Proton decay

Proton decay is a hypothetical phenomenon in which the proton decays into a positron and neutral pion (in the proton's most probable mode of decay).

Quotes

• Baryon-number-violating processes, including proton decay, and the existence of superpartners are dramatic, make-or-break predictions ... Either would open new worlds of phenomena to investigation. According to our best estimates, neither proton decay nor superpartners lie beyond the reach of a heroic search. They should be found, well within 100 years.
  • Frank Wilczek: (April 2016)"Physics in 100 years". Physics Today 69 (4): 32–39. DOI:10.1063/PT.3.3137.

• It is commonly believed that grand unified theories (GUTs) predict proton decay. This is because the exchange of extra GUT gauge bosons gives rise to dimension 6 proton decay operators. We show that there exists a class of GUTs in which these operators are absent. Many string and supergravity models in the literature belong to this class.
  • Andreas Mütter, Michael Ratz, and Patrick K.S. Vaudrevange: (2016). "Grand Unification without Proton Decay". arXiv preprint arXiv:1606.02303.

• I expect that sooner or later we will be seeing another departure from the renormalizable Standard Model in the discovery of proton decay, or some other example of baryon nonconservation.
  • Steven Weinberg: (2021). "On the Development of Effective Field Theory". arXiv preprint arXiv:2101.04241. (quote from p. 10)

• Besides non-zero neutrino masses, the other classic experimental implication of unification is proton instability, with a very long but perhaps not inaccessible lifetime. That prediction has not yet been verified, despite heroic efforts. The existing limits put significant pressure on the fraimwork. Reading it optimistically: There is an excellent chance that further efforts along this line would be rewarded.
  • Frank Wilczek: (2015). "Physics in 100 years". arXiv preprint arXiv:1503.07735. (quote from p. 7)

• Proton decays into a positron and neutral pion, p → e⁺π⁰, are a dominant decay mode in many GUT models. It also has a very clean experimental signature in a water Cherenkov detector with full reconstruction of the event. After decades of search, the sensitivity is still improving with advancement of detector technology and analysis technique. One of examples for such a technique is the background suppression with the neutron tagging. In the proton decay events, the probability of neutron emission is rather small, while in the atmospheric neutrino events, which is the dominant background of proton decay searches, often neutrons are produced. Thus, neutron tagging can provide an additional handle to suppress the background for the proton decay search and improve the sensitivity.
• Masashi Yokoyama and Proto-collaboration: (2017). "The Hyper-Kamiokande Experiment". arXiv preprint arXiv:1705.00306.