Protected Baryon Number in Models with Majorana Neutrino

Neutrino oscillation experiments reveal that neutrinos have very small but finite masses. Since neutrinos have no masses in the standard model, we need to extend the standard model to induce the neutrino masses which coincide with the results of the neutrino oscillation experiments. It is possible that neutrino can have both of Dirac mass and Majorana mass. Majorana mass has the feature that it violates the lepton number explicitly, which is conserved at classical level in the standard model. Here we consider the case that neutrinos have Majorana masses. When we minimally extend the standard model within the framework of the SU(2)L × U(1)Y gauge theory for neutrino mass generation, typical three models exist, which are seesaw[1, 2, 3], Zee[4], and SU(2)L triplet Higgs model[5]. The baryon number in the present universe is well known to be [6]