Seminar
1013 events
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Seminar
Tetra-neutron system studied by RI-beam experiments
January 17 (Tue) 13:30 - 15:00, 2023
Susumu Shimoura (Research Scientist, Spin isospin Laboratory, RIKEN Cluster for Pioneering Research (CPR))
Multi-neutron systems have attracted a long-standing attention in nuclear physics. In several decades, experimental attempts have been made with a particular focus on the tetra-neutron system. Among them, the two different experiments, the double-charge exchange reaction on 4He and the alpha-particle knockout reaction from the 8He, show a sharp peak just above the threshold in the four-neutron spectra, which could be a signature of a "resonant state", separate from a broad bump structure at higher excitation energy regions. Both the experiments have been realized by using the 8He beam above 150 A MeV at the RIKEN RI Beam Factory. Details of the two experiments including basic idea, experimental techniques, and analysis are presented as well as a historical review of previous experimental attempts. Emphasis is made for the experimental conditions for populating a kinematically isolated tetra-neutron system with very small momentum transfer. The spectral shape is discussed by means of reaction processes and correlations in the final tetra-neutron system with several recent theoretical studies.
Venue: Common Room #246-248 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Mergers of neutron star-neutron star (or black hole) binaries as r-process sites
January 13 (Fri) 14:00 - 15:00, 2023
Shinya Wanajo (Senior Scientist, Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Germany)
The discovery of an electromagnetic counterpart (kilonova) associated with GW170817 confirms that binary neutron star (NS) mergers are at least one of sites of r-process nucleosynthesis. However, there is no observational evidence that black hole (BH)-NS mergers are r-process sites. In this talk, we overview the latest work of nucleosynthesis based on long-term hydrodynamics simulations of NS-NS and BH-NS mergers covering early dynamical and late post-merger mass ejections. We also briefly discuss a possible constraint on nuclear equations of state.
Venue: via Zoom / Common Room #246-248
Event Official Language: English
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Seminar
Basic of microbial ecology and applicability of your life and research
January 12 (Thu) 16:00 - 17:00, 2023
Daiki Kumakura (Ph.D. Student, Graduate School of Life Science, Hokkaido University)
Microbial ecology is a fascinating field that examines the various environments in which microbes can thrive and their potential applications to human life. In this seminar, I will delve into four main topics:
Venue: via Zoom
Event Official Language: English
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Seminar
Searching for high-freqeuncy gravitational waves with axion detectors
January 12 (Thu) 14:00 - 15:30, 2023
Valerie Domcke (Senior Faculty, Department of Theoretical Physics, CERN, Switzerland)
Current gravitational wave (GW) experiments cover a large frequency range from nHz to kHz. Beyond that, the regime of high frequency GWs is both extremely challenging challenging, and highly motivated as a unique window to the very early Universe. In this talk I will discuss a proposal for a new type of electromagnetic GW detector which makes use of the observation that GWs generate oscillating electromagnetic effects in the vicinity of external electric and magnetic fields. This is in close analogy to the interaction of the axion with electromagnetic fields. I will discuss how existing bounds from axion searches can be recast for GWs, as well as implications for future axion searches such as the DMRadio program.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
A cell membrane model that reproduces cortical flow-driven cell migration and collective movement
January 5 (Thu) 16:00 - 17:00, 2023
Katsuhiko Sato (Associate Professor, Research Institute for Electronic Science, Hokkaido University)
Cellular migration is a key component of numerous biological processes, including the morphogenesis of multicellular organisms, wound healing, and cancer metastasis, where cells adhere to each other to form a cluster and collectively migrate. Although the mechanisms controlling single-cell migration are relatively well understood, those underlying multiple-cell migration still remain unclear. A key reason for this knowledge gap is the so-called many-body problem. That is, many forces—including contraction forces from actomyosin networks, hydrostatic pressure from the cytosol, frictional forces from the substrate, and forces from adjacent cells—contribute to cell cluster movement, making it challenging to model, and ultimately elucidate, the final result of these forces. In this talk, I provide a two-dimensional cell membrane model that represents cells on a substrate with polygons and expresses various mechanical forces on the cell surface, keeping these forces balanced at all times by neglecting cell inertia. The model is discrete but equivalent to a continuous model if appropriate replacement rules for cell surface segments are chosen. When cells are given a polarity, expressed by a direction-dependent surface tension reflecting the location dependence of contraction and adhesion on a cell boundary, the cell surface begins to flow from front to rear as a result of force balance. This flow produces unidirectional cell movement, not only for a single cell but also for multiple cells in a cluster, with migration speeds that coincide with analytical results from a continuous model. Further, if the direction of cell polarity is tilted with respect to the cluster center, surface flow induces cell cluster rotation. The reason why this model moves while keeping force balance on cell surface (i.e., under no net forces from outside) is because of the implicit inflow and outflow of cell surface components through the inside of the cell. I provide an analytical formula connecting cell migration speed and turnover rate of cell surface components.
Venue: via Zoom
Event Official Language: English
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Seminar
How to infer evolutionary history
December 22 (Thu) 16:00 - 17:00, 2022
Jeffrey Fawcett (Senior Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
One main goal of evolutionary studies is to infer the evolutionary that explains the current diversity. We want to infer the ancestral state and what kind of changes occurred from the previous ancestral state to the current state. In other words, we want to infer the phylogenetic relationship that explains the branching pattern that leads to the current diversity and infer the state at each node and the changes that occurred in each branch of the phylogeny. In this talk, I will introduce some basic concepts that are used in evolutionary biology to tackle these questions, especially how molecular data, i.e., DNA and protein sequence data, can be utilized. This talk will be introductory and aimed at non-experts including non-biologists.
Venue: via Zoom
Event Official Language: English
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Seminar
Modelling Optical Signals from Magnetar-Driven Supernovae
December 20 (Tue) 14:00 - 15:00, 2022
Conor Omand (Postdoctoral Researcher, Department of Astronomy, Stockholm University, Sweden)
Many energetic supernovae are thought to be powered by the rotational energy of a highly-magnetized, rapidly-rotating neutron star. The emission from the associated luminous pulsar wind nebula (PWN) can affect the system in different ways, including accelerating the ejecta, ionizing the ejecta, and breaking the spherical symmetry through hydrodynamic instabilities or large scale asymmetries. Modeling the observables from these processes; the light curves, spectrum, and polarization; is essential from understanding the nature of the central engine. I will present the results of a radiative transfer study looking at the effects of a PWN on the supernova nebular spectrum, and the preliminary results from a more physically motivated light curve model for parameter inference, and a study examining the polarization that arises due to hydrodynamic instabilities in the ejecta of engine-driven supernovae.
Venue: Common Room #246-248 / via Zoom
Event Official Language: English
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Seminar
The ANDES Deep Underground Laboratory in South America: status and prospects
December 19 (Mon) 12:30 - 13:30, 2022
Maria Manuela Saez (Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
The construction of the Agua Negra tunnels that will link Argentina and Chile under the Andes mountains opens the possibility of building a deep underground laboratory in the Southern Hemisphere. Dark Matter particles can be detected directly via their elastic scattering with nuclei, and next-generation experiments can eventually find physical evidence about dark matter candidates. I will show you our predictions for the expected direct dark matter signal and the ANDES site laboratory, whose location in the Southern Hemisphere should play a significant role in understanding dark matter modulation signals. Additionally, since planned next-generation large-scale direct detection experiments will measure the coherent elastic scattering of neutrinos on protons and nuclei, we have calculated the SN neutrino signal expected for the location. Finally, to study the background, we have calculated the contributions to the neutrino floor by considering the reactor’s neutrinos and geoneutrinos at the laboratory site. We hope these studies might contribute to dark matter detection strategies that maximize the future ANDES laboratory detection capabilities.
Venue: via Zoom
Event Official Language: English
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Seminar
CM minimization and special K-stability
December 16 (Fri) 14:00 - 16:30, 2022
Masafumi Hattori (Ph.D. Student, Department of Mathematics, Graduate School of Science, Kyoto University)
Odaka proposed a conjecture predicting that the degrees of CM line bundles for families with fixed general fibers are strictly minimized if the special fibers are K-stable. This conjecture is called CM minimization and a quantitative strengthening of the conjecture of separatedness of moduli spaces of K-stable varieties (K-moduli). This conjecture was already shown for K-ample (Wang-Xu), Calabi-Yau (Odaka) and Fano varieties (Blum-Xu). In this talk, we introduce a new class, special K-stable varieties, and settle CM minimization for them, which is a generalization of the above results. In addition, we would like to explain an important application of this, construction of moduli spaces of uniformly adiabatically K-stable klt trivial fibrations over curves as a separated Deligne-Mumford stack in a joint work with Kenta Hashizume to appear. This is based on arXiv:2211.03108.
Venue: via Zoom
Event Official Language: English
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Seminar
Geometric decomposition of entropy production in stochastic and chemical systems
December 16 (Fri) 13:30 - 15:00, 2022
Kohei Yoshimura (Ph.D. Student, Department of Physics, Graduate School of Science, The University of Tokyo)
Entropy production is central to understanding nonequilibrium phenomena. It is known that decomposing entropy production enables us to separately treat distinct two aspects of dynamics, nonstationarity and breaking of detailed balance. In this seminar, I talk about our recent progress on geometric decomposition of entropy production in discrete stochastic systems and deterministic chemical systems. For the audience who may not be familiar with nonequilibrium thermodynamics and linear algebraic graph theory, which the latter enables us to treat the two kinds of systems at once, I would like to start with a very basic introduction. Then I explain why and how we decompose entropy production. Specifically, I mainly focus on the "Onsager-projective decomposition" we study in arXiv:2205.15227 rather than the information geometric decomposition provided in the following paper arXiv:2206.14599. Further, several physical consequences will be discussed, including generalization of Schnakenberg's decomposition stemming from cycles in a steady system, and its relation to gradient flow expressions of a master equation and a rate equation.
Venue: via Zoom
Event Official Language: English
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Seminar
Chemophoresis Engine: Theory of ATPase-driven Cargo Transport
December 15 (Thu) 16:00 - 17:00, 2022
Takeshi Sugawara (Project Researcher, Universal Biology Institute, The University of Tokyo)
The formation of macromolecule patterns depending on chemical concentration under non-equilibrium conditions, first observed during morphogenesis, has recently been observed at the intracellular level, and its relevance as intracellular morphogen has been demonstrated in the case of bacterial cell division. These studies have discussed how cargos maintain positional information provided by chemical gradients. However, how cargo transports are directly mediated by chemical gradients remains unknown. Based on the previously proposed mechanism of chemotaxis-like behavior of cargos (referred to as chemophoresis), we introduce the chemophoresis engine as a physicochemical mechanism of cargo motion, which transforms chemical free energy to directed motion through the catalytic ATP hydrolysis [1]. We propose its possible role as a universal principle of hydrolysis-driven intracellular transport.
Venue: via Zoom
Event Official Language: English
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Seminar
Lattice gauge theory in curved spacetimes
December 15 (Thu) 14:00 - 15:30, 2022
Arata Yamamoto (Assistant Professor, Department of Physics, Graduate School of Science, The University of Tokyo)
Lattice gauge theory is a powerful computational approach in quantum field theory. It is also utilizable for investigating quantum phenomena in curved spacetimes, such as rotating frame, torsion, and gravitational backgrounds. In this talk, I would like to overview the formulation and results of lattice simulations in curved spacetimes.
Venue: Common Room #246-248 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Chiral effects on lepton transport in core-collapse supernovae
December 13 (Tue) 13:30 - 15:00, 2022
Di-Lun Yang (Assistant Research Fellow, Institute of Physics, Academia Sinica, Taiwan)
Dynamics of leptons such as electrons and neutrinos play an important role in the evolution of core-collapse supernovae (CCSN). Nevertheless, chirality as one of fundamental microscopic properties that could affect lepton transport, through e.g. weak interaction, has been mostly overlooked. In this talk, I will discuss how chiral effects such as the renowned chiral magnetic effect (CME), generating an electric charge current along magnetic fields with chirality imbalance, could result in the unstable modes of magnetic fields and inverse cascade, which potentially influence the matter evolution in CCSN and pulsar kicks. I will also show how an effective CME could be realized via the backreaction from neutrino radiation even in the absence of an axial charge characterizing an unequal number of right- and left-handed electrons.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Quantum kinetics of neutrinos in high-energy astrophysical phenomena
December 9 (Fri) 14:00 - 15:00, 2022
Hiroki Nagakura (Specially Appointed Assistant Professor (NAOJ Fellow), Division of Science, NAOJ)
Neutrinos are the most mysterious and elusive particles in the standard model of particle physics. They play important roles in core-collapse supernovae and binary neutron star mergers as driving mass-ejection, synthesizing heavy elements including r-process nuclei, and neutrino signals from these sources. This exhibits the importance of accurate modeling of neutrino radiation field in these phenomena, which will be used to connect neutrino physics to multi-messenger astronomy. It has recently been suggested that neutrino-flavor conversion (or neutrino-oscillation) can ubiquitously occur in these astrophysical environments, exhibiting the requirement of quantum kinetic treatments in the modeling of neutrino transport. In this seminar, I will give an overview of the quantum kinetics neutrino transport and then introduce its recent progress, paying a special attention to the connection to astrophysics. I will also present the latest results of our numerical simulations of collective neutrino oscillations, which can be properly accounted for only by quantum kinetic framework.
Venue: via Zoom
Event Official Language: English
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Seminar
Carrollian hydrodynamics near the black hole horizon
December 8 (Thu) 16:00 - 17:30, 2022
Puttarak Jai-akson (Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
The membrane paradigm provides a fascinating bridge between gravitational dynamics near black hole horizons (null boundaries) and fluid dynamics. One question naturally follows: what type of fluids and hydrodynamics emerged at the horizon? Contrary to the longstanding belief, it turns out that the horizon fluid is Carrollian, rather than the Galilean (Navier-Stokes) fluid. The Carroll geometries and Carrollian physics, arising originally when the speed of light goes to zero (c to 0 limit), have recently gained increasing attention in the fields of black hole physics and flat holography. In this presentation, I will talk about the Carrollian limit and the resulting Carroll geometries and this unusual kind of hydrodynamics, the Carrollian hydrodynamics. I will then present the geometrical construction of the membrane (also known as the stretched horizon) in a way that a Carroll geometry manifest, therefore allowing us to spell out precisely the dictionary between gravitational degrees of freedom on the membrane and the Carrollian fluid quantities. I will also show that the Einstein’s equations projected onto the horizon are the Carrollian hydrodynamic conservation laws. Lastly, I will discuss the covariant phase space of the horizon, symmetries, and conservation laws. The talk is based on arXiv:2209.03328 and arXiv:2211.06415.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Cosmic Birefringence: how our universe violates left-right symmetry
December 6 (Tue) 13:30 - 15:00, 2022
Tomohiro Fujita (Assistant Professor, Waseda Institute for Advanced Study, Waseda University)
Our universe is lefty: recent observations imply that the polarization plane of light that has traveled through cosmic space for 13.8 billion years rotates about 0.3 degrees to the left. A similar phenomenon is known to occur in materials such as crystals, and is called birefringence. But why does birefringence occur even in the outer space, which is supposed to be a vacuum? Dark energy, the unknown energy that fills the universe, may be responsible for it. In this seminar, I will review observations and theories of cosmic birefringence and discuss future prospects.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Efficient encoding of the Schrodinger equation on quantum computers
December 5 (Mon) 14:00 - 15:30, 2022
Ermal Rrapaj (Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
The continuous space Schrödinger equation is reformulated in terms of spin Hamiltonians. For the kinetic energy operator, the critical concept facilitating the reduction in model complexity is the idea of position encoding. A binary encoding of position produces a spin-1/2 Heisenberg-like model and yields exponential improvement in space complexity when compared to classical computing. Encoding with a binary reflected Gray code (BRGC), and a Hamming distance 2 Gray code (H2GC) reduces the model complexity down to the XZ and transverse Ising model respectively. Any real potential is mapped to a series of k-local Ising models through the fast Walsh transform. As a first step, the encoded Hamiltonian is simulated for quantum adiabatic evolution. As a second step, the time evolution is discretized, resulting in a quantum circuit with a gate cost that is better than the Quantum Fourier transform. Finally, a simple application on an ion-based quantum computer is provided as proof of concept.
Venue: Common Room #246-248 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Tricritical phenomena in holographic chiral transitions
November 29 (Tue) 13:30 - 15:00, 2022
Masataka Matsumoto (Postdoctoral Researcher, Department of Physics, Shanghai University, China)
Tricritical point (TCP) is the end-point of a line of three-phase coexistence (a triple line) at which three coexisting phases simultaneously become identical. A TCP can be observed in various systems, for example, the QCD phase diagram with the chiral limit and a metamagnet such as a FeCl2 crystal. In the AdS/CFT correspondence, a TCP associated with a chiral phase transition has been found in the D3/D7 model [1]. In this talk, I will discuss the recent study [2] of critical phenomena at a tricritical point which emerges in the D3/D7 model in the presence of a finite baryon number density and an external magnetic field. We found all the critical exponents defined in this paper take the mean-field values. I will also compare the results with our previous works about the critical phenomena at the TCP that emerges in the steady state [3,4].
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Community assembly and species coexistence in the heterogeneous world
November 28 (Mon) 16:00 - 17:00, 2022
Naoto Shinohara (Assistant Professor, Graduate School of Life Sciences, Tohoku University)
How ecological communities are assembled and maintained is a fundamental question in community ecology. To tackle this challenge in the heterogeneous world, we need to understand how community assembly patterns change with environmental gradients and how species coexist in temporally fluctuating environments. In the first of my talk, I introduce our study on how plant community assembly patterns change along with the largest global environmental gradient, the latitudinal gradient. Then, I will present how the seasonal variability of environments contributes to the coexistence of phytoplankton species in a lake. These results altogether uncover how spatiotemporal heterogeneity of environments forms ecological communities in nature.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Tropical methods in Enumerative Geometry and Mirror Symmetry
November 25 (Fri) 14:00 - 16:00, 2022
Michel Van Garrel (Assistant Professor, School of Mathematics, University of Birmingham, UK)
Abstract for the 1st hour: Enumerative Geometry has been a feature of mathematics from its beginnings, just think about the number of lines in the plane passing through 2 points. I will take you on a history of the subject and its relationship to other areas of mathematics and physics. Abstract for the 2nd hour: Many problems in mathematics are solved by taking a limit and solving the limiting problem. Tropical geometry is a key technique that allows us to do this systematically. I will talk about the following problem. Take the complex projective plane S and an elliptic curve E in S. Count algebraic maps from the affine line into the complement S \ E. This counting problem is solved via tropical geometry as I will describe in this talk.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
1013 events