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

TL;DR

This paper demonstrates that wash-in leptogenesis allows for right-handed neutrino masses as low as 7 TeV, significantly below the standard Davidson–Ibarra bound, by leveraging initial charge asymmetries and specific Boltzmann dynamics.

Contribution

It introduces wash-in leptogenesis as a framework that relaxes the traditional mass bound on right-handed neutrinos, enabling lower mass scales through initial asymmetries.

Findings

RHN mass can be as low as 7 TeV in wash-in leptogenesis.

A primordial charge asymmetry between right-handed electrons and left-handed positrons is necessary.

Implications for chiral instability in the Standard Model plasma are discussed.

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 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.