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Lifetime of electrons and protons outside the nuclues.

  1. Aug 24, 2009 #1

    MathematicalPhysicist

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    For neutron we all know, it has the 15 minutes of fame before it decays, what is the theoretical bound on proton's lifetime outside the nucleus, and to what it should it be dacayed to?
    As far as I can tell electron is a point particle and cannot be dissolved into other parts, are there any bounds on its lifetime?
     
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  3. Aug 24, 2009 #2

    Vanadium 50

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    Electrons are absolutely stable. For an electron to decay means that either there is a lighter charged particle or that charge is not conserved. Neither one has any evidence in favor of it, and there's a lot of evidence against it.

    Protons, as far as we know, are stable: measured lifetimes are around 1032 years, with some model dependence.
     
  4. Aug 24, 2009 #3

    clem

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    Whoops. That is an lower limit on the proton. There is no "measured lifetime" of the proton.
     
  5. Aug 24, 2009 #4
    But you cannot say that hadron are stable. If there is a potential disintegration, there is a potential lifetime. I also agree that the lifetime is so long that they are considered "stable".

    is the average time defore neutron decays (defined as the lifetime), which means that some of them decay much before, while others will take "forever" to decay".

    Cheers
     
  6. Aug 24, 2009 #5

    Vanadium 50

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    Yes, sloppy writing on my part. I should have said 'at lease 1032' years.
     
  7. Aug 24, 2009 #6

    MathematicalPhysicist

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    It makes me think, how can you construct a proton from quarks, if presumably quarks cannot get detached from eathother?
     
  8. Aug 24, 2009 #7
    How can the age of a proton be determined?
     
  9. Aug 24, 2009 #8
    The maximum age of all protons in the universe is about 13.7 billion years. The minimum radioactive lifetime for decay into a positron and a pi-zero meson can be determined by putting 50,000 tonnes of ultrapure water in a big tank 1000 meters underground and watch for Cerenkov radiation from pi-zero decay into two 67-Mev gammas using over 11,000 photomultipliers. See
    http://en.wikipedia.org/wiki/Proton_decay#Experimental_evidence
     
  10. Aug 24, 2009 #9

    clem

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    ...and see nothing.
     
  11. Aug 25, 2009 #10
    This is still an empirical fact. So, we should be able to translate the lack of any observed violation of charge conservation into a lower bound on the electron lifetime - even if we expect that the electron's lifetime is infinite. In fact, the particle data group's 2008 book lists the lower bound on the mean electron lifetime as [itex]4.6 \times 10^{26}\ \mathrm{yr}[/itex].
     
  12. Aug 25, 2009 #11

    Vanadium 50

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    Actually, the best measurement is from "electron disappearance", which is ~1.5x more stringent.

    However, the masslessness of the photon sets extremely stringent indirect limits. (An exactly massless photon requires exactly conserved charge)
     
  13. Aug 25, 2009 #12

    mgb_phys

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    Are there any good theories about why a neutron lasts about as long as donuts left in the break room while a proton is for ever?
     
  14. Aug 25, 2009 #13
    Any good theory should, first of all, describe neutron decay - an experimental fact. An electro-weak theory does it. The proton decay has not been observed so far, and any good theory should predict thus a very small or zero probability of proton decay. Are you not satisfied with the electro-weak model predictions?
     
  15. Aug 25, 2009 #14

    mgb_phys

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    Sorry - should have said simple theory.
    It seems that such a large difference in behaviour between up-up-down and up-down-down quarks should have an explainable reason - rather than something buried deep in the maths of some gauge group?
     
  16. Aug 25, 2009 #15
    I think it is a question of the ground and excited sates of a compound system, if you like.
     
  17. Aug 25, 2009 #16

    Vanadium 50

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    Don't be so quick to knock the simplicity of groups. There are problems where a little group theory turns a multipage problem into a two-liner.

    That said, there is a simple reason: a neutron is heavier than a proton, so it can decay into a proton, but not vice versa.
     
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