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Muon decay and internal structure of leptons.

  1. Aug 28, 2009 #1

    Could someone tell me please what is the scientific consensus(or some interesting theories) on why do muons decay and what is the mechanism of it? My understanding of particle decay is based on for example neutron wchich decays because a quark in it flips from down to up. But leptons, if they have no internal structure(that I know of) can't decay this way. So how? Is it because of interaction with some virtual particles? Are there any fairly recent articles about it? Wikipedia only informs they decay via "weak interaction" which doesn't explain a lot.

    Using my imagination I came up to a possible explanation that there is some intrisic instability in so much mass concentrated in a lepton so when a right mix of virtual particles appear nearby the muon "gets rid" of its energy for their benefit. I wonder what explanation real physicist have?
  2. jcsd
  3. Aug 28, 2009 #2
    Yes, one can think so. Even a weak interaction is a permanent interaction so the "elementary" muon is not really elementary and the virtual particles are always present with it, just like an electron in an atomic excited state. Transition of this excited compound system into the ground state is a system decay.
  4. Aug 28, 2009 #3
    Following the Wikipedia trail I've found out that the muon first converts itself to a neutrino and a W boson. Then the boson delivers its electroweak "message" to virtual particles, or decays to an electron and a neutrino. So my question becomes: Why do muons convert themselves to a neutrino and W bosons in the first place? Why are they unstable (i know it is because of their mass, but why their mass causes instability?). Is there some sort of "official" in depth explanation or is it just one of those things we have to accept as "that's the way it is"?
  5. Aug 28, 2009 #4
    The larger mass causes instability because it allows them to move to a lower energy state (electron / positron) while maintaining the right quantum numbers. Electrons are stable because there is no lighter particle for them to decay to that would allow the system to retain the correct quantum numbers.
  6. Aug 28, 2009 #5
    Hi Flynth-
    The muon decay is actually fairly complicated. It decays to a positron or electron and two neutrinos. Because of helicity, the positron/electron momentum spectrum is described by the Michel rho parameter. The negative muon lifetime and electron spectrum in matter is also determined by its being captured in a muonic atom.
    Bob S
  7. Aug 28, 2009 #6
    Thanks Hologram. Great explanation why it happens.

    Is the lifetime of the negative muon longer or shorter when it's captured by an atom? Muons prove to be quite an extensive and interesting subject. Someone should write a book about them(taking in account modern theories).
  8. Aug 28, 2009 #7
    It does not matter as long as no relativistic effects come into play.
  9. Aug 28, 2009 #8
    Hi Flynth-
    See Table IV on pages 6-10 in
    The lifetime in high Z elements is about 80 nsec, nearly independent of Z, vs. 2200 nsec in vacuum.
    The have been many lengthy articles on muons in journals like Annual Reviews.

    [Added] The negative muons cascade down to a 1S orbit in a few picoseconds, and for Hi Z targets, it is swimming around in a nuclear soup. The competing reaction to leptonic decay to an electron is capture on a proton (similar to nuclear electron capture of 1S electrons). So the lifetime is nearly independent of Z.
    Bob S
    Last edited: Aug 28, 2009
  10. Aug 28, 2009 #9
    It is due to a capture channel, not a decay one.
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