Limits on Treh for thermal leptogenesis with hierarchical neutrino masses

We make a simple observation that if one of the right-chiral neutrinos is very heavy or its Yukawa couplings to the standard lepton doublets are negligible, so that it effectively decouples from the see-saw mechanism, the prediction for the baryon asymmetry of the universe resulting from leptogenesis depends only on the masses M-1 and M-2 of the remaining two right-chiral neutrinos and the element (Y) over tilde (22)(v) of the neutrino Yukawa coupling. For M-2 greater than or similar to 5M(1) the lower bound on M-1 and also on T-reh, resulting from the requirement of 'successful leptogenesis' is then significantly increased compared to the one computed recently by Buchmuller et al. in the most general case. Within the framework of thermal leptogenesis, the only way to lower this limit is then to allow for sufficiently small mass difference M-2 - M-1. (C) 2003 Published by Elsevier B.V.