arXiv subjects

K. Kohri

Publications and source records attributed to K. Kohri.

72 records · Page 4Linked to original sources

arXiv2013

The Cherenkov Telescope Array Large Size Telescope

The two arrays of the Very High Energy gamma-ray observatory Cherenkov Telescope Array (CTA) will include four Large Size Telescopes (LSTs) each with a 23 m diameter dish and 28 m focal distance. These telescopes will enable CTA to achieve a low-energy threshold of 20 GeV, which is critical for important studies in astrophysics, astroparticle physics and cosmology. This work presents the key specifications and performance of the current LST design in the light of the CTA scientific objectives.

astro-ph.IM↗

arXiv2005

Radiative Decay of a Long-Lived Particle and Big-Bang Nucleosynthesis

The effects of radiatively decaying, long-lived particles on big-bang nucleosynthesis (BBN) are discussed. If high-energy photons are emitted after BBN, they may change the abundances of the light elements through photodissociation processes, which may result in a significant discrepancy between the BBN theory and observation. We calculate the abundances of the light elements, including the effects of photodissociation induced by a radiatively decaying particle, but neglecting the hadronic branching ratio. Using these calculated abundances, we derive a constraint on such particles by comparing our theoretical results with observations. Taking into account the recent controversies regarding the observations of the light-element abundances, we derive constraints for various combinations of the measurements. We also discuss several models which predict such radiatively decaying particles, and we derive constraints on such models.

hep-ph↗

arXiv2003

Stabilizing dilaton and baryogenesis

Entropy production by the dilaton decay is studied in the model where the dilaton acquires potential via gaugino condensation in the hidden gauge group. Its effect on the Affleck-Dine baryogenesis is investigated with and without non-renormalizable terms in the potential. It is shown that the baryon asymmetry produced by this mechanism with the higher-dimensional terms is diluted by the dilaton decay and can be regulated to the observed value.

hep-ph↗

arXiv2002

Gamma-Ray Burst Neutrino Background and Star Formation History in the Universe

We estimate the flux of the GRB neutrino background and compute the event rate at SK and TITAND in the collapsar model, assuming that GRB formation rate is proportional to the star formation rate. We find that the flux and the event rate depend sensitively on the mass-accretion rate. Although the detection of signals from GRBs seems to be difficult by SK, we find that we can detect them by TITAND (optimistically about 20 events per year in the energy range where the energy of anti-electron neutrino is larger than 50 MeV). Therefore, we show that we will obtain the informations on the mass-accretion rate of collapsar by observing the GRB neutrino background, which, in turn, should give us informations on the total explosion energy of GRBs.

astro-ph↗

arXiv2001

Oscillating scalar-field dark matter in supergravity

We show that an oscillating scalar field in supergravity of mass of the order of $\sim$ TeV with a nonzero vacuum expectation value ($\sim 10^{10}$ GeV) can be a candidate of cold dark matter (CDM). To avoid the gravitino problem, we need a low reheating temperature after the primordial inflation. Then, the energy density of the oscillating scalar field satisfies all the requirements for CDM at present in the universe.

hep-ph↗

arXiv2001

Anisotropic $e^+ e^-$ pressure due to the QED effect in strong magnetic fields and the application to the entropy production in neutrino-driven wind

We study the equation of state of electron in strong magnetic fields which are greater than the critical value $B_c \simeq 4.4 \times 10^{13}$ Gauss. We find that such a strong magnetic field induces the anisotropic pressure of electron. We apply the result to the neutrino-driven wind in core-collapse supernovae and find that it can produce large entropy per baryon, $S \sim 400 k_B$. This mechanism might successfully account for the production of the heavy nuclei with mass numbers A = 80 -- 250 through the r-process nucleosynthesis.

hep-ph↗

arXiv2001

Primordial nucleosynthesis and hadronic decay of a massive particle with a relatively short lifetime

In this paper we consider the effects on big bang nucleosynthesis (BBN) of the hadronic decay of a long-lived massive particle. If high-energy hadrons are emitted near the BBN epoch ($t \sim 10^{-2}$ -- $10^2 \sec$), they extraordinarily inter-convert the background nucleons each other even after the freeze-out time of the neutron to proton ratio. Then, produced light element abundances are changed, and that may result in a significant discrepancy between standard BBN and observations. Especially on the theoretical side, now we can obtain a lot of experimental data of hadrons and simulate the hadronic decay process executing the numerical code of the hadron fragmentation even in the high energy region where we have no experimental data. Using the light element abundances computed in the hadron-injection scenario, we derive a constraint on properties of such a particle by comparing our theoretical results with observations.

astro-ph↗

arXiv2001

Polarization tensors in a strong magnetic field

The vacuum polarization tensor in strong external magnetic fields has been evaluated numerically for various strengths of magnetic fields and momenta of photons under the threshold of the $e^{\pm}$ pair creation. The fitting formula has been obtained which reproduces the calculated results within 10 % of error. The proper time method is employed further to obtain the retarded polarization tensor for finite temperature plasmas.

astro-ph↗

arXiv2001

Rapid-Process Nucleosynthesis in Neutrino-Magneto-Centrifugally Driven Winds

We have studied whether the rotation and magnetic fields in neutrino-driven winds can be key processes for the rapid-process (r-process) nucleosynthesis. We have examined the features of a steady and subsonic wind solutions which extend the model of Weber and Davis (1967), which is a representative solar wind model. As a result, we found that the entropy per baryon becomes lower and the dynamical timescale becomes longer as the angular velocity becomes higher. These results are inappropriate for the production of the r-process nuclei. As for the effects of magnetic fields, we found that a solution as a steady wind from the surface of the proto-neutron star can not be obtained when the strength of the magnetic field becomes $\ge$ $10^{11}$ G. Since the magnetic field in normal pulsars is of order $10^{12}$ G, a steady wind solution might not be realized there, which means that the models in this study may not be adopted for normal proto-neutron stars. In this situation, we have little choice but to conclude that it is difficult to realize a successful r-process nucleosynthesis in the wind models in this framework.

astro-ph↗

arXiv2000

Radiative decay of a massive particle and the non-thermal process in primordial nucleosynthesis

We consider the effects on big bang nucleosynthesis (BBN) of the radiative decay of a long-lived massive particle. If high-energy photons are emitted after the BBN epoch ($t \sim 1 - 10^3$ sec), they may change the abundances of the light elements through photodissociation processes, which may result in a significant discrepancy between standard BBN and observation. Taking into account recent observational and theoretical developments in this field, we revise our previous study constraining the abundance of the radiatively-decaying particles. In particular, on the theoretical side, it was recently claimed that the non-thermal production of $^6$Li, which is caused by the photodissociation of $\hefour$, most severely constrains the abundance of the radiatively-decaying particle. We will see, however, it is premature to emphasize the importance of the non-thermal production of $^6$Li because (i) the theoretical computation of the $^6$Li abundance has large uncertainty due to the lack of the precise understanding of the $^6$Li production cross section, and (ii) the observational data of $^6$Li abundance has large errors.

hep-ph↗

arXiv2000

Neutralino Warm Dark Matter

In the supersymmetric (SUSY) standard model, the lightest neutralino may be the lightest SUSY particle (LSP), and it is is a candidate of the dark matter in the universe. The LSP dark matter might be produced by the non-thermal process such as heavy particle decay after decoupling of the thermal relic LSP. If the produced LSP is relativistic, and does not scatter enough in the thermal bath, the neutralino LSP may contribute as the warm dark matter (WDM) to wash out the small scale structure of O(0.1) Mpc. In this letter we calculate the energy reduction of the neutralino LSP in the thermal bath and study whether the LSP can be the WDM. If temperature of the production time T_I is smaller than 5MeV, the bino-like LSP can be the WDM and may contribute to the small-scale structure of O(0.1) Mpc. The Higgsino-like LSP might also work as the WDM if T_I< 2MeV. The wino-like LSP cannot be the WDM in the favoured parameter region.

hep-ph↗

arXiv2000

Pulsar Kick and Asymmetric Iron Velocity Distribution in SN 1987A

We have investigated the relation of the direction of the momentum among the matter, neutrino, and proto-neutron star in a collapse-driven supernova in order to discuss the pulsar kick. In particular, we have investigated the effects of the pulsar motion on the explosion, which are neglected in the previous study. As a result, it is suggested that the direction of the total momentum of the matter and neutrino is opposite to that of the momentum of the proto-neutron star in the asymmetric explosion models. This is because the center of the explosion deviates from the center of the progenitor due to the pulsar motion. This picture is common among the asymmetric explosion models. So if we assume that the pulsar motion is caused by an asymmetric supernova explosion, the neutron star born in SN 1987A, which has not been found yet, will be moving in the southern part of the remnant. In other words, if we can find one neutron star in SN 1987A on the south part of the remnant, asymmetric explosion models will be supported by the observation better than the binary models.

astro-ph↗

arXiv2000

Features of Nucleosynthesis and Neutrino Emission from Collapsars

In this study we present two indicators that will reflect the difference between collapsars and normal collapse-driven supernovae. They are products of explosive nucleosynthesis and neutrino emission. In the collapsar model, it is natural to consider that the product of explosive nucleosynthesis depends on the zenith angle because the system becomes highly asymmetric in order to generate a fire ball. We also consider the detectability of the HNRs which is located nearby our Galaxy. As a result, the number of the HNRs is estimated to be 5 $\times$ ($10^2$ -- $10^{-3}$), whose chemical composition can be spatially resolved. Using the optimistic estimate, more HNRs will be found and it will be possible to discuss on the chemical composition statistically. As for the energy spectrums of neutrinos, they are not thermalized in a collapsar because the density of the accretion disk is much lower than that of a neutron star. The energy spectrums of (anti-)electron neutrinos from hypernovae will be mainly determined by the process of electron (positron) capture on free proton (neutron). It is also noted that high energy tail is not dumped in the case of hypernovae because the density of emitting region is low. Total energy of neutrino from hypernovae will depend on a lot of physical parameters such as total accreting mass and mass accretion rate, which are quite contrary to the situation of the normal collapse-driven supernovae. Therefore there will be a large variety of total neutrino's energies among collapsars. In the case of SN 1998bw, we think that the matter around the equatorial plane might be ejected from the system, which resulted in the formation of relatively weak jets and faint GRB 980425.

astro-ph↗

arXiv2000

MeV-scale Reheating Temperature and Thermalization of Neutrino Background

The late-time entropy production by the massive particle decay induces the various cosmological effects in the early epoch and modify the standard scenario. We investigate the thermalization process of the neutrinos after the entropy production by solving the Boltzmann equations numerically. We find that if the large entropy are produced at t $\sim$ 1 sec, the neutrinos are not thermalized very well and do not have the perfect Fermi-Dirac distribution. Then the freeze-out value of the neutron to proton ratio is altered considerably and the produced light elements, especially He4, are drastically changed. Comparing with the observational light element abundances, we find that $T_R$ < 0.7 MeV is excluded at 95 % C.L. We also study the case in which the massive particle has a decay mode into hadrons. Then we find that $T_R$ should be a little higher, i.e. $T_R$ > 2.5 MeV - 4 MeV, for the hadronic branching ratio $B_h = 10^{-2} - 1$. Possible influence of late-time entropy production on the large scale structure formation and temperature anisotropies of cosmic microwave background is studied. It is expected that the future satellite experiments (MAP and PLANCK) to measure anisotropies of cosmic microwave background radiation temperature can detect the vestige of the late-time entropy production as a modification of the effective number of the neutrino species $N_ν^{\rm eff}$.

astro-ph↗

arXiv1999

Primordial Black Holes and Primordial Nucleosynthesis I: Effects of Hadron Injection from Low Mass Holes

We investigate the influence of hadron injection from evaporating primordial black holes (PBHs) in the early stage of the primordial nucleosynthesis era (t = 10^{-3} - 10^4 sec). The emitted quark-antiquark pairs or gluons immediately fragment into a lot of hadrons and scatter off the thermal plasma which is constituted by photons, electrons and nucleons. For the relatively low mass holes we point out that the dominant effect is the inter-conversion between ambient proton and neutron through the strong interaction induced by the emitted hadrons. Even after the freeze-out time of the week interactions between neutron and proton, more neutrons are produced and the synthesized light element abundances could be drastically changed. Comparing the theoretical predictions with the observational data, we constrain the PBH's density and their lifetime. We obtain the upper bound for PBH's initial mass fraction, β< 10^{-20} for 10^8 g < M < 10^{10} g, and β< 10^{-22} for 10^{10} g < M < 3 * 10^{10} g.

astro-ph↗

arXiv1999

Cosmological Constraints on Late-time Entropy Production

We investigate cosmological effects concerning the late-time entropy production due to the decay of non-relativistic massive particles. The thermalization process of neutrinos after the entropy production is properly solved by using the Boltzmann equation. If a large entropy production takes place at late time t$\simeq$ 1 sec, it is found that a large fraction of neutrinos cannot be thermalized. This fact loosens the tight constraint on the reheating temperature T_R from the big bang nucleosynthesis and T_R could be as low as 0.5 MeV. The influence on the large scale structure formation and cosmic microwave background anisotropies is also discussed.

astro-ph↗

arXiv1998

Massive Decaying Tau Neutrino and Big Bang Nucleosynthesis

Comparing Big Bang Nucleosynthesis predictions with the light element abundances inferred from observational data, we can obtain the strong constraints on some neutrino properties, e.g. number of neutrino species, mass, lifetime. Recently the deuterium abundances were measured in high red-shift QSO absorption systems. It is expected that they are close to the primordial values, however, two groups have reported inconsistent values which are different in one order of magnitude. In this paper we show how we can constrain on $τ$ neutrino mass and its lifetime in each case when we adopt either high or low deuterium data. We find that if $0.01 \sec \lesssim τ_{\nutau} \lesssim 1 \sec$ and $10\mev \lesssim m_{\nutau} \lesssim 24\mev$, the theoretical predictions agree with the low D/H abundances. On the other hand if we adopt the high D/H abundances, we obtain the upper bound of $τ$ neutrino mass, $m_{\nutau}\lesssim 20 \mev$.

astro-ph↗

arXiv1997

Big Bang Nucleosynthesis and Lepton Number Asymmetry in the Universe

Recently it is reported that there is the discrepancy between big bang nucleosynthesis theory and observations (BBN crisis). We show that BBN predictions agree with the primordial abundances of light elements, He4, D, He3 and Li7 inferred from the observational data if an electron neutrino has a net chemical potential xi_{nu_e} due to lepton asymmetry. We estimate that xi_{nu_e} = 0.043^{+0.040}_{-0.040} (95% C.L.) and Omega_bh^2 = 0.015^{+0.006}_{-0.003} (95% C.L.).

astro-ph↗