理論物理学セミナー
90 イベント
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セミナー
Compact Star Solutions Beyond General Relativity
2023年11月7日(火) 13:30 - 15:00
沼尻 光太 (名古屋大学 大学院理学研究科 博士課程)
The neutron star solutions have been gathering attention. Their high compactness enables us to observationally access the information about extreme regimes of hadron physics. On the other hand, their strong gravity features bring up another possibility, gravity beyond general relativity (GR). Although GR has been a great success until now, the present scenario for our universe still has several problems, such as dark sectors and the quantum description of gravity. To tackle these problems, the modified gravity theories have been discussed for decades. Their modifications are expected to become noticeable in strong gravity regimes like compact stars. In this talk, I will discuss the configuration of the compact star solution under the F(R) gravity, one of the most popular and simplest modifications of GR. The background hydrostatic solutions are calculated with some F(R) models, which show non-trivial influences from the additional scalar DOF in this theory. The tidal deformation phenomenon is also considered to focus on another observable, tidal deformability. I will comment on how to utilize obtained observables to determine the gravity theory and the unknown equation of state simultaneously.
会場: セミナー室 (359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
Topological Aspect of Adsorption Site Selectivity on Metal Surfaces
2023年10月24日(火) 13:30 - 15:00
辻 雄太 (九州大学 総合理工学研究院 准教授)
In this talk, the presenter will discuss which adsorption structure is preferred in the adsorption of atoms and molecules on metal surfaces based on the topology of the adsorption structures. The method of moments is used to analyze the electronic density of states of the surface. The third-order moment, which characterizes the skewness of the distribution of the electronic density of states, is related to the topology of the triangles at the adsorption interface. By further relating this to the change in energy of the system with the change in electron occupancy of the states, it is shown that it is possible to discuss the relationship between the type of metal and the topological features of the energetically stable adsorption structure.
会場: セミナー室 (359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
First X-ray polarimetry of neutron stars with strong magnetic fields
2023年9月25日(月) 13:30 - 15:00
玉川 徹 (理化学研究所 開拓研究本部 (CPR) 玉川高エネルギー宇宙物理研究室 主任研究員)
We launched the world's first highly sensitive X-ray polarimetry satellite, IXPE, in December 2021. IXPE began observations in January 2022 and detected significant X-polarization from objects in all categories. The IXPE observations have opened a new window in astrophysics. In this talk, I will introduce IXPE and present the results of X-ray polarimetry observations of neutron stars with strong magnetic fields (magnetars and neutron star binaries). The neutron star observations show results quite different from our prior expectations and await further theoretical interpretation.
会場: セミナー室 (359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
Induced gravitational waves from inflaton oscillons
2023年9月22日(金) 14:00 - 15:30
カロイアン・ディミトロフ・ロザノフ (東京大学 カブリ数物連携宇宙研究機構 (Kavli IPMU) 特任研究員)
We present a new way to study cosmic inflation with gravitational waves. The gravitational signal is generated thanks to nonlinear structures in the inflaton field, called oscillons. This novel probe allows us to test models of inflation which are challenging to test with CMB experiments.
会場: 研究本館 3階 359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
Dark matter heating vs vortex creep heating in old neutron stars
2023年8月7日(月) 13:30 - 15:00
藤原 素子 (Postdoctoral Researcher, Theoretical Particle Physics Group, Technical University of Munich, Germany)
Old isolated neutron stars have been gathering attention as targets to probe Dark Matter (DM) through temperature observations. DM will anomalously heat neutron stars through its gravitational capture and annihilation process, which predicts surface temperature as T_s ~ (1 − 3) × 10^3 K for t > 10^6 years. We may put constraints on DM-nucleon scattering cross section by finding even colder neutron stars. This story, however, assumed that there is no relevant heating source for old neutron stars. In this talk, we discuss the creep motion of vortex lines in the neutron superfluid of the inner crust as the heating mechanism. This creep mechanism is inherent in the structure of neutron stars. The heating luminosity is proportional to the time derivative of the angular velocity of the pulsar rotation, and the proportional constant J has an approximately universal value for each neutron star. If this vortex creep heating is quantitatively relevant against DM heating, this mechanism may cause a serious background to probe DM. The J parameter can be determined from the temperature observation of old neutron stars because the heating luminosity is balanced with the photon emission in the late time. We study the latest data of neutron star temperature observation and find that these data indeed give similar values of J, in favor of the assumption that these neutron stars are heated by the frictional motion of vortex lines. Besides, these values turn out to be consistent with the theoretical calculations of the vortex-nuclear interaction. Integarting all the results, we evaluate the vortex creep heating and conclude its significance against DM heating.
会場: セミナー室 (359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
Maximum Force Conjecture and Black Hole Thermodynamics
2023年6月30日(金) 14:00 - 15:30
Yen Chin Ong (Professor, Center for Gravitation and Cosmology, College of Physical Science and Technology, Yangzhou University, China)
I review the current controversial status of the so-called "maximum force conjecture" in general relativity, whose validity has recently been debated. Then I will discuss how maximum force conjecture can nevertheless be relevant for black hole thermodynamics.
会場: 研究本館 3階 359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
The classical equations of motion of quantised gauge theories
2023年6月23日(金) 13:30 - 15:00
トム・メリア (東京大学 カブリ数物連携宇宙研究機構 (Kavli IPMU) 准教授)
The Einstein and Maxwell equations are the jewels in the crown of classical physics. But classical physics is only an approximation to nature, arising as a limit of the underlying quantum mechanical description. And in the case of both general relativity and electromagnetism, owing to their gauge theory nature, the full set of classical equations of motion are not guaranteed to follow from the quantum theory. The time-time and time-space components of the Einstein equations in GR and Gauss’ law in EM are enforced ‘by hand' in the quantisation procedure—a choice so as to make the classical-like states behave as per our classical belief. But what if our universe was actually described by another classical-like state? For GR, the resulting modification of the Einstein equations can be packaged as the inclusion of an auxiliary energy-momentum tensor describing a ’shadow’ matter that adds no additional degrees of freedom to the theory. The homogeneous and isotropic background piece of this auxiliary matter contributes to expansion of the universe identical to cold dark matter, and the inhomogeneous components source curvature perturbations that grow linearly at linear order.
会場: 研究本館 3階 359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
A Spin on Wave Dark Matter
2023年6月15日(木) 16:00 - 17:30
Mustafa Amin (Associate Professor, Department of Physics and Astronomy, Rice University, USA)
What can we learn about the intrinsic spin of ultralight dark matter field from astrophysical observations? That is, is dark matter is a scalar (spin 0), (spin 1) or tensor (spin 2) field? Using analytic calculations and 3+1 dimensional simulations, I will argue that the imprint of spin can be seen via (i) the initial density power spectrum, (ii) interference patterns in the density field inside dark matter halos, and through (iii) (polarized) solitons with macroscopic intrinsic spin. Based on features in the initial power spectrum, I will provide a bound on the dark matter mass > 10^(-18) eV for post-inflationary production. With increasing intrinsic spin, interference patterns in halos are reduced (and the inner shapes of halos modified) — which can be probed by lensing and dynamical heating of stars. Finally, after introducing polarized solitons, I will show that the time-scale of emergence of solitons (within halos) increases with increasing spin, and briefly discuss electromagnetic and gravitational wave signatures from such polarized solitons. Time-permitting, I will also mention connections to “spinor" Bose-Einstein condensates in the laboratory.
会場: 研究本館 3階 359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
Termination of Superradiance from a Binary Companion
2023年6月6日(火) 13:30 - 15:00
Huiyu Zhu (Ph.D. Student, Department of Physics, Hong Kong University of Science and Technology, Hong Kong)
We study the impact of a binary companion on black hole superradiance at orbital frequencies away from the gravitational-collider-physics (GCP) resonance bands. A superradiant state can couple to a strongly absorptive state via the tidal perturbation of the companion, thereby acquiring a suppressed superradiance rate. Below a critical binary separation, this superradiance rate becomes negative, and the boson cloud gets absorbed by the black hole. This critical binary separation leads to tight constraints on GCP. Especially, a companion with mass ratio q > 10^−3 invalidates all GCP fine structure transitions, as well as almost all Bohr transitions except those from the |211> state. Meanwhile, the backreaction on the companion manifests itself as a torque acting on the binary, producing floating/sinking orbits that can be verified via pulsar timing. In addition, the possible termination of cloud growth may help to alleviate the current bounds on the ultralight boson mass from various null detections.
会場: 研究本館 3階 359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
Spectral correlations and scrambling dynamics in Sachdev-Ye-Kitaev type models
2023年5月30日(火) 13:30 - 15:00
手塚 真樹 (京都大学 大学院理学研究科 物理学・宇宙物理学専攻 助教)
Note: Due to unexpected trouble, we have made the decision to postpone the seminar scheduled for February 21 to May 30. Sorry for the trouble. Abstract: The Sachdev-Ye-Kitaev (SYK) model, proposed in 2015, is a quantum mechanical model of N Majorana or complex fermions with all-to-all random four-body interactions. The model has attracted significant attention over the years due to its features such as the existence of the large-N solution with maximally chaotic behavior at low temperatures and holographic correspondence to low-dimensional gravity. The sparse version of the SYK model reproduces essential features of the original model for reduced numbers of disorder parameters. We recently proposed [1] a further simplification, where we set the nonzero couplings to be +1 or -1 rather than sampling from a continuous distribution such as Gaussian. This binary-coupling model exhibits strong correlations in the spectrum, as observed in the spectral form factor, more efficiently in terms of the number of nonzero terms than in the Gaussian distribution case. We also discuss the scrambling dynamics with the binary-coupling sparse SYK model, comparing the model with the original model as well as the SYK model with random two-body terms [2], where the localization of the many-body eigenstates in the Fock space has been quantitatively studied [3,4].
会場: 研究本館 3階 359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
Statistical methods to probe binary stellar evolution with ZTF and LISA data
2023年5月23日(火) 13:30 - 15:00
ルーシー・マクニール (京都大学 大学院理学研究科 物理学・宇宙物理学専攻 特別研究員)
The Laser-Interferometer-Space-Antenna (LISA) will be capable of detecting all galactic double neutron star binaries (DNSBs) with orbital periods < 20 minutes, as well as 10-100’s of thousands of double white dwarf binaries. I will present our method to use LISA detections to constrain the formation frequencies of galactic DNSBs, which are determined by supernova physics and various mass exchange processes. Next, I summarise the key differences in DNSBs and white dwarf binaries (WDBs) in the context of future LISA observations, as well as the current Zwicky Transient Facility (ZTF) for the latter. Then we will examine the current catalogue of short period white dwarf binaries (orbital periods < 1 hour) detected by ZTF so far, including the first ever measurement (Burdge et al. 2023) of the temperature of a mass transferring white dwarf in a binary. Here I will make the case that taken together, these observations are in conflict with the theoretical picture commonly used in Galactic modelling- specifically related to cooling and mass transfer leading to WDB mergers. However, heating from tidal interactions may explain and mediate this inconsistency. Finally I will put these results into context regarding preparing for the unprecedented data set of galactic white dwarf binaries from LISA in the 2030s.
会場: 研究本館 3階 359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
Towards S-matrix theory of unstable particles
2023年3月15日(水) 13:30 - 15:00
青木 勝輝 (京都大学 基礎物理学研究所 基研特任助教)
The S-matrix is one of the central objects in quantum field theory and gains renewed interest recently to understand the possible structures of low-energy effective field theories and quantum gravity. However, most of the particles have finite decay widths and thus do not appear in asymptotic states. Therefore, the standard S-matrix arguments may not be directly applied to scatterings of such unstable particles and we need to formulate “the S-matrix theory of unstable particles” to properly understand the availability of the S-matrix arguments in realistic systems. In this talk, I will talk about the first steps towards this goal. In particular, I will discuss non-perturbative consequences of unitarity in a scattering amplitude of unstable particles and its analytic properties.
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
Boltzmann or Bogoliubov? A Case of Gravitational Particle Production
2023年2月9日(木) 13:30 - 15:00
金田 邦雄 (東京女子大学 理学研究科 特任講師)
Despite its weakness, gravity is the primordial source of particle production in the early Universe. All the particles, including dark matter, can inevitably be created after the end of inflation through gravity. To study this production channel, two different approaches have commonly been considered, one of which is based on the Boltzmann equation, and the other is based on the Bogoliubov transformation. The former approach has widely been used in phenomenological studies of dark matter, while the latter has been developed to describe particle production in curved spacetime. I will discuss when these two approaches are equivalent and when they are not by considering the pure gravitational production of a scalar particle.
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
Tetra-neutron system studied by RI-beam experiments
2023年1月17日(火) 13:30 - 15:00
下浦 享 (理化学研究所 開拓研究本部 (CPR) 上坂スピン・アイソスピン研究室 研究員)
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.
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
Chiral effects on lepton transport in core-collapse supernovae
2022年12月13日(火) 13:30 - 15:00
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.
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
Cosmic Birefringence: how our universe violates left-right symmetry
2022年12月6日(火) 13:30 - 15:00
藤田 智弘 (早稲田大学 高等研究所 講師)
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.
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
Tetra and pentaquarks with multi-flavor contents
2022年11月22日(火) 13:30 - 15:00
保坂 淳 (大阪大学 核物理研究センター 教授)
Recent hadron experiments keep providing evidences of exotic hadrons with multi-quark components. These multiquarks are self-arranged into various configurations such as diquarks, hadronic molecules and so on. In this seminar, we discuss possible structures of tetra and pentaquarks with multi-flavor contents including recently observed T_cc, Pc and P_cs. Based on our recent studies in the quark model and hadron models, we discuss where and how different quark structures emerge.
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
Merger and post-merger of binary neutron stars with a quark-hadron crossover equation of state
2022年11月15日(火) 15:00 - 17:01
黄 永嘉 (Ph.D. Student, University of Science and Technology of China, China)
The state of the ultra-dense matter remains one of the long-standing open questions. Neutron star (NS), as it cools down the eons ahead after the birth in the supernova explosion, provides an astrophysical laboratory to investigate the dense, strongly interacting nuclear matter at zero temperature. On the other hand, the most intense gravitational wave(GW) radiation is produced in regions of the strong gravitational field by coherent movements of masses with large compactness. Therefore, GW from binary neutron star(BNS) merger naturally contains the information from the ultra-dense matter. In this talk, I will introduce our recent work, "Merger and post-merger of binary neutron stars with a quark-hadron crossover equation of state ."Quark-hadron crossover(QHC) is one way of hadron-quark transition, which generally predicts a peak in sound speed vs. density, and so releases more pressure during the hadron-quark transition. I will first briefly summarize the features of QHC EOS and the BNS merger. I will then focus on how information on the hadron-quark transition shows in the GW and its spectrum during the BNS merger.
会場: via Zoom
イベント公式言語: 英語
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セミナー
Expanding Edges of Quantum Hall Systems in a Cosmology Language - Hawking Radiation from de Sitter Horizon in Edge Modes
2022年11月11日(金) 16:00 - 17:30
堀田 昌寛 (東北大学 大学院理学研究科物理学専攻 助教)
Expanding edge experiments are promising to open new physics windows of quantum Hall systems. In a static edge, the edge excitation, which is described by free fields decoupled with the bulk dynamics, is gapless, and the dynamics preserve conformal symmetry. When the edge expands, such properties need not be preserved. We formulate a quantum field theory in 1+1 dimensional curved spacetimes to analyze the edge dynamics. We propose methods to address the following questions using edge waveforms from the expanding region: Does the conformal symmetry survive? Is the nonlinear interaction of the edge excitations induced by edge expansion? Do the edge excitations interact with the bulk excitations? We additionally show that the expanding edges can be regarded as expanding universe simulators of two-dimensional dilaton-gravity models, including the Jackiw-Teitelboim gravity model. As an application, we point out that our theoretical setup might simulate emission of analog Hawking radiation with the Gibbons-Hawking temperature from the future de Sitter horizon formed in the expanding edge region.
会場: 研究本館 3階 345−347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
Tensor renormalization group approach to quantum fields on a lattice
2022年11月8日(火) 13:30 - 15:00
秋山 進一郎 (東京大学 量子ソフトウェア寄付講座 特任助教)
Tensor renormalization group (TRG) approach is a variant of the real-space renormalization group to evaluate the path integral defined on the thermodynamic lattice, without resorting to any probabilistic interpretation for the given Boltzmann weight. Moreover, since the TRG can directly deal with the Grassmann variables, this approach can be formulated in the same manner for the systems with bosons, fermions, or both. These advantages of the TRG approach have been confirmed by the earlier studies of various lattice theories, which suggest that the TRG enables us to investigate the parameter regimes where it is difficult to access with the standard stochastic numerical methods, such as the Monte Carlo simulation. In this talk, explaining our recent applications of the TRG approach to several lattice models, we demonstrate the efficiency of the TRG as a tool to investigate lattice theories particularly in higher dimensions and future perspective.
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
90 イベント