Proton decay is a hypothetical process in which the proton, considered a stable building block of matter, would spontaneously transform into lighter subatomic particles, such as a positron and a neutral pion. This process is not predicted by the Standard Model of particle physics, but it is a major feature of many Grand Unified Theories (GUTs) that attempt to unify the strong, weak, and electromagnetic forces into a single framework.
GUTs suggest that at extremely high energies—far beyond those accessible in current experiments—quarks inside protons could rearrange through interactions involving massive, undiscovered gauge bosons, leading to the eventual decay of the proton.
If proton decay occurs, it must be extremely rare, with a half-life greater than 10³⁴ years, far longer than the current age of the universe. Large-scale experiments like Super-Kamiokande in Japan and DUNE in the U.S. are designed to detect such incredibly rare events by monitoring vast amounts of matter for proton decay signatures.
Despite decades of searching, no confirmed proton decay has yet been observed. However, the continued absence of evidence helps constrain or rule out specific GUT models, guiding theoretical developments in the quest for a more complete understanding of fundamental forces. Detecting proton decay would be a major breakthrough in particle physics and cosmology.