Abstract
The primordial lithium abundance inferred from spectroscopic observations of metal-poor stars is times smaller than the theoretical prediction in the standard big bang nucleosynthesis (BBN) model. We assume a simple model composed of standard model particles and a sterile neutrino with mass of which decays long after BBN. We then investigate cosmological effects of a sterile neutrino decay, and check if a sterile neutrino can reduce the primordial lithium abundance. We formulate the injection spectrum of nonthermal photon induced by the decay. We take into account the generation of electrons and positrons, ’s, and active neutrinos at the decay, the primary photon production via the inverse Compton scattering of cosmic background radiation (CBR) by energetic , and electromagnetic cascade showers induced by the primary photons. The steady state injection spectrum is then derived as a function of the mass and the photon temperature. The decay produces energetic active neutrinos which are not thermalized, and ’s which are thermalized. We then derive formulas relevant to the decay rates and formulas for the baryon-to-photon ratio and effective neutrino number . The initial abundance, mass, and lifetime of are taken as free parameters. We then consistently solve (1) the cosmic thermal history, (2) nonthermal nucleosynthesis induced by the nonthermal photons, (3) the value, and (4) the value. We find that an effective destruction can occur only if the sterile neutrino decays at photon temperature . Amounts of energy injection at the decay are constrained from limits on primordial D and abundances, the value, and the CBR energy spectrum. We find that is photodisintegrated and the Li problem is partially solved for the lifetime and the mass . destruction by more than a factor of 3 is not possible because of an associated D overdestruction. In the parameter region, the value is decreased slightly, while the value is increased by a factor of . In this study, errors in photodisintegration cross sections of and that have propagated through the literature are corrected, and new functions are derived based on recent nuclear experiments. It is found that the new photodisintegration rates are 2.3 to 2.5 times smaller than the old rates. The correct cross sections thus indicate significantly smaller efficiencies of and photodisintegration. Abundances of sterile neutrino necessary for the reduction are much smaller than thermal freeze-out abundances. The relic sterile neutrino, therefore, must be diluted between the freeze-out and BBN epochs by some mechanism.
9 More- Received 29 March 2014
DOI:https://doi.org/10.1103/PhysRevD.90.083519
© 2014 American Physical Society