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Massless charged particles

  1. Jul 5, 2011 #1
    Is it possible to have a massless charged particle in nature?
     
  2. jcsd
  3. Jul 5, 2011 #2
    Re: Massless charged particles!

    There are two massless particles in nature: photons and gluons. Gluons are also charged.
     
  4. Jul 5, 2011 #3
    Re: Massless charged particles!

    Then is it possible that Gluons can move with the speed of light?
    Isn't there a contribution to the effective mass because of the charge?
     
  5. Jul 5, 2011 #4

    Bill_K

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    Re: Massless charged particles!

    There are no massless charged particles in nature. [Where else would they be?]
    Gluons have a color charge but not an electromagnetic charge. They also are not free particles. They are confined to the interior of hadrons.
    Meaningless question. You can't split the mass of a particle into contributions. The mass of an electron is 0.51 MeV/c2, and you can't say that X% of this is due to its having a charge.

    Actually at 0.51 MeV/c2 an electron is pretty close to being a massless charged particle. In the LEP collider, electrons circulated with an energy of over 100 GeV, a factor of 200,000 times their rest mass, meaning that in comparison to their energy they were effectively massless.
     
  6. Jul 5, 2011 #5

    Vanadium 50

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    I changed the topic from Massless Charged Particles! I see no reason to put an exclamation point there! Indeed, it makes it look like something new and surprising has happened! That is not the case! Additionally, and most importantly, extraneous exclamations make something hard to read!
     
  7. Jul 5, 2011 #6
    Re: Massless charged particles!

    Exuse me? Gluons are massless and carry color charge.
     
  8. Jul 5, 2011 #7
    Re: Massless charged particles!

    Read up on wiki about gluons, the article looks ok.
     
  9. Jul 5, 2011 #8

    Bill_K

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    You're excused. My impression is that the original poster was referring to electric charge, not charge in the sense of any quantum number that is different in the particle and its antiparticle.
     
  10. Jul 6, 2011 #9

    blechman

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    There is no theoretical reason why you cannot have a massless, charged particle, where by "charge" I mean ANY kind of charge (electric, color, etc). That is to say: it does not violate any fundamental law of nature.

    As for, "what about QM corrections?" Well: if the particle has spin, then there are symmetries that forbid mass from being generated by QM corrections, so there is no problem. If you actually go through the trouble of calculating the loop diagrams, none of them contribute to the mass.

    As I mentioned: this is only true for particles with spin. For spin-0 particles (scalar fields), the situation is more troubling. Generally speaking, you are correct that a massless, spinless, charged particle would be very strange (although not theoretically impossible, certainly very "unnatural"). However, as there are no such beasts in nature, we can breathe a sigh of relief!

    Of course, in some cases, even scalars can be massless (or very light) without running into theoretical trouble if there is a symmetry protecting them (see "Goldstone Boson", for example).

    I hope that helps.
     
  11. Jul 6, 2011 #10

    samalkhaiat

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    A massless particle of spin > 1/2 cannot carry a charge/energy-momentum induced by a conserved Poincare' covariant vector/tensor current. This is the Wienberg-Witten theorem.
    Question for you to think about; Does the theorem imply that gluons/gravitons donnot exist?

    sam
     
  12. Jul 6, 2011 #11

    Vanadium 50

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    There are no freely propagating gluons. Gravitons are explicitly covered by Weinberg & Witten's paper. (Have you read it?)
     
  13. Jul 6, 2011 #12

    blechman

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    true, I forgot about WW theorem. So when I said there was no theoretical reason to forbid massless charged particles, I was wrong. But the WW theorem is a funny thing: nature doesn't use it very much (as gluons/gravitons are counterexamples that do not satisfy the conditions of the theorem, as are electrically charged W bosons). That's why I didn't think of it.
     
  14. Jul 6, 2011 #13

    blechman

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    Actually, I believe the issue lies with the fact that gluons (and other Yang-Mills gauge bosons) interact with a NON-GAUGE-INVARIANT current, and therefore do not satisfy the conditions for the theorem to hold. Similar with the graviton (enegy-momentum tensor is not GCI).

    Sam: is that correct?
     
  15. Jul 6, 2011 #14

    Vanadium 50

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    You can add that to the list of why gluons don't have problems with Weinberg-Witten. (Also, for W's you can add the fact that they are massive. Which is of course related to gauge invariance)
     
  16. Jul 7, 2011 #15

    blechman

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    The W bosons are massive and therefore are not privy to the WW theorem, of course. But in theory, ANY (nonabelian) Yang-Mills theory, spontaneously broken or not, asymptotically free or not, is not subject to WW due to the non-gauge-invariant current.
     
  17. Jul 7, 2011 #16

    Haelfix

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    I would add that massless scalars or pseudo scalars are problematic in the sense that they violate the equivalence principle, and tend to lead to unobserved long range forces.

    You can kind of get around this by tuning their couplings to ridiculously small values, but then there are theoretical reasons to believe this cannot happen (related to naturalness and that gravity should always remain the weakest force).
     
  18. Jul 7, 2011 #17

    samalkhaiat

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  19. Jul 7, 2011 #18

    samalkhaiat

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  20. Jul 8, 2011 #19

    blechman

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    That is an interesting point... Phenomenologically, you probably have to worry about a bunch of things, such as how they affect BBN, etc.

    But from a purely theoretical point of view, there is nothing wrong with them (ignoring the naturalness problems, which can be avoided by imposing additional symmetries).
     
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