セミナー
1088 イベント
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セミナー
The role of fluid dynamics in microbial ecology
2023年6月22日(木) 14:00 - 15:00
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.
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
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Matrix estimation via singular value shrinkage
2023年6月21日(水) 15:30 - 16:30
松田 孟留 (理化学研究所 脳神経科学研究センター (CBS) 統計数理連携ユニット ユニットリーダー)
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.
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
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セミナー
Introduction to statistical decision theory and Stein’s paradox
2023年6月21日(水) 14:00 - 15:00
松田 孟留 (理化学研究所 脳神経科学研究センター (CBS) 統計数理連携ユニット ユニットリーダー)
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.
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
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A Spin on Wave Dark Matter
2023年6月15日(木) 16:00 - 17:30
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.
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
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Phase reduction beyond the Kuramoto model
2023年6月15日(木) 14:00 - 15:00
Iván León (東京工業大学 工学院 システム制御系 研究員)
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.
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
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セミナー
Quantum skyrmion lattices in Heisenberg ferromagnets
2023年6月8日(木) 17:00 - 18:15
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
会場: via Zoom
イベント公式言語: 英語
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Deciphering speciation processes: a mathematical modelling approach to biodiversity patterns
2023年6月8日(木) 10:00 - 11:00
山口 諒 (北海道大学 大学院先端生命科学研究院 先端融合科学研究部門 助教 / 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.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Around homogeneous spaces of complex semisimple quantum groups
2023年6月7日(水) 14:00 - 16:30
北村 侃 (東京大学 大学院数理科学研究科 博士課程)
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.
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
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セミナー
Termination of Superradiance from a Binary Companion
2023年6月6日(火) 13:30 - 15:00
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.
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
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セミナー
Modelling radiation cancer treatment with ordinary and fractional differential equations
2023年6月1日(木) 10:00 - 11:00
キャスリーン・ウィルキー (Associate Professor, Department of Mathematics, Toronto Metropolitan University, Canada)
Fractional calculus has recently been applied to mathematical modelling of tumour growth, but its use introduces complexities that may not be warranted. Mathematical modelling with differential equations is a standard approach to study and predict treatment outcomes for population-level and patient-specific responses. Here we use patient data of radiation-treated tumours to discuss the benefits and limitations of introducing fractional derivatives into three standard models of tumour growth. The fractional derivative introduces a history-dependence into the growth function, which requires a continuous death-rate term for radiation treatment. This newly proposed radiation-induced death-rate term improves computational efficiency in both ordinary and fractional derivative models. This computational speed-up will benefit common simulation tasks such as model parameterization and the construction and running of virtual clinical trials.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Quantum uncertainty of fields and its effect on entanglement generation in quantum particles
2023年5月31日(水) 14:00 - 15:15
杉山 祐紀 (九州大学 大学院理学府 物理学専攻 博士課程)
The unification of gravity and quantum mechanics is one of the important problems. To elucidate the theory of quantum gravity, it is becoming more and more important to get any hint of the quantum nature of gravity. In particular, the quantum-gravity-induced-entanglement of masses (QGEM) scenario, which is expected to observe the quantum nature of non-relativistic gravity, has recently attracted great attention. In this talk, we show the effect of relativistic fields on entanglement generation based on quantum field theory. We also discuss the relationship between the entanglement generation and quantum uncertainty of the fields.
会場: セミナー室 (359号室) / via Zoom
イベント公式言語: 英語
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セミナー
Spectral correlations and scrambling dynamics in Sachdev-Ye-Kitaev type models
2023年5月30日(火) 13:30 - 15:00
手塚 真樹 (京都大学 大学院理学研究科 物理学・宇宙物理学専攻 助教)
Note: Due to unexpected trouble, we have made the decision to postpone the seminar scheduled for February 21 to May 30. Sorry for the trouble. Abstract: The Sachdev-Ye-Kitaev (SYK) model, proposed in 2015, is a quantum mechanical model of N Majorana or complex fermions with all-to-all random four-body interactions. The model has attracted significant attention over the years due to its features such as the existence of the large-N solution with maximally chaotic behavior at low temperatures and holographic correspondence to low-dimensional gravity. The sparse version of the SYK model reproduces essential features of the original model for reduced numbers of disorder parameters. We recently proposed [1] a further simplification, where we set the nonzero couplings to be +1 or -1 rather than sampling from a continuous distribution such as Gaussian. This binary-coupling model exhibits strong correlations in the spectrum, as observed in the spectral form factor, more efficiently in terms of the number of nonzero terms than in the Gaussian distribution case. We also discuss the scrambling dynamics with the binary-coupling sparse SYK model, comparing the model with the original model as well as the SYK model with random two-body terms [2], where the localization of the many-body eigenstates in the Fock space has been quantitatively studied [3,4].
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
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セミナー
Molecular and evolutionary bases of Pieris butterflies for overcoming diverse chemical defenses in their host plants
2023年5月25日(木) 16:00 - 17:00
岡村 悠 (東京大学 大学院理学系研究科 生物科学専攻 日本学術振興会 特別研究員 PD)
In terrestrial ecosystems, plants and herbivorous insects account for more than half of the described species and play quite important ecological roles. Plants and herbivorous insects have strong chemical interaction as plants defend themselves with various defense compounds such as secondary metabolites and herbivores adapt to it by evolving detoxification mechanisms. Larvae of Pieris butterflies feed on Brassicaceae plants as the main host. Brassicaceae plants contain diverse glucosinolates (GLS) as a main chemical defense, which can be rapidly hydrolyzed into toxic isothiocyanates by a plant enzyme called myrosinase upon tissue damage. Larvae of Pieris butterflies are known to express nitrile-specifier protein in their gut and this can redirect toxic breakdown products of GLSs to less toxic metabolites. Although NSP is considered an evolutionary key innovation for Pieridae that enabled these butterflies to colonize GLS-containing plants, it has been largely unclear whether NSP is enough for Pieris butterfly larvae to overcome the diverse types of GLS they encounter in their host plants. In this seminar, I would like to introduce our recent findings showing that Pieris butterfly larvae not only use NSP but also use its ortholog major allergen (MA) to overcome the diverse types of GLS in their Brassicaceae host plants. We found that Pieris larvae show fine-tuned regulation of those two adaptive genes depending on the chemical profiles of their host plants. Furthermore, those two adaptive genes have different evolutionary trajectories in macro- and micro-evolutionary scales among Pieris species or populations associated with their pattern of host plant usage. Moreover, with an approach using CRISPR/Cas9 genome editing, we showed that both NSP and MA have different but complementary roles in disarming GLS-based defenses in their host plants and that both genes are crucial for Pieris in overcoming their host plant’s major chemical defense. Those highlight that having both NSP and MA is a key for Pieris butterflies to overcome the diverse types and GLS and, consequently, adapt to a wider range of Brassicaceae hosts. Our results illuminate that gene duplication, functional differentiation, and the evolution of gene regulation mechanisms are all crucial for herbivorous insects to overcome co-evolving chemical defenses in their host plants.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Hydrodynamic limit and the fluctuating hydrodynamics for large-scale interacting systems
2023年5月24日(水) 14:00 - 16:30
林 晃平 (理化学研究所 数理創造プログラム (iTHEMS) 訪問研究員)
In these decades, a great deal of works has been devoted to understand macroscopic phenomena, such as diffusion, aggregation or pattern formation, from the viewpoint of microscopic systems. Hydrodynamic limit, or fluctuating hydrodynamics, is a fundamental framework to explain the macroscopic behavior of physical quantities in mathematically rigorous ways from a system of the vast numbers of microscopic agents under random interactions, which system is called the large-scale interacting system. In this framework, our central aim is to derive partial differential equations (PDEs) which describe time evolution of some macroscopic quantities, starting from the large-scale interacting systems; hydrodynamic limit is a procedure to derive deterministic PDEs with help of the law of large numbers, whereas stochastic PDEs are derived under the scale of the central limit theorem by fluctuating hydrodynamics. In this talk, I would like to explain basic concepts of hydrodynamic limit and fluctuating hydrodynamics, through some simple models. In the first part, I will give a concise exposition on Markov processes as preliminaries and then state some results on scaling limits of simple exclusion processes as a pedagogical example. In the second part, I will talk about recent progress on universality which appears in fluctuating hydrodynamics. Especially, I would like to talk about the universality of the Kardar-Parisi-Zhang equation, and its mathematical background.
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
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セミナー
Statistical methods to probe binary stellar evolution with ZTF and LISA data
2023年5月23日(火) 13:30 - 15:00
ルーシー・マクニール (京都大学 大学院理学研究科 物理学・宇宙物理学専攻 特別研究員)
The Laser-Interferometer-Space-Antenna (LISA) will be capable of detecting all galactic double neutron star binaries (DNSBs) with orbital periods < 20 minutes, as well as 10-100’s of thousands of double white dwarf binaries. I will present our method to use LISA detections to constrain the formation frequencies of galactic DNSBs, which are determined by supernova physics and various mass exchange processes. Next, I summarise the key differences in DNSBs and white dwarf binaries (WDBs) in the context of future LISA observations, as well as the current Zwicky Transient Facility (ZTF) for the latter. Then we will examine the current catalogue of short period white dwarf binaries (orbital periods < 1 hour) detected by ZTF so far, including the first ever measurement (Burdge et al. 2023) of the temperature of a mass transferring white dwarf in a binary. Here I will make the case that taken together, these observations are in conflict with the theoretical picture commonly used in Galactic modelling- specifically related to cooling and mass transfer leading to WDB mergers. However, heating from tidal interactions may explain and mediate this inconsistency. Finally I will put these results into context regarding preparing for the unprecedented data set of galactic white dwarf binaries from LISA in the 2030s.
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
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セミナー
X-ray study on the synchrotron emission in Kepler's SNR
2023年5月19日(金) 14:00 - 15:15
Vincenzo Sapienza (東京大学 大学院理学系研究科 物理学専攻 博士課程)
Synchrotron X-ray emission in young supernova remnants (SNRs) is a powerful diagnostic tool to study the population of high energy electrons accelerated at the shock front. We performed a spatially resolved spectral analysis of the young Kepler's SNR, where we identify two different regimes of particle acceleration. In the north, where the shock interacts with a dense circumstellar medium (CSM), we found a more efficient acceleration than in the south, where the shock velocity is higher and there are no signs of shock interaction with dense CSM. We also studied the temporal evolution of the synchrotron flux, from 2006 to 2014. A number of regions show a steady synchrotron flux and equal cooling and acceleration times. However, we found some regions where we measured a significant decrease in flux from 2006 to 2014. Our results display a coherent picture of the different regimes of electron acceleration observed in Kepler's SNR. Also If I will have time during the seminar it will be nice to present also some preliminary results I will have in the SN 1987A project.
会場: セミナー室 (359号室) / via Zoom
イベント公式言語: 英語
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セミナー
Excursion Theory, Galton Watson Trees and their Scaling Limits
2023年5月18日(木) 16:00 - 17:00
クリスティ・コウジ・ケリー (理化学研究所 数理創造プログラム (iTHEMS) 基礎科学特別研究員)
In this talk we aim to introduce a recent perspective in probability theory that views random trees as random excursions with additional data. This perspective is particular suited to the study of the scaling limit of tree-valued random processes. Excursion theory is a useful and relatively elementary tool allowing one to derive rather explicit information about the local and global geometry of the resultant continuum trees which in turn can be used to derive information about large random trees. We illustrate these ideas in the context of the Brownian continuum random tree, the scaling limit of critical Galton-Watson trees and a structure that arises naturally in various contexts in physics; in particular the Brownian continuum random tree is a pathological model of quantum spacetime. Despite the fundamentally mathematical nature of the talk, the aim is to keep the presentation essentially heuristic emphasising key intuitions over rigorous proof. The content itself should be relevant to biologists interested in the theory of branching processes or coalescent theory.
会場: セミナー室 (359号室) / via Zoom
イベント公式言語: 英語
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セミナー
Ground-state phases of the one-dimensional SU(N)-symmetric Kondo lattice model
2023年5月11日(木) 17:00 - 18:15
戸塚 圭介 (京都大学 基礎物理学研究所 准教授)
The Kondo-lattice model and its variants (e.g., the Kondo-Heisenberg model), in which itinerant fermions interact with immobile magnetic moments via spin-exchange coupling (Kondo coupling), have been playing an important role in understanding the physics of heavy-fermion systems. In this talk, I begin by quickly explaining how the SU(N) Kondo-lattice model, in which the spin SU(2) symmetry is generalized to SU(N), is realized in actual physical systems (e.g., cold fermions and twisted bilayer graphene), and then I focus on the ground-state properties of its one-dimensional version. Specifically, when the Kondo coupling is sufficiently large, we find ferromagnetic metallic phases that can be established rigorously as well as several insulating ones. I also show that the SU(N) Kondo-lattice model provides a natural condensed-matter realization of supersymmetric [i.e., SU(N|1)] models. Various (insulating) phases at small Kondo coupling are then explored using the machinery of bosonization and various conformal field theory (CFT) techniques, and the results are compared with the predictions of the Lieb-Schultz-Mattis-type (or anomaly-matching) argument. Field: condensed matter physics Keywords: Kondo lattice model, SU(N) symmetry, supersymmetry, heavy-fermion systems, bosonization, conformal field theory
会場: via Webex
イベント公式言語: 英語
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セミナー
Conditions for maintaining pseudo-overdominance
2023年5月11日(木) 16:00 - 17:00
Diala Abu Awad (Associate Professor, Génétique Quantitative et Évolution - Le Moulon, Université Paris-Saclay, France)
Deleterious recessive mutations should purge or fix within inbred populations, yet inbred populations often retain moderate to high segregating load. However, arrays of deleterious recessives linked in repulsion could generate appreciable pseudo-overdominance, mimicking overdominant selection that would sustain segregating load. We use analytical approches and simulations to explore whether and for how long pseudo-overdominant (POD) zones can persist once created (e.g., by hybridization between populations fixed for alternative mildly deleterious mutations). Balanced haplotype loads, tight linkage, and moderate to strong cumulative selective effects all serve to maintain POD zones. Tight linkage is key, suggesting that such regions are most likely to arise and persist in low recombination regions (like inversions). Selection and drift unbalance the load, eventually eliminating POD zones, but this process is quite slow, and could influence short term evolution of populations.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Quantum transport with cold atoms
2023年5月10日(水) 13:30 - 15:00
内野 瞬 (日本原子力研究開発機構 先端基礎科学センター 研究員)
Quantum transport occurring through a mesoscopic conduction region allows us to extract interesting quantum many-body phenomena. For decades, solid-state systems have been the playground of such transport and revealed nontrivial outcomes such as the conductance quantization in a quantum point contact system and the fractional charge measurement in a fractional quantum Hall system. More recently, cold atoms trapped in the vacuum have served as the complementary system to study the quantum transport phenomena. In this seminar, I wish to discuss the recent progresses of quantum transport with cold atoms. The great advantages of those systems are that one can control quantum statistics, inter-particle interactions, dissipation, and dimensions. I try to show that such controllability enables to explore a regime of quantum transport that has yet to be reached with solid-state materials, including transport of bosons, dissipation effect in transport, and transport with synthetic dimensions.
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
1088 イベント