"Protons live a long time but perhaps not forever. Several theories predict that protons can decay, and a handful of experiments have tried to detect such an event. The Super-Kamiokande experiment in Japan has the longest track record in the search for proton decay, and its researchers have now published a new lower bound on the proton lifetime that is 2.5 times greater than their previous bound. The proton's observed stability places constraints on certain extensions of the standard model of particle physics.


"Proton decay is an expected outcome of most grand unified theories, or GUTs, which meld together the three main particle forces -- strong, weak, and electromagnetism -- at high energy. A certain class of GUTs, for example, predicts that a proton should decay into a positron and π meson with a lifetime of about 1031 years, which means roughly 1 decay per year in a sample of 1031 protons. Experiments have already ruled this possibility out.