Bounds on right-handed neutrino parameters from observable leptogenesis

Abstract

We revisit the generation of a matter-antimatter asymmetry in the minimal extension of the Standard Model with two singlet heavy neutral leptons (HNL) that can explain neutrino masses. We derive an accurate analytical approximation to the solution of the complete linearized set of kinetic equations, which exposes the non-trivial parameter dependencies in the form of parameterization-independent CP invariants. The identification of various washout regimes relevant in different regions of parameter space sheds light on the relevance of the mass corrections in the interaction rates and clarifies the correlations of baryogenesis with other observables. In particular, by requiring that the measured baryon asymmetry is reproduced, we derive robust upper or lower bounds on the HNL mixings depending on their masses, and constraints on their flavour structure, as well as on the CP-violating phases of the PMNS mixing matrix, and the amplitude of neutrinoless double-beta decay. We also find certain correlations between low and high scale CP phases. Especially emphasizing the testable part of the parameter space we demonstrate that our findings are in very good agreement with numerical results. The methods developed in this work can help in exploring more complex scenarios.

Report of the Topical Group on Physics Beyond the Standard Model at Energy Frontier for Snowmass 2021

“…The heavy neutrino mass range displayed in figure 16/2-35 allows for successful leptogenesis during heavy neutrino freeze-in [272,273] or freeze-out [274]. Updated parameter space scans [275][276][277] show that the LHC, FCC-ee and other future colliders can access the parameter region where heavy neutrinos can simultaneously explain the light neutrino masses and the baryon asymmetry of the universe. We can view the extra Higgs doublet as a new portal to the VLLs, similarly to SUSY scenarios heavy Higgs portal to electroweakino and scalar lepton sectors [278].…”

Bounds on right handed neutrinos from observable Leptogenesis

“…It is described by only 6 free parameters which are one Yukawa scale, two HNL masses and 3 phases encoding CP violation, i.e. the Dirac and Majorana PMNS phases and a high scale phase, see [4] for the parametrization. Our analytical approximations for the baryon asymmetry depends on CP flavour invariants, which can be used to derive robust connections between the generation of the baryon asymmetry and other observables, which I will discuss in the next section, but more details can be found in [4].…”

“…By employing the perturbative methods discussed in the previous section, we can derive the constraints imposed by successful baryogenesis on the masses and mixings of the HNLs as well as the CP violating phases. Here I consider two particular examples, but see [4] for further details. On the one hand, I show how to derive an absolute upper bound on the mixing of the HNLs with the active neutrinos for which leptogenesis is possible.…”

“…In this case, the relevant process to generate the BAU is via HNL oscillations during its freeze-in [1,2]. Although this model has been extensively studied in the past (see [3] for a review), an accurate analytical understanding of the parameter space that leads to successful baryogenesis was first derived in [4]. In particular, the use of parametrization-independent CP flavour invariants allows to express the analytical solutions in terms of other flavour observables.…”

“…This allows to either analytically predict the constraints on the BAU arising from putative future measurements of HNLs, CP violation in neutrino oscillations and neutrinoless double-beta decay or, alternatively, to set bounds on HNL parameters from the BAU. In the following I review the method we derived in [4] to analytically solve the complete linearized set of quantum Boltzmann equations. The solutions take into account mass effects in the interaction rates and cover all washout regimes.…”