For nearly 50 years there has been a strong belief that the proton is absolutely stable. The current experimental upper bound on its decay rate is less than one proton decay per 3 tons of matter per year, which corresponds to a mean lifetime of more than 1030 years. Even more sensitive searches for proton decay are now in progress. These are partially motivated by the development of a class of models that combine the presently accepted theories of electromagnetic, weak, and strong interactions into an elegant unified form. Some of these theories predict a proton lifetime short enough for the decays to be detectable by the proposed experiments. If the proton is unstable, a plausible explanation can be given for the apparent excess of matter over antimatter in the universe.
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