Hokkai Gakuen University

About: Hokkai Gakuen University is a education organization based out in Sapporo, Japan. It is known for research contribution in the topics: Speckle pattern & Stars. The organization has 170 authors who have published 735 publications receiving 8816 citations.

A facility to search for hidden particles at the CERN SPS: the SHiP physics case.

Abstract: This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (search for hidden particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, $\tau \to 3\mu $ and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the standard model and describe interactions between new particles and four different portals—scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the standard model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation.

A natural explanation for periodic X-ray outbursts in Be/X-ray binaries

Abstract: When applied to Be/X-ray binaries, the viscous decretion disc model, which can successfully account for most properties of Be stars, naturally predicts the truncation of the circumstellar disc. The distance at which the circumstellar disc is truncated depends mainly on the orbital parameters and the viscosity. In systems with low eccentricity, the disc is expected to be truncated at the 3:1 resonance radius, for which the gap between the disc outer radius and the critical lobe radius of the Be star is so wide that, under normal conditions, the neutron star cannot accrete enough gas at periastron passage to show periodic X-ray outbursts (type I outbursts). These systems will display only occasional giant X-ray outbursts (type II outbursts). On the other hand, in systems with high orbital eccentricity, the disc truncation occurs at a much higher resonance radius, which is very close to or slightly beyond the critical lobe radius at periastron unless the viscosity is very low. In these systems, disc truncation cannot be efficient, allowing the neutron star to capture gas from the disc at every periastron passage and display type I outbursts regularly. In contrast to the rather robust results for systems with low eccentricity and high eccentricity, the result for systems with moderate eccentricity depends on rather subtle details. Systems in which the disc is truncated in the vicinity of the critical lobe will regularly display type I outbursts, whereas those with the disc significantly smaller than the critical lobe will show only type II outbursts under normal conditions and temporary type I outbursts when the disc is strongly disturbed. In Be/X-ray binaries, material will be accreted via the first Lagrangian point with low velocities relative to the neutron star and carrying high angular momentum. This may result in the temporary formation of accretion discs during type I outbursts, something that seems to be confirmed by observations.

Viscous effects on the interaction between the coplanar decretion disc and the neutron star in Be/X-ray binaries

Abstract: We study the viscous effects on the interaction between the coplanar Be-star disc and the neutron star in Be/X-ray binaries, using a three-dimensional, smoothed particle hydrodynamics code. For simplicity, we assume the Be disc to be isothermal at the temperature of half the stellar effective temperature. In order to mimic the gas ejection process from the Be star, we inject particles with the Keplerian rotation velocity at a radius just outside the star. Both the Be star and the neutron star are treated as point masses. We find that the Be-star disc is effectively truncated if the Shakura‐Sunyaev viscosity parameter αSS � 1, which confirms the previous semi-analytical result. In the truncated disc, the material decreted from the Be star accumulates, so that the disc becomes denser more rapidly than if around an isolated Be star. The resonant truncation of the Be disc results in a significant reduction of the amount of gas captured by the neutron star and a strong dependence of the mass-capture rate on the orbital phase. We also find that an eccentric mode is excited in the Be disc through direct driving as a result of a one-armed bar potential of the binary. The strength of the mode becomes greater in the case of a smaller viscosity. In a high-resolution simulation with αSS = 0.1, the eccentric mode is found to precess in a prograde sense. The mass-capture rate by the neutron star modulates as the mode precesses.

The Be/X-ray transient 4U 0115+63/V635 Cassiopeiae - I. A consistent model

Abstract: We present photometry and high SNR spectroscopy in the classification region of V635 Cas, the optical counterpart to the transient X-ray pulsar 4U 0115+63, taken at a time when the circumstellar envelope had disappeared. V635 Cas is classified as a B0.2Ve star at a distance of $7{-}8$ kpc. We use the physical parameters derived from these observations and the orbit derived from X-ray observations to elaborate a model of the system based on the theory of decretion discs around Be stars. We show that the disc surrounding the Be star is truncated by the tidal/resonant interaction with the neutron star and cannot be in a steady state. This explains many of the observed properties of 4U 0115+63. In particular, because of this effect, under normal circumstances the neutron star cannot accrete from the disc, which explains the lack of regular type I outbursts from the source.

Higgs portal dark matter in the minimal gauged U(1) B-L model

Abstract: We propose a scenario of the right-handed neutrino dark matter in the context of the minimal gauged $U(1{)}_{B\mathrm{\text{\ensuremath{-}}}L}$ model by introducing an additional parity which ensures the stability of dark matter particle. The annihilation of this right-handed neutrino takes place dominantly through the $s$-channel Higgs boson exchange, so that this model can be called the Higgs portal dark matter model. We show that the thermal relic abundance of the right-handed neutrino dark matter with the help of Higgs resonance can match the observed dark matter abundance. In addition, we estimate the cross section with nucleon and show that the next generation direct dark matter search experiments can explore this model.