Leptogenesis With Many Neutrinos

We consider leptogenesis in scenarios with many neutrino singlets. We find that the lower bound for the reheating temperature can be significantly relaxed with respect to the hierarchical three neutrino case. We further argue that the upper bound for the neutrino mass scale from leptogenesis gets significantly lifted in these scenarios. As a specific realization, we then discuss an extra-dimensional model, where the large number of neutrinos is provided by Kaluza-Klein excitations.