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How can elementary particles spontaneously decay?

  1. Sep 22, 2012 #1
    I understand that the Tau lepton is considered to be an elementary particle. Yet, it can decay into muons and nutrinoes, etc. I can understand composit particles decaying into constituent particles. But I thought that what makes a particle elementary is the fact that it does not decay. So I'm curious as to what mechanism can cause an elementary particle, like the tau, to decay. Any help would be appreciated.
     
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  3. Sep 22, 2012 #2
    An elementary particle is not necessarily forbidden from decay. The rule of thumb is that any process that can happen, will happen given sufficient time. For example, a muon is heavier than an electron. Thus it can decay into an electron. Energy momentum conservation however, forbids the electron to decay into anything else.
     
  4. Sep 22, 2012 #3

    mfb

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    They do not have a "cause" in the classical sense - the decay can just happen, and it is possible to calculate the (expected) lifetime of the particle. From our point of view, it just decays at some specific time.

    With the many-world interpretation of quantum mechanics, it does not decay at a single point in time, but "at all times" (this description is a bit sloppy), but different decay time measurements lead to different, separate parts of the world, each with its own measurement of the decay time.
     
  5. Sep 22, 2012 #4
    It should be mentioned that "decay time" it is meant the decay time for a *single* event, the decay of one atom. The average decay of an ensemble of particles in universes with similar physics should come out the same.
     
  6. Sep 22, 2012 #5
    I was thinking in terms of possibly the tau or muon is really like an electron orbiting itself in some sense, and the extra energy that's more than the electron, comes from the speed of the orbit. Maybe the wavefunction of the tau is like an electron wavefunction whose clouds of probability has two lobes that "orbit" each other. Or something like that. Then I could see how it could break up into separate particles. This is probably not right, but is there any other theories as to how this decay process works?
     
  7. Sep 22, 2012 #6
    Don't get hung up on the word "decay"; it is jargon that does not have quite the same meaning in particle physics as it would in commons speech. In particle physics, fundamental, non-composite particles can simply transform into other particles. What happens is that a tau lepton turns into a muon, a muon antineutrino, and a tau neutrino. There is *no sense* in which these particles were "originally inside the tau" and then they came out when the tau "decayed." To drive this home, most unstable particles can "decay" into many different possible sets of products. For example the tau can also turn into an electron, an electron anti-neutrino, and a tau neutrino. Since the tau can "decay" without emitting a muon, clearly there couldn't have been a muon hiding inside the tau.

    The word "decay" is picturesque jargon that suggests a heavy particle falling apart into its lighter constituents. This is *not* what is happening. "Decay" is just the word we use to denote a heavy particle turning into a collection of lighter particles.

    In particle physics, "fundamental" particles are particles that are not composed of any constituents. Electrons, muons, taus, neutrinos, quarks, and gauge bosons are fundamental. Protons are not fundamental but composite, being composed of quarks and gluons. A particle that does not decay is called "stable." Most fundamental particles are not stable. The proton might be an example of a stable composite particle; at least, we've never seen one decay.
     
    Last edited: Sep 22, 2012
  8. Sep 22, 2012 #7
    Is the wavefunction of the tau lepton unstable in a way that makes it break down? Maybe the wavefunction has some harmonics in it that can easily decompose.
     
  9. Sep 23, 2012 #8
    Particles (excitations in quantum fields) can interact with other quantum fields and excite other particles in them. If this happens, the original particle (like tau lepton) disappears, because it gives all its energy to the other quantum field(s) and creates new particles.

    This process can happen only if the conservation laws are kept: energy, momentum, electric charge, etc. must be conserved...

    Electron would like to decay as well, but it can not decay into anything. Particles can decay only into lighter particles (to keep energy and momentum simultaneously), but there is no negatively charged (to keep charge conservation) particle lighter than electron. So the electron is imprisoned in its existence and does not decay.
     
  10. Sep 23, 2012 #9

    mfb

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    No, this violates observations, for example Z->lepton lepton decays (and many more).

    In addition, that process is not specific to decays - it happens in every interaction of any sort.
     
  11. Sep 23, 2012 #10
    That is a touch speculative...better to say we have no evidence of electron decay.

    Only in some non classical sense....hadrons [particles] have half integer spin quantum numbers....tough to imagine that in a classical view.

    All of science shares your curosity!

    If you go all the way back to just before the big bang, you'll note that spontaneous symmetry breaking[spontaneous decay] and phase transition there to a lower and more stable energy level [our universe] is what kicked off this universe.

    Just what happend, why that happened, is still unknown. But it seems that the initial conditions were able to spawn huge amounts of energy...as well as space, time, and subsequently the four fundamental forces and mass...

    Will our universe spontaneously decay?? Can time and space decay?

    Another view of 'decay' is via thermodynamic equilibrium.....
     
  12. Sep 23, 2012 #11
    Perhaps another way of saying this is that the original tau particle interacts with (scatters off) a virtual particle (of the other fields) and produces the decay products. Does that sound right? Then this is no more mysterious than a photon scattering off of an electron to produce a photon with a lower energy level.
     
  13. Sep 23, 2012 #12

    Bill_K

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    No. It does not have to scatter off anything.
     
  14. Sep 23, 2012 #13
    B ill K :
    Bill, How do we know that if we don't understand if or why an elementary particle decays??

    Are you thinking that from the sense virtual particles are an artifact of perturbation theory, and do not appear in a non-perturbative treatment??

    I've never read a clear explanation of distinctions between perturbative and non perturbative treatments except the former is a true ground state and supposedly has
    no particles but still has energy, while the latter has both. I am always a touch leery of 'different mathematical treatments' that can't be reconciled...like QM and GR for example...
     
    Last edited: Sep 23, 2012
  15. Sep 23, 2012 #14

    mfb

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    The observed decay rates are in good agreement with quantum field theory, and the main contribution for those decays does not include additional virtual particles as scattering process (it does include a virtual W, but that is produced in the decay process itself).
     
  16. Sep 23, 2012 #15
    Is that with or without gravity present??

    My understnading from forum discussions is that uniquelydefined particle states do not even in general exist in QFT on a curved spacetime. ....
     
  17. Sep 23, 2012 #16

    Bill_K

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    QFT calculations do not include gravity, which is not part of the Standard Model.

    What happens in curved spacetime is that different observers may have different vacuum states, and so they may disagree on the presence or absence of particles. But there is no disagreement on what constitutes a particle.
     
  18. Sep 24, 2012 #17

    mfb

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    Gravity is so weak that it is negligible for common particle physics experiments. Some theories with extra dimensions could change that for the LHC, but no hint of those extra dimensions was found yet.
     
  19. Sep 24, 2012 #18
    Bill...thanks, that's exactly my understanding

    but exactly what QFT says and doesn't say is a lot less clear to me.

    mfb...
    yes, but I have to wonder about existence......see Bill's previous reply....
    acceleration changes vacuum states....

    One of my favorite posts said:
    I accept that, but the subtelies of the underlying mathematics
    is not so clear to me.
     
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