Flavour in Soft Leptogenesis

Abstract

Successful Soft Leptogenesis (SL) requires a relatively low mass scale for the SU(2) singlet neutrinos of $10^5-10^8$ GeV. However, conventional SL (unflavoured) requires an unnaturally small soft supersymmetry(SUSY)-breaking bilinear $B \ll \mathcal{O}({\rm TeV})$ coupling for successful leptogenesis. On the other hand, in this regime, the interactions mediated by $\tau$, $\mu$ (and even $e$) charged lepton Yukawa interactions are in equilibrium, making the lepton number asymmetries and the washouts flavour dependent. Hence, it is crucial to take into account the flavour effects. Considering a general soft SUSY-breaking trilinear $A$ couplings, it is possible to enhance the efficiency up to $\mathcal{O}(1000)$ compared to the unflavoured case. With the enhanced efficiency, we can raise the $B$ up to TeV scale for successful leptogenesis. Taking into account the low energy constraints, we verify that the fast lepton flavour violation processes induced by the soft SUSY-breaking slepton masses would not destroy the enhancement.