections for modes close to the Hubble scale, however, these corrections are quickly suppressed for long-wavelength modes. We also show how the result of Kristiano and Yokoyama may arise when truncating the perturbative expansion, and how this result depends on the chosen gauge.

Venue: Seminar Room #359

Event Official Language: English

iTHEMS Biology Study Group April Launch Meeting (Part 1)

April 3 (Thu) at 14:00 - 15:00, 2025

Let's launch our Biology Study Group activities for the new year (Part 1 of 2). This meeting will be used to (1) say welcome to new member (SPDR Kenji Okubo, and Postdoc Lucas Sort); (2) discuss Biology seminar management in light of the new iTHEMS Centre; and (3) catch up on each other's current research. Since this will probably take us 2h, this will be Part 1 of 2 (Part 2 on 4/10). On 4/3 (Part 1) we will get a 15 min introduction talk by SPDR Kenji Okubo. This meeting is open to all RIKEN and guests. You do not need to be a member of the iTHEMS Biology Study Group.

Venue: via Zoom / 4th floor public space, Main Research Building

Event Official Language: English

Omega Meson from Lattice QCD

April 2 (Wed) at 15:00 - 16:00, 2025

Haobo Yan (Ph.D. Student, School of Physics, Peking University, China)

The three-body problem, renowned for its unsolvable nature in celestial mechanics and homonymous science fiction, is not only solvable in the quantum realm regarding spectra but also offers profound insights into QCD. In this talk, I will present the first-ever lattice calculation of the resonance parameters for the lightest hadron decaying into three particles, the -meson. By mapping finite-volume energy levels to infinite-volume scattering amplitude, a pole position trajectory is obtained that, when extrapolated to the physical point, shows good agreement with the experiment.

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

Event Official Language: English

The rarer-sex effect

March 27 (Thu) at 16:00 - 17:00, 2025

Andy Gardner (Professor, School of Biology, University of St Andrews, UK)

The study of sex allocation—that is, the investment of resources into male versus female reproductive effort—yields among the best quantitative evidence for Darwinian adaptation, and has long enjoyed a tight and productive interplay of theoretical and empirical research. The fitness consequences of an individual's sex allocation decisions depend crucially upon the sex allocation behaviour of others and, accordingly, sex allocation is readily conceptualized in terms of an evolutionary game. I will discuss the historical development of understanding of a fundamental driver of the evolution of sex allocation—the rarer-sex effect—from its inception in the writing of Charles Darwin in 1871 through to its explicit framing in terms of consanguinity and reproductive value by William D. Hamilton in 1972. I will show that step-wise development of theory proceeded through refinements in the conceptualization of the strategy set, the payoff function and the unbeatable strategy.

Venue: #445-447, 4F (Hybrid), Main Research Building / via Zoom

Event Official Language: English

Stability of nonsingular black holes

March 27 (Thu) at 15:00 - 16:30, 2025

Shinji Tsujikawa (Professor, Graduate School of Advanced Science and Engineering, Faculty of Science and Engineering, Waseda University)

We show that nonsingular black holes (BHs) realized in nonlinear electrodynamics are always prone to Laplacian instability around the center because of a negative squared sound speed in the angular direction. This is the case for both electric and magnetic BHs, where the instability of one of the vector-field perturbations leads to enhancing a dynamical gravitational perturbation in the even-parity sector. Thus, the background regular metric is no longer maintained in a steady state. We also generalize our analysis to the case in which a scalar field is present besides the U(1) gauge field and find no explicit examples of linearly stable nonsingular BHs. Our results suggest that the construction of regular BHs without instabilities is generally challenging within the scheme of classical field theories.

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

Event Official Language: English

A Century of Quantum Mechanics

March 24 (Mon) at 14:00 - 15:30, 2025

Gordon Baym (Professor Emeritus, University of Illinois, USA)

This is a RIKEN iTHEMS - The Univ. of Tokyo, Phys. Dept. Joint Seminar. This year, 2025, the "International Year of Quantum Science and Technology (IYQ)," is the 100th anniversary of the "formal" start of quantum mechanics, the description of the microscopic world. 1925 is the year in which Werner Heisenberg and others formulated "matrix mechanics," and physicists began to understand how to accurately predict microscopic phenomena. In this talk I will describe how quantum mechanics came about, starting with physicists in the late nineteenth century trying to understand the colors of hot metals and other hot objects, noting crucial advances leading to the fully developed wave and matrix quantum mechanics in the mid 1920's, to steps towards understanding real materials, culminating with spectacular applications such as smartphones, scarcely a century later.

Venue: The Univ. of Tokyo, Faculty of Science Building #4, room 1220 (Main Venue) / via Zoom

Event Official Language: English

Fast radio bursts as precursor radio emission from monster shocks

March 21 (Fri) at 16:00 - 17:15, 2025

Arno Vanthieghem (Assistant Professor, Observatoire de Paris and Sorbonne Université, France)

It has been proposed recently that the breaking of MHD waves in the inner magnetosphere of strongly magnetized neutron stars can power different types of high-energy transients. Motivated by these considerations, we study the steepening and dissipation of a strongly magnetized fast magnetosonic wave propagating in a declining background magnetic field, by means of particle-in-cell simulations that encompass MHD scales. Our analysis confirms the formation of a monster shock, that dissipates about half of the fast magnetosonic wave energy. It also reveals, for the first time, the generation of a high-frequency precursor wave by a synchrotron maser instability at the monster shock front, carrying a fraction of 0.1% of the total energy dissipated at the shock. The spectrum of the precursor wave exhibits several sharp harmonic peaks, with frequencies in the GHz band under conditions anticipated in magnetars. Such signals may appear as fast radio bursts.

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

Event Official Language: English

The puzzle of angular momentum conservation in beta decay and related processes.

March 21 (Fri) at 14:00 - 15:30, 2025

Gordon Baym (Professor Emeritus, University of Illinois, USA)

This is a iTHEMS-FQSP joint seminar. We ask the question of how angular momentum is conserved in a number of related processes, from elastic scattering of a circularly polarized photon by an atom, where the scattered photon has a different spin direction than the original photon; to scattering of a fully relativistic spin-1/2 particle by a central potential; to inverse beta decay in which an electron is emitted following the capture of a neutrino on a nucleus, where the final spin is in a different direction than that of the neutrino – an apparent change of angular momentum. The apparent non-conservation of angular momentum arises in the quantum measurement process in which the measuring apparatus does not have an initially well-defined angular momentum, but is localized in direction in the outside world. We generalize the discussion to massive neutrinos and electrons, and examine nuclear beta decay and electron-positron annihilation processes through the same lens, enabling physically transparent derivations of angular and helicity distributions in these reactions.

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

Event Official Language: English

Can we infer probability distributions from cumulants? Probabilistic approaches to inverse problems

March 18 (Tue) at 15:30 - 16:30, 2025

Yang-Yang Tan (Ph.D. Candidate, Dalian University of Technology, China)

Inverse problems, which involve estimating system inputs from outputs, are prevalent across science and engineering. Their ill-posed nature often makes finding numerically stable and unique solutions challenging. This seminar explores probabilistic methods for reconstructing distributions from a finite set of their moments or cumulants. We apply the Maximum Entropy Method (MEM) and Gaussian Process (GP) to reconstruct net-baryon number distributions across the QCD chiral crossover region using cumulant data from the STAR experiment and functional renormalization group (fRG) calculations. Our results demonstrate how higher-order cumulants shape distribution tails, while anomalous features in the reconstructed distributions provide constraints on the input cumulants. We also discuss deep learning approaches for distribution reconstruction from cumulants and present our recent work on physics-informed neural networks (PINNs) for solving fRG equations.

Venue: via Zoom

Event Official Language: English

Impact of the relativistic Cowling approximation on shear and interface modes of neutron stars

March 18 (Tue) at 11:00 - 12:30, 2025

Christian Kruger (Postdoctoral Researcher, University of Tuebingen, Germany)

Neutron stars are amongst the most compact objects known in the universe, which, therefore, require General Relativity for an accurate description. Seismic excitations of these stars may encode information about their currently unknown internal composition. As General Relativity is a mathematically complex theory, such oscillations are often considered in the Cowling approximation in which the spacetime is assumed to be static. In this talk, we will focus on shear and interface modes of neutron stars related to an elastic crust and investigate the impact of the Cowling approximation; we find that its impact on shear modes is negligible, while interface modes seem to experience some modification. Furthermore, we extend a scheme based on properties of Breit-Wigner resonances that allows to estimate the damping times of slowly damped modes. The proposed scheme is numerically robust and we compare it to estimates employing the quadrupole formula.

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

Event Official Language: English

The Golden Age of Neutron Stars

March 17 (Mon) at 15:30 - 17:00, 2025

Gordon Baym (Professor Emeritus, University of Illinois, USA)

This is a iTHEMS - Nishina Center Joint Seminar. Neutron stars were first posited in the early thirties, and discovered as pulsars in the late sixties; however we are only recently beginning to understand the matter they contain. After touching briefly on the history of neutron stars, I will describe the ongoing development of a consistent picture of the liquid interiors of neutron stars, now driven by ever increasing observations as well as theoretical advances. These include, in particular. observations of at least three heavy neutron stars of about 2.0 solar masses and higher; ongoing simultaneous inferences of masses and radii of neutron stars by the NICER telescope; and past and future observations of binary neutron star mergers, through gravitational waves as well as across the electromagnetic spectrum. I will also discuss pulsar timing arrays to detect very long wavelength gravitational waves, a remarkable role for neutron stars. Theoretically an understanding is emerging in QCD of how nuclear matter can turn into deconfined quark matter in the interior, and be capable of supporting heavy neutron stars, which I will illustrate with a discussion of modern quark-hadron crossover equations of state.

Venue: 2F Large Meeting Room, RIBF Building (E01) (Main Venue) / via Zoom

Event Official Language: English

Asymptotically flat black hole spacetimes with multiple injections

March 14 (Fri) at 15:30 - 17:00, 2025

Yuta Saito (Ph.D. Student, Graduate School of Science and Technology, Nihon University)

In quantum gravity, Hawking radiation presents several fundamental problems. One of the problems is the black hole (BH) information paradox, in which the entanglement entropy (EE), which quantifies quantum entanglement, exceeds its upper bound. In the absence of the paradox, EE follows the Page curve. Recent progress has been made in resolving this paradox using the island formula, a method for computing EE that successfully reproduces the expected Page curve. In this approach, a portion of the black hole interior is treated as part of the radiation region. Meanwhile, an alternative scenario has been proposed where multiple collapsing shells prevent the formation of a well-defined event horizon [1]. In this case, radiation is emitted throughout the collapse process, shifting dynamically the Schwarzschild radius inward, and a surface structure is formed just outside. This leads to a distinction between the conventional event horizon and the surface, introducing an intermediate region between the Schwarzschild radius and the surface. Interestingly, this model also suggests that part of the black hole interior effectively belongs to the radiation region, drawing a possible parallel to the island formula. In this talk, we explore spacetimes with multiple energy injections in asymptotically flat two-dimensional black hole backgrounds and analyze the entanglement entropy in such scenarios. Since considering backreaction in gravitational collapse in two dimensions is difficult, we instead construct a spacetime solution with multiple energy injections and analyze EE within this background. The main focus of this talk is to derive the spacetime and examine its properties. Additionally, we perform EE calculations in parallel with previous studies [2], which consider the case of α single injection, and confirm that the behavior of EE depends on the interval between energy injections.

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

Event Official Language: English

Complexity, expressivity, syntax and semantics

March 14 (Fri) at 14:00 - 14:30, 2025

Yusaku Nishimiya (Student Trainee, Natural Language Understanding Team, RIKEN Center for Advanced Intelligence Project (AIP))

I will summarise the philosophical motivations behind two research topics; 1. complexity/computability and 2. logic (structural proof theory), and discuss how they may help us understand what makes some problems harder than others, or equivalently, some knowledge more difficult to attain than others (my broad research goals). I. Complexity/computability Computational complexity and computability theory are a subfield of theoretical computer science in which we mathematically study the 'hardness' of problems. We do so by classifying algorithms or a collection of pre-defined rules that some solver can apply without ingenuity by how much time and memory space they require. II. Structural proof theory Even whilst maintaining the basic idea that a well-formed sentence, or a proposition, is either true or false, one can still make a conscious choice about what kind of principles to permit in deriving a new statement from assumptions. Structural proof theory formalises this as a logical-deduction system to study their effect on what the logic can and cannot do. III. What I do, more specifically I take advantage of equivalences between some computational complexity classes and logic, the latter of which, I hope, can serve as an interface to connect, via semantics, complexity with wider mathematics to elucidate something that can tell us what makes some computation inherently costly. IV. 'Computational view' of science I would love to discuss if time permits, how we may apply the idea of complexity to illuminate how information transfers from one thing to another in physical, biological and social systems.

Venue: 3rd floor public space, Main Research Building

Event Official Language: English

It’s about time! Daily rhythms in malaria infections matter for parasite survival and transmission

March 13 (Thu) at 17:00 - 18:00, 2025

Reece Sarah (Professor, University of Edinburgh, UK)

The Reece lab provides a unique perspective on parasites, examining their world within hosts and vectors (insects that transmit parasites). Working at the intersection of parasitology, chronobiology, and evolutionary ecology, our research asks: “what makes a successful parasite” and “what are their evolutionary limits”? Unlike most infection research, that focuses solely on genetics and molecular aspects, our approach considers parasites in their ecological and evolutionary contexts. This has enabled us to uncover the sophisticated strategies that malaria parasites possess, such as optimizing the balance between transmission and replication, strategic investment in each sex of transmission stages, and scheduling activities according to the time of day. By understanding how parasites navigate their challenging lifestyles and seize opportunities, we contribute to interventions that can outsmart parasites and reduce the risk of resistance evolution. Our findings extend beyond the laboratory, showcasing the potential of environmental research to curb the impact of parasitic infections, whether in humans, wildlife, livestock, or agriculture, and helping to protect ecosystems.

Venue: via Zoom

Event Official Language: English

RIKEN Quantum hands-on workshop on QURI SDK for creating and executing quantum algorithms on various quantum computers and simulators

March 13 (Thu) at 15:00 - 17:30, 2025

This workshop will be a hands-on session on QURI SDK, following the RIKEN Quantum seminar by Andreas Thomasen (QunaSys) on January 27. Even if you did not attend the previous seminar, please join us if you would like to learn how to use QURI SDK.

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

Event Official Language: English

From the Andes to the Lab Bench: Genomic, Evolutionary, and Functional Insights into Amylase Gene Variation and Metabolic Adaptation

March 6 (Thu) at 16:00 - 17:00, 2025

Gokcumen Omer (Professor, Department of Biological Sciences, University at Buffalo, USA)

Our laboratory investigates how genomic structural variations (SVs) uniquely drive biological diversity and evolution, surpassing smaller-scale variations like single nucleotide polymorphisms. This talk highlights our work on the amylase locus, a rapidly evolving genomic region shaped by dietary adaptations, frequent duplications, and high mutation rates. I will discuss its convergent evolution across mammals, driven by natural selection linked to starch-rich diets, and describe how long-read sequencing uncovered the mutational mechanisms behind its rapid evolution. We also examine local positive selection in indigenous Andean populations with historically starch-rich diets and how these adaptations impact metabolic health. Finally, I will summarize functional experiments in transgenic mice and diabetic-prone Western Nile rats, relevant models for human metabolism, to investigate the broader metabolic roles of amylase gene duplications. This research provides a roadmap for studying complex SVs in evolution, offering insights into human adaptation and health.

Venue: via Zoom / Seminar Room #359

Event Official Language: English

Density-dependent dispersal promotes female-biased sex allocation in viscous populations: From theory to experiment

March 4 (Tue) at 16:00 - 17:00, 2025

Chedhawat Chokechaipaisarn (Ph.D Student, School of Biology, University of St Andrews, UK)

A key prediction in sex allocation theory is that the optimal sex ratio is completely independent to the rate of dispersal. This result challenges the notion of any relationship between dispersal and sex ratio evolution. However, the invariant result is based on the assumption that an individual's dispersal behaviour is not modulated by population density. In this talk, I will explore how density-dependent dispersal impact upon the evolution of sex allocation in a viscous-population setting. Additionally, I will discuss the process of testing this prediction through experimental evolution in spider mites.

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

Event Official Language: English

Applications of Geometry of Numbers to Phyllotaxis and Crystallography

February 28 (Fri) at 14:00 - 15:30, 2025

Ryoko Oishi-Tomiyasu (Professor, Institute of Mathematics for Industry, Kyushu University)

The golden angle method, originally known from phyllotaxis in botany, has been used to generate dense point packings on surfaces of revolution. In my recent work, I have extended this method to general surfaces and higher-dimensional manifolds by employing the theories of products of linear forms in number theory, diagonalizable metrics in differential geometry, and local solutions of quasilinear hyperbolic equations. This extension suggests that any biological forms can exhibit phyllotactic patterns locally regardless of their morphology, while the overall pattern is influenced by their global properties in the embedded space. On the algebraic side, it is interesting that the same ideas used for phyllotaxis can also be applied to pseudorandom number generation over F2 = {0, 1}. This work is motivated by my previous research in crystallography. Time permitting, I will also introduce some of the research, which contributes to the analytical foundations of crystallography and is also an application of the geometry of numbers.

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

Event Official Language: English

Genome and Sex Chromosome Analyses of Japanese Frogs Carrying Both XY and ZW Chromosomes Within the Same Species

February 27 (Thu) at 16:00 - 17:00, 2025

Yukako Katsura (Assistant Professor, Center for the Evolutionary Origins of Human Behavior, Kyoto University)

The evolution of sex chromosomes, particularly sex chromosome turnover, is a complex and fascinating topic in genetics and evolutionary biology. Sex chromosome turnover refers to the process in which the sex chromosome system changes from XY to ZW (or vice versa), or in which sex chromosomes with different evolutionary origins emerge within the same system (e.g., from one XY system to another XY system). To study sex chromosome turnover, we focus on the Japanese frog (Glandirana rugosa), which possesses both XY and ZW sex chromosomes within the same species, and investigate the molecular mechanisms behind the turnover in the frog (Review: Hayashi et al. JB 2024). Previously, we sequenced the nuclear genome of the ZZ frog (Katsura et al. LSA 2021) and identified sex-linked genes in two populations of the XY and ZW frogs (Miura et al. Mol Ecol 2022). It has been suggested that sex chromosomes originating from at least three different chromosomal lineages have independently emerged within this species. The frogs have a total of 13 chromosomes, and in two populations (Tokai/Eastern Central Japan and Hokuriku-Tohoku/North-Western Japan), chromosome 7 has morphologically differentiated into both ZW and XY chromosomes. However, in other populations, sex chromosomes do not show any morphological differentiation. In this seminar, I introduce the background of our sex chromosome study and present the results of sequence comparisons of morphologically differentiated XY and ZW chromosomes, as well as findings from our analyses of populations, genome, and transcriptome.

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

Event Official Language: English

Quantum-Centric Supercomputing Software

February 26 (Wed) at 13:00 - 14:00, 2025

Hanhee Paik (Head of IBM Quantum Japan, IBM Quantum)

A quantum-centric supercomputer represents the next generation of computing, combining a quantum computer with a classical supercomputer. It leverages error mitigation and error correction techniques to deliver results within practical timeframes. When fully developed, this system relies on advanced middleware to seamlessly integrate quantum circuits with classical computing resources. In this presentation, we will introduce IBM Quantum’s middleware for quantum-centric supercomputers, highlighting collaborative projects with our research partners.

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

Event Official Language: English