Seminar

Exploring interior magnetic-field in neutron stars

July 25 (Tue) at 10:00 - 12:00, 2023

Yasufumi Kojima (Emeritus Professor, Hiroshima University)

Neutron stars are well known as a good laboratory to test high-density material. Magnetic field on the stars is relevant to some astrophysical phenomena, and understanding the effect is crucial to extract information of the stellar interior. In this talk, I start with discussing some evidence and implications for intense magnetic field, and focus on the magneto-elastic equilibrium in the solid crust. The study leads to upper limit of deformation, and beyond a threshold crustal fracture observed burst on strongly magnetized neutron stars. The model is still primitive, and further development is desirable to connect micro-physics with astrophysical observation.

Venue: via Zoom (Main Venue) / #345, 3F, Main Research Building, RIKEN Wako Campus

Event Official Language: English

Electronic instabilities emerging from higher-order van Hove singularities

July 24 (Mon) at 17:00 - 18:15, 2023

Xinloong Han (Postdoctoral Fellow, Kavli Institute for Theoretical Sciences, University of Chinese Academy of Sciences, China)

Time: 5pm ~ 6:15pm (JST); 10am ~ 11:15am (CET); 4pm ~ 5:15pm (Taiwan) Field: condensed matter physics Keywords: topological superconductor, Van Hove singularity, Hubbard model, Kagome lattices Abstract: Competing correlated electronic states are a central topic in condensed matter physics. A typical example is the close competition between spin density wave and d-wave superconductivity in the Hubbard model on the square lattice near half filling where the band structures have saddle points at which the Fermi surface topology changes from hole type to electron type. The saddle points are called van Hove singularity (VHS) points, and host diverging density of states with power-law behavior in the two dimensions. Recently, another type of VHS, namely the higher-order VHS was investigated in ABC-stacked trilayer graphene and twisted bilayer graphene. In this talk, I will first introduce the higher-order VHS, and make comparisons to the conventional VHS. Then I will discuss the enhanced nematicity driven by large flavor number with higher-order VHSs on the square and Kagome lattices. Finally, I will show that robust topological superconductivity can emerge on the square lattice due to interplay of spin-orbital coupling and higher-order VHSs.

Venue: Hybrid Format (Common Room 246-248 and Zoom)

Event Official Language: English

Evolution of dormant egg production and their hatching rate in Aedes albopictus

July 13 (Thu) at 16:00 - 17:00, 2023

Yusuke Kuwano (Ph.D. Student, The Graduate University for Advanced Studies (SOKENDAI))

Mosquitoes are important insect vectors of infectious diseases in humans, and knowledge of their population dynamics is pivotal in disease control. Some mosquito species have dormancy in their life history to survive harsh environments. However, the population dynamics of mosquitoes have not yet been well understood due to the lack of field and experimental data on dormancy. For that reason, I modeled the population dynamics of mosquitoes that face environmental fluctuations and examine the evolution of egg dormancy strategy to survive harsh periods. I found that the ESS dormancy fraction monotonically increases with the period of environmental fluctuation. Next, I analyzed evolutionary traits of the dependence of the dormancy rate and the hatching rate from dormant egg on soil moisture content and conducted evolutionary simulations using actual weather measurement in Tokyo. The results of the hatching rate from dormant egg showed that two mosquito phenotypes having distinctly different responses to soil moisture were selected.

Venue: via Zoom

Event Official Language: English

Conserved charges in the quantum simulation of integrable spin chains

July 12 (Wed) at 13:30 - 15:00, 2023

Juan William Pedersen (Ph.D. Student, Graduate School of Arts and Sciences, The University of Tokyo)

In this talk, we present the result of the quantum simulation of the spin-1/2 Heisenberg XXX spin chain. We implement the integrable Trotterization algorithm, which allows us to control the Trotter error with conserved charges remaining conserved, on a real quantum computer and classical simulators. We study the effects of quantum noise on the time evolution of several conserved charges and specifically observe the decay of the expectation values. Our work improves our understanding of quantum noises and can potentially be applied to benchmark quantum devices and algorithms.

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

Event Official Language: English

Searching for dark matter subhalos in the Fermi-LAT catalog with Bayesian neural networks

July 10 (Mon) at 16:30 - 18:00, 2023

Slivia Manconi (Marie Skłodowska-Curie Fellow, Laboratoire d'Annecy-Le-Vieux de Physique Theorique (LAPTh), CNRS, France)

Machine learning techniques are powerful tools to tackle diverse tasks in current astroparticle physics research. For example, Bayesian neural networks provide robust classifiers with reliable uncertainty estimates, and are particularly well suited for classification problems that are based on comparatively small and imbalanced data sets, such as the gamma-ray sources detected by Fermi-Large Area Telescope (LAT). About one third of the gamma-ray sources collected in the most recent catalogs remain currently unidentified. Intriguingly, some of these could be exotic objects such as dark subhalos, which are overdensities in dark matter halos predicted to form by cosmological N-body simulations. If they exist in the Milky Way, they could be detected as gamma-ray point sources due to the annihilation or decay of dark matter particles into Standard Model final states. In this talk I will discuss our recent work* in which, after training on realistic simulations, we use Bayesian neural networks to identify candidate dark matter subhalos among unidentified gamma-ray sources in Fermi-LAT catalogs. Our novel framework allows us to derive conservative bounds on the dark matter annihilation cross section, by excluding unidentified sources classified as astrophysical-like.

Venue: via Zoom

Event Official Language: English

A dynamical model for IRAS 00500+6713: the remant of a type Iax supernova SN1181 hosting a double degenerate merger product WD J005311

July 7 (Fri) at 14:00 - 15:15, 2023

Takatoshi Ko (Ph.D. Student, Research Center for the Early Universe (RESCEU), The University of Tokyo)

Iras 00500+6713 is a bright nebula in the infrared, and X-ray observations show it consists of diffuse region and strong illuminated central region. In addition, optical spectral observations have recently revealed that fast wind with about 15,000 km/s is blowing from the massive white dwarf at the center. The properties of this nebula and white dwarf are very similar to those theoretically predicted by the binary white dwarf merger. In addition, its position on the celestial sphere and the extent make it a prime candidate for the remnant of SN 1181, a historical supernova. In this study, we propose that such a multilayered structure is formed by the collision between the remnant of SN 1181 and the stellar wind blowing from the central white dwarf, and succeeded in constructing a model that is consistent with the multi-wavelength observations. The results show that the progenitor of SN 1181 is a binary white dwarf with 1.3-1.9 solar mass and that their merger triggered an explosion that ejected mass with 0.2-0.6 solar mass to form the present object. The extent of the X-ray source concentrated in the center reveals that these winds began blowing within the last 30 years, and we will discuss this property as well.

Venue: Seminar Room #359 / via Zoom

Event Official Language: English

Introduction to braid groups

July 5 (Wed) at 14:00 - 16:30, 2023

Haru Negami (Ph.D. Student, Graduate School of Science and Engineering, Chiba University)

Part 1 (14:00-15:00): Introduction to braid groups Braid groups are groups that are defined by figures formed by the entanglement of n strings. Besides this geometric realization, it is a very interesting field where algebra and analysis intersect. In the first half of this seminar, aimed mainly at those unfamiliar with braid groups, we will introduce three aspects of braid groups and review the history of the research. In particular, in the area of its relation to analysis, the relationship between KZ equations and braid groups will be introduced. Part 2 (15:30-16:30): Representations of braid groups and the relationship between monodromy representations of KZ equations In the second half of the talk, after a brief introduction to representation theory, we will introduce the Katz-Long-Moody construction, a method of constructing infinite series of representations of the semi-direct product of braid group and free group. We will also show that its special case is isomorphic to multiplicative middle convolution, a method for constructing monodromy representations of KZ equations. Lastly, we will also discuss the connection between representations of braid groups and knot invariants. The talk includes joint work with Kazuki Hiroe.

Venue: Seminar Room #359 / via Zoom

Event Official Language: English

Maximum Force Conjecture and Black Hole Thermodynamics

June 30 (Fri) at 14:00 - 15:30, 2023

Yen Chin Ong (Professor, Center for Gravitation and Cosmology, College of Physical Science and Technology, Yangzhou University, China)

I review the current controversial status of the so-called "maximum force conjecture" in general relativity, whose validity has recently been debated. Then I will discuss how maximum force conjecture can nevertheless be relevant for black hole thermodynamics.

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English

Journal Club: Inference of hydrodynamic equations for active matter

June 29 (Thu) at 16:00 - 17:00, 2023

Kyosuke Adachi (Special Postdoctoral Researcher, Nonequilibrium Physics of Living Matter RIKEN Hakubi Research Team, RIKEN Center for Biosystems Dynamics Research (BDR))

Event Official Language: English

The classical equations of motion of quantised gauge theories

June 23 (Fri) at 13:30 - 15:00, 2023

Tom Melia (Associate Professor, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)

The Einstein and Maxwell equations are the jewels in the crown of classical physics. But classical physics is only an approximation to nature, arising as a limit of the underlying quantum mechanical description. And in the case of both general relativity and electromagnetism, owing to their gauge theory nature, the full set of classical equations of motion are not guaranteed to follow from the quantum theory. The time-time and time-space components of the Einstein equations in GR and Gauss’ law in EM are enforced ‘by hand' in the quantisation procedure—a choice so as to make the classical-like states behave as per our classical belief. But what if our universe was actually described by another classical-like state? For GR, the resulting modification of the Einstein equations can be packaged as the inclusion of an auxiliary energy-momentum tensor describing a ’shadow’ matter that adds no additional degrees of freedom to the theory. The homogeneous and isotropic background piece of this auxiliary matter contributes to expansion of the universe identical to cold dark matter, and the inhomogeneous components source curvature perturbations that grow linearly at linear order.

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English

The role of fluid dynamics in microbial ecology

June 22 (Thu) at 14:00 - 15:00, 2023

Douglas R. Brumley (Senior Lecturer, Applied Mathematics, University of Melbourne, Australia)

Bacterial motility, symbioses, and marine nutrient cycling unfold at the scale of individual microbes, and are inherently dynamic. In this talk, I will discuss the role that fluid flows play in shaping the ecology of microbes, both in the open ocean as well as around coral surfaces. In each case, I will demonstrate how iteratively combining video-microscopy, image processing and mathematical modelling can resolve features of microbial lifestyles that are difficult or impossible to see otherwise, and show how single-cell measurements can be connected to bulk processes at the population-level.

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English

Matrix estimation via singular value shrinkage

June 21 (Wed) at 15:30 - 16:30, 2023

Takeru Matsuda (Unit Leader, Statistical Mathematics Collaboration Unit, RIKEN Center for Brain Science (CBS))

In this talk, I will introduce recent studies on shrinkage estimation of matrices. First, we develop a superharmonic prior for matrices that shrinks singular values, which can be viewed as a natural generalization of Stein’s prior. This prior is motivated from the Efron–Morris estimator, which is an extension of the James–Stein estimator to matrices. The generalized Bayes estimator with respect to this prior is minimax and dominates MLE under the Frobenius loss. In particular, since it shrinks to the space of low-rank matrices, it attains large risk reduction when the unknown matrix is close to low-rank (e.g. reduced-rank regression). Next, we construct a theory of shrinkage estimation under the “matrix quadratic loss”, which is a matrix-valued loss function suitable for matrix estimation. A notion of “matrix superharmonicity” for matrix-variate functions is introduced and the generalized Bayes estimator with respect to a matrix superharmonic prior is shown to be minimax under the matrix quadratic loss. The matrix-variate improper t-priors are matrix superharmonic and this class includes the above generalization of Stein’s prior. Applications include matrix completion and nonparametric estimation.

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English

Introduction to statistical decision theory and Stein’s paradox

June 21 (Wed) at 14:00 - 15:00, 2023

Takeru Matsuda (Unit Leader, Statistical Mathematics Collaboration Unit, RIKEN Center for Brain Science (CBS))

Statistical decision theory is a general framework for discussing optimality of statistical procedures such as estimation, testing and prediction. In 1956, Charles Stein found a counter-intuitive phenomenon in estimation of the mean parameter of a multivariate normal distribution. He showed that a ``shrinkage estimator” attains better estimation accuracy (smaller mean-squared error) than the maximum likelihood estimator when the dimension is greater than or equal to three. This phenomenon is related to several mathematical fields such as Markov processes and potential theory. The idea of shrinkage estimation has been employed in many statistical methods such as regularization, empirical Bayes and model selection. In this talk, I will introduce the statistical decision theory and illustrate Stein’s paradox.

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English

A Spin on Wave Dark Matter

June 15 (Thu) at 16:00 - 17:30, 2023

Mustafa Amin (Associate Professor, Department of Physics and Astronomy, Rice University, USA)

What can we learn about the intrinsic spin of ultralight dark matter field from astrophysical observations? That is, is dark matter is a scalar (spin 0), (spin 1) or tensor (spin 2) field? Using analytic calculations and 3+1 dimensional simulations, I will argue that the imprint of spin can be seen via (i) the initial density power spectrum, (ii) interference patterns in the density field inside dark matter halos, and through (iii) (polarized) solitons with macroscopic intrinsic spin. Based on features in the initial power spectrum, I will provide a bound on the dark matter mass > 10^(-18) eV for post-inflationary production. With increasing intrinsic spin, interference patterns in halos are reduced (and the inner shapes of halos modified) — which can be probed by lensing and dynamical heating of stars. Finally, after introducing polarized solitons, I will show that the time-scale of emergence of solitons (within halos) increases with increasing spin, and briefly discuss electromagnetic and gravitational wave signatures from such polarized solitons. Time-permitting, I will also mention connections to “spinor" Bose-Einstein condensates in the laboratory.

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English

Phase reduction beyond the Kuramoto model

June 15 (Thu) at 14:00 - 15:00, 2023

Iván León (Research Scientist, Department of Systems and Control Engineering, Tokyo Institute of Technology)

Many biological, engineering and natural systems can be modeled as populations of coupled oscillators where each oscillator behaves periodically. When these units are coupled to each other, emergent phenomena, as synchronization, appears. However, dealing with those systems is usually difficult due to the large number of degrees of freedom. Conditionality reduction techniques to obtain simple tractable models are usually considered. The most common method is "phase reduction" that allows to capture the dynamics of each oscillator with just one variable, the phase. The succeed of the method was clear when the Kuramoto model, derived through phase reduction, gave a simple explanation to collective synchronization. Despite this success, phase reduction is often limited to the Kuramoto model because of the challenge to obtain analytical expressions. The porpoise of this talk is to make clear that phase reduction beyond Kuramoto model is possible. On the first part of the talk we introduce phase reduction and its limitations. Then we show how it is possible to obtain analytical phase reduced model for weakly nonlinear oscillators. Finally, we talk about second order phase reduction where higher order corrections are included to capture the qualitative dynamics and improve accuracy.

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English

Quantum skyrmion lattices in Heisenberg ferromagnets

June 8 (Thu) at 17:00 - 18:15, 2023

Andreas Haller (Postdoctoral Researcher, Department of Physics and Materials Science, University of Luxembourg, Luxembourg)

Skyrmions are topological magnetic textures that can arise in noncentrosymmetric ferromagnetic materials. In most systems experimentally investigated to date, skyrmions emerge as classical objects. However, the discovery of skyrmions with nanometer length scales has sparked interest in their quantum properties. In this talk, I present our (numeric) results on the ground states of unfrustrated two-dimensional spin-1/2 Heisenberg lattices with Dzyaloshinskii-Moriya interactions, where we discovered a broad region in the zero-temperature phase diagram which hosts quantum skyrmion lattices. The simulations are based on an established variational optimization algorithm for matrix product states called density matrix renormalization group, which can faithfully approximate the ground states of small 2D clusters well beyond system sizes amenable for exact diagonalization. We argue that the quantum skyrmion lattice phase can be detected experimentally in the magnetization profile via local magnetic polarization measurements as well as in the spin structure factor via neutron scattering experiments. Deep in the skyrmion ordered phase, we find that the quantum skyrmion lattice state is only weakly entangled with ‘domain wall' entanglement between quasiparticles and environment localized near the boundary spins of the skyrmion. In this ordered regime of weakly entangled entities, large clusters of O(1000) sites can be simulated with great efficiency. Field: condensed matter physics Keywords: quantum spin systems, topology, density matrix renormalization group

Venue: via Zoom

Event Official Language: English

Deciphering speciation processes: a mathematical modelling approach to biodiversity patterns

June 8 (Thu) at 10:00 - 11:00, 2023

Ryo Yamaguchi (Assistant Professor, Department of Advanced Transdisciplinary Sciences, Faculty of Advanced Life Science, Hokkaido University / Postdoctoral Research Fellow, Biodiversity Research Centre, University of British Columbia, Canada)

The grandeur and complexity of Earth’s biodiversity present a challenge to comprehend the intricate mechanisms underlying speciation. Once dubbed by Darwin as the “mystery of mysteries,” speciation remains a frontier in biology, with much still cloaked in obscurity. Applying mathematical models inspired by population genetics and individual-based simulations, I aim to shed light on the complex mechanisms underlying speciation. In this talk, I focus on the concept of a “speciation cycle,” a recurring pattern integral to the formation of biodiversity. In contrast to traditional views that focus solely on a single speciation event, our approach argues for the necessity of multiple intertwined processes. These include the coexistence of closely related species, ongoing diversification, and the accumulation of new species, all while avoiding extinction. By overviewing mathematical models of each evolutionary and ecological process, I will introduce their basic ideas, and examine under what conditions the formation and coexistence of new species are promoted. Then we further explore the temporal and spatial dimensions of speciation, looking closely at the intervals between speciation events and the steady buildup of biodiversity over geological timescales. By bridging the gap between microevolutionary processes and macroevolutionary patterns, I hope to enable the prediction of biodiversity patterns based on a deeper understanding of speciation mechanisms.

Venue: via Zoom

Event Official Language: English

Around homogeneous spaces of complex semisimple quantum groups

June 7 (Wed) at 14:00 - 16:30, 2023

Kan Kitamura (Ph.D. Student, Graduate School of Mathematical Sciences, The University of Tokyo)

Murray and von Neumann initiated the study of operator algebras motivated by the mathematical foundations of quantum physics. Operator algebras give good language to treat quantum symmetries, such as quantum groups. In this talk, I would like to give an overview of this topic first. Then, I discuss the q-deformations of complex semisimple Lie groups. From an operator algebraic viewpoint, we can treat them as "locally compact" quantum groups. Especially, I will focus on its homogenous spaces coming from discrete quantum subgroups with a motivation toward the quantum analog of lattices. Unlike the classical setting, we can obtain a complete classification of its discrete quantum subgroups.

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

Event Official Language: English

Termination of Superradiance from a Binary Companion

June 6 (Tue) at 13:30 - 15:00, 2023

Huiyu Zhu (Ph.D. Student, Department of Physics, Hong Kong University of Science and Technology, Hong Kong)

We study the impact of a binary companion on black hole superradiance at orbital frequencies away from the gravitational-collider-physics (GCP) resonance bands. A superradiant state can couple to a strongly absorptive state via the tidal perturbation of the companion, thereby acquiring a suppressed superradiance rate. Below a critical binary separation, this superradiance rate becomes negative, and the boson cloud gets absorbed by the black hole. This critical binary separation leads to tight constraints on GCP. Especially, a companion with mass ratio q > 10^−3 invalidates all GCP fine structure transitions, as well as almost all Bohr transitions except those from the |211> state. Meanwhile, the backreaction on the companion manifests itself as a torque acting on the binary, producing floating/sinking orbits that can be verified via pulsar timing. In addition, the possible termination of cloud growth may help to alleviate the current bounds on the ultralight boson mass from various null detections.

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English