Scalar triplet flavored leptogenesis: a systematic approach

Abstract

Type-II seesaw is a simple scenario in which Majorana neutrino masses are generated by the exchange of a heavy scalar electroweak triplet. When endowed with additional heavy fields, such as right-handed neutrinos or extra triplets, it also provides a compelling framework for baryogenesis via leptogenesis. We derive in this context the full network of Boltzmann equations for studying leptogenesis in the flavored regime. To this end we determine the relations which hold among the chemical potentials of the various particle species in the thermal bath. This takes into account the SM Yukawa interactions of both leptons and quarks as well as sphaleron processes which, depending on the temperature, may be classified as faster or slower than the Hubble rate. We find that when leptogenesis is enabled by the presence of an extra triplet, lepton flavor effects allow the production of the $B-L$ asymmetry through lepton number conserving CP asymmetries. This scenario becomes dominant as soon as the triplets couple more to leptons than to SM scalar doublets. In this case, the way the $B-L$ asymmetry is created through flavor effects is novel: instead of invoking the effect of $L$-violating inverse decays faster than the Hubble rate, it involves the effect of $L$-violating inverse decays slower than the Hubble rate. We also analyze the more general situation where lepton number violating CP asymmetries are present and actively participate in the generation of the $B-L$ asymmetry, pointing out that as long as $L$-violating triplet decays are still in thermal equilibrium when the triplet gauge scattering processes decouple, flavor effects can be striking, allowing to avoid all washout suppression effects from seesaw interactions. In this case the amount of $B-L$ asymmetry produced is limited only by a universal gauge suppression effect, which nevertheless goes away for large triplet decay rates.