Seminar
385 events
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Seminar
Introduction to stability conditions 1
March 2 (Wed) at 16:00 - 17:30, 2022
Dr. Naoki Koseki (Postdoctoral Research Associate, School of Mathematics, University of Edinburgh, UK)
In 2002, Bridgeland defined the notion of stability conditions on a triangulated category, motivated by string theory and mirror symmetry. Since then, Bridgeland stability conditions have been found very useful not only in Mathematical Physics, but also in various areas of Pure Mathematics. In the first part, I will review basic background and open problems in the theory of Bridgeland stability conditions. In the second part, I will explain recent developments of the theory, especially its applications to algebraic geometry.
Venue: via Zoom
Event Official Language: English
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Seminar
How to understand Earth science system using data science
February 25 (Fri) at 16:00 - 18:00, 2022
Dr. Kaman Kong (Postdoctoral Researcher, Computational Climate Science Research Team, RIKEN Center for Computational Science (R-CCS))
Hi everyone, my name is Kaman Kong. After I graduated from Nagoya University last April, I joined the computational climate science research team, R-CCS at Kobe. Although I have still not yet had the important results now, I would like to share my idea and future plan here. In this talk, different from the previous seminar, I would like to highlight how to use data science approaches to understand our Earth system science. In the first 60 minutes, I would like to share my research experiences in ecosystems, dust outbreaks, and atmospheric sciences and try to discuss their limitation in my study. After a 10-minute break, the 30 minutes will be spent discussing the potential methodology to overcome these limitations and new opportunities and challenges in Earth system science. (Part 1) In the first 60 minutes, I would like to talk about the relationships among ecosystems, dust outbreaks, and atmospheric conditions. I used the models of dust and ecosystem to explore seasonal variations of threshold wind speed, an index of soil susceptibility to dust outbreak, and its relations with land surface conditions, such as plant growth and soil moisture and temperature changes, in the Mongolian grasslands. On the other side, I am improving the weather forecast model to accurately predict dust emission and discuss its effects on the Earth system. Meanwhile, I am integrating the dust model into the ecosystem model. During this period, I realized there are many uncertainties of simulation. (Part 2) In the second 30 minutes, I will explain these limitations as I mentioned before and try to discuss how to solve these problems. For example, using deep learning to identify the green and brown plants separately for discussing their different effect on the dust model. And, used data assimilation (e.g., EnKF and Bayesian calibration) to improve the simulated performance of land surface parameters (e.g., soil moisture and vegetation).
Venue: via Zoom
Event Official Language: English
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Seminar
How is turbulence born: Spatiotemporal complexity and phase transition of transitional fluids
February 24 (Thu) at 17:00 - 18:15, 2022
Dr. Hong-Yan Shih (Assistant Research Fellow, Institute of Physics, Academia Sinica, Taiwan)
How a laminar flow becomes turbulence has been an unsolved problem for more than a century and is important in various industrial applications. Recently precise measurements in pipe flow experiments showed non-trivial spatiotemporal complexity at the onset of turbulence. Based on numerical evidence from the hydrodynamics equations, we discovered the surprising fact that the fluid behavior at the transition is governed by the emergent predator-prey dynamics of the important long-wavelength mode, leading to the mathematical prediction that the laminar-turbulent transition is analogous to an ecosystem on the edge of extinction. This prediction demonstrates that the laminar-turbulent transition is a non-equilibrium phase transition in the directed percolation universality class, and provides a unified picture of transition to turbulence emerging in systems ranging from turbulent convection to magnetohydrodynamics. *Detailed information about the seminar refer to the email.
Venue: via Zoom
Event Official Language: English
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Seminar
Stator dynamics of the bacterial flagellar motor
February 24 (Thu) at 17:00 - 18:00, 2022
Dr. Ashley Nord (Researcher, CNRS, Centre de Biologie Structurale, France)
Dr. Rubén Pérez-Carrasco (Lecturer in Theoretical Systems Biology, Faculty of Natural Sciences, Department of Life Sciences, Imperial College London, UK)The bacterial flagellar motor is the membrane-embedded rotary molecular motor which turns the flagellum that provides thrust to many bacteria for swimming, swarming, and chemotaxis. This large multimeric complex, composed of a few dozen constituent proteins, is a hallmark of dynamic subunit exchange. The stator units are inner-membrane ion channels which dynamically bind to the cell wall and convert electrochemical energy into torque which is applied to the rotor. The dynamic exchange of stator units is a function of the viscous load on the flagellum, allowing the bacterium to adapt to its local environment, though the molecular mechanisms of this mechanosensitivity remain unknown. Previously, we have shown that stator units behave as a catch bond, a counterintuitive bond which becomes stronger under applied tension. Here, by actively perturbing the steady-state stator stoichiometry of individual motors, we reveal a stoichiometry-dependent asymmetry in stator remodeling kinetics. We interrogate the potential effect of next-neighbor interactions and local stator unit depletion and find that neither can explain the observed asymmetry. We then simulate and fit two mechanistically diverse models which recapitulate the asymmetry, finding assembly dynamics to be particularly well described by a two-state catch-bond mechanism.
Venue: via Zoom
Event Official Language: English
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Seminar
iTHEMS - R-CCS(FTRT) Joint Online Seminar: Second order chiral phase transition in three flavor quantum chromodynamics?
February 18 (Fri) at 16:30 - 18:00, 2022
Prof. Gergely Fejos (Assistant Professor, Institute of Physics, Eötvös Loránd University, Hungary)
We calculate the renormalization group flows of all renormalizable interactions in the three dimensional Ginzburg--Landau potential for the chiral phase transition of three flavor quantum chromodynamics [1]. On the contrary to the common belief we find a fixed point in the system that is able to describe a second order phase transition in the infrared. This shows that longstanding assumptions on the transition order might be false. If the transition is indeed of second order, our results can also be interpreted as indirect evidence that the axial anomaly restores at the transition temperature.
Venue: via Zoom
Event Official Language: English
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Seminar
Spin transport in ultracold atomic gases
February 18 (Fri) at 14:00 - 15:00, 2022
Dr. Yuta Sekino (Postdoctoral Researcher, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))
In condensed matter physics, transport measurement has played crucial roles in understanding fascinating phenomena such as superconductivity and quantum Hall and Kondo effects. In this talk, we discuss the usefulness of spin transport as a probe for many-body properties in ultracold atoms. In the first part, we focus on the conductivity of alternating spin current, which includes information on superfluid gap, pseudogap, and topological phase transition. In the latter part, we consider mesoscopic spin transport between two Fermi gases weakly connected with each other. Our analysis suggests that the spin current is sensitive to whether the gases have pseudogaps, which are gap-like structures in densities of states just above the superfluid transition temperature. In this talk, we also mention similarities of ultracold atoms to neutron star matter.
Venue: via Zoom
Event Official Language: English
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Seminar
Hilbert transform and its applications to biology
February 17 (Thu) at 10:00 - 11:00, 2022
Dr. Shingo Gibo (Postdoctoral Researcher, iTHEMS)
In chronobiology, the estimation of phase dynamics is important for measuring period and phase shift. However, it is difficult to precisely estimate the phase from time-series data when the frequency and the amplitude are nonstationary. Hilbert transform has been known as a signal processing method for decomposing time-series into the phase and the amplitude dynamics. This method allows us to analyze the phase from nonstationary time-series data. In this talk, I would like to introduce the basic concept of Hilbert transform and a few examples of its applications.
Venue: via Zoom
Event Official Language: English
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Seminar
False vacuum decay in the Lorentzian path integral
February 15 (Tue) at 13:30 - 15:00, 2022
Mr. Takumi Hayashi (Ph.D. Student, Research Center for the Early Universe (RESCEU), The University of Tokyo)
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.
Venue: via Zoom
Event Official Language: English
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Seminar
Stochastic operators: properties and applications
February 10 (Thu) at 10:00 - 11:00, 2022
Dr. Gilberto Nakamura (Postdoctoral Researcher, iTHEMS)
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.
Venue: via Zoom
Event Official Language: English
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Seminar
Mixed dark matter scenarios consisting of primordial black hole dark matter and WIMPs
January 31 (Mon) at 11:00 - 12:00, 2022
Dr. Kenji Kadota (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.
Venue: via Zoom
Event Official Language: English
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Seminar
Recent progress on dualities in W-superalgebras
January 28 (Fri) at 16:00 - 18:00, 2022
Dr. Shigenori Nakatsuka (JSPS Fellow, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)
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.
Venue: via Zoom
Event Official Language: English
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Seminar
Galactic archaeology with r-process elements
January 28 (Fri) at 10:00 - 11:30, 2022
Dr. Yutaka Hirai (JSPS Research Fellow, Graduate School of Science Department of Astronomy, Tohoku University / 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.
Venue: via Zoom
Event Official Language: English
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Seminar
On Flow and Form at Low Reynolds Number
January 27 (Thu) at 10:00 - 11:00, 2022
Prof. Kenta Ishimoto (Associate Professor, Research Institute for Mathematical Sciences (RIMS), Kyoto University)
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.
Venue: via Zoom
Event Official Language: English
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Seminar
Bethe ansatz and quantum computing
January 26 (Wed) at 22:00 - 23:15, 2022
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.
Venue: via Zoom
Event Official Language: English
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Seminar
A simple XY model for cascade transfer
January 20 (Thu) at 13:30 - 15:00, 2022
Mr. Tomohiro Tanogami (Graduate School of Science, Kyoto University)
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].
Venue: via Zoom
Event Official Language: English
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Seminar
A study of biological systems from topological point of view
January 20 (Thu) at 10:00 - 11:00, 2022
Dr. Hiroyasu Miyazaki (Senior Research Scientist, iTHEMS)
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.
Venue: via Zoom
Event Official Language: English
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Seminar
Axion-like particles from core-collapse supernovae
January 17 (Mon) at 11:00 - 12:00, 2022
Dr. Kanji Mori (Research Institute of Stellar Explosive Phenomena (REISEP), Fukuoka University)
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.
Venue: via Zoom
Event Official Language: English
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Seminar
The Ohsawa-Takegoshi $L^2$ extension theorem and variations of Bergman kernels
January 14 (Fri) at 16:00 - 18:00, 2022
Dr. Genki Hosono (Mathematical Institute, Graduate School of Science, Tohoku University)
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.
Venue: via Zoom
Event Official Language: English
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Seminar
A comprehensive view of the SARS-CoV-2 infection process
January 13 (Thu) at 10:00 - 11:00, 2022
Dr. Wataru Nishima (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.
Venue: via Zoom
Event Official Language: English
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Seminar
Physics of nuclear bodies
January 6 (Thu) at 10:00 - 11:00, 2022
Prof. Tetsuya Yamamoto (Specially Appointed Associate Professor, Institute for Chemical Reaction Design and Discovery, Hokkaido University)
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.
Venue: via Zoom
Event Official Language: English
385 events