Editing Proton decay

{{short description|Hypothetical particle decay process of a proton}}
{{about|the hypothetical decay of protons|the type of radioactive decay in which a nucleus ejects a proton|Proton emission|the radioactive decay where a proton within a nucleus converts to a neutron|positron emission}}
{{redirect|Nucleon decay|decay of neutrons|neutron decay (disambiguation){{!}}neutron decay}}

[[File:Proton decay.svg|upright=1.6|right|thumb|The pattern of [[weak isospin]]s, [[weak hypercharge]]s, and [[color charge]]s for particles in the [[Georgi–Glashow model]]. Here, a proton, consisting of two up quarks and a down, decays into a pion, consisting of an up and anti-up, and a positron, via an X&nbsp;boson with electric charge &minus;{{sfrac|4|3}}''e''.]]

In [[particle physics]], '''proton decay''' is a [[Hypothesis|hypothetical]] form of [[particle decay]] in which the [[proton]] decays into lighter [[subatomic particle]]s, such as a neutral [[pion]] and a [[positron]].<ref>{{Citation |last=Ahmad |first=Ishfaq |title=Radioactive decays by Protons. Myth or reality? |date=1969 |work=The Nucleus |pages=69–70 |author-link=Ishfaq Ahmad Khan}}</ref> The proton decay hypothesis was first formulated by [[Andrei Sakharov]] in 1967.  Despite significant experimental effort, proton decay has never been observed. If it does decay via a positron, the proton's half-life is constrained to be at least {{val|1.67|e=34|u=years}}.<ref name="Bajc">{{cite journal |arxiv=1603.03568 |bibcode= 2016NuPhB.910....1B|doi=10.1016/j.nuclphysb.2016.06.017|title= Threshold corrections to dimension-six proton decay operators in non-minimal SUSY SU(5) GUTs|journal= Nuclear Physics B|volume= 910|page= 1|year= 2016|last1= Bajc|first1= Borut|last2= Hisano|first2= Junji|last3= Kuwahara|first3= Takumi|last4= Omura|first4= Yuji|s2cid= 119212168}}</ref>

According to the [[Standard Model]], the proton, a type of [[baryon]], is stable because [[baryon number]] ([[quark number]]) is [[conservation of baryon number|conserved]] (under normal circumstances; see ''[[Chiral anomaly]]'' for an exception). Therefore, protons will not decay into other particles on their own, because they are the lightest (and therefore least energetic) baryon. [[Positron emission]] and [[electron capture]]—forms of [[radioactive decay]] in which a proton becomes a neutron—are not proton decay, since the proton interacts with other particles within the atom.

Some beyond-the-Standard-Model [[Grand Unified Theory|grand unified theories]] (GUTs) explicitly break the baryon number symmetry, allowing protons to decay via the [[Higgs particle]], [[magnetic monopoles]], or new [[X boson]]s with a half-life of 10{{sup|31}} to 10{{sup|36}} years. For comparison, the [[Age of the universe|universe is roughly {{val|1.38|e=10}} years old]].<ref>{{Cite web|last=Francis|first=Matthew R.|title=Do protons decay?|url=https://www.symmetrymagazine.org/article/do-protons-decay|access-date=2020-11-12|website=symmetry magazine|date=22 September 2015 |language=en}}</ref> To date, all attempts to observe new phenomena predicted by GUTs (like proton decay or the existence of [[magnetic monopoles]]) have failed.

[[Quantum tunnelling]] may be one of the mechanisms of proton decay.<ref name="url[nucl-th/9809006] Time-dependent properties of proton decay from crossing single-particle metastable states in deformed nuclei">{{cite journal |title=Time-dependent properties of proton decay from crossing single-particle metastable states in deformed nuclei |year=1998 |doi=10.1103/PhysRevC.58.3280 |arxiv=nucl-th/9809006 |last1=Talou |first1=P. |last2=Carjan |first2=N. |last3=Strottman |first3=D. |journal=Physical Review C |volume=58 |issue=6 |pages=3280–3285 |bibcode=1998PhRvC..58.3280T |s2cid=119075457 }}</ref><ref name="dadicus 1982">{{Cite journal |last1=Dicus |first1=D. A. |last2=Letaw |first2=J. R. |last3=Teplitz |first3=D. C. |last4=Teplitz |first4=V. L. |date=January 1982 |title=Effects of proton decay on the cosmological future |journal=[[The Astrophysical Journal]] |language=en |volume=252 |pages=1 |bibcode=1982ApJ...252....1D |doi=10.1086/159528 |issn=0004-637X}}</ref><ref name="urlQuantum Tunnelling to the Origin and Evolution of Life">{{cite journal |title=Quantum Tunnelling to the Origin and Evolution of Life |year=2013 |pmc=3768233 |last1=Trixler |first1=F. |journal=Current Organic Chemistry |volume=17 |issue=16 |pages=1758–1770 |doi=10.2174/13852728113179990083 |pmid=24039543 }}</ref>

[[Quantum gravity]]<ref>{{cite journal |title=Dangerous implications of a minimum length in quantum gravity |year=2008 |doi=10.1088/0264-9381/25/19/195013 |arxiv=0803.0749 |last1=Bambi |first1=Cosimo |last2=Freese |first2=Katherine |journal=Classical and Quantum Gravity |volume=25 |issue=19 |page=195013 |bibcode=2008CQGra..25s5013B |hdl=2027.42/64158 |s2cid=2040645 }}</ref> (via [[virtual black hole]]s and [[Hawking radiation]]) may also provide a venue of proton decay at magnitudes or lifetimes well beyond the GUT scale decay range above, as well as extra dimensions in [[supersymmetry]].<ref name="urlProton Decay, Black Holes, and Large Extra Dimensions - NASA/ADS">{{cite journal |url=https://ui.adsabs.harvard.edu/abs/2001IJMPA..16.2399A/abstract |title=Proton Decay, Black Holes, and Large Extra Dimensions - NASA/ADS |format= |journal= International Journal of Modern Physics A|year=2001 |volume=16 |pages=2399–2410 |doi=10.1142/S0217751X0100369X |bibcode=2001IJMPA..16.2399A |accessdate=|last1=Adams |first1=Fred C. |last2=Kane |first2=Gordon L. |last3=Mbonye |first3=Manasse |last4=Perry |first4=Malcolm J. |issue=13 |arxiv=hep-ph/0009154 |s2cid=14989175 }}</ref><ref name="url[1903.02940] Proton Decay and the Quantum Structure of Spacetime">{{cite journal |title=Proton decay and the quantum structure of space–time |year=2019 |doi=10.1139/cjp-2018-0423 |arxiv=1903.02940 |last1=Al-Modlej |first1=Abeer |last2=Alsaleh |first2=Salwa |last3=Alshal |first3=Hassan |last4=Ali |first4=Ahmed Farag |journal=Canadian Journal of Physics |volume=97 |issue=12 |pages=1317–1322 |bibcode=2019CaJPh..97.1317A |hdl=1807/96892 |s2cid=119507878 }}</ref><ref>{{cite arXiv |title=The black hole information paradox |eprint=hep-th/9508151 |author1-link=Steven Giddings |last1=Giddings |first1=Steven B. |year=1995 }}</ref><ref>{{cite journal |url=https://www.researchgate.net/publication/315696398 |doi=10.1209/0295-5075/118/50008 |title=Virtual black holes from the generalized uncertainty principle and proton decay |year=2017 |last1=Alsaleh |first1=Salwa |last2=Al-Modlej |first2=Abeer |last3=Farag Ali |first3=Ahmed |journal=Europhysics Letters |volume=118 |issue=5 |page=50008 |arxiv=1703.10038 |bibcode=2017EL....11850008A |s2cid=119369813 }}</ref>

There are theoretical methods of baryon violation other than proton decay including interactions with changes of baryon and/or lepton number other than 1 (as required in proton decay). These included [[Baryon number|''B'']] and/or [[Lepton number|''L'']] violations of 2, 3, or other numbers, or [[B − L|''B''&nbsp;&minus;&nbsp;''L'']] violation. Such examples include neutron oscillations and the electroweak [[sphaleron]] [[chiral anomaly|anomaly]] at high energies and temperatures that can result between the collision of protons into antileptons<ref>{{Cite journal |doi = 10.1103/PhysRevD.92.045005 |title = Bloch wave function for the periodic sphaleron potential and unsuppressed baryon and lepton number violating processes |year = 2015 |last1 = Tye |first1 = S.-H. Henry |last2 = Wong |first2 = Sam S. C. |journal = Physical Review D |volume = 92 |issue = 4 |page = 045005 |arxiv = 1505.03690 |bibcode = 2015PhRvD..92d5005T |s2cid = 73528684 }}</ref> or vice versa (a key factor in [[leptogenesis (physics)|leptogenesis]] and non-GUT [[baryogenesis]]).

== Baryogenesis ==
{{Main| Baryogenesis}}
{{unsolved|physics|Do protons [[Radioactive decay|decay]]? If so, then what is the [[half-life]]? Can [[nuclear binding energy]] affect this?}}
One of the outstanding problems in modern physics is the predominance of [[matter]] over [[antimatter]] in the [[universe]]. The universe, as a whole, seems to have a nonzero positive baryon number density &ndash; that is, there is more matter than antimatter. Since it is assumed in [[physical cosmology|cosmology]] that the particles we see were created using the same physics we measure today, it would normally be expected that the overall baryon number should be zero, as matter and antimatter should have been created in equal amounts. This has led to a number of proposed mechanisms for [[symmetry breaking]] that favour the creation of normal matter (as opposed to antimatter) under certain conditions. This imbalance would have been exceptionally small, on the order of 1 in every 10<sup>10</sup> particles a small fraction of a second after the Big Bang, but after most of the matter and antimatter annihilated, what was left over was all the baryonic matter in the current universe, along with a much greater number of [[boson]]s.

Most grand unified theories explicitly break the baryon number symmetry, which would account for this discrepancy, typically invoking reactions mediated by very massive [[X boson]]s {{nowrap|({{SubatomicParticle|X boson}})}} or massive [[Higgs boson]]s ({{SubatomicParticle|Higgs boson}}). The rate at which these events occur is governed largely by the mass of the intermediate {{SubatomicParticle|X boson}} or {{SubatomicParticle|Higgs boson}} particles, so by assuming these reactions are responsible for the majority of the baryon number seen today, a maximum mass can be calculated above which the rate would be too slow to explain the presence of matter today. These estimates predict that a large volume of material will occasionally exhibit a spontaneous proton decay.

== Experimental evidence ==
Proton decay is one of the key predictions of the various grand unified theories (GUTs) proposed in the 1970s, another major one being the existence of [[magnetic monopoles]]. Both concepts have been the focus of major experimental physics efforts since the early 1980s. To date, all attempts to observe these events have failed; however, these experiments have been able to establish lower bounds on the half-life of the proton.  Currently, the most precise results come from the [[Super-Kamiokande]] water [[Cherenkov radiation]] detector in Japan:<ref>
{{cite journal
 |last1 = Mine           |first1 = Shunichi
 |year = 2023
 |title = Nucleon decay: theory and experimental overview
 |journal = Zenodo
 |doi = 10.5281/zenodo.10493165
}}
</ref> a lower bound on the proton's half-life of {{val|2.4|e=34|u=years}} via positron decay, and similarly, {{val|1.6|e=34|u=years}} via [[antimuon]] decay, close to a supersymmetry (SUSY) prediction of 10<sup>34</sup>–10<sup>36</sup>&nbsp;years.<ref>{{Cite web |date=25 November 2009 |title=Proton lifetime is longer than 10<sup>34</sup> years |url=http://www-sk.icrr.u-tokyo.ac.jp/whatsnew/new-20091125-e.html |url-status=dead |archive-url=https://web.archive.org/web/20110716144726/http://www-sk.icrr.u-tokyo.ac.jp/whatsnew/new-20091125-e.html |archive-date=16 July 2011 |website=[[Kamioka Observatory]]}}</ref> An upgraded version, [[Hyper-Kamiokande]], probably will have sensitivity 5–10 times better than Super-Kamiokande.

== 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.

== Projected proton lifetimes ==
{| class="wikitable"
|-
! Theory class
! Proton lifetime (years)<ref>{{Cite journal |last1=Bueno |first1=Antonio |last2=Melgarejo |first2=Antonio J |last3=Navas |first3=Sergio |last4=Dai |first4=Zuxiang |last5=Ge |first5=Yuanyuan |last6=Laffranchi |first6=Marco |last7=Meregaglia |first7=Anselmo |last8=Rubbia |first8=André |date=2007-04-11 |title=Nucleon decay searches with large liquid Argon TPC detectors at shallow depths: atmospheric neutrinos and cosmogenic backgrounds |url=http://stacks.iop.org/1126-6708/2007/i=04/a=041?key=crossref.289d2df8e7c5228ed3c2136a08194b62 |journal=Journal of High Energy Physics |volume=2007 |issue=4 |pages=041 |doi=10.1088/1126-6708/2007/04/041 |issn=1029-8479|arxiv=hep-ph/0701101 |bibcode=2007JHEP...04..041B |s2cid=119426496 }}</ref>
! Ruled out experimentally?
|-
| Minimal SU(5) ([[Georgi–Glashow model|Georgi–Glashow]])
| 10<sup>30</sup>–10<sup>31</sup>
| {{success|Yes}}
|-
| Minimal [[Supersymmetry|SUSY]] SU(5)
| 10<sup>28</sup>–10<sup>32</sup>
| {{success|Yes}}
|-
| [[Supergravity|SUGRA]] SU(5)
| 10<sup>32</sup>–10<sup>34</sup>
| {{success|Yes}}
|-
| SUSY [[SO(10)]]
| 10<sup>32</sup>–10<sup>35</sup>
| {{partial success|Partially}}
|-
| SUSY SU(5) ([[Minimal Supersymmetric Standard Model|MSSM]])
| ~10<sup>34</sup>
| {{partial success|Partially}}
|-
| SUSY SU(5) – 5 dimensions
| 10<sup>34</sup>–10<sup>35</sup>
| {{partial success|Partially}}
|-
| SUSY SO(10) MSSM G(224)
| {{val|2|e=34}}
| {{failure|No}}
|-
| Minimal (Basic) SO(10) – Non-SUSY
| < ~10<sup>35</sup> (maximum range)
| {{failure|No}}
|-
| [[Flipped SU(5)]] (MSSM)
| 10<sup>35</sup>–10<sup>36</sup>
| {{failure|No}}
|-
|}

The lifetime of the proton in vanilla SU(5) can be naively estimated as <math display="inline">\tau_p\sim M_X^4/m_p^5 </math>.<ref>{{cite journal |last1=Chanowitz |first1=Michael S. |last2=Ellis |first2=John |last3=Gaillard |first3=Mary K. |title=The price of natural flavour conservation in neutral weak interactions |journal=Nuclear Physics B |date=3 October 1977 |volume=128 |issue=3 |pages=506–536 |doi=10.1016/0550-3213(77)90057-8 |issn=0550-3213|bibcode=1977NuPhB.128..506C |s2cid=121007369 |url=https://cds.cern.ch/record/878518 }}</ref>  Supersymmetric GUTs with reunification scales around {{math|µ ~}}&nbsp;{{val|2e16|ul=GeV/c2}} yield a lifetime of around {{val|e=34|u=yr}}, roughly the current experimental lower bound.

== Decay operators ==

=== Dimension-6 proton decay operators ===
The [[classical scaling dimension|dimension]]-6 proton decay operators are <math display="inline"> qqq\ell/\Lambda^2, </math> <math display="inline"> d^c u^c u^c e^c/\Lambda^2, </math> <math display="inline"> \overline{e^c}\overline{u^c}qq/\Lambda^2, </math> and <math display="inline"> \overline{d^c}\overline{u^c}q\ell/\Lambda^2, </math> where <math>\Lambda</math> is the [[Cutoff (physics)|cutoff scale]] for the [[Standard Model]]. All of these operators violate both baryon number ({{mvar|B}}) and [[lepton number]] ({{mvar|L}}) conservation but not the combination [[B - L|{{mvar|B}}&nbsp;&minus;&nbsp;{{mvar|L}}]].

In <abbr title="Grand Unified Theory">GUT</abbr> models, the exchange of an [[X and Y bosons|X or Y&nbsp;boson]] with the mass {{math|Λ}}{{sub|GUT}} can lead to the last two operators suppressed by <math display="inline"> 1/\Lambda_\text{GUT}^2 </math>. The exchange of a triplet Higgs with mass {{mvar|M}} can lead to all of the operators suppressed by <math display="inline"> 1/M^2 </math>. See ''[[Doublet–triplet splitting problem]]''.

<gallery caption="Proton decay. These graphics refer to the [[X and Y bosons|X bosons]] and [[Higgs boson]]s." widths="250px" heights="300px" perrow="3">
Image:Proton decay2.svg|Dimension-6 proton decay mediated by the '''X boson (3,2){{su|b=−{{frac|5|6}}}}''' in SU(5) GUT
Image:proton decay3.svg|Dimension-6 proton decay mediated by the<br>'''X boson (3,2){{su|b={{frac|1|6}}}}''' in flipped SU(5) GUT
Image:proton decay4.svg|Dimension-6 proton decay mediated by the<br>'''triplet Higgs T (3,1){{su|b=−{{frac|1|3}}}}''' and the<br>'''anti-triplet Higgs {{overline|T}} ({{overline|3}},1){{su|b={{frac|1|3}}}}''' in SU(5) GUT
</gallery>

=== Dimension-5 proton decay operators ===
In supersymmetric extensions (such as the [[Minimal Supersymmetric Standard Model|MSSM]]), we can also have dimension-5 operators involving two fermions and two [[sfermion]]s caused by the exchange of a tripletino of mass {{mvar|M}}. The sfermions will then exchange a [[gaugino]] or [[Higgsino]] or [[gravitino]] leaving two fermions. The overall [[Feynman diagram]] has a loop (and other complications due to strong interaction physics). This decay rate is suppressed by <math display="inline"> 1/ M M_\text{SUSY} </math> where {{math|''M''{{sub|SUSY}}}} is the mass scale of the [[superpartner]]s.

=== Dimension-4 proton decay operators ===

[[Image:R-parity violating decay.svg|frame|right]]

In the absence of [[matter parity]], supersymmetric extensions of the Standard Model can give rise to the last operator suppressed by the inverse square of [[sdown]] quark mass. This is due to the dimension-4 operators {{math|{{SubatomicParticle|Quark}}{{SubatomicParticle|Lepton}}{{SubatomicParticle|Down squark}}{{sup|c}}}} and {{math|{{SubatomicParticle|Up quark}}{{sup|c}}{{SubatomicParticle|Down quark}}{{sup|c}}{{SubatomicParticle|Down squark}}{{sup|c}}}}.

The proton decay rate is only suppressed by <math display="inline"> 1/M_\text{SUSY}^2 </math> which is far too fast unless the couplings are very small.

== See also ==
* [[Age of the universe]]
* [[B − L]]
* [[Virtual black hole]]
* [[Weak hypercharge]]
* [[X and Y bosons]]
* [[Iron star]]

== References ==
{{reflist}}

== Further reading ==
* {{Cite journal |last=Amsler |first=C. |display-authors=etal |date=September 2008 |collaboration=[[Particle Data Group]] |title=Review of Particle Physics – ''N'' Baryons |url=http://pdg.lbl.gov/2008/tables/rpp2008-tab-baryons-N.pdf |journal=[[Physics Letters B]] |language=en |volume=667 |issue=1–5 |pages=1–6 |bibcode=2008PhLB..667....1A |doi=10.1016/j.physletb.2008.07.018 |s2cid=227119789 |hdl-access=free |hdl=1854/LU-685594}}
* {{Cite journal |last=Hagiwara |first=K. |display-authors=etal |date=July 2002 |collaboration=[[Particle Data Group]] |title=Review of Particle Physics – ''N'' Baryons |url=http://pdg.lbl.gov/2002/bxxxn.pdf |journal=[[Physical Review D]] |language=en |volume=66 |issue=1 |page=010001 |bibcode=2002PhRvD..66a0001H |doi=10.1103/PhysRevD.66.010001 |issn=0556-2821}}
* {{Cite book |last1=Adams |first1=Fred |title=The five ages of the universe: inside the physics of eternity |last2=Laughlin |first2=Greg |date=2000 |publisher=[[Touchstone Books]] |isbn=978-0-684-86576-8 |location=London}}
* {{Cite book |last=Krauss |first=Lawrence Maxwell |url=https://archive.org/details/atomodysseyfromt00krau |title=Atom: an odyssey from the big bang to life on earth-- and beyond |publisher=[[Little Brown & Company]] |year=2001 |isbn=978-0-316-49946-0 |location=Boston |url-access=registration}}
* {{Cite journal |last1=Wu |first1=Dan-di |last2=Li |first2=Tie-zhong |year=1985 |title=Proton decay, annihilation or fusion? |journal=[[Zeitschrift für Physik]] |language=en |volume=27 |issue=2 |pages=321–323 |bibcode=1985ZPhyC..27..321W |doi=10.1007/BF01556623 |issn=0170-9739 |s2cid=121868029}}
* {{Cite journal |last1=Nath |first1=Pran |last2=Fileviez Pérez |first2=Pavel |date=April 2007 |title=Proton stability in grand unified theories, in strings and in branes |journal=[[Physics Reports]] |language=en |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}}
== External links ==
{{wikiquote}}
* [http://hep.bu.edu/~superk/pdk.html Proton decay at Super-Kamiokande]
* [http://www-personal.umich.edu/~jcv/imb/imb.html Pictorial history of the IMB experiment]
* {{cite conference
 |author=Luciano Maiani
 |author-link=Luciano Maiani
 |date=8 February 2006
 |title=The problem of proton decay
 |url=http://axpd24.pd.infn.it/NO-VE2006/talks/NOVE_Maiani.pdf
 |conference-url=http://axpd24.pd.infn.it/NO-VE2006/
 |conference=Third NO-VE International Workshop on Neutrino Oscillations in Venice
 |location=Venice
}}

{{Proton decay experiments}}

{{DEFAULTSORT:Proton decay}}
[[Category:Proton]]
[[Category:Nuclear physics]]
[[Category:Physics beyond the Standard Model]]
[[Category:Grand Unified Theory]]
[[Category:Supersymmetric quantum field theory]]
[[Category:Hypotheses in physics]]
[[Category:Ultimate fate of the universe]]
[[Category:1967 in science]]

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