Probing grand unification with fermion masses,neutrino oscillations and proton decay

It is noted that a set of facts points to the relevance in four dimensions of conventional supersymmetric unification based on minimally a string-unifiedG(224) symmetry, or maximallySO(10). These include: (i) the observed family structure, (ii) quantization of electric charge, (iii) meeting of the three gauge couplings, (iv) neutrino oscillations (in particular the value of δm ²(νμ−δ _τ), suggested by SuperK), (v) the intricate pattern of the masses and mixings of the fermions, including the smallness ofV _cb and the largeness ofθ _μνμτ ^osc , and (vi) the need for B-L as a generator to implement baryogenesis (via leptogenesis). A concrete proposal is presented within a predictiveSO(10)/G(224) framework that successfully describes the masses and mixings of all fermions, including the neutrinos — with eight predictions, all in agreement with observation. Within this framework, a systematic study of proton decay is carried out, which (a) pays special attention to its dependence on the fermion masses, (b) limits the threshold corrections so as to preserve natural coupling unification, and (c) uses recently improved values of the matrix element and renormalization effects. Allowing for both minimal supersymmetric standard model (MSSM) and its proposed variant, the so-called extended supersymmetric standard model (ESSM), as effective low-energy the ories, the study shows that a conservative upper limit on the proton lifetime is about (l–2)× 10³⁴ years, with ·⁻K⁺ being the dominant decay mode, and quite possibly μ⁺ K ⁰ ande ⁺π⁰ being prominent. This in turn strongly suggests that an improvement in the current sensitivity by a factor of five to ten ought to reveal proton decay. For comparison, some alternatives to the conventional approach to unification pursued here are mentioned at the end. Invited paper presented at the International Summer School held at ICTP, Trieste (June, 2001) and at WHEPP-7 Conference, Allahabad, India (January, 2002). This is an updated version of the paper presented at the Erice School (September, 2000), hep-ph/0106082.