A lower bound on the right-handed neutrino mass from wash-in leptogenesis

Abstract

Leptogenesis is an attractive scenario for the generation of the baryon asymmetry of the Universe that relies on the dynamics of right-handed neutrinos (RHNs) in the seesaw extension of the Standard Model. In standard thermal leptogenesis, the RHN mass scale $M_N$ is subject to the Davidson--Ibarra bound, $M_N\gtrsim 10^9\text{GeV}$, which builds on the assumption that RHN decays are responsible for the violation of both charge-parity invariance ($CP$) and baryon-minus-lepton number ($B-L$). In this paper, we relax this assumption in the context of the more flexible framework of wash-in leptogenesis, in which $CP$ violation is encoded in the initial conditions and the only remaining task of the RHN decays is to violate $B-L$. Solving the relevant set of Boltzmann equations for vanishing initial baryon and lepton numbers (i.e., $B=L_e=L_{\mu }=L_{\tau }=0$ initially), we find that, in wash-in leptogenesis, the RHN mass scale can be as low as 7~TeV. Wash-in leptogenesis at such low RHN masses requires the presence of a primordial charge asymmetry between right-handed electrons and left-handed positrons. We discuss several possibilities for the origin of such an asymmetry and comment on its implications for the chiral instability of the Standard Model plasma.

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