Leptogenesis as the origin of matter

▪ Abstract We explore in some detail the hypothesis that the generation of a primordial lepton-antilepton asymmetry (Leptogenesis) early on in the history of the Universe is the root cause for the origin of matter. After explaining the theoretical conditions for producing a matter-antimatter asymmetry in the Universe we detail how, through sphaleron processes, it is possible to transmute a lepton asymmetry—or, more precisely, a (B – L)-asymmetry—into a baryon asymmetry. Because Leptogenesis depends in detail on properties of the neutrino spectrum, we review briefly existing experimental information on neutrinos as well as the seesaw mechanism, which offers a theoretical understanding of why neutrinos are so light. The bulk of the review is devoted to a discussion of thermal Leptogenesis, and we show that for the neutrino spectrum suggested by oscillation experiments, one obtains the observed value for the baryon to photon density ratio in the Universe, independently of any initial boundary conditions. In ...