Kazunori Kohri

Big-Bang nucleosynthesis and hadronic decay of long-lived massive particles

We study the big-bang nucleosynthesis (BBN) with the long-lived exotic particle, called X. If the lifetime of X is longer than \sim 0.1 sec, its decay may cause non-thermal nuclear reactions during or after the BBN, altering the predictions of the standard BBN scenario. We pay particular attention to its hadronic decay modes and calculate the primordial abundances of the light elements. Using the result, we derive constraints on the primordial abundance of X. Compared to the previous studies, we have improved the following points in our analysis: The JETSET 7.4 Monte Carlo event generator is used to calculate the spectrum of hadrons produced by the decay of X; The evolution of the hadronic shower is studied taking account of the details of the energy-loss processes of the nuclei in the thermal bath; We have used the most recent observational constraints on the primordial abundances of the light elements; In order to estimate the uncertainties, we have performed the Monte Carlo simulation which includes the experimental errors of the cross sections and transfered energies. We will see that the non-thermal productions of D, He3, He4 and Li6 provide stringent upper bounds on the primordial abundance of late-decaying particle, in particular when the hadronic branching ratio of X is sizable. We apply our results to the gravitino problem, and obtain upper bound on the reheating temperature after inflation.

New cosmological constraints on primordial black holes

We update the constraints on the fraction of the Universe going into primordial black holes in the mass range 10{sup 9}-10{sup 17} g associated with the effects of their evaporations on big bang nucleosynthesis and the extragalactic photon background. We include for the first time all the effects of quark and gluon emission by black holes on these constraints and account for the latest observational developments. We then discuss the other constraints in this mass range and show that these are weaker than the nucleosynthesis and photon background limits, apart from a small range 10{sup 13}-10{sup 14} g, where the damping of cosmic microwave background anisotropies dominates. Finally we review the gravitational and astrophysical effects of nonevaporating primordial black holes, updating constraints over the broader mass range 1-10{sup 50} g.

Hadronic decay of late-decaying particles and big-bang nucleosynthesis

Abstract We study the big-bang nucleosynthesis (BBN) scenario with late-decaying exotic particles with lifetime longer than ∼1 s. With a late-decaying particle in the early universe, predictions of the standard BBN scenario can be significantly altered. Therefore, we derive constraints on its primordial abundance. We pay particular attention to hadronic decay modes of such particles. We see that the non-thermal production process of D, 3 He and 6 Li provides a stringent upper bound on the primordial abundance of late-decaying particles with hadronic branching ratio.

Big-bang nucleosynthesis and gravitinos

We derive big-bang nucleosynthesis (BBN) constraints on both unstable and stable gravitino taking account of recent progress in theoretical study of the BBN processes as well as observations of primordial light-element abundances. In the case of unstable gravitino, we set the upper limit on the reheating temperature assuming that the primordial gravitinos are mainly produced by the scattering processes of thermal particles. For stable gravitino, we consider B-ino, stau, and sneutrino as the next-to-the-lightest supersymmetric particle and obtain constraints on their properties. Compared with the previous works, we improved the following points: (i) we use the most recent observational data, (ii) for gravitino production, we include contribution of the longitudinal component, and (iii) for the case with unstable long-lived stau, we estimate the bound-state effect of stau accurately by solving the Boltzmann equation.

Big-bang nucleosynthesis with unstable gravitino and upper bound on the reheating temperature

We study the effects of unstable gravitino on big-bang nucleosynthesis. If the gravitino mass is smaller than {approx}10 TeV, primordial gravitinos produced after inflation are likely to decay after big-bang nucleosynthesis starts, and light-element abundances may be significantly affected by hadro and photodissociation processes as well as by p{r_reversible}n conversion process. We calculate the light-element abundances and derive upper bounds on the reheating temperature after inflation. In our analysis, we calculate decay parameters of the gravitino (i.e. lifetime and branching ratios) in detail. In addition, we perform a systematic study of the hadron spectrum produced by the gravitino decay, taking account of all the hadrons produced by the decay products of the gravitino (including the daughter superparticles). We discuss model dependence of the upper bound on the reheating temperature.

Can Neutrino-cooled Accretion Disks Be an Origin of Gamma-Ray Bursts?

It is often proposed that a massive torus with approximately solar mass surrounding a stellar-mass black hole could be a central engine of gamma-ray bursts. We study the properties of such massive accretion tori (or disks) based on the α viscosity model. For surface density exceeding about 1020 g cm-2, which is realized when ~1 M☉ of material is contained within a disk of size ~5 × 106 cm, we find that (1) the luminosity of photons is practically zero because of significant photon trapping, (2) neutrino cooling dominates over advective cooling, (3) the pressure of degenerate electrons dominates over the pressure of gas and photons, and (4) the magnetic field strength exceeds the critical value of about 4 × 1013 G, even if we take 0.1% of the equipartition value. The possible observable quantum electrodynamical (QED) effects arising from supercritical fields are discussed. Most interestingly, photon splitting may occur, producing a significant number of photons of energies below ~511 keV, thereby possibly suppressing e± pair creation.

MeV-scale reheating temperature and thermalization of the 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

Neutrino-dominated accretion and supernovae

\sim

COSMOLOGICAL CONSTRAINTS ON LATE-TIME ENTROPY PRODUCTION

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

TeV γ‐rays from old supernova remnants

T_R