Proton decay

The most spectacular prediction of all Grand Unified Theories - and perhaps the only one to be confronted with experiment - is beyond doubt the instability of the proton. Employing for the decays into pseudoscalar mesons the soft meson technique - which has been developed within the framework of current algebra under the hypothesis of PCAC - and for the decays into vector mesons the assumption of pole dominance, the nucleon decay widths are reduced to a set of effective baryon-lepton transition amplitudes, which provide a very convenient tool for the treatment of the phenomenology of baryon number non-conserving processes. The hadronic matrix elements of the baryon and lepton number violating interactions formulated in terms of quark field operators are evaluated within the Bethe-Salpeter formalism, which guarantees a fully relativistic covariant description of hadrons as bound states of constituent quarks. The resulting branching ratios for the two-body nuclear decays allow for an unambiguous discrimination between the different Grand Unified Models by the experiments dedicated to the investigation of the proton instability. The nucleon lifetime is predicted to some 4.1030 yr in the minimal Grand Unified Model, at the very edge of the experimentally allowed region but still somewhat larger than the values obtained by recent calculations based on different approaches. (Author)