text: In particle physics , proton decay is a hypothetical form of radioactive decay in which the proton decays into lighter subatomic particles , such as a neutral pion and a positron . There is currently no experimental evidence that proton decay occurs .
simplified: Proton decay is a theory where some physicists believe that radioactive decay could also be that a proton decays into two particles smaller than an atom , a pion and a positron .