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Neutrino decay

  1. Jan 27, 2005 #1
    some theorist predicts proton can decay, since the decay of proton does not violate any fundemantal conservation law. My question is, could neutrino decay? How come I have never heard of anyone say neutrino could decay?
  2. jcsd
  3. Jan 27, 2005 #2
    Well a neutrino sitting there all on it's own could only turn into another lepton or else it would violate conservation of lepton number, and it couldn't turn into a charged lepton since that would violate conservation of charge, so a neutrino can only really turn into another flavour of neutrino- which just happens to be what they're doing all the time. Google for 'neutrino oscillation'.
  4. Jan 27, 2005 #3
    conservation of lepton's number?
    I am not talking about standard model here, as i said b4, some theorist has already predicted the proton will decay, wouldn't the decay of proton violate the conservation of baryon's number?
    all of the most fundamental conservation law is closely related to a symmetry principle... however, the conservation of baryon's and lepton's number don't have a symmetry behind them..
    could anyone anwser me why people always talk about proton's decay but seldomly say a word about neutrino's decay
  5. Jan 27, 2005 #4
    Well, that depends on what the proton is decaying into...This law predicts what would qualify as "a valid decay-product", if you will

  6. Jan 27, 2005 #5


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    Baryon number and lepton number are accidental symmetries of perturbation series in the standard model. Things like protons should remain stable, unless you enlarge the standard model gauge group or somesuch. You can postulate such an enlargement, but nothing like that has ever been observed.

    Now if lepton number is not conserved, by some extension of the sm, then yes neutrinos should be able to decay. Except that they are extremely light, and no lighter leptons have ever been observed, hence by phase space arguments the electron neutrino should remain stable under decays.
  7. Jan 27, 2005 #6
    let's say, if the lepton number is not conserved, why couldn't neutrino decay into 2 photon like neutral pion?
  8. Jan 27, 2005 #7


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    Take the neutrino massive and apply the law of energy-momentum conservation...

  9. Jan 27, 2005 #8
    I'd guess that in those theories it goes [tex]p \rightarrow n + e^+ + v[/tex], which doesn't violate anything, and happens in beta-plus decay.
  10. Jan 27, 2005 #9
    if neutrino has mass, it won't violate the conservation of energy+momentum when it decay into 2 photon

    doesn't violate anything?? I will give you one more post to correct what you said....
    Last edited: Jan 27, 2005
  11. Jan 27, 2005 #10


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    What??I'm giving you one more post to correct your mistake...


    P.S.What theory of the ones known admits a 2-photonic vertex??
  12. Jan 27, 2005 #11
    i am not talking about feynman diagram or standard model here...... i knew in the standard model, neutrino won't decay, so do proton... my question is, how come the theorist predicted the proton will decay into positron, but they didn't predict neutrino will decay into photon
  13. Jan 28, 2005 #12


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    Oh ok I see the misconception, beta plus decay is not a pure proton decay mode. Its a nuclear decay mode. This apparent weirdness in energy nonconservation occurs because there are lots of orbiting protons and neutrons surrounding the reaction (actually lots of protons and very few neutrons).. The entire configuration is more energetically favorable with an extra neutron. You can think of it as the proton borrowing energy from the surroundings and converting itself into a neutron.

    But in a pure vacuum, like in particle physics, this will never happen... Except by GUT theories, which are not observed as of yet.
  14. Jan 28, 2005 #13
    What do you mean by these words ?

    What energy non-conservation ? Besides, how do you know all this ? Really, i think i am lost here or i am havig a bad day :tongue2: Please elaborate

  15. Jan 28, 2005 #14


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    Beta plus decay commonly means the basic process p->n + e++v. It is a nuclear decay mode in that it can only happen if the proton is inside a heavier nucleus and the final state nucleus is more tightly bound; the process is forbidden in free space by energy conservation since a neutron alone is heavier than a proton.

    Going back to neutrino decays, there is another possibility not mentioned so far that does not require GUT extensions. If the neutrinos are not massless as evidence suggests, then the heavier mass eigenstate can decay to a lighter mass eigenstate by emitting a photon. The amplitude for such processes is very low since it requires an electron-W virtual pair and a photon coupling to one of them; three vertices + W and e propagators = very tiny amplitude.
  16. Jan 28, 2005 #15

    thanks for the explanaition

  17. Jan 30, 2005 #16


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    Um, without a model it's kind of hard for a theorist to predict anything! :confused:

    In the standard model, neutrinos interact only via the weak interaction, and photons are involved only with the electromagnetic interaction. The weak interaction is a lot weaker than the electromagnetic interaction. If neutrinos interacted electromagnetically, it would surely have been noticed experimentally a long time ago. People have been studying neutrinos via their weak interactions for several decades now.
  18. Jan 30, 2005 #17
    oh come on, i said it was a guess, we were talking about lepton and baryon numbers. Anyway, googling for proton decay gives a GUT prediction of p-> e^+ + pi0, which violates both, so there you go.
  19. Jan 31, 2005 #18


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    There's precious little for a neutrino to decay into. It has spin 1/2 and lepton number +/- 1. that means a pair, or a photon (unlikely indeed) and another lepton -- for decay must be an electron, muon etc.. But there's not enough energy. The best that can happen is, as has been suggested, a transition into another neutrino, and a photon, like gamma decay. The photon, probably on the order of ev/1000, will have a very, very slow frequency. It's not clear to me if the requisite neutrinos exist.
    Reilly Atkinson
  20. Feb 13, 2005 #19


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    1 meV that's about 10 Kelvin, no ?
    Probably it isn't the case: we should have seen that emission line in the microwave background, no ?

  21. Feb 13, 2005 #20
    it could never turn into a tau, muon or electron because that would violate conservation of charge (unless a weak boson comes along). But what about electron neutrinos? They can't possibly "decay" into a tau neutrino because that would violate conservation of mass/energy quite a fair bit, isn't it?
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