iTHEMS Theoretical Physics Seminar
41 events
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SeminarSpectral correlations and scrambling dynamics in Sachdev-Ye-Kitaev type models
May 30 (Tue) at 13:30 - 15:00, 2023
Dr. Masaki Tezuka (Assistant Professor, Division of Physics and Astronomy, Graduate School of Science, Kyoto University)
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].
Venue: Common Room #246-248 / via Zoom
Event Official Language: English
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Seminar
Towards S-matrix theory of unstable particles
March 15 (Wed) at 13:30 - 15:00, 2023
Dr. Katsuki Aoki (Research Assistant Professor, Yukawa Institute for Theoretical Physics, Kyoto University)
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.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Boltzmann or Bogoliubov? A Case of Gravitational Particle Production
February 9 (Thu) at 13:30 - 15:00, 2023
Dr. Kunio Kaneta (Lecturer, Graduate School of Science, Tokyo Woman's Christian University)
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.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Tetra-neutron system studied by RI-beam experiments
January 17 (Tue) at 13:30 - 15:00, 2023
Prof. Susumu Shimoura (Research Scientist, Spin isospin Laboratory, RIKEN Cluster for Pioneering Research (CPR))
Multi-neutron systems have attracted a long-standing attention in nuclear physics. In several decades, experimental attempts have been made with a particular focus on the tetra-neutron system. Among them, the two different experiments, the double-charge exchange reaction on 4He and the alpha-particle knockout reaction from the 8He, show a sharp peak just above the threshold in the four-neutron spectra, which could be a signature of a "resonant state", separate from a broad bump structure at higher excitation energy regions. Both the experiments have been realized by using the 8He beam above 150 A MeV at the RIKEN RI Beam Factory. Details of the two experiments including basic idea, experimental techniques, and analysis are presented as well as a historical review of previous experimental attempts. Emphasis is made for the experimental conditions for populating a kinematically isolated tetra-neutron system with very small momentum transfer. The spectral shape is discussed by means of reaction processes and correlations in the final tetra-neutron system with several recent theoretical studies.
Venue: Common Room #246-248 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Chiral effects on lepton transport in core-collapse supernovae
December 13 (Tue) at 13:30 - 15:00, 2022
Dr. Di-Lun Yang (Assistant Research Fellow, Institute of Physics, Academia Sinica, Taiwan)
Dynamics of leptons such as electrons and neutrinos play an important role in the evolution of core-collapse supernovae (CCSN). Nevertheless, chirality as one of fundamental microscopic properties that could affect lepton transport, through e.g. weak interaction, has been mostly overlooked. In this talk, I will discuss how chiral effects such as the renowned chiral magnetic effect (CME), generating an electric charge current along magnetic fields with chirality imbalance, could result in the unstable modes of magnetic fields and inverse cascade, which potentially influence the matter evolution in CCSN and pulsar kicks. I will also show how an effective CME could be realized via the backreaction from neutrino radiation even in the absence of an axial charge characterizing an unequal number of right- and left-handed electrons.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Cosmic Birefringence: how our universe violates left-right symmetry
December 6 (Tue) at 13:30 - 15:00, 2022
Dr. Tomohiro Fujita (Assistant Professor, Waseda Institute for Advanced Study, Waseda University)
Our universe is lefty: recent observations imply that the polarization plane of light that has traveled through cosmic space for 13.8 billion years rotates about 0.3 degrees to the left. A similar phenomenon is known to occur in materials such as crystals, and is called birefringence. But why does birefringence occur even in the outer space, which is supposed to be a vacuum? Dark energy, the unknown energy that fills the universe, may be responsible for it. In this seminar, I will review observations and theories of cosmic birefringence and discuss future prospects.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Tetra and pentaquarks with multi-flavor contents
November 22 (Tue) at 13:30 - 15:00, 2022
Prof. Atsushi Hosaka (Professor, Research Center for Nuclear Physics, Osaka University)
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.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Merger and post-merger of binary neutron stars with a quark-hadron crossover equation of state
November 15 (Tue) at 15:00 - 17:01, 2022
Dr. Yongjia Huang (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.
Venue: via Zoom
Event Official Language: English
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Seminar
Expanding Edges of Quantum Hall Systems in a Cosmology Language - Hawking Radiation from de Sitter Horizon in Edge Modes
November 11 (Fri) at 16:00 - 17:30, 2022
Dr. Masahiro Hotta (Assistant Professor, Department of Physics, Graduate School of Science, Tohoku University)
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.
Venue: #345-347, 3F, Main Research Building (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Tensor renormalization group approach to quantum fields on a lattice
November 8 (Tue) at 13:30 - 15:00, 2022
Dr. Shinichiro Akiyama (Assistant Professor, Quantum Software Project, The University of Tokyo)
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.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Ultra-Light Axion Dark Matter: Bose-Einstein condensates and superfluids in the sky
October 11 (Tue) at 13:30 - 15:00, 2022
Prof. Elisa G.M. Ferreira (Project Assistant Professor, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)
The nature of dark matter (DM) is one of the biggest mysteries in cosmology. There are many different models to explain the nature of this elusive component. In this talk I will present a class of dark matter models: ultra-light dark matter (ULDM) or ultra-light axions (ULA). I will show the different models present in the literature and classify them according to the way they behave on small scales. One of the most interesting features of this class of DM models is that it might condense in the interior of the halos of galaxies forming a Bose-Einstein condensate (BEC) or superfluid. This interesting quantum phenomena on macroscopic scales, and the wave nature of ULDM leads to different and interesting astrophysical consequences that can be probed on small scales. I will quickly review first the fuzzy dark matter model, one of the most well studied ULA models, where I will present its description, predictions and current bounds. Then I will introduce the DM superfluid model, where, upon condensation in the interior of galaxies, DM dynamics represents that of MOdified Newtonian Dynamics (MOND) on galactic scales. This behaviour can address some of the curiosities of the behaviour of DM on small scales. I plan to show the theoretical description of this model and its interesting phenomenology.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Test of the Cosmological principle by observing the primordial gravitational waves
July 27 (Wed) at 13:30 - 15:00, 2022
Dr. Yuko Urakawa (Associate Professor, High Energy Accelerator Research Organization (KEK))
In this talk, using the generalized deltaN formalism, which dramatically facilitates a computation of the primordial density perturbation and the primordial GWs (PGWs), we address a violation of the Cosmological principle, namely a violation of the global isotropy in the Universe. It’s turned out that measuring the PGWs provides a powerful tool to explore a violation of the global isotropy. If time permits, I will also discuss some prospects on LiteBIRD.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Gradient flow exact renormalization group 2
July 6 (Wed) at 13:30 - 17:00, 2022
Prof. Hiroshi Suzuki (Professor, Graduate School of Science, Kyushu University)
Wilson’s exact renormalization group (ERG), which tells how a system changes under the scale transformation, provides a fundamental framework to define quantum field theory even beyond the perturbation theory. It has however been known that it is difficult to preserve a manifest gauge symmetry in ERG because of the usage of the momentum cutoff in ERG. Here, we propose a possible modification of ERG, the gradient flow exact renormalization (GFERG), which preserves a manifest gauge symmetry being based on a gauge-covariant diffusion equation. I explain the basic idea and properties of GFERG. If time permits, I want to present a possible application of GFERG to the consideration of the axial anomaly.
Venue: via Zoom
Event Official Language: English
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Seminar
Gradient flow exact renormalization group 1
July 5 (Tue) at 13:30 - 17:00, 2022
Prof. Hiroshi Suzuki (Professor, Graduate School of Science, Kyushu University)
Wilson’s exact renormalization group (ERG), which tells how a system changes under the scale transformation, provides a fundamental framework to define quantum field theory even beyond the perturbation theory. It has however been known that it is difficult to preserve a manifest gauge symmetry in ERG because of the usage of the momentum cutoff in ERG. Here, we propose a possible modification of ERG, the gradient flow exact renormalization (GFERG), which preserves a manifest gauge symmetry being based on a gauge-covariant diffusion equation. I explain the basic idea and properties of GFERG. If time permits, I want to present a possible application of GFERG to the consideration of the axial anomaly.
Venue: via Zoom
Event Official Language: English
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Seminar
Dress code for infrared safe S-matrix in QED
June 22 (Wed) at 13:30 - 15:00, 2022
Dr. Sotaro Sugishita (Designated Assistant Professor, Institute for Advanced Research (IAR), Nagoya University)
We consider the infrared (IR) aspects of the gauge invariant S-matrix in QED. I will review the problem of IR divergences in QED, and introduce the dressed state formalism to obtain IR-safe S-matrix elements. I will show a condition for dressed states to obtain IR-safe S-matrix elements, and explain that this condition can be interpreted as the memory effect and is related to asymptotic symmetry. I also explain that IR divergences are necessary to prohibit the violation of asymptotic symmetry. We also argue that the difference between dressed and undressed states can be observed, even if we are able to observe an inclusive cross-section summing over soft photons.
Venue: Hybrid Format (Common Room 246-248 and 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
Dr. 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
S-matrix Unitarity toward UV Completion
September 13 (Mon) at 13:30 - 15:00, 2021
Prof. Keisuke Izumi (Assistant Professor, Kobayashi-Maskawa Institute for the Origin of Particles and the Universe (KMI), Nagoya University)
Einstein gravity is not renormalizable and does not hold perturbative unitarity at high energy. This is the main reason why the construction of quantum gravity is difficult. A conjecture was proposed by Llewellyn Smith, "renormalizablility and tree-unitarity at high energy give the same conditions for theories". This conjecture would be important because it shows that, if a theory is constructed s.t. unitarity is satisfied, renormalizablility holds automatically, and vice versa. Unfortunately, a counterexample was pointed out. If a theory involves higher derivatives, there exists a theory which is renormalizable but does not satisfy tree-unitarity. A candidate of quantum gravity, the quadratic gravity (R_{\mu\nu}^2 gravity), is one of the examples. Therefore, Llewellyn Smith's conjecture would not be useful for the discussion of quantum gravity. Then, we introduce a new conjecture, "renormalizablility and S-matrix unitarity (or often called pseudo-unitarity) at high energy give the same conditions for theories". In this talk, Llewellyn Smith's conjecture and our contribution to it will be explained. Then, our new conjecture will be introduced. Finally, it will be shown that our conjecture works well even in theories with higher derivatives.
Venue: via Zoom
Event Official Language: English
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Seminar
Application of AdS/CFT to non-equilibrium phenomena in external electric fields
August 16 (Mon) at 13:00 - 15:00, 2021
Dr. Shunichiro Kinoshita (Collaborative Researcher, Faculty of Science and Engineering, Chuo University)
The AdS/CFT correspondence is a useful tool for studying strongly-coupled gauge theories. According to this correspondence, the D3/D7 brane system in string theory is one of the simplest toy model dual to supersymmetric QCD-like gauge theory. In the dual field theory, the mesons, i.e., the quark-antiquark bound states are stable in vacuum when the quark is massive, while the dielectric breakdown occurs by pair production of quark-antiquark under strong electric fields. In this talk, I will review a series of our works of D3/D7 systems and show time-dependent, non-equilibrium phenomena driven by external electric fields such as suddenly increasing or rotating electric fields.
Venue: via Zoom
Event Official Language: English
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Seminar
Non-Unitary TQFTs from 3d N=4 Rank-0 SCFTs
July 5 (Mon) at 13:00 - 14:30, 2021
Dr. Myungbo Shim (Kyung Hee University, Republic of Korea)
We propose a novel procedure of assigning a pair of non-unitary topological quantum field theories (TQFTs), TFT_\pm[T_0], to a (2+1)D interacting N=4 superconformal field theory (SCFT) T_0 of rank 0, i.e. having no Coulomb and Higgs branches. The topological theories arise from particular degenerate limits of the SCFT. Modular data of the non-unitary TQFTs are extracted from the supersymmetric partition functions in the degenerate limits. As a non-trivial dictionary, we propose that F = max{ -log |S^{(+)}_{0\alpha}| } = max{ -log |S^{(-)}_{0\alpha}| }, where F is the round three-sphere free energy of T_0 and S^{(\pm)}_{0\alpha} is the first column in the modular S-matrix of TFT_\pm. From the dictionary, we derive the lower bound on F, F > -log(\sqrt{(5-\sqrt{5})/10}) \simeq 0.642965, which holds for any rank 0 SCFT. The bound is saturated by the minimal N=4 SCFT proposed by Gang-Yamazaki, whose associated topological theories are both the Lee-Yang TQFT. We explicitly work out the (rank 0 SCFT)/(non-unitary TQFTs) correspondence for infinitely many examples. Before going to the technical part, we provide some background materials including some peculiar features in 3d gauge theories, some supersymmetries, anyons, and some modular data of MTC in this talk.
Venue: via Zoom
Event Official Language: English
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Seminar
Toward QCD-based description of dense baryonic matter
June 29 (Tue) at 13:00 - 14:30, 2021
Dr. Yuki Fujimoto (Postdoctoral Scholars, Department of Physics, University of Washington, USA)
The equation of state (EoS) of dense baryonic/quark matter is the crucial ingredient for understanding neutron stars. I briefly review the current state of the high-density matter EoS based on the QCD perspectives. In this talk, I particularly focus on the perturbative QCD (pQCD) EoS, which was previously thought to be useless at realistic density because it is plagued by the large uncertainty. I introduce our recent analysis of the EoS calculated within the pQCD framework with the resummation [Fujimoto & Fukushima, 2011.10891]. I discuss our scheme for the Hard Dense Loop resummation, which turns out to reduce the uncertainty compared with the conventional pQCD estimate without resummation. Our result apparently extends the applicability of the QCD-based EoS down to densities realized inside neutron stars and infers a smooth matching with the baryonic EoS.
Venue: via Zoom
Event Official Language: English
41 events