All materials in this space are made of atoms, which consist of nucleus and electrons. Furthermore, nucleus is a composite of protons and neutrons. Neutron is slightly heavier than proton and neutron can decay into proton, electron, and neutrino (beta decay), but it has been thought that proton is eternally stable because it is the ligIs it really true? Grand Unified Theory, which unifies strong, weak, and electromagnetic interactions, predicts proton will decay into lighter particle like mesons and leptons. The dominant decay mode is that proton decays into a neutral pion and electron. The neutral pion immediately decays into two gamma rays, thus we can observe three electron-like rings in Super-Kamiokande (Fig.1). If protons decay, all materials in the world will be broken in future. But, don't worry! The predicted proton lifetime is much longer than age of the universe.
Then, how can we measure such long lifetime? Lifetime of a particle is defined as the time which number of the particle decrease to 1/2.7 from start time of measurement. If we can collect many protons and some of them decay, we can estimate proton lifetime unless waiting for so long time. Super-Kamiokande uses 50,000 tons of pure water and it contains 7x1033 protons. We are measuring proton lifetime with huge number of protons.
Super-Kamiokande has started measurement since 1996 and is running more than 10 year, however, we have not observed any evidence of proton decay yet. From this result, proton lifetime is estimated to be more than 1034 years (age of the universe ~1010 years). If we find proton decay, it will be key of a door for Grand Unified Theory beyond the Standard Theory. Super-Kamiokande will keep running towards a new horizon of the world of particle physics.