iTHEMS Theoretical Physics Seminar
67 events

Seminar
Boundaryinduced transitions in Möbius quenches of holographic BCFT
May 15 (Wed) at 16:00  17:30, 2024
Dongsheng Ge (Project Researcher, Department of physics, Osaka University)
Boundary effects play an interesting role in finitesize physical systems. In this work, we study the boundaryinduced properties of 1+1dimensional critical systems driven by inhomogeneous Möbiuslike quenches. We focus on the entanglement entropy in BCFTs with a large central charge and a sparse spectrum of lowdimensional 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.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
Role of selfgravity on the central halo structure of fuzzy dark matter
April 30 (Tue) at 13:30  15:00, 2024
Yusuke Manita (Affiliated Scientist, Yukawa Institute for Theoretical Physics, Kyoto University)
Fuzzy dark matter (FDM) is a dark matter model that is characterized by the ultralight masses around 10−22 eV. As FDM has the wavelike nature, the selfgravitating structure is described by the SchrödingerPoisson equation. Previous simulations based on the SchrödingerPoisson equation have demonstrated that solitonlike structure having a highdensity 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 NavarroFrenkWhite (NFW) profile. In this paper, we study the role of the selfgravity of the soliton core, and its relation to the bulk halo properties by solving the SchrödingerPoisson equation in a simplified setup. The findings indicate that the contribution from the soliton selfgravity must dominate over the NFW potential in order to sustain the soliton.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Seminar
A night out with ghosts
April 24 (Wed) at 16:00  17:30, 2024
Veronica Errasti Diez (Research Fellow, Faculty of Physics, LudwigMaximiliansUniversität 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 nonlinear and/or higherorder 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 illbehaved theories, ghosts are not intrinsically pathological. As we will see, goodnatured ghosts open the door to multidisciplinary tantalizing opportunities…! And ghosts make excellent partygoers, so make sure not to miss this appointment!
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Seminar
ShortLived Hawking Radiation Under Stringy Effects
April 11 (Thu) at 13:30  15:00, 2024
WeiHsiang Shao (Ph.D. Student, Department of Physics, National Taiwan University, Taiwan)
A UV theory is required in order to describe the origin of latetime Hawking radiation. In this talk, I will explore Hawking radiation in a nonlocal 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 spacetime uncertainty relation. As a result, the characteristics of transPlanckian field modes differ significantly from that in the standard lowenergy effective theory, and I will argue that this ultimately results in the termination of Hawking radiation around the scrambling time of the black hole.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Seminar
Brane field theory with higherform symmetry
March 12 (Tue) at 14:00  15:30, 2024
Kiyoharu Kawana (Research Fellow, Korea Institute for Advanced Study (KIAS), Republic of Korea)
We propose field theory for branes with higherform symmetry as a generalization of ordinary Landau theory. The field \psi[C_p^{}] becomes a functional of pdimensional 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 higherform transformation, we first generalize the concept of “derivative” for higherdimensional objects. Then, we discuss various fundamental properties of the brane field based on the higherform invariant action. It is shown that the classical solution exhibits the area law in the unbroken phase of U(1) pform symmetry, while it indicates a constant behavior in the broken phase for the large volume limit of Cp. In the latter case, the lowenergy effective theory is described by the pform Maxwell theory. If time permits, we also discuss branefield theories with a discrete higherform symmetry and show that the lowenergy effective theory becomes a BFtype topological field theory, resulting in topological order.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Seminar
Stochastic tunneling in de Sitter spacetime
February 28 (Wed) at 16:00  17:30, 2024
Taiga Miyachi (Ph.D. Student, Institute of Cosmophysics, Department of Physics, Graduate School of Science, Kobe University)
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 shortwavelength modes are stretched and transformed into longwavelength modes. In the stochastic approach, such UVIR transition is incorporated as quantum noise, and the dynamics of the longwavelength modes are described by stochastic differential equations. In this talk, we construct a SchwingerKeldysh path integral that reproduces this stochastic differential equation and reformulate the tunneling probability. We also reproduce the HawkingMoss tunneling probabilities by using the saddle point approximation.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
Recent advances in nuclear Density Functional Theory and applications to the nuclear response
February 6 (Tue) at 13:30  15:00, 2024
Gianluca Colò (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 symmetryrestored calculations are nowadays of common use in the study of groundstate properties and lowlying 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 SkyrmeRandom 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 welldeformed 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 cohosted by Nuclear Manybody Theory Laboratory and Fewbody Systems in Physics Laboratory, RIKEN Nishina Center for AcceleratorBased Science.
Venue: 2F Large Meeting Room, RIBF Building, RIKEN Wako Campus (Main Venue) / via Zoom
Event Official Language: English

Seminar
Nuclear EnergyDensity Functional Approach to Bridging NeutronRich Nuclei and Neutron Stars
February 5 (Mon) at 13:30  15:00, 2024
Kenichi Yoshida (Associate Professor, Research Center for Nuclear Physics, Osaka University)
Understanding the properties of neutronrich nuclei has been a central subject in lowenergy 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 neutronstar observation techniques, the structure of neutron stars has been becoming better understood. The data accumulated from these observations unveil properties of neutronrich matter that are otherwise inaccessible through terrestrial experiments. In this talk, I will introduce an attempt to construct a nuclear energydensity 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 spintriplet 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 cohosted by UKAKUREN.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Quantum features in cosmological perturbations?
January 18 (Thu) at 14:15  15:00, 2024
Amaury Micheli (Postdoctoral Researcher, iTHEMS)
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.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Seminar
Gravitational lensing on superposed curved spacetime
January 18 (Thu) at 13:30  14:15, 2024
Youka Kaku (Ph.D. Student, Graduate School of Science, Nagoya University)
In 2017, Bose et al. proposed a tabletop experiment to observe the gravitational effect induced by a spatially superposed mass source, particularly gravityinduced 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 semiclassical gravity case. This work is currently in progress and is a collaboration with Yasusada Nambu.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Seminar
Symmetry Topological field theory for Subsystem symmetry
January 9 (Tue) at 15:00  16:00, 2024
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 2foliated 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 KramersWannier and JordanWigner 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.
Venue: via Zoom / Seminar Room #359
Event Official Language: English

Seminar
Inflationary Cosmology with a scalarcurvature mixing term $\frac{1}{2} \xi R \phi^2$
December 20 (Wed) at 16:00  17:30, 2023
Payel Sarkar (Visiting Researcher, Kyoto University)
We use the PLANCK 2018 and the WMAP data to constraint inflation models driven by a scalar field $\phi$ in the presence of the nonminimal scalarcurvature 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 nonminimal gravity theory. We calculate the potential slowroll parameters, predict the scalar spectral index $n_s$, tensortoscalar ratio $r$, leading and higher order nonGaussianity 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 nonminimal coupling parameter $\xi$. We have compared our results with the ones existing in the literature, and this indicates the present status of nonminimal inflation after the release of the PLANCK 2018 data.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building (Main Venue) / via Zoom
Event Official Language: English

Seminar
Gravitational Lensing in Black Hole Spacetimes of the PlebanskiDemianski Class
December 6 (Wed) at 16:00  17:30, 2023
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 socalled PlebanskiDemianski spacetime. Besides the Schwarzschild and Kerr spacetimes, the PlebanskiDemianski spacetime also includes other solutions as special cases, among them the Cmetric 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 Cmetric, 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 latitudelongitude 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.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
Rotating discs on the Kerr black hole background
December 5 (Tue) at 15:00  16:30, 2023
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).
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
Gravity of Accretion Discs and Black Holes
December 5 (Tue) at 14:00  15:00, 2023
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.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
Breaking down the magnonic WiedemannFranz law in the hydrodynamic regime
December 4 (Mon) at 15:00  16:30, 2023
Ryotaro Sano (Ph.D. Student, Division of Physics and Astronomy, Graduate School of Science, Kyoto University)
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 quasiparticles 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 noninteracting 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 socalled 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.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Seminar
Compact Star Solutions Beyond General Relativity
November 7 (Tue) at 13:30  15:00, 2023
Kota Numajiri (Ph.D. Student, Graduate School of Science, Nagoya University)
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 nontrivial 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.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
Topological Aspect of Adsorption Site Selectivity on Metal Surfaces
October 24 (Tue) at 13:30  15:00, 2023
Yuta Tsuji (Associate Professor, Faculty of Engineering Sciences, Kyushu University)
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 thirdorder 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.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
First Xray polarimetry of neutron stars with strong magnetic fields
September 25 (Mon) at 13:30  15:00, 2023
Toru Tamagawa (Chief Scientist, High Energy Astrophysics Laboratory, RIKEN Cluster for Pioneering Research (CPR))
We launched the world's first highly sensitive Xray polarimetry satellite, IXPE, in December 2021. IXPE began observations in January 2022 and detected significant Xpolarization 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 Xray 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.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
Induced gravitational waves from inflaton oscillons
September 22 (Fri) at 14:00  15:30, 2023
Kaloian Dimitrov Lozanov (Project Researcher, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)
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.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English
67 events
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