Grand unified theories and proton decay

Grand unified theories, in which the strong and electroweak interactions are embedded into an underlying theory with a single gauge coupling constant, are reviewed. A detailed description is given of many of the necessary background topics, including gauge theories, spontaneous symmetry breaking, the standard SU₂ x U₁ electroweak model and its modifications and extensions, Majorana and Dirac neutrino masses, the induced cosmological term, CP violation, quantum chromodynamics and its symmetries, and dynamical symmetry breaking. The Georgi-Glashow SU₅ model is examined in detail. Models based on unitary, orthogonal, exceptional, and semi-simple groups and general constraints on model building are surveyed. Phenomenological aspects of grand unified theories are described, including the determination of the unification mass, the prediction of sin² θw in various models, existing and planned nucleon decay experiments, the predictions for the proton lifetime and branching ratios, general baryon number violating interactions, and the possible explanation of the matter-antimatter asymmetry of the universe. Other aspects of grand unified theories are discussed, including horizontal symmetries, neutrino and fermion masses, topless models, asymptotic freedom, implications for the neutral current, CP violation, superheavy magnetic monopoles, dynamical symmetry breaking, and the hierarchy problem.