In the seesaw model, the baryon asymmetry of the Universe can be generated by the decay of the lightest right-handed neutrino, nu_R. For a hierarchical spectrum of right-handed neutrinos, we show that there is a model independent upper bound on the CP asymmetry produced in these decays: epsilon < 3 m_{ nu_3} M_{ nu_R}/(8 pi <H_u>^2). This implies that epsilon and the mass M_{ nu_R} of the lightest right-handed neutrino are not independent parameters, as is commonly assumed. If m_{ nu_3} = sqrt{ Delta m^2_{ atm}} and the nu_R are produced thermally, then leptogenesis requires a reheat temperature of the Universe T_{ reh} > M_{ nu_R} > 10^8 GeV. Reasonable estimates of nu_R production and the subsequent washout of the asymmetry, as made by Buchmuller and Plumacher, imply M_{ nu_R} > 10^9 GeV, and T_{ reh} > 10^{ 10} GeV. Implications for the gravitino problem are also discussed.
Categories: hep-ph