Electroweak Baryon Number Violation and Constraints on Left-handed   Majorana Neutrino Masses
  During a large period of time, the anomalous baryon number violating interactions are in equilibrium, when the $(B+L)$ asymmetry is washed out. If there is any lepton number violation during this period, that will also erase the $(B-L)$ asymmetry. As a result, survival of the baryon asymmetry of the universe pose strong constraints on lepton number violating interactions. We review here the constraints on the left-handed Majorana neutrino masses arising from this survival requirement of the baryon asymmetry of the universe. We then briefly review models of leptogenesis, where lepton number violation is used to generate a baryon asymmetry of the universe and hence the constraints on the Majorana neutrino mass is relaxed. 
Utpal Sarkar