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 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., $B = L_e = L_μ= L_τ= 0$ 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.

Figures (2)

• Figure 1: Example solutions of the set of Boltzmann equations in Eqs. \ref{['eq:BE1']} to \ref{['eq:BE4']} in the weak / strong wash-in regime (top / bottom row). Solid / dashed lines denote positive / negative comoving asymmetries $Q$. The $Q_{B-L}$ values at $z \gg 1$ allow one to read off the efficiency factor defined in Eq. \ref{['eq:kappa']}, $Q_{B-L}\left(z\gg 1\right) = C_{\rm win}\,\kappa$. The solutions in the first / second column correspond to positive / negative $\kappa$ values. In all four plots, the RHN branching ratios $p_{e,\mu,\tau}$ are set to the optimal values that maximize $\left|\kappa\right|$ for the given neutrino masses $\widetilde{m}_1$ and $M$.
• Figure 2: $\kappa_+ > 0$ (solid red contours) and $\kappa_- < 0$ (dashed blue contours) solutions for the efficiency factor, which assume optimal RHN branching ratios $p_{e,\mu,\tau}$, as functions of the neutrino masses $\widetilde{m}_1$ and $M$. The transparent bands in the zoomed-in portion on the right-hand side indicate the ranges $\left|\kappa_\pm\right| = 2\cdots4\times 10^{-4}$, with the central lines corresponding to the $\left|\kappa_\pm\right| = 3\times 10^{-4}$ contours.