Editing Proton decay (section)

{{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]]).