Baryon asymmetry and lower bound on right handed neutrino mass in fast expanding Universe: an analytical approach

Abstract

The expansion rate of the Universe deviates from its standard value when the total energy density includes contribution from a new scalar field apart from the radiation energy density. The non-trivial modifications incurred in the Boltzmann equations render the well known analytical solutions unsuitable in non standard scenario. In the present study we derive analytical expressions for the efficiency factor (which is nothing but solution of set of Boltzmann equations) using certain legible approximations. A fair degree of accuracy of these formulas have been observed by juxtaposing the analytical results with that obtained through numerical solution of Boltzmann equations. Faster expansion of the Universe results in decrement of the effective decay parameter which brings down the amount of washout of asymmetry due to inverse decay. Thus in non-standard cosmology scenario, a larger fraction of the asymmetry (generated at early epoch) is expected to survive till present epoch. Alteration of the cosmology does not affect the underlying particle physics model responsible for the generation of the CP asymmetry. Therefore starting from an identical particle physics model we will end up with a larger final baryon asymmetry in the non-standard scenario. It hints towards the possible relaxation of the lower bound of the lightest right handed neutrino mass required to produce adequate asymmetry which is in agreement with current experimental data.