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The wide disparity between the decay tif a neutron and a protonme o

  1. Mar 3, 2010 #1
    Despite sharing similar physical properties(neutron and proton having little variation in masses, protons and neutron being made up of quarks, both elementary particles are held together by the strong force) why is the decay time for like a neutron 10 minutes and the decay time for like a proton 10^32 years? (Sorry the title of the thread is suppose to read: (the wide disparity between the decay time of a neutron and a proton)
  2. jcsd
  3. Mar 3, 2010 #2


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    The neutron is heavier than the proton, and so can decay into the proton, an electron, and an antineutrino. The proton is the lightest baryon, so baryon number conservation prevents its decay into anything lighter. The number you quote for the proton is not its lifetime, but is an experimental lower limit on its lifetime. There are some speculative theories that predict proton decay with a long lifetime, but proton decay has never been observed. The proton may be completely stable.
  4. Mar 4, 2010 #3
    Yes , it is the lightest baryon but it is not the lightest particle for leptons and mesons are generally much lighter than the mass of a typical baryon and leptons , a electron and positron for instance can combine and react together to form and decay into a photon. Yes it is not observed and it is the most stable particle , but why does the baryon number have to be conserved and but the lepton number and meson number are conserved? That cannot be the only explanaton for why protons are very stable and therefore tend not to decay, although conservation laws for lepton number and baryon number tend to be approximate rather than exact . Perhaps this is the reason why scientists have such a difficult time finding quarks by themselves in nature.
  5. Mar 4, 2010 #4


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    You should also compare the huge disparity between the decay rate of the neutron and the beta decay of any other particle. In this sense, the neutron and proton are alike. Check the plot here: http://dftuz.unizar.es/~rivero/research/nonstrong.jpg [Broken]
    Last edited by a moderator: May 4, 2017
  6. Mar 4, 2010 #5
    As clem pointed out, the fact that a proton can't decay without violating baryon number conservation is exactly the reason. A baryon number violating interaction has never been detected -- in the Standard Model there are none and the proton never decays -- and as you note there is a very stringent bound on any baryon number violating process that would lead to proton decay. I don't think I can give you a good reason why baryon number HAS to be conserved. You can say it's due to a symmetry of QCD, but there's no particular reason that this symmetry should be respected in nature. It just appears to be the case.

    I believe lepton number is also exactly conserved in the Standard Model unless neutrinos turn out to be Majorana particles (each lepton family is not conserved because of neutrino oscillations, but I think total lepton number is still conserved.)

    Note, however, that "meson number" is not a conserved quantity, or really even a useful concept as far as I'm aware.
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