The elementary building blocks of ordinary matter are quarks and electrons. Quarks have never been observed in isolation, and only as multi-quark bound states such as baryons. Some baryons, the well-known protons and neutrons that we collectively call nucleons, combine to form the atomic nuclei. In turn, atomic nuclei combine with electrons to form the atoms and molecules we are made of. Protons, as well as neutrons bound in the atomic nuclei, are so far believed to be stable particles, as no process involving their decay has ever been observed. As a consequence, the conservation of baryon number (B) has been introduced in particle physics as an empirical selection rule to explain the observed pattern of particle decay modes and scattering rates.
But why is it interesting to search for proton decay processes, or equivalently for B violations? The main reason is that cosmological observations have unequivocally demonstrated that the Universe is almost entirely made of matter today, as a result of an asymmetry between matter and antimatter that would have developed in the early Universe, and of subsequent matter-antimatter annihilation, strongly suggesting the existence of baryon number violating processes at some point in Universe’s evolution. In addition, in the Standard Model (SM) of particle physics, the conservation of B stems from an accidental global symmetry, and SM extensions generally predict baryon number violation. Also, measurements of the running of the electric, weak and strong gauge couplings at low energies hint at their unification at some high energy. Grand-unified theories (GUTs), a family of well-motivated SM extensions, unify the three interactions into a single one, and predict baryon number violation.
However, if proton decay processes occur in Nature at all, they would be exceedingly rare, with lifetimes far exceeding the age of the Universe, and B violations would be tiny. As such, proton decay searches demand massive detectors, located underground to shield against cosmic rays, and require cutting-edge detector technologies capable of further mitigating the background processes that could mimic this rare signal. The world-leading LAr TPC technology that will be employed by the DUNE far detector modules at SURF is ideally suited to perform sensitive nucleon decay searches over a broad range of potential decay modes.
