セミナー 2014年

Andrew Akeroyd, University of Southampton

Decay of charged Higgs bosons into charm and bottom quarks in multi-Higgs doublet models

Meeting room 1, Kenkyu honkan 1F
A scalar particle has been discovered at the Large Hadron Collider (LHC), but it is not yet known if this particle corresponds to the Higgs boson of the Standard Model or if it is the first of many Higgs bosons which are waiting to be discovered. Higgs bosons with electric charge (“charged Higgs bosons” or “charged scalars”) are predicted in many extensions of the Standard Model. Searches for such particles are being carried out at the LHC, in particular in the decays of the top quark, and these searches assume that two specific decay modes of the charged Higgs boson are dominant: In this talk I show that a third decay mode, into charm and bottom quarks, can be dominant in models with more than two Higgs isospin doublets. So far there has been no dedicated search for this decay to charm and bottom quarks at the LHC, but it could readily be performed as an extension of an ongoing search which assumes that the charged Higgs boson decays into a charm quark and a strange quark.

Francesco Knechtli, University of Wuppertal

Non-perturbative Gauge-Higgs Unification on the Lattice

Meeting room 1, Kenkyu honkan 1F
Gauge-Higgs Unification models are theories where the Higgs field is identified with some of the extra-dimensional components of the gauge field.
One motivation for these models is that the extra-dimensional gauge symmetry protects the Higgs potential from divergences in the ultra-violet cut-off.
Perturbative studies indicate that fermions are necessary to trigger the Higgs mechanism. Given the non-renormalizability of gauge theories in dimensions higher than four, we study non-perturbatively the simplest model which is a five-dimensional SU(2) gauge theory on a orbifold.
The starting point is the formulation on an anisotropic Euclidean lattice.
Results using a semi-analytic mean-field expansion and Monte Carlo simulations show that the Higgs mechanism takes place without fermions and support an alternative view of the lattice orbifold as a five-dimensional bosonic superconductor.

Tsunehide Kuroki, Nagoya U

SUSY breaking by nonperturbative dynamics in a matrix model for 2D type IIA superstrings

Seminar room, Kenkyu honkan 3F
We explicitly compute nonperturbative effects in a supersymmetric double-well matrix model corresponding to two-dimensional type IIA superstring theory on a nontrivial Ramond-Ramond background. We analytically determine the full one-instanton contribution to the free energy and one-point function, including all perturbative fluctuations around the one-instanton background. The leading order two-instanton contribution is determined as well. We see that supersymmetry is spontaneously broken by instantons, and that the breaking persists after taking a double scaling limit which realizes the type IIA theory from the matrix model. The result implies that spontaneous supersymmetry breaking occurs by nonperturbative dynamics in the target space of the IIA theory. We also discuss recent progress in summing up all instanton contributions using knowledge of random matrix theory

Masakiyo Kitazawa, Osaka U

Thermal fluctuations in relativistic heavy ion collisions

Seminar room, Kenkyu honkan 3F
Event-by-event fluctuations of conserved charges in relativistic heavy ion collisions are useful observables to investigate primordial thermodynamics of the hot medium generated by the collisions. In particular, they are promising observables to investigate the global structure of QCD phase diagram in experiments, and have been actively measured in the beam-energy scan program at RHIC. In this talk, I will give a review on the present status of the experimental measurement of the fluctuations observables. I will also explian why the fluctuations are interesting and important observables. Some new methods to use these observables more efficiently will also be discussed.

Philipp Gubler, RIKEN

How does the measurement of the phi-meson mass shift in nuclear matter constrain the strangeness content of the nucleon?

Seminar room, Kenkyu honkan 3F
In this seminar, I will present our latest results of our study on the behavior of the phi-meson at finite density [1], which makes use of a QCD sum rule approach in combination with the maximum entropy method [2]. As earlier works have already shown, we find a strong correlation between a possible mass shift of the phi-meson in nuclear matter and the strangeness content of the nucleon, which is proportional to the strange sigma term, sigma_{sN}. In contrast to other studies, we however find that, depending on the value of sigma_{sN}, the phi-meson could receive both a positive or negative mass shift at nuclear matter density. I will discuss the relevance of our results for the interpretation of the E325 experiment, which was performed at KEK and for the future E16 experiment to be carried out at the J-PARC facility.
If time allows, I will also briefly describe a few possible other applications of our method, such as investigating the behavior of in hadrons in a magnetic field or the study of some basic properties of the unitary fermi gas.
[1] P. Gubler and K. Ohtani, arXiv:1404.7701 [hep-ph].
[2] P. Gubler and M. Oka, Prog. Theor. Phys. 124, 995 (2010), arXiv:1005.2459 [hep-ph].

Roberto Emparan, University of Barcelona, YITP

The large D limit of General Relativity

Meeting room 1, Kenkyu honkan 1F
General Relativity is an old theory but its dynamics remains difficult to solve and understand, in particular when strongly-gravitating objects such as black holes are involved. Although at first sight it may seem an odd idea, I will argue that it is actually quite natural to investigate the properties of this theory and its black holes in the limit in which the number of spacetime dimensions D grows to infinity. The gravitational field localizes very strongly near the black hole horizons, which simplifies dramatically the description of their interactions and allows efficient calculational approaches in an expansion in 1/D. There are also hints that this limit may lead to a reformulation of the theory in terms of low-dimensional strings.

Yuji Omura, Nagoya University

Study on two-Higgs-doublet models (with gauged U(1) symmetry)

Meeting room 1, Kenkyu honkan 1F
It is well known that generic two-Higgs-doublet models (2HDMs) suffer from potentially large Higgs-mediated flavor-changing neutral current (FCNC) problem, unless additional symmetries are imposed on the Higgs fields thereby respecting the Natural Flavor Conservation Criterion (NFC) by Glashow and Weinberg. A common way to respect the NFC is to impose Z_2 symmetry which is softly broken by a dim-2 operator. Another new way is to introduce local U(1)_H Higgs flavor symmetry that distinguishes one Higgs doublet from the other. In this talk, we discuss the phenomenology in 2HDMs with not only the Z_2 but also U(1)_H symmetry. We also study dark matter (DM) physics in the Type-I inert 2HDM with local U(1)_H Higgs gauge symmetry. The lightest neutral scalar component H of the U(1)_H-charged Higgs doublet, which does not have Yukawa couplings with the Standard-Model (SM) fermions, is stable because of the remnant discrete symmetry, and it interacts with the SM particles through the U(1)_H gauge boson (Z_H) exchange as well as the SM boson exchange. We investigate the constraints on DM: thermal relic density, and direct/indirect detections. The additional U(1)_H gauge interaction plays a crucial role in reducing the DM thermal relic density. The most important result within the inert DM model with local U(1)_H symmetry is that ~ O(10) GeV dark matter scenario, which is strongly disfavored in the usual Inert Doublet Model (IDM) with Z_2 symmetry, is revived in our model because of newly open channels, H H -> Z_H Z_H , Z_H Z. Exotic Higgs decays, h -> Z_H Z_H, Z Z_H, would be distinctive signatures of the inert 2HDM with local U(1)_H symmetry. I also give a comment on the type-II 2HDM inspired by E_6 GUT, and introduce some results on DM physics.

飯沼昌隆, 広島大学


Seminar room, Kenkyu honkan 3F

Yuan Qiang, IHEP

Millisecond pulsar interpretation of the Galactic center gamma-ray excess

Meeting room 2, Kenkyu honkan 1F
It was found in the Fermi-LAT data that there is an extended $gamma$-ray excess in the Galactic center region. The proposed sources to be responsible for the excess include the dark matter annihilation or an astrophysical alternative from a population of millisecond pulsars (MSPs). Whether or not the MSP scenario can explain the data self-consistently has very important implications for the detection of particle dark matter, which is however, subject to debate in the literature. In this work we study the MSP scenario in detail, based on the detected properties of the MSPs by Fermi-LAT. We build a model of the Milky Way MSPs which can reproduce the $gamma$-ray properties of the Fermi-LAT MSPs, and derive the intrinsic luminosity function of the MSPs. The model is then applied to a bulge population of MSPs. We find that the extended $gamma$-ray excess can be well explained by the bulge MSPs without violating the detectable flux distribution of MSPs by Fermi-LAT. The spatial distribution of the bulge MSPs as implied by the distribution of low mass X-ray binaries follows a $r^{-2.4}$ profile, which is also consistent with the $gamma$-ray excess data. We conclude that the MSP model can explain the Galactic center $gamma$-ray excess self-consistently, satisfying all the current observational constraints.

Yuta Hamada, Kyoto University

Naturalness and maximum entropy principle

Meeting room 3, Kenkyu honkan 1F
The theory of wormholes and multiverse suggests that the parameters of the Standard Model are fixed in such a way that the total entropy at the late stage of the universe is maximized, which we call the maximum entropy principle. In this talk, We show that Higgs vacuum expectation value is determined in order to satisfy maximum entropy principle. We assume that the baryon number is produced by the sphaleron process, and that the current quark masses, the gauge couplings and the Higgs self coupling are fixed when we vary Higgs vacuum expectation value. It turns out that the existence of the atomic nuclei plays a crucial role to maximize the entropy.

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