Neutrino Masses: Shedding Light on Unification and Our Origin

Abstract

In the first part of the talk, three key ideas proposed in the 1970s, and in particular their combined role in providing an understanding of the neutrino-masses as well as of the baryon-asymmetry of the universe, are expounded. The ideas in question include: (i) The symmetry SU(4)-color, which introduces the right-handed neutrino as an essential member of each family and also provides (rather reliably) the Dirac mass of the tau-neutrino by relating it to the top quark mass; (ii) SUSY grand unification together with the scale of the meeting of the three gauge couplings, which provides the scale for the superheavy Majorana masses of the RH neutrinos; and (iii) the seesaw mechanism, which combines the Dirac and the superheavy Majorana masses of the neutrinos obtained as above to yield naturally light LH neutrinos and in particular the right magnitude for m(nu-tau). In the second part, an attempt is made, based in part on recent works, to show how a set of diverse phenomena including (a) fermion masses, (b) neutrino oscillations, (c) CP and flavor violations, and (d) baryogenesis via leptogenesis can fit together neatly within a single predictive framework based on an effective symmetry group G(224) = SU(2)_L x SU(2)_R x SU(4)-color or SO(10), possessing supersymmetry. CP and flavor violations arising within this framework include enhanced rates (often close to observed limits) for mu -> e + gamma and tau -> mu + gamma and also measurable electric dipole moments of the neutron and the electron. Expectations arising within the same framework for proton decay are summarized at the end. It is stressed that the two notable missing pieces of this framework, which is otherwise so successful, are supersymmetry and proton decay.