セミナー
751 イベント
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セミナー
Axion electrodynamics in neutron stars
2022年9月27日(火) 14:00 - 15:00
Filippo Anzuini (Postdoc Fellow, Department of Astronomy, Monash University, Australia)
Axions are pseudo-Goldstone bosons that provide a solution to the strong CP problem, and are prominent candidates for dark matter. In neutron stars, it has been shown recently that the potential of the QCD axion acquires finite density corrections that shift the axion field expectation value, which can be large compared to the vanishing expectation value in vacuo. Such a shift leaves an imprint on typical neutron star observables such as the redshifted thermal luminosity, which can be used to constrain the axion parameter space. In this talk we focus on the coupling of axions with photons, which modifies Maxwell’s equations and alters the neutron star magnetic field. By performing state-of-the-art magneto-thermal simulations, we calculate the axion-induced perturbations to the neutron star’ magnetic field, and show that they grow on relatively short time-scales. At the same time, intense electric currents form, leading to enhanced ohmic dissipation, which increases the stars’ observable thermal luminosity. The activation of such mechanisms depends on the axion decay constant and the axion mass, two long-sought parameters at the center of several experimental and theoretical investigations. Both parameters can be constrained by comparing our simulations to observations of thermally-emitting neutron stars. The latter do not exhibit uncontrolled growth of the magnetic field that causes enhanced ohmic dissipation, allowing us to place bounds on axion parameters. Our results open a new astrophysical avenue to constrain axions, extending significantly the parameter range that can be probed with direct axion searches.
会場: コモンルーム 246-248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
Discrete Stochastic Model for the Co-infection Dynamics with Defective Interfering Particles
2022年9月26日(月) 16:00 - 17:00
ジジュウ・リ (理化学研究所 数理創造プログラム (iTHEMS) 特別研究員)
Defective interfering particle (DIP) in the context of influenza A virus is a virion with a significantly shortened RNA segment substituting one of its eight full-length parent RNA segments, such that it is preferentially amplified during replications. Therefore, a cell co-infected by standard viruses (STVs) and DIPs will produce mainly DIPs, suppressing the STV yield and displaying nontrivial co-infection dynamics. An important approach to quantifying the co-infection dynamics is mathematical modeling with ordinary differential equations (ODEs), which treat relevant quantities (such as numbers of target cells, STVs, and DIPs) as continuous numbers evolving with prescribed physical laws. However, the ODE models are mean-field in nature that is only valid for scenarios with large numbers of STVs and DIPs. For small-number scenarios, the infection outcomes can be dominated by random fluctuations and stochasticity, which cannot be captured by ODE models. In this week’s biology seminar, we introduce a new Discrete Stochastic Model (DSM) aimed to rectify the shortcomings of the ODEs by treating the co-infection dynamics as stochastic processes. As we will show, the new DSM is consistent with the ODE model in the large number regime. In the low number regime, the DSM yields bi-modal distributions for the infection outcomes (extinct vs established infections) that are otherwise unattainable by ODE models.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Hyperons in neutron stars: fast cooling, Joule heating and hyperon superfluidity
2022年9月26日(月) 14:00 - 15:00
Filippo Anzuini (Postdoc Fellow, Department of Astronomy, Monash University, Australia)
Neutron stars challenge current models of highly dense matter. Despite be- ing the targets of numerous observational campaigns (e.g. gravitational-wave searches and X-ray observations), their equation of state is still unknown. One of the most exciting possibilities is that “unconventional” particles such as hy- perons may appear in neutron star cores. Hyperons have a major impact on the observed thermal luminosity, because they accelerate the cooling rate via direct Urca processes, which copiously increase the neutrino emission from the core. Such mechanism is often considered to be a key signature of hyperon concentrations at high densities. Hyperon superfluidity plays a major role as well, because it can suppress the neutrino emissivity exponentially. The hope is that a comparison of the theoretical cooling curves against the available data of thermally-emitting neutron star can hint towards the existence of hyperons and their superfluidity. There is one ingredient, however, that is often neglected in neutron star cooling models: internal heating. The magnetic field of neutron stars decays due to the dissipation of the electric currents circulating in the crust, generating substantial Joule heating in the shallower layers. The ther- mal power generated by this process can counterbalance hyperon fast cooling, making it difficult to infer the presence of hyperons from the available thermal luminosity data, and complicating the link between measured thermal emission and internal composition. We show that this is the case for magnetars, because their crustal temperature is almost independent of hyperon direct Urca cooling in the core, regardless of whether hyperons are superfluid or not. Likewise, ther- mal luminosity data of moderately magnetized neutron stars are not suitable to extract information about the internal composition, as long as hyperons are superfluid.
会場: コモンルーム 246-248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
Genetic Drift and Gnatural Selection
2022年9月22日(木) 16:00 - 17:00
トーマス・ヒッチコック (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)
Understanding how the various evolutionary forces of mutation, selection, and drift collectively shape the genetic composition of populations is a key goal of population genetics research. One classic method of study has been to compare different inheritance systems, and particularly popular has been the within genome comparison of autosomes and sex chromosomes. However, inferences from such comparisons can be limited by the fact that multiple factors differ between sex chromosomes and autosomes (e.g. ploidy and transmission genetics). Here, we study a group of black winged fungus gnats with a peculiar type of reproduction “digenic PGE” in which X and autosomes are inherited equally from females and males, but the X remains expressed in a haploid state in males compared to a diploid state in females. I first explain what is known about their inheritance system, and then show how we can extend classic theory to the various inheritance systems that coexists within the fungus gnats.
会場: via Zoom
イベント公式言語: 英語
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セミナー
MeV gamma-ray all sky simulation
2022年9月16日(金) 14:00 - 15:00
辻 直美 (神奈川大学 理学部 特別助教)
The MeV gamma-ray domain is the only unexplored window among recent multiwavelength observations in astrophysics, often referred to as the "MeV gap". To fulfill this gap, there are several ongoing and planned projects of MeV gamma-ray telescopes. The measurement of MeV gamma rays (both continuum and line emission) would give us new insight into many topics in astrophysics, such as relativistic jets, particle acceleration, and origin of matter. In advance of the future MeV gamma-ray missions, we have been working on prediction of the MeV gamma-ray sky, which is helpful to determine what kinds of sources can be detectable with the future telescopes. In order to explore the MeV gamma-ray sources, we performed a catalog cross-matching between the hard X-ray (Swift/BAT) and GeV gamma-ray (Fermi/LAT) catalogs, resulting in 145 firmly cross-matched sources. Combined with the Galactic diffuse emission, which is calculated by GALPROP to reconcile the cosmic-ray and gamma-ray spectra with observations by AMS-02, Voyager, and Fermi-LAT, the all-sky maps in the MeV gamma-ray band can be produced. This is also used to investigate a long-standing problem in the MeV gamma-ray astrophysics: the origin of the diffuse emission from the inner Galaxy, measured by COMPTEL. I will report the analysis and results in detail, and introduce future missions of the MeV gamma-ray detectors.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Phantom Bethe excitations and spin helices in integrable spin chains
2022年9月15日(木) 17:00 - 18:15
Vladislav Popkov (University Wuppertal, Germany)
We demonstrate the existence of a special chiral “phantom” mode with some analogy to a Goldstone mode in the anisotropic quantum XXZ Heisenberg spin chain. The phantom excitations contribute zero energy to the eigenstate, but a finite fixed quantum of momentum. The mode exists not due to symmetry principles, but results from nontrivial scattering properties of magnons with momentum k given by the anisotropy via cos (k) = Jz/Jx. The mode originates from special string-type solutions of the Bethe ansatz equations with unbounded rapidities, the phantom Bethe roots. All such Bethe states are chiral (the simplest representative being factorized state with helicoidal magnetization profile) and exist in both periodic and open XXZ spin chain under fine-tuning. I show how phantom Bethe states can be generated dissipatively, by setting a polarization gradient via coupling the ends of the open spin chain to suitable dissipative baths. Spin helix eigenstates were observed and used in recent cold atom experiments, and led to further surprising findings.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Hessian Geometric Structure of Equilibrium and Nonequilibrium Chemical Reaction Newtworks
2022年9月8日(木) 16:00 - 17:00
小林 徹也 (東京大学 生産技術研究所 准教授)
Cells are the basic units of all living things, and their functions are realized by circuits and networks of chemical reactions. Thus, understanding the mechanism how various cellular functions are implemented by chemical reaction networks (CRN) is the central challenge in biophysics and quantitive biology. Among various aspects of CRN, its thermodynamic property is particularly important because most of biological functions are energy-consuming nonequilibrium phenomena. However, even though the equilibrium chemical thermodynamics and kinetics of chemical reactions were founded more than one century ago, the nonequilibrium theory of CRN is still immature. One reason is the nonlinearity in the constitutive equation between chemical force and flux, which prevents us from associating the tangent and cotangent spaces of the dynamics by the usual inner product structure. In this work, we show that the nonlinear relation between chemical force and flux can be captured by Legendre transformation and the geometric aspects of CRN dynamics can be characterized by Hessian geometry. Hessian geometry is the geometry generated by Legendre dual pairs of convex functions and is the basis of dually flat structure of information geometry and also equilibrium thermodynamics. Thus, we have dually flat structures in CRN dynamics, one on the state-potential space where equilibrium and energetic aspect is formulated (1,2), and the other on the force-flux space where nonequilibrium and kinetics aspect is characterized(3). Two of them are consistently connected by topological property of the underlying hypergraph structure of CRN. We discuss potential applications of this structure not only for CRN but also for other phenomena and problems(4,5).
会場: via Zoom
イベント公式言語: 英語
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セミナー
Search for Galactic SNR PeVatrons: γ-ray observations in the vicinity of SNRs G106.3+2.7 & HB9
2022年9月2日(金) 14:00 - 15:00
岡 知彦 (京都大学 大学院理学研究科 物理学・宇宙物理学専攻 博士課程)
Supernova remnants (SNRs) are believed to be the site of cosmic ray acceleration up to PeV (called PeVatron), but there is no conclusive observational evidence. The possible reason is that only young SNRs (t_age < 1 kyr) can accelerate CRs up to PeV, and then the particles escape at the early stage, thus, the opportunity to observe them is limited. To investigate this scenario, we observed and analyzed the following two SNRs. First, we focused on SNR G106.3+2.7, the most promising SNR as a PeVatron, since 100 TeV gamma rays have been detected with air shower experiments. With the gamma-ray observation results, we discussed the origin of the PeV CR in the vicinity of this middle-aged SNR (t_age = 5-10 kyr) and then obtained the following interpretation: CRs accelerated at the SNR in the past are illuminating the molecular cloud and producing gamma rays at present. Second, we analyzed the observation data around SNR HB9 and newly found gamma-ray emissions outside the SNR shell at the molecular cloud region. The gamma-ray emission can be explained by the protons accelerated and escaped from the SNR in the past. Therefore, we have attempted to measure the time evolution of the maximum acceleration energy at the SNR by comparing the gamma-ray spectra at the SNR shell and cloud regions. In this seminar, I will report the analysis results of those two SNRs.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Loop structure via one sided loop extrusion with twist deformation
2022年9月1日(木) 16:00 - 17:00
横田 宏 (理化学研究所 数理創造プログラム (iTHEMS) 特別研究員)
During cell division, a chromatin fiber condenses into the rod-like shape which is so called chromosome. The chromosome is composed of consecutive loop structures. Many researchers have been interested in the loop formation mechanism. The loop extrusion is the one of the promising hypotheses. However, the only loop extrusion does not completely explain the chromosome condensation dynamics. In order to tackle this problem, we constructed a mechanical model of the loop formation dynamics by focusing on the twist and writhe structures in DNA or chromatin. In this talk, I would like to explain the loop extrusion mechanism and our model.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Study of the eta' meson in nuclei in the LEPS2/BGOegg experiment
2022年8月26日(金) 16:30 - 18:00
冨田 夏希 (京都大学 大学院理学研究科 附属サイエンス連携探索センター (SACRA) 特定助教)
A large mass reduction of eta'(958) meson in nuclear medium owing to its UA(1) anomaly is expected in several model calculations. We carried out the LEPS2/BGOegg experiment at SPring-8 to study eta' meson properties in nuclei. If there is a large mass reduction in nuclei, an eta' meson and a nucleus may form a bound state. We searched for the eta'-nucleus bound states via missing mass spectroscopy of the 12C(gamma, p) reaction. To suppress background events from multi-meson production, we simultaneously measured decay products from the eta'-nucleus system for the first time. We also carried out the missing mass spectroscopy simultaneously detecting an eta' meson escaping from a nucleus for the first time. We will show the experimental results and comparisons with theoretical calculations. We will also show the preliminary results of the line shape analysis of the invariant mass spectra of the eta'->2gamma decay in the gamma+12C->eta'+X reaction.
会場: コモンルーム (246-248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
Testing Astrophysical Models from the Shadow of the Galactic Center Black Hole
2022年8月26日(金) 14:00 - 15:00
水野 陽介 (T.D. Lee Fellow / Associate Professor, Tsung-Dao Lee Institute, Shanghai Jiao Tong University, China)
We present the first Event Horizon Telescope (EHT) observations of Sagittarius A* (Sgr A*), the Galactic center source associated with a supermassive black hole. These observations were conducted in 2017 using a global interferometric array of eight telescopes operating at a wavelength 1.3 mm. A variety of imaging and modeling analyses all support an image that is dominated by a bright, thick ring with a diameter of ~50 micro-arcsecond. Using a large suite of numerical simulations, we demonstrate that the EHT images of Sgr A* are consistent with the expected appearance of a Kerr black hole with mass ∼4 million solar mass, which is inferred to exist at this location based on previous infrared observations of individual stellar orbits, as well as maser proper-motion studies. Our model comparisons disfavor scenarios where the black hole is viewed at high inclination (i > 50 deg), as well as non-spinning black holes and those with retrograde accretion disks. Our results provide direct evidence for the presence of a supermassive black hole at the center of the Milky Way. In this talk, I will focus on more theoretical interpretation and model comparison to understand the accretion flow properties nearby Sgr A*.
会場: via Zoom
イベント公式言語: 英語
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A rooted phylogeny of bacteria resolves early evolution
2022年8月25日(木) 16:00 - 17:00
Adrian Davin (東京大学 大学院理学系研究科 生物科学専攻 JSPS海外特別研究員)
Bacteria are the most diverse life forms on Earth and yet, we know surprisingly little about their early evolution. In this talk, I will explain how phylogenetic reconciliations and models of genome evolution can be used to answer some of the most interesting open questions in biology, such as the nature of the last bacterial common ancestor or whether a tree is a meaningful representation of evolution in the presence of abundant lateral gene transfer.
会場: via Zoom
イベント公式言語: 英語
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セミナー
How does subatomic matter organize itself? A low-energy nuclear physics perspective
2022年8月3日(水) 16:30 - 18:00
Xavier Roca-Maza (Associate Professor, Department of Physics, University of Milan, Italy / Sezione di Milano, INFN, Italy)
This seminar is a part of the RCNP workshop (RCNP研究会「低エネルギー核物理と高エネルギー天文学で読み解く中性子星」). Those who want to attend this seminar are required to register for the workshop by July 31. This seminar is supported by Gravitational Wave and Equation of State Working Group (GW-EOS WG), RIKEN iTHEMS.
会場: 大阪大学 核物理研究センター (RCNP) (メイン会場) / via Zoom
イベント公式言語: 英語
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Product Replacement Algorithm, Semidefinite Programming, and Operator Algebras
2022年8月2日(火) 16:00 - 17:00
小澤 登高 (京都大学 数理解析研究所 (RIMS) 教授)
Suppose you are given a large finite set G and want to estimate the size |G| or see how a typical element x in G looks like. In this talk, G will be a finite group generated by g_1,...,g_d. The "Product" Replacement Algorithm" is a popular algorithm for random sampling in the group G. The PRA shows outstanding performance in practice, but the theoretical explanation has remained mysterious. I will talk how an infinite-dimensional topological-algebraic analysis (operator algebra theory) connects this problem to a convex (semidefinite) optimization problem that can be rigorously solved by computer. This talk is intended for a general audience.
会場: コモンルーム 246-248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
Test of the Cosmological principle by observing the primordial gravitational waves
2022年7月27日(水) 13:30 - 15:00
浦川 優子 (高エネルギー加速器研究機構 (KEK) 准教授)
In this talk, using the generalized deltaN formalism, which dramatically facilitates a computation of the primordial density perturbation and the primordial GWs (PGWs), we address a violation of the Cosmological principle, namely a violation of the global isotropy in the Universe. It’s turned out that measuring the PGWs provides a powerful tool to explore a violation of the global isotropy. If time permits, I will also discuss some prospects on LiteBIRD.
会場: コモンルーム 246-248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 instanton knot homology
2022年7月25日(月) 16:00 - 18:00
井森 隼人 (京都大学 大学院理学研究科 数学・数理解析専攻 博士課程)
Floer theory is an infinite-dimensional version of Morse theory and has provided powerful invariants in the study of low-dimensional topology. In the context of Yang-Mills gauge theory, some versions of Floer homology groups for knots have been developed. These knot invariants are called instanton knot homology groups and are strongly related to representations of the fundamental group of the knot complement. In this talk, the speaker introduces basic constructions of instanton knot homology groups and recent developments related to the equivariant version of instanton knot homology theory.
会場: コモンルーム 246-248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
Superconducting-like heat current: Effective cancellation of current-dissipation trade-off by quantum coherence
2022年7月25日(月) 13:30 - 15:00
田島 裕康 (電気通信大学 大学院情報理工学研究科 助教)
Recent developments in statistical mechanics have revealed a tradeoff between heat current and dissipation [1,2]. In various situations, this current-dissipation tradeoff represents a relationship between thermal energy flow and entropy increase, similar to Joule’s law W=RI^2. On the other hand, the coherence effect on the current-dissipation tradeoff has not been thoroughly analyzed. Here, we systematically analyze how coherence affects the current-dissipation tradeoff [3]. The results can be summarized in the following three rules:
会場: via Zoom
イベント公式言語: 英語
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セミナー
Bayesian optimization of multivariate genomic prediction models based on secondary traits for improved accuracy gains and phenotyping costs
2022年7月21日(木) 16:00 - 17:00
濱崎 甲資 (東京大学 大学院農学生命科学研究科 博士課程)
In recent years, the genomic prediction that predicts phenotypic values from marker genotype data has attracted much more attention in the area of breeding. Especially, genomic selection using prediction values based on genomic prediction models has been contributing to more efficient and rapid breeding. In genomic prediction, it is important to construct the prediction model so that its accuracy becomes higher. Thus, multivariate genomic prediction models with secondary traits, such as data from various omics technologies including high-throughput phenotyping (e.g., unmanned aerial vehicle-based remote sensing), have started to be applied to many datasets because it offers improved accuracy gains compared with genomic prediction based only on marker genotypes. Although there is a trade-off between accuracy gains and phenotyping costs of secondary traits, no attempt has been made to optimize these trade-offs. In this study, we propose a novel approach to optimize multivariate genomic prediction models with secondary traits measurable at early growth stages for improved accuracy gains and phenotyping costs. The proposed approach employs Bayesian optimization for efficient Pareto frontier estimation, representing the maximum accuracy at a given cost. The proposed approach successfully estimated the optimal secondary trait combinations across a range of costs while providing genomic predictions for only about 20% of all possible combinations. The simulation results reflecting the characteristics of each scenario of the simulated target traits showed that the obtained optimal combinations were reasonable. Analysis of real-time target trait data showed that the proposed multivariate genomic prediction model had significantly superior accuracy compared to the univariate genomic prediction model.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Seiberg-Witten Floer homotopy
2022年7月15日(金) 14:00 - 16:30
今野 北斗 (東京大学 大学院数理科学研究科 助教)
I will survey a mathematical object called the Seiberg-Witten Floer homotopy type introduced by Manolescu. This is a machinery that extracts interesting aspects of 3- and 4-dimensional manifolds through the Seiberg-Witten equations. This framework assigns a 3-manifold to a "space" (more precisely, the stable homotopy type of a space), and this space contains rich information that is strong enough to recover the monopole Floer homology of the 3-manifold, which is known already as a strong invariant. I shall sketch how this theory is constructed along Manolescu's original work, and introduce major applications. If time permits, I will also explain recent developments of Seiberg-Witten Floer homotopy theory. If you are not familiar with the mathematical formulation of TQFT and categorification, I recommended you to watch Dr. Sano's recent talk in advance (see related links).
会場: コモンルーム 246-248号室 とZoomのハイブリッド開催
イベント公式言語: 英語
751 イベント