S3 flavor symmetry and leptogenesis

We consider leptogenesis in a minimal S3 extension of the standard model with an additional Z2 symmetry in the leptonic sector. It is found that the CP phase appearing in the mass matrix of the left-handed neutrinos is the same as that for the CP asymmetries responsible for leptogenesis. Because of the discrete S3 x Z2 flavor symmetries, the CP asymmetries are strongly suppressed. To obtain a realistic size of the baryon number asymmetry in the universe, we therefore have to assume that resonant enhancement of the CP asymmetries takes place, and that three degenerate right-handed neutrino masses of O(10) TeV are present. (orig.)