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Can a quark emit a photon

  1. Jan 16, 2013 #1
    In quark-level diagrams of decays, I often see, for instance, a baryon with one quark emitting a gluon and then the gluon goes on to decay into two new quarks. What I'm wondering is, if the gluon is color-neutral, can a photon play this same role in theory? Is this impossible due to some conservation law or other principle, or just very unlikely?

  2. jcsd
  3. Jan 16, 2013 #2
    Yes, quark-antiquark annihilation can produce photons (though with lower probability than gluons) and quark-quark and quark-antiquark scattering can be mediated by photon exchange. Quarks are electrically charged which is enough to tell you they interact with photons.

    Edit: Though note that quark-antiquark annihilation into photons can only occur if the incoming particles have the same flavour and are anti-colour pairs. So a blue up quark can annihilate with an anti-blue up antiquark into photons.
    Last edited: Jan 16, 2013
  4. Jan 16, 2013 #3
    Hmm well I know that two quarks can annihilate to produce photons, but I guess I'm wondering about a more specific process here---where a single quark emits a photon, so that the original quark and the photon both go flying off, and then the photon decays into two quarks.
  5. Jan 16, 2013 #4


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    Yes, its possible. The photon can be considered a "long distance effect" while the strong a "short distance" effect. (all about energy scales).

    Here are the first order diagrams for an up going to an up and a down-antidown pair:

    \text{WEAK} & \text{PHOTON}\\
    \text{GLUON} & \text{Z BOSON}

  6. Jan 16, 2013 #5
    Wow thanks!
  7. Jan 17, 2013 #6


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    Staff: Mentor

    Just one detail:
    There are no color-neutral quarks.
    If a quark emits a photon, it simply keeps its color charge.
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