Proton decay was a surprising prediction made by various attempts at a Grand Unified Theory of the strong and electroweak forces. It turns out that, given a timescale of about 10^30 or so years (depending on which GUT you use), a quark can spontaneously decay into a lepton. The simplest form of predicted proton decay results in a positron and a neutral pi meson. Proton decay does not exist in the standard model. Current experimental evidence with the Super-Kamiokande apparently pins the lower limit of proton lifetime at 10^33 years, which rules out most of the simpler GUTs. Whether or not protons actually decay is one of the big questions in modern physics. | 
