The standard model of particle physics states that protons are stable, i.e., that the laws of physics do not allow a proton (which is baryonic matter) to spontaneously decay into a positron and photons (non-baryonic matter) because of conservation of the baryon number. However, it has recently been suggested that the predominance of matter over antimatter in the universe is the result of a very slight imbalance in the ratio that occurred very early in its formation. This imbalance would have been exceptionally small, on the order of 1 in every 10,000 particles, but after most of the matter and antimatter annihilated, what was left over was all the baryonic matter in our current universe. This means that in essence, rather than breaking the law of conservation of the baryon number, proton decay could actually be the inevitable mechanism for bringing the baryon number back to equilibrium—in a sense correcting the original imbalance that made all current matter in our universe possible.

Experimental Evidence

Recent experiments at the Super-Kamiokande water Cherenkov radiation detector in Japan indicate a lower boundary for the proton half-life of 10³⁵ years. Since this is a lower bound, it is consistent with the nonexistence of proton decay.

Theoretical Motivation

Despite the lack of observational evidence for proton decay, some Grand Unified Theories require it. According to some such theories, the proton would have have a half-life of 10³⁶ years), and would decay into a positron and a pion that itself immediately decays into photons in the range of gamma radiation.

p → e⁺π⁰

This process has not been observed experimentally.

Early GUTs (which were in fact the first sound theories to suggest proton decay) postulated that the proton's half-life would be at least 10³¹ years. As further experiments and calculations were performed in the 1990s, it became clear that the proton half-life could not lie below 10³² years. Many books from that period refer to this figure for the possible decay time for baryonic matter.

Although the phenomenon is referred to as "proton decay," the effect would also be seen in neutrons bound inside atomic nuclei.

Further reading

• Particle Data Group current best estimates of proton lifetime;
  • http://pdg.lbl.gov/2002/bxxxn.pdf - K. Hagiwara et al., Phys. Rev. D 66, 010001 (2002)
• Adams, Fred and Laughlin, Greg The Five Ages of the Universe : Inside the Physics of Eternity ISBN 0684865769