セミナー
359 イベント
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セミナー
False vacuum decay in the Lorentzian path integral
2022年2月15日13:30 - 15:00
林 拓未 (東京大学 大学院理学系研究科附属 ビッグバン宇宙国際研究センター (RESCEU) 博士課程)
False vacuum decay is a non-perturbative phenomenon in quantum field theory and important quantum process in cosmology. It has relied on the Euclidean formalism developed by Coleman, but there are several subtle issues in cosmological application as a negative mode problem or ambiguity in the definition of the decay rate in the presence of the gravity. Instead of the Euclidean path integral, we directly evaluate the Lorentzian path integral to discuss false vacuum decay and estimate the decay probability. To make the Lorentzian path integral convergent, the deformation of an integral contour is performed on the basis of the Picard-Lefschetz theory. We show that the nucleation probability of a critical bubble, for which the corresponding bounce action is extremized, has the same exponent as the Euclidean approach. We also extend our computation to the nucleation of a bubble larger or smaller than the critical one to which the Euclidean formalism is not applicable.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Stochastic operators: properties and applications
2022年2月10日10:00 - 11:00
ジルベルト・ナカムラ (数理創造プログラム 特別研究員)
Stochastic processes are widely used to model systems in which one or more variables fluctuate randomly. Problems arise when large sets of random variables are allowed to interact with each other, as is often the case with physical and biological systems. Stochastic operators provide a convenient framework for describing the interactions and evolution of the random variables. In this talk, I will discuss techniques and methods typically used in spin systems to deal with stochastic operators and their spectral analysis in the context of random processes. I will briefly review their properties and applications to biological systems. As practical examples, I will present some results of my research in infectious diseases and migration of glioma cells.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Mixed dark matter scenarios consisting of primordial black hole dark matter and WIMPs
2022年1月31日11:00 - 12:00
門田 健司 (Senior faculty scientist, Hangzhou Institute for Advanced Study at University of Chinese Academy of Sciences (HIAS-UCAS), International Center for Theoretical Physics-Asia Pacific (ICTP-AP) Hangzhou Branch, China)
While the possibility for the primordial black holes (PBHs) to constitute all of the dark matter (DM) is being narrowed by the astrophysical observations such as the gravitational microlensing, the PBH as a partial DM component is still an intriguing possibility. I will discuss the scenarios where the rest of the dark matter consists of the widely discussed weakly interacting massive particles (WIMPs) and show that PBH and WIMP cannot co-exist with an emphasis on the astrophysical probes including the gamma ray, 21cm and CMB observations.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Recent progress on dualities in W-superalgebras
2022年1月28日16:00 - 18:00
中塚 成徳 (東京大学 カブリ数物連携宇宙研究機構 (Kavli IPMU) 東京大学特別研究員)
Vertex superalgebras are algebras which describe the chiral part of two dimensional superconformal field theory. A rich and fundamental class is provided by the affine vertex superalgebras associated with simple Lie superalgebras and the W-superalgebras obtained from them by cohomology parametrized by nilpotent orbits. Historically, the W-algebras associated with simple Lie algebras and principal nilpotent orbit have been studied intensively and are well-known to play an essential role in the quantum geometric Langlands program. In particular, they enjoy a duality, called the Feigin-Frenkel duality, which is a chiral analogue of the isomorphism between centers of the enveloping algebras of simple Lie algebras in Langlands duality. Recently, physicists found a suitable generalization for other types of nilpotent orbits from study on four dimensional supersymmetric gauge theory. In this talk, I will report the recent progress on our understanding of dualities in W-superalgebras since then in terms of several aspects: algebras, modules, and fusion rules.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Galactic archaeology with r-process elements
2022年1月28日10:00 - 11:30
平居 悠 (東北大学 大学院理学研究科 天文学専攻 / JSPS Research Fellow (Visiting Scholar), Department of Physics, University of Notre Dame, USA)
Galactic archaeology studies the evolutionary histories of galaxies using information preserved in stars. Abundances of elements in stars are keys to understanding how the galaxies were evolved. It is, therefore, crucial to making it clear the origin of elements and the cycle of materials in galaxies. This talk will show the enrichment of heavy elements, including r-process elements, in dwarf galaxies and the Milky Way. Our high-resolution simulations of galaxies suggest that binary neutron star mergers play an important role in enriching r-process elements in dwarf galaxies and the Milky Way. I will also show that r-process enhanced stars in the Milky Way tend to form in dwarf galaxies previously accreted to the Milky Way. I will demonstrate that the abundance of r-process elements in stars can be used as an indicator for the early evolution of the Milky Way.
会場: via Zoom
イベント公式言語: 英語
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セミナー
On Flow and Form at Low Reynolds Number
2022年1月27日10:00 - 11:00
石本 健太 (京都大学 数理解析研究所 (RIMS) 准教授)
Cell locomotion is mechanically restricted by surrounding viscous fluids. With a focus on swimming cells in a low-Reynolds-number flow, I will give a brief introduction to microbiological fluid dynamics and present a 'hydrodynamic shape' theory at the cellular scale.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Bethe ansatz and quantum computing
2022年1月26日22:00 - 23:15
Prof. Rafael I. Nepomechie (Professor, Physics Department, University of Miami, Florida, USA)
We begin with a brief review of the Heisenberg quantum spin chain and its remarkable solution found by Bethe. We then review a probabilistic algorithm for preparing exact eigenstates of this model on a quantum computer. An exact formula for the success probability is presented, and the computation of correlation functions is discussed. A generalization of the algorithm to open chains with boundaries is also noted.
会場: via Zoom
イベント公式言語: 英語
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セミナー
A simple XY model for cascade transfer
2022年1月20日13:30 - 15:00
田之上 智宏 (京都大学 大学院理学研究科)
Cascade transfer is the phenomenon that an inviscid conserved quantity, such as energy or enstrophy, is transferred conservatively from large (small) to small (large) scales. As a consequence of this cascade transfer, the distribution of the transferred quantity obeys a universal scaling law independent of the details of large (small) scales. For example, in the energy cascade in fluid turbulence, the energy spectrum follows Kolmogorov's power law [1]. Such behavior is observed even in systems different from ordinary fluids, such as quantum fluid, elastic body, and spin systems. Here, we aim to establish the concept of a universality class for cascade transfer. As a first step toward this end, we propose a simple model representing one universality class [2]. In doing so, we regard cascade transfer as a cooperative phenomenon of unidirectional transport across scales and ask how it emerges from spatially local interactions. The constructed model is a modified XY model with amplitude fluctuations, in which the spin is regarded as the “velocity” of a turbulent field in d dimensions. We show that the model exhibits an inverse energy cascade with the non-Kolmogorov energy spectrum. We also discuss the relation to spin turbulence [3,4] and atmospheric turbulence [5].
会場: via Zoom
イベント公式言語: 英語
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セミナー
A study of biological systems from topological point of view
2022年1月20日10:00 - 11:00
宮﨑 弘安 (数理創造プログラム 上級研究員)
A biological body can be regarded as a complicated network of chemical reactions. The chemical reaction network (CRN) is a (hyper)graph-theoretic model of such biological networks. Recently, in the joint work with Yuji Hirono, Takashi Okada and Yoshimasa Hidaka, we applied a topological method to the study of CRNs, and found a suitable way to simplify the networks. Since Professor Hirono has already explained our work in this seminar, I will try to explain it from a slightly different point of view. In the first half of the talk, I will review the entire work. In the second half, I will try to give a rough sketch of the mathematical method we used in the work.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Axion-like particles from core-collapse supernovae
2022年1月17日11:00 - 12:00
森 寛治 (福岡大学 基盤研究機関 爆発天体研究所)
Axion-like particles (ALPs) are a class of hypothetical pseudoscalar particles which feebly interact with ordinary matter. The hot plasma of stars and core-collapse supernovae is a possible laboratory to explore physics beyond the standard model including ALPs. Once produced in a supernova, some of the ALPs can be absorbed by the supernova matter and affect energy transfer. We recently calculated the ALP emission in core-collapse supernovae and the backreaction on supernova dynamics consistently. It is found that the stalled bounce shock can be revived if the coupling between ALPs and photons is as high as g_{a gamma} ~ 10^{-9} GeV^{-1} and the ALP mass is 40-400 MeV. In this talk, I will briefly review stellar and supernova constraints on ALPs and then discuss our recent results.
会場: via Zoom
イベント公式言語: 英語
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セミナー
The Ohsawa-Takegoshi $L^2$ extension theorem and variations of Bergman kernels
2022年1月14日16:00 - 18:00
細野 元気 (東北大学 大学院理学研究科 数学専攻)
In complex analysis and geometry, $L^2$ methods are very important and widely used. Recent studies show that the $L^2$ theory and the variational theory are closely related. In particular, the (optimal) $L^2$ extension theorem can be proved by subharmonicity of variations of Bergman kernels and vice versa. In this talk, I will explain the background, results, and key ideas of the proof. *Please contact Keita Mikami mailing address to get access to the Zoom meeting room.
会場: via Zoom
イベント公式言語: 英語
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セミナー
A comprehensive view of the SARS-CoV-2 infection process
2022年1月13日10:00 - 11:00
二島 渉 (Scientist, New Mexico Consortium, Mexico)
Nishima et al. recently published a paper about a computational model of SARS-CoV-2 Spike Protein [1]. Although it is still a hypothesis due to the lack of direct experimental evidence, the story comprehensively explains the initial infection process of SARS-CoV-2 consistent with most of the empirical evidence. In the presentation, I would like to explain the overview of the infection process for the non-expert audience and how the hypothesis influences the current COVID-19 situation. If time permits, I would like to briefly explain the current plan of the iTHEMS-NMC COVID project, which is going to be the first case of undergoing an interdisciplinary collaboration framework between Japan and the US.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Physics of nuclear bodies
2022年1月6日10:00 - 11:00
山本 哲也 (北海道大学 創成研究機構化学反応創成研究拠点 特任准教授)
Eukaryotic nucleus is not a uniform solution of DNA, but there are a number of nuclear bodies in the interchromatin spaces. There are growing number of experiments that suggest that nuclear bodies are assembled by liquid-liquid phase separation (LLPS). Condensates assembled by LLPS show coarsening or coalescence to decrease the surface energy. However, in some nuclear bodies, such as paraspeckles, nuclear stress bodies, and fibrillar centers in nucleoli, multiple condensates are stably dispersed and are not likely assembled by LLPS. The assembly mechanism of nuclear bodies is relevant to the regulation of the area of condensate surfaces, which are functional in some nuclear bodies, and the mobility of nuclear bodies. Hirose group (Osaka Univ.) has elucidated that nuclear bodies are scaffolded by a class of RNA, called architectural RNA (arcRNA), which forms complexes with RNA binding proteins. This implies that the assembly of nuclear bodies is governed RNA dynamics, such as transcription, degradation, and processing, and the sequence of bases of arcRNA. In the seminar, I will show how the base sequences and the dynamics of RNA are involved in the assembly of paraspeckles and fibrillar centers in nucleoli.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Hidden Markov Models and their applications
2021年12月23日10:00 - 11:00
岡田 崇 (数理創造プログラム 上級研究員)
The Hidden Markov models (HMM) have been used in a variety of fields for different purposes. I am going to review statistical inference methods associated with HMM & related biological problems. As an example of their applications, I'll also present my research on the SARS-CoV-2 evolution.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Quantum metric of topological and non-topological insulators in AMO and other systems
2021年12月20日13:30 - 15:00
小澤 知己 (東北大学 材料科学高等研究所 (AIMR) 准教授)
Recently, the concept of quantum geometry is attracting great interests in various areas of condensed matter and AMO physics. Quantum geometry tells how much the quantum states "change" as one moves in a parameter space, and is closely related to the topology of the quantum states. Quantum geometric tensor is often used to characterize the geometry, whose real part is the quantum metric and the imaginary part is the Berry curvature. Although Berry curvature is rather well-studied in the context of topological insulators and superconductors, less has been known about the quantum metric. However, experiments detecting the quantum metric have appeared in the past couple of years and interest in quantum metric is indeed growing. In this talk, I first explain basics of quantum metric and its recent experimental observations. I then discuss various aspects of quantum metric, including its relation to localization, topology, and the Kähler geometry.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Revisiting Standard Methods for Phylogenetic Tree Inference
2021年12月16日10:00 - 11:00
松井 求 (東京大学 大学院理学系研究科 助教)
Phylogenetic tree inference is the foundation to answer any biological questions, for example, how the living systems were established. However, the existing methods show poor performance to infer the phylogenetic tree when constructing an informative multiple sequence alignment (MSA) is difficult. In this talk, I will first review the current problems in phylogenetics, then introduce the graph splitting (GS), and edge perturbation (EP) method. The GS method rapidly reconstructs a protein superfamily-scale phylogenetic tree using a graph-based approach; evolutionary simulation showed that the GS method can accurately reconstruct phylogenetic trees when sequences substantially diverge. The EP method is the bootstrap-like method using pairwise sequence alignment (PSA) instead of MSA, which can provide reliable measurements on the estimated branches. In addition, we can rapidly and reliably reconstruct a phylogenetic tree with problematic MSA switching NJ+EP and GS+EP methods, because the EP method can be applied to the NJ method. These methods not only improve the accuracy of phylogenetic tree inference, but they also could open the door for revisiting phylogenetics.
会場: via Zoom
イベント公式言語: 英語
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セミナー
The FASER experiment
2021年12月15日17:00 - 18:00
音野 瑛俊 (九州大学 先端素粒子物理研究センター 助教)
FASER, the ForwArd Search ExpeRiment, is an experiment dedicated to searching for light, extremely weakly-interacting particles at the LHC. Such particles may be produced in the LHC’s high-energy collisions and then decay to visible particles in FASER, which is placed 480 m downstream of the ATLAS interaction point. FASER, also includes a sub-detector, FASER$\nu$, designed to detect neutrino’s produced in the LHC collisions and to study their properties. This seminar will describe the physics motivations, detector design, expected performance of FASER, and current status, as well as the physics prospects.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Cosmological particle production as Stokes phenomena
2021年12月15日13:30 - 15:00
山田 悠介 (東京大学 大学院理学系研究科附属 ビッグバン宇宙国際研究センター (RESCEU) 学振特別研究員)
Particle production from “vacuum” takes place in time-dependent backgrounds. In very early universe, particularly just after inflation, expanding metric as well as oscillating scalar fields play the role of such backgrounds. Mathematically, “particle production from vacuum” can be understood as “Stokes phenomena”, and such understanding enables us to estimate the amount of produced particles in a systematic way. In this talk, I will review the relation between Stokes phenomena and particle production. Then, from the Stokes phenomena viewpoint, I will (re)consider particle production associated with expanding universe, an oscillating scalar field, or both of them. I will also discuss the time evolution of particle number, and its relation to the ambiguity of “vacuum states”.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Magnetic field dependence of neutrino-driven core-collapse supernova models
2021年12月10日14:00 - 15:00
松本 仁 (慶應義塾大学 理工学研究科 慶應義塾基礎科学・基盤工学インスティテュート 助教)
Massive stars can explode and release huge energy (typically 10^51 erg) at the end of their life. It is one of the most energetic explosions in the Universe and is called a core-collapse supernova. The impact of the magnetic field on the explosion mechanisms of the core-collapse supernova is a long-standing mystery. Recently, we have updated our neutrino-radiation-hydrodynamics supernova code (3DnSNe, Takiwaki et al. 2016) to include magnetohydrodynamics (MHD). Using this code, we have performed three-dimensional MHD simulations for the evolution of non-rotating stellar cores focusing on the difference in the magnetic field of the progenitors. Initially, 20 and 27 solar mass pre-supernova progenitors are threaded by only the poloidal component of the magnetic field, which strength is 10^10 (weak) or 10^12 (strong) G. We find that the neutrino-driven explosion occurs in both the weak and strong magnetic field models. The neutrino heating is the main driver for the explosion in our models, whereas the strong magnetic field slightly supports the explosion. In my talk, I will introduce the details of this mechanism.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Selective inference for testing trees and edges in hierarchical clustering and phylogeny
2021年12月9日10:00 - 11:00
下平 英寿 (京都大学 大学院情報学研究科 教授 / 理化学研究所 革新知能統合研究センター (AIP) 数理統計学チーム チームリーダー)
Bootstrap resampling is quite useful for computing “confidence values” or “p-values” of trees and edges. However, they are biased and may lead to false positives (too many wrong discoveries) or false negatives (too few correct discoveries) depending on the “curvature” of the boundary surface of a hypothesis region in the data space. In addition, we face the issue of selection bias because we tend to use the dataset twice for hypothesis selection and its evaluation. I will explain these two types of bias and show methods to adjust the confidence values.
会場: via Zoom
イベント公式言語: 英語
359 イベント