Editing Proton decay (section)

== Theoretical motivation ==
Despite the lack of observational evidence for proton decay, some [[grand unification theory|grand unification theories]], such as the [[SU(5)]] Georgi–Glashow model and [[SO(10)]], along with their supersymmetric variants, require it. According to such theories, the proton has a [[half-life]] of about {{10^|31}}~{{10^|36}}&nbsp;years and decays into a [[positron]] and a neutral [[pion]] that itself immediately decays into two [[gamma radiation|gamma ray]] [[photon]]s:

<math display=block> \begin{align} 
  \rm p^+ \longrightarrow e^+ + & \ \pi^0 \\
  & \downarrow \\
  & 2\gamma 
\end{align}</math>

Since a positron is an [[lepton|antilepton]] this decay preserves {{nobr| {{mvar|[[B &minus; L]]}} }} number, which is conserved in most <abbr title="Grand Unified Theory">GUT</abbr>s.

Additional decay modes are available (e.g.: {{nobr| {{SubatomicParticle|Proton+}} → {{math| {{SubatomicParticle|link=yes|Muon+}} }} + {{math|{{SubatomicParticle|link=yes|Pion0}} }} }}), both directly and when catalyzed via interaction with <abbr title="Grand Unified Theory">GUT</abbr>-predicted [[magnetic monopole]]s.<ref>
{{cite journal
 |first=B.V. |last=Sreekantan  |author-link=B. V. Sreekantan
 |date=1984
 |title=Searches for proton decay and superheavy magnetic monopoles
 |url=http://www.ias.ac.in/jarch/jaa/5/251-271.pdf
 |journal=[[Journal of Astrophysics and Astronomy]]
 |volume=5 |issue=3 |pages=251–271
 |doi=10.1007/BF02714542 |bibcode=1984JApA....5..251S  |s2cid=53964771
}}</ref> Though this process has not been observed experimentally, it is within the realm of experimental testability for future planned very large-scale detectors on the megaton scale. Such detectors include the [[Hyper-Kamiokande]].

Early [[grand unification theory|grand unification theories]] (GUTs) such as the Georgi–Glashow model, which were the first consistent theories to suggest proton decay, postulated that the proton's half-life would be at least {{val|e=31|u=years}}. As further experiments and calculations were performed in the 1990s, it became clear that the proton half-life could not lie below {{val|e=32|u=years}}. Many books from that period refer to this figure for the possible decay time for baryonic matter. More recent findings have pushed the minimum proton half-life to at least {{10^|34}}–{{10^|35}}&nbsp;years, ruling out the simpler GUTs (including minimal SU(5) / Georgi–Glashow) and most non-SUSY models. The maximum upper limit on proton lifetime (if unstable), is calculated at {{val|6|e=39|u=years}}, a bound applicable to SUSY models,<ref name=Nath-Perez-2007>
{{cite journal
 |first1=Pran  |last1=Nath   
 |first2=Pavel |last2=Fileviez Pérez
 |year=2007
 |title=Proton stability in grand unified theories, in strings and in branes
 |journal=Physics Reports
 |volume=441  |issue=5–6  |pages=191–317
 |arxiv=hep-ph/0601023  |bibcode=2007PhR...441..191N
 |doi=10.1016/j.physrep.2007.02.010  |s2cid=119542637
}}</ref> with a maximum for (minimal) non-SUSY GUTs at {{val|1.4|e=36|u=years}}.<ref name=Nath-Perez-2007/>{{rp|style=ama|at=part&nbsp;5.6}}

Although the phenomenon is referred to as "proton decay", the effect would also be seen in [[neutron]]s bound inside atomic nuclei. Free neutrons—those not inside an atomic nucleus—are already known to decay into protons (and an electron and an antineutrino) in a process called [[beta decay]]. Free neutrons have a half-life of 10&nbsp;minutes ({{val|610.2|0.8|u=s}})<ref name="RPP">
{{cite journal
 |first1=K. A. |last1=Olive
 |collaboration=Particle Data Group
 |display-authors=etal
 |date=2014
 |title=Review of Particle Physics – N Baryons
 |url=http://pdg.lbl.gov/2015/tables/rpp2015-sum-baryons.pdf
 |journal=[[Chinese Physics C]]
 |volume=38 |issue=9 |page=090001
 |doi=10.1088/1674-1137/38/9/090001
 |arxiv=astro-ph/0601168
 |bibcode=2014ChPhC..38i0001O
 |s2cid=118395784
 }}
</ref> due to the [[weak interaction]]. Neutrons bound inside a nucleus have an immensely longer half-life – apparently as great as that of the proton.