The Neutrino Mass Bound from Leptogenesis Revisited

Recent years have seen a great improvement in the computation of $CP$-conserving and $CP$-violating equilibration rates for leptogenesis. These are relevant for the relativistic regime of the sterile Majorana fermions and the dynamics of the Standard Model particles acting as spectator processes. In order to probe the regime of large $({\cal O}(10^2))$ washout parameters, we add $\Delta L = 2$ washout processes, which we derive in the CTP-formalism. To demonstrate their significance, we apply state-of-the-art computational techniques to a simple yet well-motivated phenomenological scenario: unflavored leptogenesis in a hierarchical type-I seesaw model. We then perform a parameter scan of the final baryon asymmetry and find a constraint $m_\text{lightest} \lesssim 0.15 \, \text{eV}$ on the absolute neutrino mass scale, which is slightly less stringent than previously reported bounds obtained without the aforementioned improvements. The relaxation of the bounds is mainly due to partially equilibrated spectator fields, which protect part of the asymmetry from washout and lead to larger final asymmetries. While this might seem like a minor correction, the actual dynamics of the fields is substantially altered by these effects. Even though we focused on a particularly simple scenario for leptogenesis, the methods employed here can and should be extended to other models, thus giving us a more accurate picture of the different leptogenesis scenarios.