From Particle Physics to Astroparticle Physics: Proton Decay and the Rise of Non-accelerator Physics

The search for proton decay was motivated by simple questions about the content of the observable universe. Why is matter so stable and why do we not see antimatter of primordial origin? The symmetry of the standard model of particle physics would have required that matter and antimatter annihilated in the early universe. In 1968, Sacharov showed that the matter–antimatter asymmetry could have formed in a state of thermal non-equilibrium of the universe, as given in big bang cosmology, together with the well-confirmed C and CP violations, and proton decay. The latter phenomenon could be only investigated in large none-accelerator experiments. The SU(5) extension of the standard model implied a proton lifetime of about 1029 years. With detectors consisting of 1 000 tons of matter and located deep under the Earth surface, such as the French–German Frejus iron-calorimeter, in the mid 1980s one expected to detect several proton decays per year. Here, we report on the way leading from accelerator laboratories to underground physics, which paradoxically enough turned out to studying cosmic rays. There has not been any evidence for the instability of protons, and lifetime limits of more than 1034 years have been obtained. However, great progress in particle physics and in the physics of cosmic rays could be achieved with neutrinos.