Seminar
782 events
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Seminar
Axions around rotating black holes
November 12 (Fri) at 14:00 - 16:00, 2021
Hirotaka Yoshino (Institute of Cosmophysics, Department of Physics, Graduate School of Science, Kobe University)
String theories indicate the existence of many axionlike scalar fields with light masses in addition to the QCD axion. If this is the case, an axion field around a rotating black hole extracts the energy of the black hole by the mechanism called the “superradiant instability”. Then, every astrophysical black hole is expected to wear a cloud of the axion. In this talk, I would like to give an overview on this topic, and introduce our numerical studies on the phenomena caused by the axion cloud at the last stage of the superradiant instability where the self-interaction of axions becomes important.
Venue: via Zoom
Event Official Language: English
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Seminar
Toward mathematical medicine: development of a new drug and digital medicine for sleep disorders
November 11 (Thu) at 12:30 - 13:30, 2021
Jae Kyoung Kim (Associate Professor, Department of Mathematical Sciences, KAIST, Republic of Korea)
In this talk, I will illustrate collaborative stories between our math group and medical researchers to treat disrupted circadian rhythms and sleep. First, I will illustrate the key molecular mechanism for robust circadian rhythms against spatio-temporal noise, which we identified by analyzing spatio-temporal timeseries data of clock molecules. This explains why Alzheimer's diseases, obesity, and aging cause unstable circadian rhythms and sleep-wake cycle: cytoplasmic traffic jam, which provides a new paradigm to treat sleep disorders. Next, I will talk about our collaboration story with Pfizer Inc: how we used mathematical modeling to help the development of a new drug modulating the circadian phase. Finally, I will introduce our collaboration with Samsung medical center: how we used mathematical modeling to analyze complex sleep patterns of shift workers measured with wearables to find optimal sleep patterns minimizing sleep disorders. This opens the chance for the development of an app providing a personalized sleep schedule for shift workers.
Venue: via Zoom
Event Official Language: English
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Seminar
Hunting hypernuclei by machine learning in nuclear emulsions
November 8 (Mon) at 14:00 - 15:00, 2021
Takehiko Saito (Chief Scientist, High Energy Nuclear Physics Laboratory, RIKEN Cluster for Pioneering Research (CPR))
A hypernuclus is a subatomic systems with strange quark(s). They have been studied already for seven decades for understanding the fundamental baryonic interaction and nuclear matters inside the core of neutron stars. The hypertriton is the lightest hypernucleus with a neutron, a proton and a Lambda hyperon, and it is the benchmark in hypernuclear studies. However, recent experimental studies with heavy ion beams have revealed that the nature of the hypertriton is unclear, especially on its biding energy and lifetime. The most urgent issue is to measure its binding energy very precisely. Measurements with nuclear emulsion have provided the best precision for the hypernuclear binding energy, however, it requires a huge human load on visual image analyses. We have developed machine learning models to detect events associated with production and decay of hypertriton in nuclear emulsions data, and we have already discovered hypertriton events [1]. In the seminar, we’ll discuss the challenges and developments of our machine learning models as well as the outcomes and perspectives of our works.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Geometry and Physics of Mirror Symmetry
November 5 (Fri) at 16:00 - 18:00, 2021
Naichung Conan Leung (Professor of Mathematics, Department of Mathematics, The Institute of Mathematical Sciences, The Chinese University of Hong Kong, Hong Kong)
In the first half of this talk, I will describe the geometry and physics behind mirror symmetry in layman's terms. In the second half of this talk, I will provide a more mathematical explanation of the concepts involved in this mysterious conjecture. *Please contact Keita Mikami's mail address to get access to the Zoom meeting room.
Venue: via Zoom
Event Official Language: English
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Seminar
Boolean algebras and operator algebras
November 4 (Thu) at 15:00 - 16:30, 2021
Michiya Mori (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
The concept of Boolean algebra was introduced by George Boole in 1847. It plays a fundamental role in the theory of propositional logic. The theory of operator algebras was initiated by John von Neumann in around 1930. A keyword of the latter theory is "noncommutativity". In this talk, I will first explain basics of Boolean algebras and some ideas in operator algebra theory. Then I will talk about my recent attempt to give a new formulation of the concept of "noncommutative Boolean algebras" in an operator algebraic framework.
Venue: via Zoom
Event Official Language: English
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Seminar
Protein structure modeling from cryo-electron microscopy data
November 4 (Thu) at 13:30 - 14:30, 2021
Takaharu Mori (Senior research scientist, Theoretical Molecular Science Laboratory, RIKEN Cluster for Pioneering Research (CPR))
Recent advances in cryo-electron microscopy (cryo-EM) have enabled us to determine three-dimensional structures of biomolecules at near-atomic resolution. Protein structure modeling from experimental cryo-EM data can be achieved using a molecular dynamics (MD) simulation, called flexible fitting. We have developed MD-based flexible fitting algorithms for efficient and reliable protein structure modeling. In this seminar, I would like to talk about our recent contributions to this field, and propose perspectives towards next-generation structural biology.
Venue: via Zoom
Event Official Language: English
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Seminar
Is the Standard Model in the Swampland? Consistency Requirements from Gravitational Scattering
November 2 (Tue) at 14:30 - 16:00, 2021
Katsuki Aoki (Research Assistant Professor, Yukawa Institute for Theoretical Physics, Kyoto University)
Underlying assumptions on ultraviolet completion can impose constraints on its low-energy effective field theories (EFTs). The swampland program aims to clarify consistent and inconsistent EFTs with quantum gravity and aims to understand quantum gravity from low-energy physics and vice versa. One of the most well-established constraints is called positivity bounds, provided that general assumptions such as Poincare invariance and unitarity are satisfied at all scales. I will first explain how these consistency conditions arise especially in the presence of gravity. I will then show that the positivity bound is violated if the Standard Model of particle physics coupled to General Relativity is extrapolated up to 10^16 GeV, requiring new physics there or below. The precise value of the cutoff is determined by hadronic physic while it is insensitive from non-gravitational physics beyond the Standard Model. This is a signal from established physics for the necessity of quantum gravity below 10^16 GeV.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Nonlinear dynamics in frog choruses
October 28 (Thu) at 10:00 - 11:00, 2021
Ikkyu Aihara (Associate Professor, Institute of Systems and Information Engineering, University of Tsukuba)
Male frogs produce sounds to attract conspecific females as well as advertise their own territories to other males. Subsequently male frogs acoustically interact with each other, which induces various types of synchronized behavior. In this seminar, I will introduce the synchronized calling behavior of actual male frogs as well as theoretical studies using a phase oscillator model [1, 2]. Next, I will introduce our recent projects on the identification of the phase oscillator model from empirical data [3] and the extension of the model to a hybrid dynamical system in which male frogs intermittently switch their behavioral mode based on internal condition and also the interaction with other males [4].
Venue: via Zoom
Event Official Language: English
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Seminar
Geometry of hyperkahler 4 manifolds
October 22 (Fri) at 13:00 - 15:00, 2021
Song Sun (Associate Professor, Department of Mathematics, University of California, Berkeley, USA)
An n dimensional Riemannian metric g defines a holonomy group, which is a subgroup of SO(n) given by parallel transport along all contractible loops (with respect to the Levi-Civita connection). According to the Berger classification we know that if a complete Riemannian metric is not locally symmetric and not locally reducible then its holonomy group is either the entire SO(n) (generic case), or U(n) (Kahler), or is special and belongs to a small list. Riemannian metrics with special holonomy are very interesting geometric objects to study, with many connections to analysis and physics. The simplest model is given by a 4 dimensional hyperkahler metric. We will explain the general background and discuss recent progress on understanding the geometry of hyperkahler 4 manifolds. *Please contact Keita Mikami's mail address to get access to the Zoom meeting room.
Venue: via Zoom
Event Official Language: English
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Seminar
Evaluation of origin of driving force for loop formation in a chromatin fiber
October 21 (Thu) at 10:00 - 11:00, 2021
Hiroshi Yokota (Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
During cell division, chromatin fiber is condensed into the rod-like shape which is called chromosome. The rod-like shape of the chromosome is constructed by consecutive chromatin loop structures which are formed by the protein complex named condensin. In this talk, by calculating the driving force for the loop formation, we discuss the mechanism of loop formation which is the one of the controversial issues on chromosome condensation. The driving force is evaluated based on the free energy of the chromatin loop formation by constructing the polymer model. Based on the free energy, the loop growth length in the unit time is also evaluated. These evaluations also lead to the time evolution of the loop length and the mechanism of the loop formation.
Venue: via Zoom
Event Official Language: English
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Seminar
Nonlinear response in strongly correlated systems
October 20 (Wed) at 17:00 - 18:15, 2021
Robert Peters (Lecturer, Department of Physics, Graduate School of Science, Kyoto University)
Nonlinear responses in condensed matter are intensively studied because they provide rich information about materials and hold the possibility of being applied in diodes or high-frequency optical devices [1-4]. While nonlinear responses in noninteracting models have been explored widely, the effect of strong correlations on the nonlinear response is still poorly understood. This talk will introduce a Green's function method to calculate nonlinear conductivities in strongly correlated materials [5-6]. Correlation effects are thereby included by the self-energy of the material. I will then use this method to study the nonlinear conductivities in noncentrosymmetric f-electron systems. The first system is a heavy Fermion system, where a nonreciprocal conductivity appears in the ferromagnetic phase. The nonreciprocal conductivity thereby always occurs perpendicular to the magnetization of the system and has a strong spin dependence, which might be advantageous for spintronic applications. The second system is a model corresponding to the Weyl-Kondo semimetal Ce3Bi4Pd3, in which a giant spontaneous Hall effect without time-reversal symmetry breaking has been observed [7]. This Hall effect can be explained as a nonlinear Hall effect in an inversion-symmetry broken Weyl-semimetal. It has been shown that the nonlinear Hall effect is related to the Berry curvature dipole [4]. Our study shows that the magnitude of the experimentally observed nonlinear Hall effect can be explained by the strong correlations inherent in this f-electron material [8]. *Detailed information about the seminar refer to the email.
Venue: via Zoom
Event Official Language: English
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Seminar
Floquet vacuum engineering: laser-driven chiral soliton lattice in the QCD vacuum
October 20 (Wed) at 13:30 - 15:00, 2021
Akihiro Yamada (Master's Student, School of Fundamental Science and Technology, Graduate School of Science and Technology, Keio University)
What happens to the QCD vacuum when a time-periodic circularly polarized laser field with a sufficiently large intensity and frequency is applied? Based on the Floquet formalism for periodically driven systems and the systematic low-energy effective theory of QCD, we show that for a sufficiently large frequency and above a critical intensity, the QCD vacuum is unstable against the chiral soliton lattice of pions, a crystalline structure of topological solitons that spontaneously breaks parity and continuous translational symmetries. In the chiral limit, in particular, the QCD vacuum is found unstable by the laser with an arbitrary small intensity. Our work would pave the way for novel “Floquet vacuum engineering.”
Venue: via Zoom
Event Official Language: English
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Seminar
Distinctive signals of boosted dark matter from semi-annihilations
October 20 (Wed) at 10:00 - 11:30, 2021
Takashi Toma (Assistant Professor, Institute of Liberal Arts and Science, Kanazawa University)
The recent dark matter direct detection experiments impose the stringent upper bound on the elastic scattering cross section with nucleons. This implies that the cross section is suppressed by small dark matter velocity. However such dark matter can be probed if it is boosted by some mechanism. In this talk, we show that the specific semi-annihilation channel where two dark matter particles annihilate into a pair of anti-dark matter and neutrino indicates signals distinctive from the other semi-annihilation and standard dark matter annihilation processes. Since the boosted dark matter produced by this semi-annihilation is regarded as a high energy neutrino, the total flux of the dark matter and the accompanying neutrino yields double peaks at the energy close to the dark matter mass. Both of the particles can be detectable at large volume neutrino detectors.
Venue: via Zoom
Event Official Language: English
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Seminar
Recent progress on the r-process in the era of gravitational-wave astronomy
October 15 (Fri) at 16:00 - 18:00, 2021
Nobuya Nishimura (Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))
The r-process, the rapid neutron-capture process, is a major origin of heavy nuclei beyond iron in the universe, occurring in explosive astrophysical phenomena with very neutron-rich environments. In the studies of r-process nucleosynthesis, there are several unsolved problems in nuclear physics and astrophysics. In this talk, I will briefly summarize recent progress on the studies of the r-process, mainly focusing on neutron star mergers. We will see that the scenario of neutron star mergers is consistent with several observations, e.g., GW170817 with a kilonova, chemical evolution of r-process elements. In addition, nevertheless, there are several remaining (or newly realized) problems on the origin of r-process elements in the universe. Focusing on our own research, I will introduce attempts to address these issues.
Venue: via Zoom
Event Official Language: English
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Seminar
The branched deformations of special Lagrangian submanifolds
October 15 (Fri) at 10:00 - 12:00, 2021
Siqi He (Research Assistant Professor, Simons Center for Geometry and Physics, Stony Brook University, USA)
Special Lagrangian submanifolds are a distinguished class of real calibrated submanifolds defined in a Calabi-Yau manifold, which are calibrated by the real part of the holomorphic volume form. Given a compact, smooth special Lagrangian submanifold, Mclean proved that the space of nearby special Lagrangian submanifolds of it could be parametrized by the harmonic 1-forms. In this talk, we will discuss some recent progress on generalizing Mclean’s result to the branched deformations. We will describe how to use multi-valued harmonic functions to construct branched nearby deformations. In the first one hour, we will introduce the background of special Lagrangian submanifold and explain the motivations to study this problem. In this second one hour, we will discuss the technical details and interesting new phenomenon in this branching deformation problem. *Please contact Keita Mikami's mail address to get access to the Zoom meeting room.
Venue: via Zoom
Event Official Language: English
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Seminar
Understanding the effect of defective, interfering influenza virus
October 14 (Thu) at 10:00 - 11:00, 2021
Catherine Beauchemin (Deputy Program Director, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Defective interfering virus particles (DIPs) are viruses that are defective in a very specific way that allows them to out-compete standard, non-defective virus. It is difficult to count DIPs because they can look too similar to standard virus. So instead, people are counting them based on their effect on suppressing the standard virus population. In this talk, I will explain the basic biology of virus replication, what are DIPs, and how they compete with standard virus. I will present our mathematical model (ordinary differential equation) that describes co-infection competition with DIPs and standard virus. I will use the mathematical model to show how experiments to count DIPs can give incorrect results, and I will propose some solutions.
Venue: via Zoom
Event Official Language: English
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Seminar
Symmetry-based analysis for unconventional superconductors: Diagnosis of topological and nodal superconductivity
October 12 (Tue) at 16:00 - 17:15, 2021
Seishiro Ono (Department of Applied Physics, School of Engineering, The University of Tokyo)
The physics of unconventional superconductors has gained a new dimension in the past decade, thanks to the bloom in the understanding of topological quantum materials. Keeping in mind the success of the symmetry-based diagnosis in the large-scale discovery of topological insulator and semimetal candidates [1], it is natural to ask whether the approach can be generalized to superconducting systems. In this talk, I provide a unified way to diagnose topology and superconducting nodes in unconventional superconductors. First, I review symmetry-indicator theory for the topological insulators [2]. Also, I also discuss how to generalize the theory to superconductors [3,4,5]. Next, I show that the symmetry-based approach can extensively classify superconducting nodes pinned to high-symmetric momenta [6]. Finally, I show that these results enable us to derive the comprehensive correspondences between pairing symmetries and topological/nodal superconducting nature for each material [7]. *Detailed information about the seminar refer to the email.
Venue: via Zoom
Event Official Language: English
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Seminar
Geography of varieties of general type
October 8 (Fri) at 16:00 - 18:10, 2021
Chen Jiang (Associate Professor, Shanghai Math Center, Fudan University, China)
We study birational invariants in order to study birational classifications of varieties. Geography is the study of relations among different invariants. We will focus on two fundamental invariants: canonical volume and geometric genus. For surfaces there are classical results such as Miyaoka-Yau inequality and Noether inequality. I will discuss higher dimensional analogue of them, and introduce our recent work on the optimal Noehter inequality for 3-folds joint with Jungkai Chen and Meng Chen. *Please contact Keita Mikami's mail address to get access to the Zoom meeting room.
Venue: via Zoom
Event Official Language: English
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Seminar
Wave function geometry and anomalous Landau levels of flat bands
October 7 (Thu) at 16:00 - 17:15, 2021
Bohm-Jung Yang (Associate Professor, Department of Physics and Astronomy, Seoul National University, Republic of Korea)
Semiclassical quantization of electronic states under magnetic field describes not only the Landau level spectrum but also the geometric responses of metals under a magnetic field. However, it is unclear whether this semiclassical idea is valid in dispersionless flat-band systems, in which an infinite number of degenerate semiclassical orbits are allowed. In this talk, I am going to show that the semiclassical quantization rule breaks down for a class of flat bands including singular flat bands [1-5] and isolated flat bands [6]. The Landau levels of such a flat band develop in the empty region in which no electronic states exist in the absence of a magnetic field. The total energy spread of the Landau levels of flat bands is determined by the quantum geometry of the relevant Bloch states, which is characterized by their Hilbert–Schmidt quantum distance and fidelity tensors. The results indicate that flat band systems are promising platforms for the direct measurement of the quantum geometry of wavefunctions in condensed matter. *Detailed information about the seminar refer to the email.
Venue: via Zoom
Event Official Language: English
782 events