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Spin Polarization and Momentum in Particle Decay

  1. Feb 19, 2008 #1
    In an elementary particle decay, such as the decay of a positive pion into a positive muon and a muon neutrino, are the spin polarizations of either product always parallel (or anti-parallel) to their momentum? If so why?
     
  2. jcsd
  3. Feb 19, 2008 #2

    pam

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    The muon neutrino is so close to massless that its spin must be aligned with its momentum (helicity=+1/2). Since the pi is spinless, the muon must also have +1/2 helicity. For other decays, the polarization may not be 100%.
    The momentum direction makes a good z axis to describe polarization because then L_z=0.
     
  4. Feb 20, 2008 #3
    Hi,

    I think neutrinos are Left-handed so helicity = -1/2 ?
     
  5. Feb 20, 2008 #4

    pam

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    You are right about neutrinos, but a positive muon is an anti-lepton, so the "neutrino" in this case is really a right-handed anti-neutrino.
     
  6. Feb 20, 2008 #5

    kdv

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    I thought that if the muon emitted is an antilepton, the muon neutrino had to be a neutrino, not an antineutrino. no?
     
  7. Feb 21, 2008 #6

    pam

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    I am greatly embarrassed by making the same silly mistake twice, and apologize to all.
    I must have been sleepthinking. Thank you Barmecides and kdv for your corrections.
     
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