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Proton decay
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<h2 id="baryogenesis">Baryogenesis</h2> <p class="note">Main article: <a href="/facts/Baryogenesis/6PdRkNBN">Baryogenesis</a></p> Unsolved problem in physics Do protons <a href="/facts/Radioactive_decay/Ya6FVjt4">decay</a>? If so, then what is the <a href="/facts/Half-life/kGVH8BAB">half-life</a>? Can <a href="/facts/Nuclear_binding_energy/PU8bREHW">nuclear binding energy</a> affect this? <a href="/facts/List_of_unsolved_problems_in_physics/vJ3d1A83">More unsolved problems in physics</a> <p>One of the outstanding problems in modern physics is the predominance of <a href="/facts/Matter/osyc04UN">matter</a> over <a href="/facts/Antimatter/NvpGDLof">antimatter</a> in the <a href="/facts/Universe/lfoqc5m6">universe</a>. The universe, as a whole, seems to have a nonzero positive baryon number density – that is, there is more matter than antimatter. Since it is assumed in <a href="/facts/Physical_cosmology/ww4ChbEq">cosmology</a> 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 <a href="/facts/Symmetry_breaking/q3yqVlNQ">symmetry breaking</a> 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 1010 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 <a href="/facts/Boson/XtPFcjb5">bosons</a>. </p><p>Most grand unified theories explicitly break the baryon number symmetry, which would account for this discrepancy, typically invoking reactions mediated by very massive <a href="/facts/X_boson/TJwvcbE3">X bosons</a> (X) or massive <a href="/facts/Higgs_boson/3LAYXfft">Higgs bosons</a> (H0). The rate at which these events occur is governed largely by the mass of the intermediate X or H0 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. </p> <h2 id="experimental-evidence">Experimental evidence</h2> <p>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 <a href="/facts/Magnetic_monopoles/R1TSbfMY">magnetic monopoles</a>. Both concepts have been the focus of major <a href="/facts/Experimental_physics/2Ile53Iq">experimental physics</a> 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 <a href="/facts/Super-Kamiokande/pvvQ3ogI">Super-Kamiokande</a> water <a href="/facts/Cherenkov_radiation/lSPJAC3U">Cherenkov radiation</a> detector in Japan:<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> a lower bound on the proton's half-life of 2.4×1034 years via positron decay, and similarly, 1.6×1034 years via <a href="/facts/Antimuon/BP2fLVIR">antimuon</a> decay, close to a supersymmetry (SUSY) prediction of 1034–1036 years.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> An upgraded version, <a href="/facts/Hyper-Kamiokande/MqNowWLj">Hyper-Kamiokande</a>, probably will have sensitivity 5–10 times better than Super-Kamiokande. </p> <h2 id="theoretical-motivation">Theoretical motivation</h2> <p>Despite the lack of observational evidence for proton decay, some <a href="/facts/Grand_unification_theory/UVHj76XX">grand unification theories</a>, such as the <a href="/facts/SU(5)/Qgy2orcu">SU(5)</a> Georgi–Glashow model and <a href="/facts/SO(10)/7c1IQPtt">SO(10)</a>, along with their supersymmetric variants, require it. According to such theories, the proton has a <a href="/facts/Half-life/kGVH8BAB">half-life</a> of about 1031~1036 years and decays into a <a href="/facts/Positron/HgV4zBzA">positron</a> and a neutral <a href="/facts/Pion/x5HamzG3">pion</a> that itself immediately decays into two <a href="/facts/Gamma_radiation/5Drf913J">gamma ray</a> <a href="/facts/Photon/mFNhAEzA">photons</a>: </p><p> p + ⟶ e + + π 0 ↓ 2 γ {\displaystyle {\begin{aligned}{\rm {p^{+}\longrightarrow e^{+}+}}&\ \pi ^{0}\\&\downarrow \\&2\gamma \end{aligned}}} </p><p>Since a positron is an <a href="/facts/Lepton/X4a6MGop">antilepton</a> this decay preserves <a href="/facts/B_%25E2%2588%2592_L/jMoTgrXW">B − L</a> number, which is conserved in most GUTs. </p><p>Additional decay modes are available (e.g.: p+ → <a href="/facts/Antimuon/BP2fLVIR">μ+</a> + <a href="/facts/Pion/x5HamzG3">π0</a> ), both directly and when catalyzed via interaction with GUT-predicted <a href="/facts/Magnetic_monopole/R1TSbfMY">magnetic monopoles</a>.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> 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 <a href="/facts/Hyper-Kamiokande/MqNowWLj">Hyper-Kamiokande</a>. </p><p>Early <a href="/facts/Grand_unification_theory/UVHj76XX">grand unification theories</a> (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 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. More recent findings have pushed the minimum proton half-life to at least 1034–1035 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 6×1039 years, a bound applicable to SUSY models,<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> with a maximum for (minimal) non-SUSY GUTs at 1.4×1036 years.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a>(part 5.6) </p><p>Although the phenomenon is referred to as "proton decay", the effect would also be seen in <a href="/facts/Neutron/oRCM1geb">neutrons</a> bound inside <a href="/facts/Atomic_nucleus/orpIm5Xl">atomic nuclei</a>. 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 <a href="/facts/Beta_decay/vxTBlckp">beta decay</a>. Free neutrons have a half-life of 10 minutes (610.2±0.8 s)<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> due to the <a href="/facts/Weak_interaction/f6upmKMs">weak interaction</a>. Neutrons bound inside a nucleus have an immensely longer half-life – apparently as great as that of the proton. </p> <h2 id="projected-proton-lifetimes">Projected proton lifetimes</h2> <table><tbody><tr><th>Theory class</th><th>Proton lifetime (years)<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a></th><th>Ruled out experimentally?</th></tr><tr><td>Minimal SU(5) (<a href="/facts/Georgi%25E2%2580%2593Glashow_model/BbqhoGhU">Georgi–Glashow</a>)</td><td>1030–1031</td><td>Yes</td></tr><tr><td>Minimal <a href="/facts/Supersymmetry/hkWGrMet">SUSY</a> SU(5)</td><td>1028–1032</td><td>Yes</td></tr><tr><td><a href="/facts/Supergravity/jeNk3aAB">SUGRA</a> SU(5)</td><td>1032–1034</td><td>Yes</td></tr><tr><td>SUSY <a href="/facts/SO(10)/7c1IQPtt">SO(10)</a></td><td>1032–1035</td><td>Partially</td></tr><tr><td>SUSY SU(5) (<a href="/facts/Minimal_Supersymmetric_Standard_Model/kQwWa2hW">MSSM</a>)</td><td>~1034</td><td>Partially</td></tr><tr><td>SUSY SU(5) – 5 dimensions</td><td>1034–1035</td><td>Partially</td></tr><tr><td>SUSY SO(10) MSSM G(224)</td><td>2×1034</td><td>No</td></tr><tr><td>Minimal (Basic) SO(10) – Non-SUSY</td><td>< ~1035 (maximum range)</td><td>No</td></tr><tr><td><a href="/facts/Flipped_SU(5)/Er1hc1vF">Flipped SU(5)</a> (MSSM)</td><td>1035–1036</td><td>No</td></tr></tbody></table> <p>The lifetime of the proton in vanilla SU(5) can be naively estimated as τ p ∼ M X 4 / m p 5 {\textstyle \tau _{p}\sim M_{X}^{4}/m_{p}^{5}} .<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> Supersymmetric GUTs with reunification scales around µ ~ 2×1016 <a href="/facts/Electronvolt/YMT1fqX9">GeV/<i>c</i>2</a> yield a lifetime of around 1034 yr, roughly the current experimental lower bound. </p> <h2 id="decay-operators">Decay operators</h2> <h3>Dimension-6 proton decay operators</h3> <p>The <a href="/facts/Classical_scaling_dimension/fBE7kpo2">dimension</a>-6 proton decay operators are q q q ℓ / Λ 2 , {\textstyle qqq\ell /\Lambda ^{2},} d c u c u c e c / Λ 2 , {\textstyle d^{c}u^{c}u^{c}e^{c}/\Lambda ^{2},} e c ¯ u c ¯ q q / Λ 2 , {\textstyle {\overline {e^{c}}}{\overline {u^{c}}}qq/\Lambda ^{2},} and d c ¯ u c ¯ q ℓ / Λ 2 , {\textstyle {\overline {d^{c}}}{\overline {u^{c}}}q\ell /\Lambda ^{2},} where Λ {\displaystyle \Lambda } is the <a href="/facts/Cutoff_(physics)/T3lw9fK0">cutoff scale</a> for the <a href="/facts/Standard_Model/LhVGf9ow">Standard Model</a>. All of these operators violate both baryon number (B) and <a href="/facts/Lepton_number/vvocnLqR">lepton number</a> (L) conservation but not the combination <a href="/facts/B_-_L/jMoTgrXW">B − L</a>. </p><p>In GUT models, the exchange of an <a href="/facts/X_and_Y_bosons/TJwvcbE3">X or Y boson</a> with the mass ΛGUT can lead to the last two operators suppressed by 1 / Λ GUT 2 {\textstyle 1/\Lambda _{\text{GUT}}^{2}} . The exchange of a triplet Higgs with mass M can lead to all of the operators suppressed by 1 / M 2 {\textstyle 1/M^{2}} . See <i><a href="/facts/Doublet%25E2%2580%2593triplet_splitting_problem/46ALyfot">Doublet–triplet splitting problem</a></i>. </p> <h3>Dimension-5 proton decay operators</h3> <p>In supersymmetric extensions (such as the <a href="/facts/Minimal_Supersymmetric_Standard_Model/kQwWa2hW">MSSM</a>), we can also have dimension-5 operators involving two fermions and two <a href="/facts/Sfermion/vhU95umA">sfermions</a> caused by the exchange of a tripletino of mass M. The sfermions will then exchange a <a href="/facts/Gaugino/0QRX2D7s">gaugino</a> or <a href="/facts/Higgsino/bSYSM8Hk">Higgsino</a> or <a href="/facts/Gravitino/YpPgoEMj">gravitino</a> leaving two fermions. The overall <a href="/facts/Feynman_diagram/BRFNvDdr">Feynman diagram</a> has a loop (and other complications due to <a href="/facts/Strong_interaction/JjRsKcLg">strong interaction</a> physics). This decay rate is suppressed by 1 / M M SUSY {\textstyle 1/MM_{\text{SUSY}}} where <i>M</i>SUSY is the mass scale of the <a href="/facts/Superpartner/L7T76mve">superpartners</a>. </p> <h3>Dimension-4 proton decay operators</h3> <p>In the absence of <a href="/facts/Matter_parity/6UWnvwvV">matter parity</a>, supersymmetric extensions of the Standard Model can give rise to the last operator suppressed by the inverse square of <a href="/facts/Sdown/vhU95umA">sdown</a> quark mass. This is due to the dimension-4 operators qℓd͂c and ucdcd͂c. </p><p>The proton decay rate is only suppressed by 1 / M SUSY 2 {\textstyle 1/M_{\text{SUSY}}^{2}} which is far too fast unless the couplings are very small. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Age_of_the_universe/BaByputq">Age of the universe</a></li> <li><a href="/facts/B_%25E2%2588%2592_L/jMoTgrXW">B − L</a></li> <li><a href="/facts/Virtual_black_hole/nXc0XpRu">Virtual black hole</a></li> <li><a href="/facts/Weak_hypercharge/XlYXbQj7">Weak hypercharge</a></li> <li><a href="/facts/X_and_Y_bosons/TJwvcbE3">X and Y bosons</a></li> <li><a href="/facts/Iron_star/1Qgt5h4d">Iron star</a></li></ul> <h2 id="further-reading">Further reading</h2> <ul><li>Amsler, C.; et al. (<a href="/facts/Particle_Data_Group/e89W3u8f">Particle Data Group</a>) (September 2008). <a href="http://pdg.lbl.gov/2008/tables/rpp2008-tab-baryons-N.pdf">"Review of Particle Physics – <i>N</i> Baryons"</a> (PDF). <i><a href="/facts/Physics_Letters_B/6EkxhxM5">Physics Letters B</a></i>. 667 (1–5): 1–6. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2008PhLB..667....1A">2008PhLB..667....1A</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1016%2Fj.physletb.2008.07.018">10.1016/j.physletb.2008.07.018</a>. <a href="/facts/Hdl_(identifier)/rdebSxmC">hdl</a>:<a href="https://hdl.handle.net/1854%2FLU-685594">1854/LU-685594</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:227119789">227119789</a>.</li> <li>Hagiwara, K.; et al. (<a href="/facts/Particle_Data_Group/e89W3u8f">Particle Data Group</a>) (July 2002). <a href="http://pdg.lbl.gov/2002/bxxxn.pdf">"Review of Particle Physics – <i>N</i> Baryons"</a> (PDF). <i><a href="/facts/Physical_Review_D/QxekSF11">Physical Review D</a></i>. 66 (1): 010001. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2002PhRvD..66a0001H">2002PhRvD..66a0001H</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1103%2FPhysRevD.66.010001">10.1103/PhysRevD.66.010001</a>. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/0556-2821">0556-2821</a>.</li> <li>Adams, Fred; Laughlin, Greg (2000). <i>The five ages of the universe: inside the physics of eternity</i>. London: <a href="/facts/Touchstone_Books/pESq8Dxl">Touchstone Books</a>. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-684-86576-8.</li> <li>Krauss, Lawrence Maxwell (2001). <a href="https://archive.org/details/atomodysseyfromt00krau"><i>Atom: an odyssey from the big bang to life on earth-- and beyond</i></a>. Boston: <a href="/facts/Little_Brown_%2526_Company/aJ9JSCnn">Little Brown & Company</a>. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-316-49946-0.</li> <li>Wu, Dan-di; Li, Tie-zhong (1985). "Proton decay, annihilation or fusion?". <i><a href="/facts/Zeitschrift_f%25C3%25BCr_Physik/B0VsWLSC">Zeitschrift für Physik</a></i>. 27 (2): 321–323. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/1985ZPhyC..27..321W">1985ZPhyC..27..321W</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1007%2FBF01556623">10.1007/BF01556623</a>. <a href="/facts/ISSN_(identifier)/DPAflDvU">ISSN</a> <a href="https://search.worldcat.org/issn/0170-9739">0170-9739</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:121868029">121868029</a>.</li> <li>Nath, Pran; Fileviez Pérez, Pavel (April 2007). "Proton stability in grand unified theories, in strings and in branes". <i><a href="/facts/Physics_Reports/AjNs5QIf">Physics Reports</a></i>. 441 (5–6): 191–317. <a href="/facts/ArXiv_(identifier)/H6EtgnBe">arXiv</a>:<a href="https://arxiv.org/abs/hep-ph/0601023">hep-ph/0601023</a>. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2007PhR...441..191N">2007PhR...441..191N</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1016%2Fj.physrep.2007.02.010">10.1016/j.physrep.2007.02.010</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:119542637">119542637</a>.</li></ul> <h2 id="external-links">External links</h2> Wikiquote has quotations related to <i>Proton decay</i>. <ul><li><a href="http://hep.bu.edu/~superk/pdk.html">Proton decay at Super-Kamiokande</a></li> <li><a href="http://www-personal.umich.edu/~jcv/imb/imb.html">Pictorial history of the IMB experiment</a></li> <li><a href="/facts/Luciano_Maiani/3hgcWyvK">Luciano Maiani</a> (8 February 2006). <a href="http://axpd24.pd.infn.it/NO-VE2006/talks/NOVE_Maiani.pdf"><i>The problem of proton decay</i></a> (PDF). <a href="http://axpd24.pd.infn.it/NO-VE2006/">Third NO-VE International Workshop on Neutrino Oscillations in Venice</a>. Venice.</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Ahmad, Ishfaq (1969), "Radioactive decays by Protons. Myth or reality?", The Nucleus, pp. 69–70 <a href="/wiki/Ishfaq_Ahmad_Khan" target="_blank">/wiki/Ishfaq_Ahmad_Khan</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Bajc, Borut; Hisano, Junji; Kuwahara, Takumi; Omura, Yuji (2016). "Threshold corrections to dimension-six proton decay operators in non-minimal SUSY SU(5) GUTs". Nuclear Physics B. 910: 1. arXiv:1603.03568. Bibcode:2016NuPhB.910....1B. doi:10.1016/j.nuclphysb.2016.06.017. S2CID 119212168. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Francis, Matthew R. (22 September 2015). "Do protons decay?". symmetry magazine. Retrieved 2020-11-12. <a href="https://www.symmetrymagazine.org/article/do-protons-decay" target="_blank">https://www.symmetrymagazine.org/article/do-protons-decay</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Talou, P.; Carjan, N.; Strottman, D. (1998). "Time-dependent properties of proton decay from crossing single-particle metastable states in deformed nuclei". Physical Review C. 58 (6): 3280–3285. arXiv:nucl-th/9809006. Bibcode:1998PhRvC..58.3280T. doi:10.1103/PhysRevC.58.3280. S2CID 119075457. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Dicus, D. A.; Letaw, J. R.; Teplitz, D. C.; Teplitz, V. L. (January 1982). "Effects of proton decay on the cosmological future". The Astrophysical Journal. 252: 1. Bibcode:1982ApJ...252....1D. doi:10.1086/159528. 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