Quintessential kination and leptogenesis

Abstract

Thermal leptogenesis induced by theCP-violating decay of a right-handed neutrino (RHN) is discussed against the background ofquintessential kination, i.e., in a cosmological model where the energy density of the earlyUniverse is assumed to be dominated by the kinetic term of a quintessence field during someepoch of its evolution. This assumption may lead to very different observational consequencescompared to the case of a standard cosmology where the energy density of the Universe isdominated by radiation. We show that, depending on the choice of the temperatureTr above which kination dominates over radiation, all situations between the strong and thesuperweak wash-out regimes are equally viable for leptogenesis, even with the RHNYukawa coupling fixed to provide the observed atmospheric neutrino mass scale∼0.05 eV. For , i.e., when kination stops to dominate at a time which is not much laterthan when leptogenesis takes place, the efficiency of the process, definedas the ratio between the produced lepton asymmetry and the amount ofCP asymmetry in the RHN decay, can be larger than in the standard scenario of radiationdomination. This possibility is limited to the case when the neutrino mass scale is largerthan about 0.01 eV. The superweak wash-out regime is obtained for , and includes the case when Tr is close to the nucleosynthesis temperature,∼1 MeV. In this latter case the efficiency for leptogenesis is strongly suppressed, but canstill explain the baryon asymmetry observed in the Universe when a resonantCP asymmetry of order 1 is assumed. Irrespective ofTr, we always find a sufficient window above the electroweak temperatureT∼100 GeV for the sphaleron transition to thermalize, so that the lepton asymmetry can alwaysbe converted to the observed baryon asymmetry.