理論物理学セミナー
89 イベント
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セミナー
Worldline Path Integrals for the Graviton and 1-Loop Divergences in Quantum Gravity
2024年6月28日(金) 16:00 - 17:20
Fiorenzo Bastianelli (Professor, University of Bologna, Italy)
In this talk, I will discuss perturbative quantum gravity at the 1-loop level by reviewing and systematizing old results on UV divergences and presenting new findings along with new methods for their calculation. The traditional approach to this problem employs the Schwinger-DeWitt heat kernel method. We extend this approach by incorporating worldline path integrals to compute the perturbative expansion at small proper time. In addition, we explore a more principled approach that utilizes the BRST path integral quantization of the N=4 spinning particle, which describes the graviton in first quantization. Using these methods, we calculate the one-loop divergences in quantum gravity with a cosmological constant in arbitrary dimensions. When evaluated on-shell, these calculations yield a set of gauge-invariant coefficients that characterize pure quantum gravity with a cosmological constant. These coefficients may serve as benchmarks for comparing various approaches to quantum gravity.
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
Magnonic spin current and shot noise in an itinerant Fermi gas
2024年6月25日(火) 13:30 - 15:00
テイユ・チョウ (東京大学 大学院理学系研究科 物理学専攻 博士課程)
Spin transport phenomena at strongly-correlated interfaces play central roles in fundamental physics as well as spintronic applications. Although the spin-flip tunneling process, a key mechanism of spin transport, has been extensively studied in solid-state systems, its behavior in itinerant Fermi gases remains elusive. In this regard we study the spin tunneling in a repulsively interacting ultracold Fermi gas based on the conventional quasiparticle tunneling process. we investigate the spin current induced by quasiparticle and spin-flip tunneling processes to see their bias dependence and interaction dependence. To anatomize spin carriers, we propose the detection of the spin current noise in the system. The Fano factor, which is defined as the ratio between the spin current and its noise can serve as a probe of elementary carriers of spin transport. The change of the Fano factor microscopically evinces a crossover from the quasiparticle transport to magnon transport in itinerant fermionic systems.
会場: セミナー室 (359号室) 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
Grassmann Tensor Renormalization Group for two-flavor Schwinger model with a theta term
2024年6月24日(月) 16:00 - 17:00
菅野 颯人 (理化学研究所 仁科加速器科学研究センター (RNC) 理研BNL研究センター 理論研究グループ 基礎科学特別研究員)
QCD has been understood through numerical calculations by the Monte Carlo method. However, this method does not work for some parameter regions because of the sign problem. For example, QCD with a theta term has a sign problem, so the nature of QCD with a finite theta parameter is unknown. The theta dependence is also important to axion physics. To reveal such systems, tensor network methods are powerful tools. Tensor network methods have been developed by condensed matter theorists. Furthermore, recently there have been some attempts to apply them to high energy physics. In particular, the tensor renormalization group (TRG) method is remarkable for its applicability to higher dimensions. The Schwinger model is known as a two-dimensional toy model of QCD. It has the chiral symmetry and theta term as the same as QCD. In this study, the free energy of the two-flavor Schwinger model is calculated in a broad range of mass and theta parameters. We use TRG to calculate it, with obvious 2pi periodicity of theta parameter. We check the consistency with analytical values in large and small mass limits.
会場: via Zoom / セミナー室 (359号室)
イベント公式言語: 英語
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セミナー
Boundary-induced transitions in Möbius quenches of holographic BCFT
2024年5月15日(水) 16:00 - 17:30
Dongsheng Ge (大阪大学 大学院理学研究科 特任研究員)
Boundary effects play an interesting role in finite-size physical systems. In this work, we study the boundary-induced properties of 1+1-dimensional critical systems driven by inhomogeneous Möbius-like quenches. We focus on the entanglement entropy in BCFTs with a large central charge and a sparse spectrum of low-dimensional operators. We find that the choice of boundary conditions leads to different scenarios of dynamical phase transitions. We also derive these results in a holographic description in terms of intersecting branes in AdS3, and find a precise match.
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
Role of self-gravity on the central halo structure of fuzzy dark matter
2024年4月30日(火) 13:30 - 15:00
間仁田 侑典 (京都大学 基礎物理学研究所 協力研究員)
Fuzzy dark matter (FDM) is a dark matter model that is characterized by the ultralight masses around 10−22 eV. As FDM has the wave-like nature, the self-gravitating structure is described by the Schrödinger-Poisson equation. Previous simulations based on the Schrödinger-Poisson equation have demonstrated that soliton-like structure having a high-density flat core is formed at the central region of the FDM halos, and the size of such a core is typically determined by the de Broglie wavelength. Away from the central core, the density profile of the FDM halos resembles that of the cold dark matter halos on average, and is shown to be described by the Navarro-Frenk-White (NFW) profile. In this paper, we study the role of the self-gravity of the soliton core, and its relation to the bulk halo properties by solving the Schrödinger-Poisson equation in a simplified setup. The findings indicate that the contribution from the soliton self-gravity must dominate over the NFW potential in order to sustain the soliton.
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 night out with ghosts
2024年4月24日(水) 16:00 - 17:30
Veronica Errasti Diez (Research Fellow, Faculty of Physics, Ludwig-Maximilians-Universität München, Germany)
Field theories are the chief theoretical framework for physics. For instance, the Standard Model and General Relativity are widely accepted as accounting for subatomic particle and gravitational behavior, respectively. Nonetheless, even such acclaimed field theories have their limitations, such as the mysterious neutrino masses and dark sector. A natural and popular way around the hurdles consists in generalizations of field theories, via the inclusion of non-linear and/or higher-order corrections. Unless painstakingly avoided, these corrections lead to the propagation of negative kinetic energy modes, or ghosts for short. Ghosts have earned an appalling fame: kill, exorcise, avoid… No efforts are spared to guarantee their absence. In this talk, we will delve into the root causes for the ill name of ghosts. As a result, we will take up the cudgels for ghosts. While they do have a strong tendency to yield ill-behaved theories, ghosts are not intrinsically pathological. As we will see, good-natured ghosts open the door to multi-disciplinary tantalizing opportunities…! And ghosts make excellent party-goers, so make sure not to miss this appointment!
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
Short-Lived Hawking Radiation Under Stringy Effects
2024年4月11日(木) 13:30 - 15:00
ウェイシャン・シャオ (Ph.D. Student, Department of Physics, National Taiwan University, Taiwan)
A UV theory is required in order to describe the origin of late-time Hawking radiation. In this talk, I will explore Hawking radiation in a non-local model of the radiation field inspired by Witten's open string field theory. An attempt at extracting the correlators of this theory will be discussed, which leads to a space-time uncertainty relation. As a result, the characteristics of trans-Planckian field modes differ significantly from that in the standard low-energy effective theory, and I will argue that this ultimately results in the termination of Hawking radiation around the scrambling time of the black hole.
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
Brane field theory with higher-form symmetry
2024年3月12日(火) 14:00 - 15:30
川名 清晴 (Research Fellow, Korea Institute for Advanced Study (KIAS), Republic of Korea)
We propose field theory for branes with higher-form symmetry as a generalization of ordinary Landau theory. The field \psi[C_p^{}] becomes a functional of p-dimensional closed brane Cp embedded in a spacetime. As a natural generalization of ordinary field theory, we call this theory brane field theory. In order to construct an action that is invariant under higher-form transformation, we first generalize the concept of “derivative” for higher-dimensional objects. Then, we discuss various fundamental properties of the brane field based on the higher-form invariant action. It is shown that the classical solution exhibits the area law in the unbroken phase of U(1) p-form symmetry, while it indicates a constant behavior in the broken phase for the large volume limit of Cp. In the latter case, the low-energy effective theory is described by the p-form Maxwell theory. If time permits, we also discuss brane-field theories with a discrete higher-form symmetry and show that the low-energy effective theory becomes a BF-type topological field theory, resulting in topological 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
Stochastic tunneling in de Sitter spacetime
2024年2月28日(水) 16:00 - 17:30
宮地 大河 (神戸大学 大学院理学研究科 物理学専攻 宇宙論研究室 博士課程)
The formulation of tunneling in real time formalism is discussed. In the case of de Sitter spacetime, there is a method called the stochastic approach, which is known to reproduce the tunneling predicted by Hawking and Moss in the imaginary time formalism. In the case of accelerated expansion of space, the short-wavelength modes are stretched and transformed into long-wavelength modes. In the stochastic approach, such UV-IR transition is incorporated as quantum noise, and the dynamics of the long-wavelength modes are described by stochastic differential equations. In this talk, we construct a Schwinger-Keldysh path integral that reproduces this stochastic differential equation and reformulate the tunneling probability. We also reproduce the Hawking-Moss tunneling probabilities by using the saddle point approximation.
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
Recent advances in nuclear Density Functional Theory and applications to the nuclear response
2024年2月6日(火) 13:30 - 15:00
ジャンルカ・コロ (Professor, Department of Physics, University of Milan, Italy / Professor, Sezione di Milano, INFN, Italy)
In this contribution, I will give an overall (and, of course, biased) view of the general status of DFT. I will stress that, in contrast to ab initio methods, DFT is the only framework that allows the study of excited states, including those lying at relatively high energy. Accordingly, I will focus on the nuclear response. After a reminder on the nuclear Giant Resonances and the link with the nuclear equation of state, I will discuss the projection methods to restore symmetries in the calculations of deformed systems. While symmetry-restored calculations are nowadays of common use in the study of ground-state properties and low-lying excitations, similar realistic investigations for the nuclear response are essentially missing in the literature. Recently, we have implemented an exact Angular Momentum Projection (AMP) on top of Skyrme-Random Phase Approximation (RPA) calculations in a projection after variation (PAV) scheme, for the first time. The results will be critically analysed in the case of the monopole response, also taking into account the experimental investigations that can be envisioned for well-deformed systems. If time allows, the nuclear response will be also discussed as a way to improve the current density functionals and ground them on ab initio nuclear theory. This seminar is co-hosted by Nuclear Many-body Theory Laboratory and Few-body Systems in Physics Laboratory, RIKEN Nishina Center for Accelerator-Based Science.
会場: 理化学研究所 和光キャンパス RIBF棟 2階 大会議室 (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
Nuclear Energy-Density Functional Approach to Bridging Neutron-Rich Nuclei and Neutron Stars
2024年2月5日(月) 13:30 - 15:00
吉田 賢市 (大阪大学 核物理研究センター 准教授)
Understanding the properties of neutron-rich nuclei has been a central subject in low-energy nuclear physics. The great interest lies not only in the pursuit of a variety of structures and the elucidation of the mechanisms of their occurrence but also in obtaining insights into the structure of the inner crust of neutron stars. With advances in neutron-star observation techniques, the structure of neutron stars has been becoming better understood. The data accumulated from these observations unveil properties of neutron-rich matter that are otherwise inaccessible through terrestrial experiments. In this talk, I will introduce an attempt to construct a nuclear energy-density functional (EDF) inspired by the observations and then demonstrate its applicability to nuclear structure problems, including mass and deformation. One intriguing aspect of neutron stars is the emergence of superfluidity, especially the occurrence of spin-triplet pairing. I will discuss the unconventional pairing in nuclei within the nuclear EDF framework and give perspectives on the study of the phase diagram of the superfluidity in neutron stars. This seminar is co-hosted by UKAKUREN.
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
Quantum features in cosmological perturbations?
2024年1月18日(木) 14:15 - 15:00
アモリ・ミケリ (数理創造プログラム 特別研究員)
The statistical properties of the CMB anisotropies, reflecting the curvature inhomogeneities in the very early Universe, are very well accounted for by assuming that the inhomogeneities come from amplified vacuum fluctuations. This scenario makes the cosmological perturbations a possible observational window on the interplay between quantum degrees of freedom and gravity. I will review the discussions on the current presence or absence of quantum features in the perturbations, emphasising the quantum information approaches to this question, and comment on the observability of these features.
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
Gravitational lensing on superposed curved spacetime
2024年1月18日(木) 13:30 - 14:15
郭 優佳 (名古屋大学 大学院理学研究科 博士課程)
In 2017, Bose et al. proposed a tabletop experiment to observe the gravitational effect induced by a spatially superposed mass source, particularly gravity-induced entanglement. This experiment is expected to be the first step in exploring the quantum nature of gravity. Also, there are ongoing efforts to extend their proposal to the relativistic region to observe the unique quantum nature of gravity. In this talk, I will investigate gravitational lensing in a weak gravitational field induced by a spatially superposed mass source. I will show the Einstein ring image of a quantum scalar field propagated on a superposed curved spacetime and compare it with the image of the semi-classical gravity case. This work is currently in progress and is a collaboration with Yasusada Nambu.
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
Symmetry Topological field theory for Subsystem symmetry
2024年1月9日(火) 15:00 - 16:00
Qiang Jia (Research Fellow, School of Physics, Korea Institute for Advanced Study (KIAS), Republic of Korea)
We generalize the idea of symmetry topological field theory (SymTFT) to subsystem symmetry. We propose the 2-foliated BF theory with level N in (3+1)d as subsystem SymTFT for subsystem Z_N symmetry in (2+1)d. Focusing on N=2, we investigate various topological boundaries. The subsystem Kramers-Wannier and Jordan-Wigner dualities can be viewed as boundary transformations of the subsystem SymTFT and are included in a larger duality web from the subsystem SL(2,Z_2) symmetry of the bulk foliated BF theory.
会場: via Zoom / セミナー室 (359号室)
イベント公式言語: 英語
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セミナー
Inflationary Cosmology with a scalar-curvature mixing term $\frac{1}{2} \xi R \phi^2$
2023年12月20日(水) 16:00 - 17:30
Payel Sarkar (京都大学 客員研究員)
We use the PLANCK 2018 and the WMAP data to constraint inflation models driven by a scalar field $\phi$ in the presence of the non-minimal scalar-curvature mixing term $\frac{1}{2}\xi R \phi^2$. We consider four distinct scalar field potentials $\phi^p e^{-\lambda\phi},~(1 - \phi^{p})e^{-\lambda\phi},~(1-\lambda\phi)^p$ and $\frac{\alpha\phi^2}{1+\alpha\phi^2}$ to study inflation in the non-minimal gravity theory. We calculate the potential slow-roll parameters, predict the scalar spectral index $n_s$, tensor-to-scalar ratio $r$, leading and higher order non-Gaussianity parameters ($f_{NL},~\tau_{NL}$ and $g_{NL}$) and the amplitude of the scalar spectrum $A_s$ in the parameter ($\lambda, p, \alpha$) space of the potentials corresponding to different values of the non-minimal coupling parameter $\xi$. We have compared our results with the ones existing in the literature, and this indicates the present status of non-minimal inflation after the release of the PLANCK 2018 data.
会場: 研究本館 3階 359号室とZoomのハイブリッド開催 (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
Gravitational Lensing in Black Hole Spacetimes of the Plebanski-Demianski Class
2023年12月6日(水) 16:00 - 17:30
Torben Christian Frost (Postdoctoral Researcher, Kavli Institute for Astronomy and Astrophysics, Peking University, China)
Einstein's field equations allow various different black hole solutions. Among these solutions, the most famous are most likely the Schwarzschild and the Kerr spacetimes, which are both special cases of the so-called Plebanski-Demianski spacetime. Besides the Schwarzschild and Kerr spacetimes, the Plebanski-Demianski spacetime also includes other solutions as special cases, among them the C-metric and the NUT metric. They describe a linearly accelerating black hole and a black hole with gravitomagnetic charge, respectively. The question is now how we can determine if an astrophysical black hole can be described by one of these spacetimes. We will address this question using gravitational lensing for the three spacetimes with the most salient lensing features, namely the C-metric, the NUT metric, and the Kerr metric. For this purpose, we will first outline how to solve the equations of motion analytically using elementary and Jacobi's elliptic functions as well as Legendre's elliptic integrals. Then we will fix an observer in the domain of outer communication and relate the constants of motion of the lightlike geodesics to latitude-longitude coordinates on the observer's celestial sphere. We will use the analytic solutions to write down the lens equations, calculate the redshift, and the travel time. Finally, we will discuss and compare the results and comment on how we can use them to place constraints on the spin parameter, the acceleration parameter, and the gravitomagnetic charge of a black hole.
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
Rotating discs on the Kerr black hole background
2023年12月5日(火) 15:00 - 16:30
David Kofroň (Postdoctoral Researcher, Institute of Theoretical Physics, Charles University, Czechia)
Analytical solution of a rotating black hole surrounded by accretion disc in full GR is, so far, unknown. The Ernst equation is nonlinear. In this talk, we will provide a framework in which the solutions of linearised Ernst equations can be obtained from the linear perturbations of Kerr black hole treated in the formalism of the Debye potentials. In this way, we recover all the metric perturbations in term of a single complex scalar function (which solves the Laplace equation).
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
Gravity of Accretion Discs and Black Holes
2023年12月5日(火) 14:00 - 15:00
Petr Kotlařík (Ph.D. Student, Institute of Theoretical Physics, Charles University, Czechia)
The typical black hole solutions describe only isolated black holes. However, in astrophysics, such a condition is never strictly satisfied. As matter accretes onto the black hole, disc structures are often formed. In this talk, I will summarize our recent attempts to find the gravitational field of such a nonisolated black hole. We start from the simplest case of static and axially symmetric metric. Although it is a somewhat "rough" approximation in the astrophysical context, this idealization may already help us to understand some interesting implications of the disc's gravity. Moreover, with such a simplification, we can obtain exact analytical "superpositions" of the Schwarzchild black hole and a disc. When some rotation is present, dragging effects complicate the situation dramatically. Then, one typically has to resort to numerical relativity or some approximate methods, e.g., perturbations. In the talk, I also address the stationary case and demonstrate what we can do on the level of the direct metric perturbation.
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
Breaking down the magnonic Wiedemann-Franz law in the hydrodynamic regime
2023年12月4日(月) 15:00 - 16:30
佐野 涼太郎 (京都大学 大学院理学研究科 物理学・宇宙物理学専攻 博士課程)
Quantum transport has attracted a profound growth of interest owing to its fundamental importance and many applications in condensed matter physics. Recent significant developments in experimental techniques have further boosted the study of quantum transport. Notably in ultraclean systems, strong interactions between quasi-particles drastically affect the transport properties, resulting in an emergent hydrodynamic behavior. Recent experiments on ultrapure ferromagnetic insulators have opened up new pathways for magnon hydrodynamics. Hydrodynamic magnon transport implies exhibiting extraordinary features and has a potential for innovative functionalities beyond the conventional non-interacting magnon picture. However, the direct observation of magnon fluids remains an open issue due to the lack of probes to access the time and length scales characteristics of this regime. In this work, we derive a set of coupled hydrodynamic equations for a magnon fluid and study the spin and thermal conductivities by focusing on the most dominant time scales [1]. As a hallmark of the hydrodynamic regime, we reveal that the ratio between the two conductivities shows a large deviation from the so-called magnonic WF law. We also identify an origin of the drastic breakdown of the magnonic WF law as the difference in relaxation processes between spin and heat currents, which is unique to the hydrodynamic regime. Therefore, our results will become key evidence for an emergent hydrodynamic magnon behavior and lead to the direct observation of magnon fluids.
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
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
89 イベント