Probing Grand Unification Through Neutrino Oscillations, Leptogenesis, and Proton Decay

Abstract

Evidence in favor of supersymmetric grand unification including that based on the observed family multiplet-structure, gauge coupling unification, neutrino oscillations, baryogenesis, and certain intriguing features of quark-lepton masses and mixings is noted. It is argued that attempts to understand (a) the tiny neutrino masses (especially Delta m^2 (nu_2 -nu_3)), (b) the baryon asymmetry of the universe (which seems to need leptogenesis), and (c) the observed features of fermion masses such as the ratio m_b/m_tau, the smallness of V_cb and the maximality of theta_{nu_mu-nu_tau}, seem to select out the route to higher unification based on an effective string-unified G(224) = SU(2)_L x SU(2)_R x SU(4)^c or SO(10)-symmetry, operative in 4D, as opposed to other alternatives. A predictive framework based on an effective SO(10) or G(224) symmetry possessing supersymmetry is presented that successfully describes the masses and mixings of all fermions including neutrinos. It also accounts for the observed baryon asymmetry of the universe by utilizing the process of leptogenesis, which is natural to this framework. It is argued that a conservative upper limit on the proton lifetime within this SO(10)/G(224)-framework, which is so far most successful, is given by (1/3-2) x 10^34 years. This in turn strongly suggests that an improvement in the current sensitivity by a factor of five to ten (compared to SuperK) ought to reveal proton decay. Implications of this prediction for the next-generation nucleon decay and neutrino-detector are noted.