Proton decay

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Proton decay is the hypothetical decay of a free proton into other particles.

Proton decay is predicted by some variants of the Great Unified Theory (GUT) of particle physics . There it is assumed that a free proton is not stable, but only has a very long half-life . In contrast, the proton is regarded as stable in the standard model , since the baryon number is a conserved quantity here . There are currently no experimental observations that support the proton decay hypothesis.


Numerous types of proton decay are conceivable. A possible example is the decay mediated by a hypothetical X boson into a positron e + and a neutral pion π 0 , which then further decays into radiation ( photons γ) in a known way :

p → e + + π 0
π 0 → 2γ

Since the positron is an antilepton, the difference between the number of baryons and number of leptons (B - L) is retained. This is a prediction of most variants of the GUT.

Quark model

In a simplified view (without the virtual quarks in p and π 0 ) one can imagine the decay of a proton as follows:

The proton consists of the quark components and the neutral pion is a quantum mechanical mixed state of d d and u u . According to GUT, the decay of the proton to the two components of the pion occurs via an intermediate hypothetical X or Y boson as follows:

uu → X → e + + d

with the 'uninvolved' down quark we get:

uud → e + + d d

or for the other component of π 0 :

ud → Y → e + + u

with the 'uninvolved' up-quark we get:

uud → e + + u u


Despite an intensive search, no proton decay has been observed to date. The theories predict a half-life of 10 31 to 10 36 years. Experiments on the Super Kamiokande detector in Japan indicate (as of 2017) a lower limit of at least 1.6 · 10 34 years. Similar particle detectors are used for the detection as for the detection of neutrinos. Some GUT candidates could already be excluded through the experiments, the minimal SU (5) theory as early as the mid-1990s.

As Rubakow discovered in 1981, magnetic monopoles - if they exist - could also cause proton decay through catalysis . Such magnetic monopoles must (according to the theories predicting them) be of great mass; they have not yet been proven.


  • Fred Adams, Greg Laughlin: The Five Ages of the Universe. A physics of eternity. , dtv, ISBN 3-423-33086-4 . 280 p. (2002)

Web links

Proton decay at the Super-Kamiokande (English)

Individual evidence

  1. ^ Stephan Kreppner: Seminar: Proton decay . University of Erlangen, 2003 ( seminar presentation ( MS PowerPoint ; 1.0 MB)).
  2. Holger Motz: Seminar Astro and Particle Physics WS 2003/2004: Physics beyond the Standard Model . University of Erlangen, 2003 ( PDF ).  ( Page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Toter Link /  
  3. ^ Girtler: Seminar Astro-Particle Physics: Proton Decay . University of Innsbruck, 2003 ( PDF ).
  4. M. Miura et al. a. (Super-Kamiokande), Phys. Rev. D, Volume 95, 2017, p. 012004, Arxiv
  5. Natalie Wolchover, Grand Unification Dream Kept at Bay , Quanta Magazine, December 15, 2016