Proton decay is a hypothetical baryon-number-violating transition of the proton into lighter particles. Its nonobservation places strong lower bounds on the scale, or strong upper bounds on the couplings, of baryon-number-violating interactions.
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Proton decay is a hypothetical form of particle decay in which a proton, a fundamental constituent of atomic nuclei, transforms into lighter particles. This process would imply that protons are not absolutely stable, as is commonly assumed in the Standard Model of particle physics, which treats them as stable particles under normal conditions.