Varying Entropy Degrees of Freedom Effects in Low-Scale Leptogenesis

We analyse in detail the effect of varying entropy degrees of freedom on low-scale leptogenesis models. As an archetypal model, we consider the Tri-Resonant Leptogensis${}$ (TRL) scenario introduced recently by the authors, where the neutrino-Yukawa coupling matrix is dictated by an approximate $\mathbb{Z}_n$ discrete symmetry (with $n=3,6$). TRL models exhibit no preferred direction in the leptonic flavour space and have the remarkable feature that leptogenesis can successfully take place even if all light neutrinos are strictly massless up to one-loop order. Most interestingly, for TRL scenarios with heavy Majorana neutrinos lighter than 100 GeV, temperature varying degrees of freedom associated with the entropy of the plasma have a dramatic impact on the predictions of the Baryon Asymmetry in the Universe (BAU), and sensitively depend on the freeze-out sphaleron temperature $T_{\rm sph}$. We find that this is a generic feature of most freeze-out low-scale leptogenesis models discussed in the literature. In the same context, we consider heavy-neutrino scenarios realising dynamics related to critical unstable qudits in the thermal plasma and assess their significance in generating the BAU. The phenomenological implications of TRL scenarios at the intensity and high-energy frontiers are analysed.