## 2015年度のセミナー

 Date/Place Feb. 16 (Tues.) 15:30-16:30 / H711 Kenzo Ishikawa (Hokkaido U.) Forward scattering anomaly in photon and neutrino transitions: New macroscopic quantum phenomena (Slides) In the scattering or decay in the extreme forward direction, the initial and final waves overlap and the standard S-matrix defined under the adiabatic switch off of the interaction is not applicable, which was pointed out by Goldberger and Watson in 1964. Due to a small phase space and the difficulty of the experiments, serious study on this problem has not been made. In this talk, I will report our works on the quantum physics in this region. A new component arises in the transition probability as a correction to the Fermi's golden rule, and reveals rich phenomena in wide area. This component is enormously enhanced in the neutrino radiative transition in the solar corona condition. A sufficient heat is transferred from the neutrino and causes the steep rising of the temperature from 6000K to 1,000,000 K at 2000Km height. The solar neutrino in the earth is affected by this process in the corona and in the earth ionosphere and this mechanism can be directly tested with the daily variation of the B8 neutrino.

 Date/Place Jan. 26 (Tues.) 15:30-16:30 / H711 Masato Nishi (Kyoto U.) de Sitter thin brane model (Slides) There is an extremely large hierarchy between the Plank scale and the weak scale. We discuss this hierarchy problem in a 5D braneworld model which represents our acceleratively expanding universe. The Randall-Sundrum model can naturally explain the problem, but cannot describe our expanding universe. In this talk, we study instead the de Sitter thin brane model, where the 4-dimensional space-time on the branes is dS4. We show that, for the negative 5-dimensional cosmological constant case, the large hierarchy is naturally realized in the 5-dimensional theory in the same manner as in the RS model. Moreover, we find that the lightest mass of the massive Kaluza-Klein modes and the intervals of the mass spectrum are of order 1TeV. [Ref. arXiv:1508.06371]

 Date/Place Jan. 12 (Tues.) 15:30-16:30 / H711 Yutaro Shoji (U. of Tokyo) Fate of false vacua at one-loop (Slides) It can be the case that we are in a meta-stable vacuum and its decay time is longer than the age of universe. In most papers, the decay rate is estimated without calculating the pre-exponential factor because they believe that it is much less significant than the exponential suppression factor. What we point out is that this estimate can involve a large error owing to the uncertainty in the renormalization scale. Since the potential itself is scale dependent, it modifies the bounce solution and thus the decay rate. The uncertainty can be comparable to the exponential factor and it should be controlled to get a consistent result. We explicitly calculate the pre-exponential factor and show that it is greatly reduced.

 Date/Place Dec. 22 (Tues.) 15:30-16:30 / H711 Shin-ichi Sasa (Kyoto U.) Thermodynamic entropy as a Noether invariant We study a classical many-particle system with an external control represented by a time dependent parameter in a Lagrangian. We show that thermodynamic entropy of the system is the Noether invariant associated with a symmetry for an infinitesimal non-uniform time translation t→t+ηℏβ, where η is a small parameter, ℏ is the Planck constant, β is the inverse temperature that depends on the energy, and trajectories in the phase space are restricted to those consistent with quasi-static processes in thermodynamics. This study was done in collaboration with Yuki Yokokura and see Ref. arXiv:1509.08943.

 Date/Place Dec. 15 (Tues.) 15:30-16:30 / H711 Masaki Oshikawa (U. of Tokyo) Symmetry protection of critical phases and global anomaly in 1+1 dimensions While there has been a significant progress in classification of gapped quantum phases, classification of gapless quantum phases remains very much open. Symmetries are naturally expected to play an important role here, as in the case of gapped quantum phases. In this talk, we argue that there is a protection of bulk gapless critical phases by discrete symmetry. We demonstrate this for the SU(2)-symmetric quantum antiferromagnetic chains and their effective field theory, SU(2) Wess-Zumino-Witten (WZW) theory as an example. The SU(2) WZW theory is characterized by a natural number k, which is called level. In the presence of the SU(2) and a certain discrete Z_2 symmetry, they are classified into the two "symmetry-protected'' categories: one corresponds to even levels and the other to odd levels. Reference: S. C. Furuya and M. O., arXiv:1503.07292

 Date/Place Dec. 8 (Tues.) 15:30-16:30 / H711 Yutaka Ookouchi (Kyushu U.) Decay of False Vacuum via Fuzzy Monopole in String Theory (Slides) I will talk about dielectric branes in false vacua in Type IIA and IIB string theories. The dielectric branes are supported against collapsing by lower energy vacua inside spherical or tube-like branes. We claim that such branes can be seeds for semi-classical (or quantum mechanical) decay of the false vacua, which makes the life-time of the false vacua shorter. Also, we will show a topology change of a bubble corresponding to the fuzzy monopole triggered by dissolving fundamental strings.

 Date/Place Dec. 1 (Tues.) 15:30-16:30 / H711 Yasuhiro Yamamoto (Osaka U.) Unitarity bounds on the dark matter effective interactions at LHC We study the compatibility of the unitarity bound and the 8TeV LHC on the effective theory of the dark matter. In several signals of effective interactions, mono-jet with missing energy events are studied. We found that, at least, if the dark matter mass is about several hundred GeV or heavier, contributions of events violating the unitarity are not negligible. The unitarity conditions in the 14TeV LHC are also calculated.

 Date/Place Nov. 24 (Tues.) 15:30-16:30 / H711 Masazumi Honda (Weizmann Institute) The Interpolating function (Slides) In theoretical physics, we sometimes have two perturbative expansions of physical quantity around different two points in parameter space, e.g. in theory with S-duality, lattice gauge theory with weak and strong coupling expansions, field theory with gravity dual, and statistical system with low and high temperature expansions, etc. In terms of these two perturbative expansions, we can construct smooth interpolating functions, which are consistent with the both expansions up to some orders. In my talk, I will report recent progress on such interpolating functions.

 Date/Place Nov. 10 (Tues.) 15:30-16:30 / H711 Atsushi Watanabe (Maskawa Institute) New physics and astrophysical neutrinos in IceCube (Slides) IceCube has recently detected neutrino events whose energies are larger than 100 TeV, heralding a new era of high-energy astronomy. After a brief review of the IceCube experiment and the astrophysical neutrinos, we discuss recent results on IceCube, focusing especially on the flavor composition. We also discuss the ability of the IceCube experiment to probe new physics, for example Z' in light of the muon g-2.

 Date/Place Oct. 27th (Tues.) 15:30-16:30 / H711 Yasuaki Hikida (Rikkyo U.) Higher spin AdS_3 holography and superstring theory (Slides) Higher spin gauge theory is expected to describe the tensionless limit of superstring theory, and it attracts a lot of attention due to the application for AdS/CFT correspondence. As examples, a 4d higher spin gauge theory is proposed to be dual to 3d O(N) vector model, and a 3d higher spin gauge theory is claimed to correspond to 2d large N minimal model. Recently, it becomes possible to discuss concrete relations between higher spin gauge theory and superstring theory by extending these dualities. In this talk, I will review recent developments particularly focusing on AdS_3 case, and our works on the extensions of duality and relations to superstring theory.

 Date/Place Oct. 20th (Tues.) 15:30-16:30 / H711 Ryuichiro Kitano (KEK) Strong CP problem and axion on the lattice (Slides) In QCD, while the phase in the quark masses are physical through the axial anomaly, it must be extremely small to be consistent with the measurement of the electric dipole moment of the neutron. We review and discuss this mystery, called the strong CP problem, and present an approach from the lattice QCD. Especially, we discuss the temperature dependence of the topological property of QCD which has physical significance in the estimation of the QCD axion in the Universe.

 Date/Place Oct. 13th (Tues.) 15:30-16:30 / H711 Tatsuma Nishioka (U. of Tokyo) Aspects of Renormalized Entanglement Entropy (Slides) Entanglement entropy is sensitive to the ultraviolet structure of quantum field theory and becomes divergent without regularization. Renormalized entanglement entropies (REEs) can be defined in two and three dimensions, being independent of a regularization scheme. They are known to play roles of a measure of degrees of freedom under renormalization group flows in an analogous manner to Zamolodchikov’s c-function. In this talk, I will review the constructions of REEs and comment on their behaviors under a relevant perturbation of conformal field theory.

 Date/Place Oct. 6th (Tues.) 15:30-16:30 / H711 Etsuko Itou (KEK) Entropic c-function of the SU(3) gauge theory in four dimensions (Slides) We have investigated the quantum entanglement entropy for the four dimensional Euclidean SU(3) gauge theory. The entropic c-function is nonperturbatively calculated based on the replica method using the lattice Monte Carlo numerical simulation. The value of C(l), where l is the length of subspace in a dimension, is a constant in the short l region, while it smoothly decreases around l=¥Lambda_{QCD}^{-1}. It becomes consistent with zero in the long l limit. There is no clear UV cutoff dependence in the c-function. We compare our results with the other works.

 Date/Place July. 28th (Tues.) 15:00-16:00 / H711 Wen Yu Wen (Chung Yuan Christian U.) Twisted fate of BTZ (Slides) It is known that a solution of remnant were suggested for black hole ground state after surface gravity is corrected by loop quantum effect. On the other hand, a Schwarzschild black hole in asymptotic Anti-de Sitter space would tunnel into thermal soliton solution known as the Hawking-Page phase transition. In this letter, we investigate these competing phases at low temperature.

 Date/Place July. 23th (Thur.) 15:00-16:00 / H711 Toshinori MATSUI (Toyama U.) Higgs inflation by flat potential and a radiative seesaw mechanism The standard model is established by discovering the Higgs boson. However, the problems of dark matter (DM), neutrino oscillation and baryon asymmetry of the Universe are known as the beyond the-standard-model phenomena. In the radiative seesaw mechanism, neutrino mass generation is forbidden at the tree level and DM stability is explained by Z2 symmetry. For such multi-Higgs models, the constraint of vacuum stability can be relaxed. If the vacuum is stable up to the inflation scale, Higgs boson can behave as inflaton in the Higgs inflation scenario. For the case of flat potential, Higgs inflation may be tested by future cosmic microwave background (CMB) experiments such as LiteBIRD. In this talk, we investigate Higgs inflation by flat potential in the framework of radiative seesaw mechanism. Exploring the effect of the inflation constraints on TeV scale, we discuss testability at the DM direct detections and the collider experiments in addition to the CMB observations. This talk is based on the paper Phys. Lett. B723, 126 (2013), with S.Kanemura and T.Nabeshima, including some recent developments.

 Date/Place July. 22th (Wed.) 13:30-14:30 / H711 Weonjong Lee (Seoul National University) Current status of $\varepsilon_K$ and B meson spectrum test We will present recent progress in determining $\varepsilon_K$ using the lattice QCD inputs: $B_K$, $V_{us}$, $V_{cb}$ and $\xi_0$. The Standard Model evaluation of $\varepsilon_K$ with lattice QCD inputs has 3.4$\sigma$ tension from the experimental value. The dominant error source is $V_{cb}$. Hence, we take the first step to calculate $V_{cb}$ in the exclusive semi-leptonic decay. We perform the B-meson spectrum test to check the improvement of the binding energy calculation using the OK action, which is a next generation fermilab formulation for heavy quarks. The results will be reported.

 Date/Place July. 21th (Tues.) 15:00-16:00 / H711 Akihiko Sekine (Tohoku U.) Stability of Topological Semimetals against Strong Long-Range Coulomb Interactions (Slides) Dirac and Weyl semimetals have attracted much attention as three-dimensional (3D) topological phases. They were experimentally observed recently [1,2]. The low-energy effective models of Dirac (Weyl) semimetals are described by 3D massless Dirac (Weyl) fermions. Hence they can be regarded as 3D analogs of graphene. In this talk, based on the U(1) lattice gauge theory, we study the stability of Dirac and Weyl semimetals against strong 1/r Coulomb interactions. Namely, we discuss whether energy gaps open in the lattice fermion models for Dirac and Weyl semimetals. It is shown that the semimetal phases survive in the strong coupling limit [3,4]. Our results suggest that 3D topological phases are stable against 1/r Coulomb interactions. [1] Z. K. Liu et al., Science 343, 864 (2014). [2] S.-Y. Xu et al., arXiv:1502.03807. [3] A. Sekine & K. Nomura, Phys. Rev. B 90, 075137 (2014). [4] A. Sekine & K. Nomura, J. Phys. Soc. Jpn. 83, 094710 (2014).

 Date/Place July. 14th (Tues.) 15:00-16:00 / H711 Muneto Nitta (Keio U.) Triplet neutron superfluidity in neutron stars It is generally believed that neutron superfluids are realized in neutron stars. Above the normal nuclear matter density, a triplet superfluids have been predicted to occur instead of the conventional singlet superfluids. Based on the Ginzburg-Landau theory, we discuss rotation and magnetic responses of the triplet superfluids and find vortex structures such as spontaneous magnetization in the vortex core. We comment on possible effects on neutron stars. This talk is besed on the work in preparation in collaboration with Kota Masuda.

 Date/Place July. 7th (Tues.) 15:00-16:00 / H711 Sotaro Sugishita (Kyoto U.) Random volumes from matrices (Slides) We propose a new class of models generating three-dimensional random volumes. As the Feynman diagrams of matrix models can be interpreted as triangulations of two-dimensional surfaces, the diagrams of our models can be regarded as collection of triangles glued together along multiple hinges. These "triangle-hinge models" are characterized by semisimple associative algebras, and the set of possible diagrams can be reduced by an appropriate "large N limit" such that they represent only and all of the tetrahedral decompositions of three-dimensional manifolds. Thus, the models give discretized description of three-dimensional quantum (pure) gravity. We also give a prescription to put local matters on simplices of any dimensions (tetrahedra, triangles, edges and vertices). [Ref. arXiv:1503.08812, arXiv:1504.03532]

 Date/Place July. 3th (Fri.) 11:00-12:00 / H711 Taro Kimura (Keio U.) Lattice, SUSY, and topological phases The topological phase, which is typically observed in topological insulators/superconductors, is one of the recent hot topics in not only theoretical, but experimental condensed-matter physics. In this talk, we would like to discuss (i) how the Ginsparg-Wilson (domain-wall/overlap) fermion is realized in topological insulators, and (ii) the SUSY localization analysis helps us to characterize the topological phase. This talk is based on a work in progress with T. Morimoto (RIKEN) and A. Tanaka (RIKEN).

 Date/Place June. 30th (Tues.) 15:00-16:00 / H711 Masaki Yang (KEK) Higgs boson as a gauge field in discrete spaces (Slides) In the context of the noncommutative geometry (NCG), some models treat the Higgs boson as a Yang-Mills field between discrete spaces. Without complicated knowledge of NCG, we can simply reformulate this kind of model as a discrete extra dimension theory with the gauge-Higgs unification. In this talk, I will review basic formalization, phenomenological applications, and recent result of the SU(5) Orbifold GUT in this "discrete gauge-Higgs unification". [Ref. arXiv:1501.03888, arXiv:1506.03166]

 Date/Place June. 23th (Tues.) 15:00-16:00 / H711 Kunio Kaneta (ICRR) Spontaneous thermal leptogenesis (Slides) Although cosmological and astrophysical observations have found the baryon asymmetry of the universe, the standard model of particle physics hardly explain its origin, and thus new physics beyond the standard model is required. Leptogenesis is one of convincing explanations for baryogenesis, in which tiny neutrino masses can be explained by the presence of right-handed Majorana neutrinos via the seesaw mechanism. There, Majorana masses of the right-handed neutrinos would be originated from a spontaneous breakdown of $B-L$ symmetry, which can lead to a Nambu-Goldstone boson called Majoron. We discuss a possibility that leptogenesis takes place in the primordial thermal bath due to the background Majoron field in motion.

 Date/Place June. 16th (Tues.) 15:00-16:00 / H711 Norisuke Sakai (Keio U.) Resurgence and Bions in Grassmann sigma models (Slides) We review recent findings on resurgence: how bions can make (divergent) perturbation series unambiguous. We also give a full characterization of bions in Grassmann sigma model. Perturbation series of quantum field theories are known to be divergent, which can be made meaningful by means of the Borel summation. However, asymptotically free theories give ambiguities associated with the so-called renormalon singularities. If one spatial direction is compactified, one can find fractional instantons which satisfy twisted boundary conditions. Composite of fractional instantons and anti-instantons are called bions. By taking nonperturbative contributions of neutral bions, the ambiguities in perturbation series are shown to be cancelled. This implies that the perturbation series can be defined on rigorous footing only with the nonperturbative bion contributions, which conspire with the perturbation series at large orders. We have given a complete classification and a method of construction for neutral and charged bions in the Grassmann sigma model on R^1 x S^1. As a result, we find that there are no charged bions in the simplest of Grassmann models, namely in CP^N model. This talk is mainly based on our work published in JHEP 1406, 164 (2014) [arXiv:1404.7225 [hep-th]] and PTEP 2014, 033B02 (2014) [arXiv:1409.3444 [hep-th]].

 Date/Place June. 9th (Tues.) 15:00-16:00 / H711 Masatoshi Sato (YITP) Topological quantum phenomena and gauge theories (Slides) Recently, there has been much interest in topological quantum phases in condensed matter physics. However, the concept of topological phase is not specific to condensed matter physics, but it is a rather general idea applicable to much wider ranges of quantum theory. In this talk, I will introduce the concept of topological phase and present related topics in gauge theories.

 Date/Place June. 2nd (Tues.) 15:00-16:00 / H711 Yoshihito Kuno (Nagoya Institute of Technology) Atomic Quantum Simulation of U(1) lattice gauge-Higgs model (Slides) Cold atoms in an optical lattice have been used as versatile quantum simulators for various many body quantum systems. Recently, there have been several proposals to realize lattice gauge theories in cold atom systems. In this seminar, we explain basic cold atom experiments to create optical lattice and also introduce a feasible experimental method to construct Quantum Simulator of U(1) lattice gauge-Higgs model in cold atom system in an optical lattice. Furthermore, we show by using Gross-Pitaevskii numerical method, the real-time dynamics of confinement electric flux between static charges. The difference of the flux behavior between confinement and Higgs phase is shown.

 Date/Place May. 29th (Fri.) 15:30-16:30 / H711 Anupam Mazumdar (Lancaster U.) The holographic discussion of gravity with infinite derivatives I will discuss properties of ghost free construction of infinite derivative theory of classical and quantum gravity.

 Date/Place May. 26th (Tues.) 15:00-16:00 / H711 Sunil Mukhi (IISER) Modular Invariance and Entanglement Entropy (Slides) The Renyi and entanglement entropies for free 2d CFT's at finite temperature and finite size will be discussed, with emphasis on their properties under modular transformations of the torus. The issue of how to sum over fermion spin structures in the replica trick is addressed. Our results are compared with those for a free boson at unit radius in the two limits and complete agreement is found, supporting the view that entanglement respects Bose-Fermi duality. We extend our computations to multiple free Dirac fermions having correlated spin structures, dual to free bosons on the Spin(2d) weight lattice.

 Date/Place May. 19 (Tues.) 15:00-16:00 / H711 Masahito Yamazaki (IPMU) Theory space In conventional studies of quantum field theories, we often pick up a particular quantum field theory and then study its properties. There are, however, old and new indications that we should rather study *the space of quantum field theories*. In this talk I will discuss several examples where this apparently far-fetched idea can be discussed in concrete research problems.

 Date/Place May. 12 (Tues.) 15:00-16:00 / H711 Yusuke Taniguchi (Tsukuba U.) QCD phase transition at real chemical potential with canonical approach We study the finite density phase transition in the lattice QCD at real chemical potential. We adopt canonical approach and the canonical partition function is constructed for Nf=2 QCD. After derivation of the canonical partition function we calculate observables like the pressure, the quark number density, its second cumulant and the chiral condensate as a function of the real chemical potential. We covered a wide range of temperature region starting from the confining low to the deconfining high temperature. We observe signals for the deconfinement and the chiral restoration phase transition at real chemical potential below Tc starting from the confining phase.

 Date/Place Apr. 28 (Tues.) 12:10-13:00 / H711 Akihiko Sonoda (Osaka U.) Universal turbulence on branes in holography (slides) We are interested in a phase transition between a confining phase and a deconfining phase at strongly coupled gauge theories. At the phase transition, we found that the mesons at high excited states become turbulent in holographic QCD. The turbulent mesons have a power-law energy distribution as K. Hashimoto, S. Kinoshita, K. Murata and T. Oka were mentioned in their paper(arXiv:1408.6293 [hep-th]). In this talk, we explain that the turbulence power is universal, irrespective of how the transition is driven, by numerically calculating the power in various static brane setups at criticality. We also find that the power depends only on the cone dimensions of the probe D-branes.

 Date/Place Apr. 21 (Tues.) 15:00-16:00 / H711 Jisuke Kubo (Kanazawa U.) Nambu-Goldstone Dark Matter in a Scale Invariant Bright Hidden Sector We consider a classically scale invariant extension of the standard model, in which the scale invariance is broken by dynamical chiral symmetry breaking in a QCD-like hidden sector. Because of unbroken flavor symmetries, the quasi Nambu-Goldstone bosons are stable so that they can become dark matter particles. In this model all the gravitational masses in the Universe have their origin in dynamical chiral symmetry breaking. The hidden sector is bright, because the hidden fermions have a non-vanishing hypercharge. We study dark matter properties using the Nambu-Jona-Lasinio theory in the self-consistent mean field approximation.

 Date/Place Apr. 14 (Tues.) 15:00-16:00 / H711 Kohsaku Tobioka (KEK, Tel Aviv U., Weizmann Institute of Science) Probing Higgs-charm coupling, for current and future LHC (slides) We introduce four different types of data-driven analyses that constrain the size of the Higgs-charm Yukawa coupling: (i) recasting the vector-boson associated, Vh, analyses that search for bottom-pair final state; (ii) the direct measurement of the total width; (iii) the search to h-> J/\psi\gamma; (iv) a global fit to the Higgs signal strengths. A comparison with t\bar{t}h data allows us to show that the Higgs does not couple to quarks in a universal way. Finally, we demonstrate how the experimental collaborations can further improve our direct bound (i) using charm-tagging. Ref: arXiv:1503.00290

 Date/Place Apr. 7 (Tues.) 15:00-16:00 / H711 Toshifumi Noumi (RIKEN) Effective field theory approach for spacetime symmetry breaking (slides) Spacetime symmetry breaking often appears in condensed matter physics and cosmology. In contrast to the internal symmetry case, it is known that the number of broken global spacetime symmetries does not coincide with that of physical Nambu-Goldstone (NG) fields in general. Correspondingly, the coset construction of effective action has to be implemented with the so-called inverse Higgs constraints to compensate such a mismatch. In this talk, I revisit the effective action construction for spacetime symmetry breaking from the local symmetry point of view. Since NG fields are generated by local transformations of order parameters, such a local picture provides a correct identification of physical NG fields. I will illustrate the importance of such a local picture and revisit the coset construction. I will also discuss applications to condensed matter physics and cosmology. Ref: arXiv:1412.5601