Proton decay
In particle physics, proton decay is a hypothetical form of particle decay in which the proton decays into lighter subatomic particles, such as a neutral pion and a positron.[1] 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 1.67×1034 years.[2]
This article is about the hypothetical decay of nucleons (protons or neutrons) into other subatomic particles. For the type of radioactive decay in which a nucleus ejects a proton, see Proton emission. For the radioactive decay where a proton within a nucleus converts to a neutron, see positron emission.
According to the Standard Model, the proton, a type of baryon, is stable because baryon number (quark number) is 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 theories (GUTs) explicitly break the baryon number symmetry, allowing protons to decay via the Higgs particle, magnetic monopoles, or new X bosons with a half-life of 1031 to 1036 years. For comparison, the universe is roughly 1.38×1010 years old.[3] 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.[4][5][6]
Quantum gravity[7] (via virtual black holes 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.[8][9][10][11]
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 B and/or L violations of 2, 3, or other numbers, or B − L violation. Such examples include neutron oscillations and the electroweak sphaleron anomaly at high energies and temperatures that can result between the collision of protons into antileptons[12] or vice versa (a key factor in leptogenesis and non-GUT baryogenesis).
Experimental evidence[edit]
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: [13] a lower bound on the proton's half-life of 2.4×1034 years via positron decay, and similarly, 1.6×1034 years via antimuon decay, close to a supersymmetry (SUSY) prediction of 1034–1036 years.[14] An upgraded version, Hyper-Kamiokande, probably will have sensitivity 5–10 times better than Super-Kamiokande.
Decay operators[edit]
Dimension-6 proton decay operators[edit]
The dimension-6 proton decay operators are and where is the cutoff scale for the Standard Model. All of these operators violate both baryon number (B) and lepton number (L) conservation but 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 . See Doublet–triplet splitting problem.
