Observable consequences of partially degenerate leptogenesis

In the context of the seesaw mechanism, it is natural that the large solar and atmospheric neutrino mixing angles originate separately from large 2 x 2 mixings in the neutrino and charged-lepton sectors, respectively, and large mixing in the neutrino couplings is in turn more plausible if two of the heavy singlet neutrinos are nearly degenerate. We study the phenomenology of this scenario, calculating leptogenesis by solving numerically the set of coupled Boltzmann equations for out-of-equilibrium heavy singlet neutrino decays in the minimal supersymmetric seesaw model. The near-degenerate neutrinos may weigh less than or equal to 10(8) GeV, avoiding the cosmological gravitino problem. This scenario predicts that Br(mu --> egamma) should be strongly suppressed, because of the small singlet neutrino masses, whilst Br(tau --> mugamma) may be large enough to be observable in B-factory or LHC experiments. If the light neutrino masses are hierarchical, we predict that the neutrinoless double-beta decay parameter m(ee) approximate to rootDeltam(sol)(2) sin(2)theta(12). (C) 2002 Elsevier Science B.V. All rights reserved.