The smallest neutrino mass

We consider models where one Majorana neutrino is massless at tree level (like the see-saw with two right-handed neutrinos), and compute the contribution to its mass m generated by two-loop quantum corrections. The result is m∼10−13 eV in the SM and m∼10−10 eV⋅(tanβ/10)4 in the MSSM, compatible with the restricted range suggested by Affleck-Dine baryogenesis.