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Upcoming Seminar

  • 2023.01.17, 13:30–15:00 (JST) @ Hybrid Format (Common Room 246-248 and Zoom)
    Susumu Shimoura (RIKEN/CNS, U. Tokyo/RCNP, Osaka U.):
    "Tetra-neutron system studied by RI-beam experiments"

    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.

iTHEMS-phys Seminar Archive

  • 2022.12.13, 13:30–15:00 (JST) @ Hybrid Format (Common Room 246-248 and Zoom)
    Di-Lun Yang (Academia Sinica):
    "Chiral effects on lepton transport in core-collapse supernovae"

    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.
  • 2022.12.06, 13:30–15:00 (JST) @ Hybrid Format (Common Room 246-248 and Zoom)
    Tomohiro Fujita (Waseda Institute for Advanced Study):
    "Cosmic Birefringence: how our universe violates left-right symmetry"

    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.
  • 2022.11.22, 13:30–15:00 (JST) @ Hybrid Format (Common Room 246-248 and Zoom)
    Atsushi Hosaka (RCNP, Osaka University):
    "Tetra and pentaquarks with multi-flavor contents"

    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.
  • 2022.11.15, 15:00–16:30 (JST) @ Hybrid Format (Common Room 246-248 and Zoom)
    Yongjia Huang (RIKEN iTHEMS/U. Sci. Tech. China):
    "Merger and post-merger of binary neutron stars with a quark-hadron crossover equation of state"

    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.
  • 2022.11.10, 10:30–17:00 (JST) @ Hybrid Format (Room 345-347 and Zoom)
    2022.11.11, 10:30–15:00 (JST) @ Hybrid Format (Room 345-347 and Zoom)
    2022.11.12, 10:30–12:00 (JST) @ Hybrid Format (Room 345-347 and Zoom)
    Masahiro Hotta (Tohoku University):
    "An Introduction to Quantum Measurement Theory for Physicists"

    In this lecture, basic concepts in quantum measurement theory are introduced, including measurement operators and POVM's. The related topics are also picked up.
  • 2022.11.08, 13:30–15:00 (JST) @ Hybrid Format (Common Room 246-248 and Zoom)
    Shinichiro Akiyama (The University of Tokyo):
    "Tensor renormalization group approach to quantum fields on a lattice"

    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.
  • 2022.10.18, 13:30–15:00 (JST) @ Hybrid Format (Common Room 246-248 and Zoom)
    Yukimi Goto (Kyushu University/RIKEN iTHEMS):
    "格子 Nambu-Jona-Lasino 模型における質量生成とカイラル対称性 (In Japanese)"

    量子色力学において, 相互作用が無ければクォークは質量を持たず, 保存量カイラリティーを持つ. 現実にはクォークは質量を持ち, カイラル対称性も破れている. これは相互作用によって真空の対称性が自発的に破れた結果であると考えられている. 本講演では, Kogut-Susskind 型の格子フェルミオンのハミルトニアンを考える. 相互作用は4つのクォークの有効相互作用とし, その強結合領域を扱う. 十分低い温度で空間次元が3以上, または基底状態で空間次元2以上のとき, この模型において無限体積極限をとることにより, 質量項の期待値が非自明な値をとることを証明する. これは連続極限がうまく取れれば, カイラル対称性が自発的に破れることを意味する. 証明においてはフェルミオンの鏡映正値性が本質的になる. 本講演は高麗徹氏との共同研究に基づく.
    Yukimi Goto and Tohru Koma. arXiv:2209.06031.
  • 2022.10.11, 13:30–15:00 (JST) @ Hybrid Format (Common Room 246-248 and Zoom)
    Elisa Ferreira (Kavli IPMU):
    "Ultra-Light Axion Dark Matter: Bose-Einstein condensates and superfluids in the sky"

    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.
  • 2022.07.27, 13:30–15:00 (JST) @ Hybrid Format (Common Room 246-248 and Zoom)
    Yuko Urakawa (KEK):
    "Test of the Cosmological principle by observing the primordial gravitational waves"

    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.
  • 2022.07.05, 13:30–17:00 (JST) @ Zoom
    2022.07.06, 13:30–17:00 (JST) @ Zoom
    Hiroshi Suzuki (Kyushu University):
    "Gradient flow exact renormalization group"

    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.
  • 2022.06.22, 13:30–15:00 (JST) @ Hybrid Format (Common Room 246-248 and Zoom)
    Sotaro Sugishita (Nagoya University):
    "Dress code for infrared safe S-matrix in QED"

    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.
  • 2021.11.02, 14:30–16:00 (JST) @ Zoom
    Katsuki Aoki (YITP, Kyoto University):
    "Is the Standard Model in the Swampland? Consistency Requirements from Gravitational Scattering"

    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.
  • 2021.09.13, 13:30–15:00 (JST) @ Zoom
    Keisuke Izumi (Nagoya University):
    "S-matrix Unitarity toward UV completion"

    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.
  • 2021.08.16, 13:00–15:00 (JST) @ Zoom
    Shunichiro Kinoshita (Chuo University):
    "Application of AdS/CFT to non-equilibrium phenomena in external electric fields"

    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.
  • 2021.07.05, 13:00–14:30 (JST) @ Zoom
    Myungbo Shim (Kyung Hee University):
    "Non-Unitary TQFTs from 3d N=4 Rank-0 SCFTs"

    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.
    D. Gang, S. Kim, K. Lee, M. Shim and M. Yamazaki, arXiv:2103.09283.
  • 2021.06.29, 13:00–14:30 (JST) @ Zoom
    Yuki Fujimoto (Univ. of Tokyo):
    "Toward QCD-based description of dense baryonic matter"

    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.
  • 2021.06.21, 13:00–14:30 (JST) @ Zoom
    Kanato Goto (RIKEN iTHEMS):
    "Black Hole Information Paradox and Wormholes"

    In this talk, I will explain about the recent progress in the black hole information paradox that I am involved with. The information paradox arises when a black hole evaporates by emitting Hawking radiation due to the quantum effects. Time dependence of the entropy of Hawking radiation is diagnosis of information loss caused by the black hole evaporation. If information is not lost, the entropy of Hawking radiation should obey the so-called Page curve. In recent research developments, it was found that “the quantum extremal islands” reproduce the unitary Page curve in an evaporating black hole. I will argue about how the quantum extremal islands are derived from the computation of the entropy of Hawking radiation using the gravitational path-integral.
    K. Goto, T. Hartman, A. Tajdini, JHEP 04 (2021) 289.
  • iTHEMS math-phys joint seminar
    2021.04.13, 16:00–18:10 (JST) @ Zoom
    Taksu Cheon (Kochi University of Technology):
    "Self-adjoint extension in quantum mechanics and non-Rydberg spectra of one-dimensional hydrogen atom"

    We offer a beginner’s guide to the functional-analytical techniques in quantum mechanics, and cover its application to the 1D Coulomb problem. It is shown that the wave function at the diverging point of the Coulomb potential is mathematically described by three-parameter family of generalized connection conditions. A scheme is devised to physically implement the generalized conditions, which provides the way to experimentally realize non-Rydberg spectra in 1D Hydrogen atom.
    - Part 1, Self-adjoint extension of Hilbert space operator
    - Part 2, 1D Coulomb problem
    Axel Pérez-Obio and Taksu Cheon, Physica E: Low-dim. Nano. 119 (2020) 114013.
  • Joint iTHEMS - ABBL seminar
    2021.02.22, 10:00–11:30 (JST) @ Zoom
    Gordon Baym (UIUC/iTHEMS):
    "The Evolution of Primordial Neutrino Helicities under Gravitational and Magnetic Fields and Implications for their Detection"

    Primordial neutrinos decoupled in the early universe in helicity eigenstates. As I will discuss, two effects -- dependent on neutrinos having a non-zero mass -- can modify their helicities as they propagate through the cosmos. First, finite mass neutrinos have a magnetic moment and thus their spins, but not their momenta, precess in cosmic and galactic magnetic fields. The second is the propagation of neutrinos past cosmic matter density fluctuations, which bend their momenta, and bend their spins by a smaller amount. (The latter is a general relativistic effect.) Both effects turn a fraction of left-handed neutrinos into right-handed neutrinos, and right-handed antineutrinos into left-handed. If neutrino magnetic moments approach that suggested by the XENON1T experiment as a possible explanation of their excess of low energy electron events -- a value well beyond the moment predicted by the standard model -- helicities of relic Dirac (but not Majorana) neutrinos could be considerably randomized. I finally will discuss the implications of neutrino helicity rotation, as well as their Dirac vs. Majorana nature, on their detection rates via the Inverse Tritium Beta Decay reaction.
    G. Baym, J-C. Peng, arXiv:2012.12421.
  • 2021.02.16, 13:00–14:30 (JST) @ Zoom
    Masaaki Tokieda (Tohoku University):
    "Quantum mechanical description of energy dissipation and application to heavy-ion fusion reactions"

    For theoretical description of heavy-ion fusion reactions, two different models have been used depending on the incident energy. At energies above the Coulomb barrier, importance of energy dissipation and fluctuation has been deduced from scattering experiments. To describe them phenomenologically, the classical Langevin equation has successfully been applied. At energies below the Coulomb barrier, on the other hand, the quantum coupled-channels method with a few number of internal states has been applied, and it has succeeded in explaining sub-barrier fusion reactions. While each method succeeds in each energy range, a unified description of heavy-ion fusion reactions from sub-barrier energies to above barrier energies is still missing. To achieve this, we need to treat dissipation and fluctuation quantum mechanically. In order to describe dissipation and fluctuation quantum mechanically, we have applied ideas of open quantum systems to heavy-ion fusion reactions. I will talk about recent development in this talk. First I will introduce a model Hamiltonian to treat dissipation and fluctuation quantum mechanically, and explain its character and a strategy for numerical studies. I will then apply the model to a fusion problem, and discuss a role of energy dissipation during quantum tunneling. Finally I will discuss a possible future direction for a unified description of heavy-ion fusion reactions.
  • 2021.02.04, 13:00–14:30 (JST) @ Zoom
    Di-Lun Yang (Keio University):
    "Quantum kinetic theory for chiral and spin transport in relativistic heavy ion collisions and core-collapse supernovae"

    Recently, the anomalous transport phenomena of relativistic fermions associated with chirality and spin induced by external fields have been greatly explored in different areas of physics. Notably, such phenomena are in connection to various quantum effects such as quantum anomalies and spin-orbit interaction. The quark gluon plasmas produced from relativistic heavy ion collisions (HIC) and the core-collapse supernovae (CCSN) are both the systems in extreme conditions with high temperature or density and the presence of strong magnetic and vortical fields. Meanwhile, the abundance of light quarks and neutrinos as relativistic fermions created therein accordingly makes these two systems ideal test grounds for studying such exotic transport phenomena. Inversely, the anomalous transport may also give rise to unexpected impacts on the evolution of both systems. However, to analyze such dynamical quantum effects, a novel quantum transport theory delineating the evolution of chirality imbalance and spin has to be introduced. In this talk, I will discuss recent developments and applications of the quantum kinetic theory for chiral and spin transport in the context of HIC and CCSN.
  • 2021.01.28, 13:30–15:00 (JST) @ Zoom
    Nobutoshi Yasutake (Chiba Institute of Technology/JAEA):
    "Many body problems from quarks to stellar evolutions"

    The many-body problems are major problems that need to be clarified not only in nuclear physics, but also in astronomy. In this seminar, I introduce stelar evolutions as gravitational many-body problems, and also hadronic matter as quantum many-body problems, based on the Lagrangian schemes. The macroscopic stars and the microscopic hadronic matter look completely different issues. But in this seminar, I introduce the similarities between the two problems. For hadronic matter, we adopt the color molecular dynamics to understand the behaviors and properties of hadronic matter in the framework of QCD. Although molecular dynamics can not be the first-principle, they are sometimes useful to understand many-body quantum properties. In this talk, we introduce the current status of our color molecular dynamics.
  • 2020.12.17, 13:00–14:30 (JST) @ Zoom
    Takeru Yokota (ISSP, The University of Tokyo):
    "Classical liquids and functional renormalization group"

    Development of methods for classical statistical mechanics is desired for accurate predictions of the structures and thermodynamic properties of liquids. A powerful framework to describe classical liquids is density functional theory (DFT). In the quantum case, there have been recent attempts to develop accurate methods with combining DFT and the functional renormalization group (FRG), which is another framework to deal with many-body systems utilizing evolution equations, and such approaches are expected to work also in the classical case. In this presentation, I will talk about a new approach for classical liquids aided by FRG. The formalism and some ideas to incorporate higher-order correlation functions to systematically improve the accuracy will be shown. I will also present a numerical demonstration in a one-dimensional exactly solvable system and discuss the results by comparing to other conventional methods such as the hypernetted chain.
  • iTHEMS math-phys joint seminar
    2020.12.14, 16:00–18:00 (JST) @ Zoom
    Naotaka Kubo (YITP, Kyoto University):
    "Non-perturbative tests of duality cascades in three-dimensional supersymmetric gauge theories"

    M2-brane is an interesting object in M-theory and string theory. A three-dimensional N=6 superconformal Chern-Simons theory with gauge group U(N_1 )×U(N_2 ), called ABJ theory, describes the low energy behavior of M2-brane. On the one hand, it has been considered that when |N_1-N_2 | is larger than the absolute value of Chern-Simons level, the supersymmetry is broken. On the other hand, it was predicted that an interesting phenomenon called duality cascade occurs, and supersymmetry is not broken in some cases. Motivated by this situation, we performed non-perturbative tests by focusing on the partition function on S^3. The result strongly suggests that the duality cascade indeed occurs. We also proposed that the duality cascade occurs in theories with more general gauge groups, and we performed non-perturbative tests in the same way. I will review and explain our physical prediction in the first half of my talk. In the second half of my talk, I will explain the non-perturbative tests. This part is mathematical because the partition function reduces to a matrix model by using the supersymmetric localization technique.
    M. Honda, N. Kubo, arXiv:2010.15656.
  • 2020.12.10, 13:00–14:30 (JST) @ Zoom
    Sinya Aoki (YITP, Kyoto University):
    "Conserved charges in gravity and entropy"

    We propose a manifestly covariant definition of a conserved charge in gravity. We first define a charge density from the energy momentum tensor with a Killing vector, if exists in the system, and calculate the energy (and angular momentum) of the black hole by a volume integral. Our definition of energy leads to a correction of the known mass formula of a compact star, which includes the gravitational interaction energy and is shown to be 68\% of the leading term in some case. Secondly we propose a new method to define a conserved charge in the absence of Killing vectors, and argue that the conserved charge can be regarded as entropy, by showing the 1st law of thermodynamic for a special case. We apply this new definition to the expanding universe, gravitational plane waves and the black hole. We discuss future directions of our research.
    [1] S. Aoki, T. Onogi, S. Yokoyama, arXiv:2005.13233.
    [2] S. Aoki, T. Onogi, S. Yokoyama, arXiv:2010.07660.
  • 2020.12.03, 13:00–14:30 (JST) @ Zoom
    Yusuke Nishida (Tokyo Institute of Technology):
    "KPZ equation, attractive bosons, and the Efimov effect"

    The Kardar-Parisi-Zhang (KPZ) equation for surface growth has been a paradigmatic model in nonequilibrium statistical physics. In particular, it in dimensions higher than two undergoes a roughening transition from smooth to rough phases with increasing the nonlinearity. It is also known that the KPZ equation can be mapped onto quantum mechanics of attractive bosons with a contact interaction, where the roughening transition corresponds to a binding transition of two bosons with increasing the attraction. Such critical bosons in three dimensions actually exhibit the Efimov effect, where a three-boson coupling turns out to be relevant under the renormalization group so as to break the scale invariance down to discrete one. On the basis of these facts linking the two distinct subjects in physics, we predict that the KPZ roughening transition in three dimensions shows either the discrete scale invariance or no intrinsic scale invariance.
    Y. Nakayama, Y. Nishida, arXiv:2010.15161.
  • 2020.11.12, 10:30–12:00 (JST) @ Zoom
    Yuya Tanizaki (YITP, Kyoto University):
    "Some idea on quantum tunneling via Lefschetz thimbles"

    In this talk, I will explain my previous study with Takayuki Koike on a possible approach to quantum tunneling via Lefschetz thimbles. We classified all the complex saddle points for the real-time path integral for the symmetric double-well quantum mechanics. We looked at various properties of those complex solutions, which motivated us to conclude that the computation of tunneling amplitudes for the symmetric double well requires the interference of infinitely many Lefschetz thimbles. I would also like to talk about some speculations, admittingly being very optimistic.
    Y. Tanizaki, T. Koike, Annals Phys. 351 (2014) 250.
  • iTHEMS math-phys joint seminar
    2020.11.10, 16:00–18:10 (JST) @ Zoom
    Naoto Shiraishi (Gakushuin university):
    "Mathematics of thermalization in isolated quantum systems"

    If an isolated macroscopic quantum system is left at a nonequilibrium state, then this system will relax to the unique equilibrium state, which is called thermalization. Most of quantum many-body systems thermalize, while some many-body systems including integrable systems do not thermalize. What determines the presence/absence of thermalization and how to understand thermalization from microscopic quantum mechanics are profound long-standing problems. In the first part of my talk, I briefly review some established results of quantum thermalization. I first clarify the problem of thermalization in a mathematical manner, and then introduce several important results and insights: typicality of equilibrium states [1], relaxation caused by large effective dimension [2], and eigenstate thermalization hypothesis (ETH) [3,4] and weak-ETH [5]. In the second part of my talk, I explain some of my results. First, I introduce a model which is non-integrable and thermalizes but does not satisfy the ETH [6,7]. This finding disproves the conjectures that all nonintegrable systems satisfy the ETH and that the ETH is a necessary condition for thermalization. I also discuss the hardness of the problem of thermalization from the viewpoint of computational science [8]. Then, I move to an analytical approach to a concrete model, and prove that S=1/2 XYZ chain with a magnetic field is nonintegrable [9]. This is the first example of proof of nonintegrability in a concrete quantum many-body system, which will help a mathematical approach to thermalization.
    [1] S. Popescu, A. Short, A. Winter, Nat. Phys. 2, 754 (2006)
    [2] P. Reimann, Phys. Rev. Lett, 101, 190403 (2008)
    [3] M. Srednicki, Phys. Rev. E 50, 888 (1994)
    [4] M. Rigol, V. Dunjko & M. Olshanii, Nature 452, 854 (2008)
    [5] T. Mori, arXiv:1609.09776 (2016)
    [6] N. Shiraishi and T. Mori, Phys. Rev. Lett. 119, 030601 (2017)
    [7] T. Mori and N. Shiraishi, Phys. Rev. E 96, 022153 (2017)
    [8] N. Shiraishi and K. Matsumoto, in preparation
    [9] N. Shiraishi, Europhys. Lett. 128, 17002 (2019)
  • 2020.10.23, 17:00–18:00 (JST) @ Zoom
    Masanori Hanada (University of Surrey):
    "Toward simulating Superstring/M-theory on a Quantum Computer"

    We present a framework for simulating superstring/M-theory on a quantum computer, based on holographic duality. Because holographicduality maps superstring/M-theory to quantum field theories (QFTs), we can study superstring/M-theory if we can put such QFTs on a quantum computer --- but it still looks like a complicated problem, if we use a usual lattice regularization. Here we propose an alternative approach, which turns out to be rather simple: we map the QFT problems to matrix models, especially the supersymmetric matrix models such as the Berenstein-Maldacena-Nastase (BMN) matrix model. Supersymmetric matrix models have natural applications to superstring/M-theory and gravitational physics, in an appropriate limit of parameters. Furthermore, for certain states in the Berenstein-Maldacena-Nastase (BMN) matrix model, several supersymmetric quantum field theories dual to superstring/M-theory can be realized on a quantum device. It is straightforward to put the matrix models on a quantum computer, because they are just quantum mechanics of matrices, and the construction of QFTs is mapped to the preparation of certain states. We show the procedures are conceptually rather simple and efficient quantum algorithms can be applied. In addition, as a (kind of) byproduct, we provide a new formulation of pure Yang-Mills on quantum computer.
  • 2020.10.22, 13:30–15:00 (JST) @ Zoom
    Tokuro Fukui (YITP, Kyoto University):
    "Realistic shell model and chiral three-body force"

    We show an evolution to derive the effective Hamiltonian in the shell-model framework starting from two- and three-body interactions based on the chiral effective field theory. A new way to calculate three-body matrix elements of the chiral interaction with the nonlocal regulator is proposed. We apply our framework to the p-shell nuclei and perform benchmark calculations to compare our results with those by an ab initio no-core shell-model. We report that our results are satisfactory and the contribution of the three-body force is essential to explain experimental low-lying spectra of the p-shell nuclei. We discuss the contribution of the three-body force on the effective single-particle energy extracted from the monopole interaction. Next, we investigate the shell evolution on the pf-shell nuclei. We show that the monopole component of the shell-model effective Hamiltonian induced by the three-body force plays an essential role to account for the experimental shell evolution. Moreover, we present our latest results on the investigation of the possible neutron dripline of the Ca isotopes. Finally, we discuss very neutron-rich systems, namely, the oxygen isotopes at the dripline and beyond, where the interplay between the three-body force and continuum states plays an important role.
  • iTHEMS math-phys joint seminar
    2020.10.02, 16:00–18:00 (JST) @ Zoom
    Toshihiro Ota (Osaka University/RIKEN iTHEMS):
    "TQFT, integrable lattice model, and quiver gauge theories"

    - 1st part (math): In physics literature, “lattice models” appear quite often as mathematical models of physical systems, e.g. Ising model, vertex models, lattice gauge theory. The aim of the 1st part is to introduce ‘what is (T)QFT,’ ‘what is lattice model,’ and ‘what does integrability mean’ in the language of mathematics. In turn, they will play a crucial role in the 2nd part of my talk. I also hope that this will lead to a good exchange among us, especially between physicists and mathematicians.
    - 2nd part (physics): In the 2nd part, I would like to explain where an integrable lattice model may come from, especially for people in the physics background. I will show a certain class of integrable lattice models is realized by Wilson-’t Hooft lines in 4d quiver gauge theories. I will also explain a bit how these gauge theories are constructed from brane configurations in string theory. String dualities allow us to relate the original 4d setups to 4d partially topological Chern-Simons theory, which is a partial TQFT and generates integrable lattice models.
  • iTHEMS math-phys joint seminar
    2020.08.31, 13:00–15:00 (JST) @ Zoom, Makiko Sasada (The Univesity of Tokyo)
    2020.09.01, 13:00–15:00 (JST) @ Zoom, Kenichi Bannai (Keio University)
    "Geometric Perspective for the Theory of Hydrodynamic Limits"

    One of the fundamental problems in the natural and social sciences is to explain macroscopic phenomena that we can observe from the rules governing the microscopic system giving rise to the phenomena. Hydrodynamic limit provides a rigorous mathematical method to derive the deterministic partial differential equations describing the time evolution of macroscopic parameters, from the stochastic dynamics of a microscopic large scale interacting system.
    In the article "Topological Structures of Large Scale Interacting Systems via Uniform Locality" joint with Yukio Kametani, we introduce a general framework encompassing a wide variety of interacting systems in order to systematically investigate various microscopic stochastic large scale interacting systems in a unified fashion. In particular, we introduced a new cohomology theory called the uniformly local cohomology to investigate the underlying geometry of the interacting system. Our theory gives a new interpretation of the macroscopic parameters, the role played by the group action on the microscopic system, and the origin of the diffusion matrix associated to the macroscopic deterministic partial differential equation obtained via the space-time scaling limit of the microscopic system.
    The purpose of the series of lectures is to introduce to the audience the theory of hydrodynamic limits, especially the relation between the macroscopic observables and the microscopic interacting system. We then explain our new perspective of how geometry comes into play in investigating the interacting system, and introduce the ideas and results of our article.
  • 2020.08.21, 13:00–14:30 (JST) @ Zoom
    Takuya Shimazaki (The University of Tokyo):
    "Lefschetz-thimble inspired analysis of the Dykhne–Davis–Pechukas method and an application for the Schwinger Mechanism"

    Dykhne–Davis–Pechukas (DDP) method is a common approximation scheme for the transition probability in two-level quantum systems, as realized in the Landau–Zener effect, leading to an exponentially damping form comparable to the Schwinger pair production rate. We analyze the foundation of the DDP method using a modern complex technique inspired by the Lefschetz-thimble method. We derive an alternative and more adaptive formula that is useful even when the DDP method is inapplicable. As a benchmark, we study the modified Landau–Zener model and compare results from the DDP and our methods. We then revisit a derivation of the Schwinger Mechanism of particle production under electric fields using the DDP and our methods. We find that the DDP method gets worse for the Sauter type of short-lived electric pulse, while our method is still a reasonable approximation. We also study the Dynamically Assisted Schwinger Mechanism in two methods.
    K. Fukushima, T. Shimazaki, Annals Phys. 415 (2020) 168111.
  • 2020.08.07, 13:00–14:30 (JST) @ Zoom
    Hiroyuki Tajima (Kochi University):
    "Nambu-Goldstone fermion in a Bose-Fermi mixture with an explicitly broken supersymmetry"

    Supersymmetry, which is a symmetry associated with interchange between bosons and fermions, is one of the most important symmetries in high-energy physics but its evidence has never been observed yet. Apart from whether supersymmetric partners such as squark exist or not in our world, it is an interesting problem to explore the consequences of the supersymmetry in an ultracold atomic gas. In this study, we address the Nambu-Goldstone mode called Goldstino associated with the spontaneous supersymmetry breaking in a Bose-Fermi mixture. While the explicit supersymmetry breaking is unavoidable even in cold atomic systems, the energy gap in Goldstino spectra can be measured in such atomic systems. By comparing the energy gaps obtained from the Gell-Mann-Oakes-Renner relation and the random phase approximation, we elucidate how the Goldstino acquires the energy gap due to the explicit breakings. We also show effects of Goldstino pole on the fermionic single-particle spectral functions, which can be measured in the recent experiments.
    H. Tajima, Y. Hidaka, D. Satow, arXiv:2001.08507.
  • 2020.07.10, 13:30–14:30 (JST) @ Zoom
    Shoichiro Tsutsui (RIKEN QHP):
    "Complex Langevin study of an attractively interacting two-component Fermi gas in 1D with population imbalance"

    We investigate an attractively interacting two-component Fermi gas in 1D described by the Gaudin-Yang model with population imbalance. While the Gaudin-Yang model is known as a solvable model based on the thermodynamic Bethe ansatz, the binding energy and mass of poralon at finite temperature and moderate impurity density are still unknown. Moreover, in such a system, quantum Monte Carlo simulation suffers from the sign problem because the population imbalance makes the fermion determinant non-positive definite. In this study, we apply complex Langevin method, a holomorphic extension of the stochastic quantization to overcome the sign problem. We first confirm our numerical results satisfy a criteria for correct convergence [1], and present how the polaron energy depends on temperature and density of impurity. We also compare our results with a recent study based on a diagrammatic approach [2].
    [1] K. Nagata, J. Nishimura, S. Shimasaki, Phys. Rev. D 94, 114515 (2016).
    [2] H. Tajima, S. Tsutsui, T. M. Doi, arXiv:2005.12124.
  • 2020.06.12, 13:00–14:30 (JST) @ Zoom
    Masaru Hongo (UIC/RIKEN iTHEMS):
    "Field theoretical approach to relativistic hydrodynamics"

    Hydrodynamics is a low-energy effective theory of a conserved charge density, which describes a long-distance and long-time behavior of many-body systems. It is applicable not only to a non-relativistic weakly-interacting dilute gas but also a relativistic strongly-interacting dense liquid like a quark-gluon plasma. The main purpose of this seminar is to explain how we can derive the hydrodynamic equation from the underlying field-theoretical description of systems [1-3]. Our derivation is based on the recent development of non-equilibrium statistical mechanics, and we show that the procedure to derive hydrodynamic equations is similar to the so-called renormalized/optimized perturbation theory. Also, to describe transport phenomena in local thermal equilibrium, we give a path-integral formula for a thermodynamic functional, which results in the emergence of thermally induced curved spacetime [2]. These results enable us to derive hydrodynamic equation based on quantum field theories.
    [1] T. Hayata, Y. Hidaka, M. Hongo, and T. Noumi, Phys. Rev. D 92, 065008 (2015).
    [2] M. Hongo, Annals of Physics, 383, 1 (2017).
    [3] M. Hongo, K. Hattori, arXiv: 2005.10239 [hep-th].
  • 2020.05.29, 15:00–16:30 (JST) @ Zoom
    Ryusuke Hamazaki (RIKEN):
    "Localization and universality in non-Hermitian many-body systems"

    Recent study on isolated quantum many-body systems have revealed two different phases distinguished by their dynamics and spectral statistics. One is an ergodic phase whose spectral statistics exhibit universality of random matrices, and the other is a many-body localized phase where dynamics is constrained due to strong disorder. In this talk, we show that novel and rich physics concerning such localization and universality appears in non-Hermitian many-body systems, which have been utilized in diverse scientific disciplines from open quantum systems to biology. As a first topic, we analyze non-Hermitian quantum many-body systems in the presence of interaction and disorder [1]. We demonstrate that a novel real-complex transition occurs upon many-body localization of non-Hermitian interacting systems with asymmetric hopping that respect time-reversal symmetry. As a second topic, we show that “Dyson’s threefold way,” a threefold symmetry classification of universal spectral statistics of random matrices, is nontrivially extended to non-Hermitian random matrices [2]. We report our discovery of two distinct universality classes characterized by transposition symmetry, which is distinct from time-reversal symmetry due to non-Hermiticity. We show that the newly found universality classes indeed manifest themselves in dissipative quantum many-body ergodic systems described by Lindblad equations.
    [1] R. Hamazaki, K. Kawabata, and M. Ueda, Phys. Rev. Lett. 123, 090603 (2019).
    [2] R. Hamazaki, K. Kawabata, N. Kura and M. Ueda, Phys. Rev. Research 2, 023286 (2020).
  • 2020.05.15, 13:30–15:00 (JST) @ Zoom
    Kengo Kikuchi (RIKEN iTHEMS):
    "Gradient Flow and Its Applications"

    Gradient flow is the one of the methods to suppress the ultraviolet divergence in gauge theories. The any correlation functions in terms of the flowed field, which is defined by the gradient flow equation, are finite without additional renormalizations. Because of this surprising property, the methods has been studied widely, especially in the lattice field theory. In this seminar, we introduce what the gradient flow is briefly. And we show our work, “generalized gradient flow equation”, which is the gradient flow equation for field theories with nonlinearly realized symmetry. Applying the formalism to a supersymmetric theory and O(N) non linear sigma model, we obtain the SUSY gradient flow and the Large N gradient flow. We also refer to the current research, the gradient flow of the supersymmetric theory with the non-renormalization theorem and the new formalism to obtain the sphalerons, which is one of the static classical solutions, using gradient flow methods, if time allows.