- Proton decay
particle physics, proton decay is a hypothetical form of radioactive decayin which the protondecays into lighter subatomic particles, usually a neutral pionand a positron. Proton decay has not been observed. There is currently no evidence that proton decay occurs.
Standard Model, protons, a type of baryon, are theoretically stable because baryon numberis approximately conserved. That is, they will not decay into other particles on their own because they are the lightest (and therefore least energetic) baryon.
Some beyond-the-Standard Model grand unified theories (GUTs) explicitly break the
baryon numbersymmetry, allowing protons to decay via new X bosons. Proton decay is one of the few observable effects of the various proposed GUTs. To date, all attempts to observe these events have failed.
One of the outstanding problems in modern physics is the predominance of
matterover antimatterin the universe. The universe, as a whole, has a nonzero baryon number density — that is, matter exists. Since it is assumed in 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 breakingthat 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 10,000,000,000 (1010) particles a split 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 bosons.
Most grand unified theories (GUTs) explicitly break the
baryon numbersymmetry, which would account for this discrepancy, typically invoking reactions mediated by very massive X bosons ("X" below) or massive Higgs bosons ("T"). The rate that these events occur is governed largely by the mass of the intermediate "X" or "T" 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 periodically exhibit spontaneous proton decay even given the much reduced energies available today.Dubious|date=March 2008Fact|date=November 2007
Proton decay is one of the few observable effects of the various proposed GUTs, the other major one being
magnetic monopoles. Both became the focus of major experimental physics efforts starting in the early 1980s. Proton decay was, for a time, an extremely exciting area of experimental physics research. To date, all attempts to observe these events have failed. Recent experiments at the Super-Kamiokandewater Cherenkov radiationdetector in Japanindicate that if protons decay at all, their half-lifemust be at least 1035 years.
Despite the lack of observational evidence for proton decay, some grand unification theories require it. According to some such theories, the proton has a
half-lifeof about 1036 years, and decays into a positronand a neutral pionthat itself immediately decays into 2 gamma ray photons:
Additional decay modes are available, both directly and when catalyzed via interaction with GUT-predicted
magnetic monopoles. [B.V. Sreekantan, [http://www.ias.ac.in/jarch/jaa/5/251-271.pdf "Searches for Proton Decay and Superheavy Magnetic Monopoles"] ( [http://adsabs.harvard.edu/abs/1984JApA....5..251S Abstract] ), "Journal of Astrophysics and Astronomy" (ISSN 0250-6335), vol. 5, Sept. 1984, p. 251-271.] 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 theories, 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.
Although the phenomenon is referred to as "proton decay", the effect would also be seen in
neutrons bound inside atomic nuclei. Free neutrons—those not inside an atomic nucleus—are already known to decay into protons (and an electron and an anti-neutrino) in a process called beta decay. Free neutrons have a half lifeof 15.4 minutes due to the weak interaction. Neutrons bound inside a nucleus have an immensely longer half-life - apparently as great as that of the proton - and there is some speculation that free protons might be more likely to decay over the eons than bound ones. [John A. Gowan, [http://people.cornell.edu/pages/jag8/proton.html The Half-Life of Proton Decay and its Relation to the "Heat Death" of the Universe] ]
Dimension-6 proton decay operators
They are , , and where Λ is the
cutoff scalefor the Standard Model. All of these operators violate both baryon numberand lepton numberbut not the combination B−L.
In GUT models, the exchange of an X or Y boson with the mass ΛGUT can lead to the last two operators suppressed by . The exchange of a triplet Higgs with mass "M" can lead to all of the operators suppressed by 1/"M"2. See
doublet-triplet splitting problem.
Dimension-5 proton decay operators
In supersymmetric extensions (such as the MSSM), we can also have dimension-5 operators involving two fermions and two
sfermions caused by the exchange of a tripletinoof mass "M". The sfermions will then exchange a gauginoor Higgsinoor gravitinoleaving two fermions. The overall Feynman diagram has a loop (and other complications due to strong interaction physics). This decay rate is suppressed by where "M"SUSY is the mass scale of the superpartners.
Dimension-4 proton decay operators
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 sdownquark mass. This is due to the dimension-4 operators
The proton decay rate is only suppressed by which is far too fast unless the couplings are very small.
Grand unification theory
X and Y bosons
* K. Hagiwara et al., [http://pdg.lbl.gov/2002/bxxxn.pdf "Particle Data Group current best estimates of proton lifetime"] , Phys. Rev. D 66, 010001 (2002) ISBN 978-0684865768
* Adams, Fred and Laughlin, Greg "The Five Ages of the Universe : Inside the Physics of Eternity" ISBN 0-684-86576-9
* Krauss, Lawrence M. "Atom : An Odyssey from the Big Bang to Life on Earth" ISBN 0-316-49946-3
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