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Photons vs. Neutrinos in electromagnetic fields

  1. Sep 27, 2011 #1
    I have been wondering why photons can't go through matter and electromagnetic fields mostly unaffected while neutrinos can. Neither of them have an electric charge as a particle, and the basic description I see about neutrinos is always that "they are unaffected because they have zero electric charge".
    I understand, on a basic level, that the photons are gauge bosons and they mediate the electromagnetic force, but I could not quite find a clear description as to how this leads to EM fields interacting with photons but not interacting with neutrinos.

    Any (also short) replies or links will be appreciated.
    Thanks in advance.
  2. jcsd
  3. Sep 27, 2011 #2


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    Staff: Mentor

    The neutrino and the photon interact via different fundamental interactions: weak interaction for the neutrino, and electromagnetic interaction for the photon. The weak interaction is basically much weaker than the electromagnetic interaction.
  4. Sep 27, 2011 #3


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    Science Advisor

    The Wikipedia neutrino page mentions that neutrinos might have an extremely small magnetic dipole moment, and if so they would interact electromagnetically also.
  5. Sep 27, 2011 #4
    To clarify : in the standard model, the neutrino has zero magnetic moment. We do not take it for granted, as some extensions of the SM predict a small moment, and so we test it :
    GEMMA experiment
    We find a very, very tiny upper value : less than a billionth of that of an electron. As a consequence, even if we attribute such a tiny magnetic moment to the neutrino, its interaction rate through electromagnetic interaction would not be substantially higher than with the weak interaction only. There is a direct comparison of neutrino-electron cross-section for the two processes in the paper above.
  6. Sep 27, 2011 #5
    Let me elaborate a little on jtbells response. As you say, photons mediate the electromagnetic force, which means that things with charge exchange many many photons as they approach each other. This is a high-strength interaction because the photon "couples" to charged particles with a lot of strength. There is a number which describes this, it is the electromagnetic coupling constant, related to the fine structure constant. So if a photon passes by a charged particle, there is a high probability that it can "crash" into it and exchange some energy with it. This number is much lower for neutrinos

    In addition, if you are prepared to consider some Feynman diagrams, in the Standard Model you are allowed a vertex which has a charged particle going in and out and a photon coming off. There is no such diagram with a neutrino replacing the photon. See this page for actual diagrams: http://www.lhc-closer.es/php/index.php?i=1&s=6&p=4&e=0. The first diagram on the page shows the vertex I describe, while the second shows what neutrinos ARE allowed to do.

    So this means that neutrinos are just not allowed to interact directly with charged particles in the same way that photons are. We say they only interact via the weak force, but even through the weak force there does not exist such a simply way for neutrinos to interact with charged particles as there is for photons. So they are doubly penalised in some sense.
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