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, $M_N \gtrsim 10^9\,\textrm{GeV}$, 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 general 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, we find that, in wash-in leptogenesis, the RHN mass scale can be as low as 7 TeV.