In particle physics, proton decay is a hypothetical form of radioactive decay in which the proton decays into lighter subatomic particles, usually a neutral pion and a positron. Proton decay has not been observed. There is currently no experimental evidence that proton decay occurs.

In the Standard Model, protons, a type of baryon, are theoretically stable because baryon number is conserved. Therefore protons 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 number symmetry, 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.

Contents 1 Baryogenesis 2 Experimental evidence 3 Theoretical motivation 4 Decay operators 4.1 Dimension-6 proton decay operators 4.2 Dimension-5 proton decay operators 4.3 Dimension-4 proton decay operators 5 See also 6 References 7 Further reading

[edit] Baryogenesis

One of the outstanding problems in modern physics is the predominance of matter over antimatter in 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 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 10,000,000,000 (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 number symmetry, which would account for this discrepancy, typically invoking reactions mediated by very massive X bosons (X) or massive Higgs bosons (T). The rate at which 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 occasionally exhibit a spontaneous proton decay.

[edit] Experimental evidence

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-Kamiokande water Cherenkov radiation detector in Japan give a lower limit of the proton half-life of 6.6×10³³ years^[1]

[edit] Theoretical motivation

Despite the lack of observational evidence for proton decay, some grand unification theories, such as the Georgi–Glashow model, require it. According to some such theories, the proton has a half-life of about 10³⁶ years, and decays into a positron and a neutral pion that itself immediately decays into 2 gamma ray photons:

p	→	e+	+	π0
π0	→	2γ

Since a positron is an antilepton this decay preserves B-L number, which is conserved in most GUT theories.

Additional decay modes are available, both directly and when catalyzed via interaction with GUT-predicted magnetic monopoles.^[2] 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, 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.

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 antineutrino) in a process called beta decay. Free neutrons have a half life of just under 15 minutes (885.7 ± 0.8 s)^[3] 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.

[edit] Decay operators

[edit] Dimension-6 proton decay operators

The dimension-6 proton decay operators are , , and where Λ is the cutoff scale for the Standard Model. All of these operators violate both baryon number and lepton number but not the combination B−L.

In GUT models, the exchange of an X or Y boson with the mass Λ_GUT 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/M². See doublet-triplet splitting problem.

Proton Decay. These graphics refer to the X bosons and Higgs bosons.

Dimension 6 proton decay mediated by the X boson in SU(5) GUT

Dimension 6 proton decay mediated by the X boson in flipped SU(5) GUT

Dimension 6 proton decay mediated by the triplet Higgs and the anti-triplet Higgs in SU(5) GUT

[edit] 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 tripletino of mass 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 where M_SUSY is the mass scale of the superpartners.

[edit] 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 sdown quark 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.

[edit] See also

• Weak hypercharge
• B−L
• X and Y bosons

[edit] References

1. ^ The Super-Kamiokande Collaboration, H. Nishino, S. Clark and et al "Search for Proton Decay via p -> e^+ pi^0 and p -> mu^+ pi^0 in a Large Water Cherenkov Detector",March 2009
2. ^ B.V. Sreekantan, "Searches for Proton Decay and Superheavy Magnetic Monopoles" (Abstract), Journal of Astrophysics and Astronomy (ISSN 0250-6335), vol. 5, Sept. 1984, p. 251–271.
3. ^ Particle Data Group's Review of Particle Physics 2006

[edit] Further reading

• latest data from the Particle Data Group
• K. Hagiwara et al., "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