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Particle composition vs particle decay

  1. Jul 1, 2012 #1
    Ive read that elementary particles can decay. Im trying to understand how this can be with a particle that has no composition. So i have two questions:

    If elementary particle A decays into particles B and C, then why cant we say that A is composed of B and C?

    If an elementary particle can decay, does it mean it can be created also? Is there a specific term for this process?
  2. jcsd
  3. Jul 1, 2012 #2


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    Are particles B and C elementary particles?
  4. Jul 1, 2012 #3
    Yes, they are elementary particles. A muon decays to 1 electron + 2 neutrinos.
  5. Jul 1, 2012 #4


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    Ah, I think I misunderstood a little. Surely A WOULD be composed of B and C?
  6. Jul 1, 2012 #5
    Thats what you would think, but A is also an elementary particle, it has no composition.
  7. Jul 1, 2012 #6


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    pftest, For one thing, elementary particles appear to have no structure - they are to the best of our knowledge pointlike, and therefore not made of any smaller pieces.

    For another, there is no consistent way to assign the pieces.

    e+ + e- → γ + γ

    Where did the photons come from? And where did the electrons go? Which is made of what?

    p + p → p + p + π+ + π-

    Where did the pions come from? Protons would seem to have an indefinite supply of pions.

    π+ → μ+ + νμ

    But sometimes

    π+ → e+ + νe

    What is the pion "made of", an electron or a muon?

    There is no way to assign the pieces consistently. One has to conclude that there are no pieces - particles simply turn into other particles.
  8. Jul 1, 2012 #7

    Vanadium 50

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    Indeed, there are a few rare cases where both the decays A -> B + C and B -> A + C are allowed. (A and B are very broad, and C is a pion)
  9. Jul 2, 2012 #8


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    After some thinking, it seems that it is not so much decaying so much as metamorphosis. It looks as if, for instance, the muon transforms into an electron and 2 neutrinos. This would also explain your other question, if turned around. Remember Einstein's equation, e=mc2, this implies that mass can become energy, and vice-versa, so I believe that this might have something to do with it.
  10. Jul 2, 2012 #9
    everything is composed of energy and field balances. any set of partcles and matching antiparticles may be made from a source of free energy, such as kinetic energy of an accelerated electron.
    furthermore, as long as all the balances are met, there is no reason that any group of particles adding up to the total mass-energy, spin, color, etc... can be created from nothing.

    "elementary" merely means it is indivisible in its rest state; if its rest state is excited in some manner, then it will decay (like a strange quark, or a Tau lepton) but it is still a fundamental and indivisible particle before it decays.
  11. Jul 2, 2012 #10
    What's weird is that there is little difference between decay and scattering in Quantum field theory (or at least in the Feynman diagram approach). You can connect interaction vertices like you want and many and more combinations are valid physical phenomena. Many are forbidden by kinematics or some conservation law, but I find it deeply fascinating and quite intriguing that you can basically play Lego with particles.
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