A fuller flavour treatment of leptogenesis

Abstract

The theoretical foundations of the seesaw mechanism and baryogenesis are developed, prior to a discussion of leptogenesis - the idea that the baryon asymmetry of the universe (BAU) originates in the physics of the seesaw mechanism. Particular emphasis is placed upon several novel lepton flavour effects arising from decays of the next-to-lightest right-handed neutrino (RHN), N₂. Two new effects are identified: the "flavour swap scenario" and "phantom leptogenesis", either of which can give local order-of-magnitude enhancements to a BAU generated via leptogenesis.

Leptogenesis is next reformulated in terms of the density matrix, where decoherence and gauge boson themalisation are taken into account and flavour-basis-independent expressions for the BAU are derived and subsequently specialised to the two RHN model in a two-flavoured basis. This model is then studied extensively, for both standard model and supersymmetric realisations, with N₂ decays fully taken into account. In both cases new N₂-dominated regions are identified, corresponding to models with light sequential dominance. It is shown that these regions get enlarged upon consistent inclusion of the "phantom terms". Results are altered in the supersymmetric case where tan beta becomes large and it is shown that new allowed regions emerge, strikingly so for the case of an inverted hierarchy.

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