The Calculation of the Decay Rate of the Proton
All attempts which have been made to construct a model which unifies the strong, weak, and electromagnetic interactions predict the existence of currents which transform quarks into leptons and this generally leads to the prediction that the proton is unstable. Now usually when a theorist predicts the existence of an exotic interaction for which there is no experimental evidence, he keeps one step ahead of the experimentalists by arguing that the mass of the particles which mediate this exotic interaction are arbitrarily massive so that as the experimentalists decrease the upper bound on this interaction the theorist increases these masses. In a certain class of grand unified theories this option is closed since the masses of the particles mediating the interactions which lead to proton decay are exactly calculable. This class of grand unified theories is the class obeying the “desert hypothesis” namely the hypothesis that between the threshold for production of W’s and Z’s (~100 GeV) and grand unification there are no new degrees of freedom which open up. The simplest such model is the SU(5) model of Georgi and Glashow1, and since the point of unification of the strong, weak and electromagnetic interactions is very insensitive† to the exact details of the model provided it obeys the desert hypothesis, I shall work with the SU(5) model.
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