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Parton Distributions in PP collisions

  1. May 9, 2009 #1
    I read some notes answering a question about how a Z boson is made in a proton anti-proton collision and it said that the quark antiquark collision is a very rare event because the antiquark has a small parton distribution function. Surely the up anti up(or down antidown) parton distributions are the same? The general context is the discovery of the W+ W- and Z bosons. Were these all quite difficult processes to observe?

  2. jcsd
  3. May 9, 2009 #2
    I can see why their integrals should cancel, but I doubt they must be equal in all variables one can cook.

    On top of that, for up or down you have contributions from the sea and the valence as well. The valence is defined as this part which does not cancel when integrated. Depending on your kinematics, the valence may contribute a lot.
  4. May 9, 2009 #3


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    Neither the integrals nor the distributions of ubar and dbar are equal.
    The "Gottfried sum rule" which assumes they are equal has been shown to be violated in several experiments. A good reason for their difference is that there are more pi+ mesons than there are pi- in the proton pion cloud. Use "surely" carefully.
  5. May 9, 2009 #4
    It was about the equality of u_sea with ubar_sea (on one hand, and on the other hand d_sea with dbar_sea) if I understand the question.
  6. May 9, 2009 #5
    Lol I'm out my depth already, cheers for the replies but I think I will have to hunt my elusive lecturer down...
  7. May 10, 2009 #6

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    Those aren't equal either, I'm afraid.
  8. May 10, 2009 #7
    That's what I tried to convey in #2
  9. May 10, 2009 #8
    Can you give us the specific quote? I think there may be some misunderstanding here. It is an obvious consequence of CPT theorem that the spin-averaged pdf of the up-quark in proton is equal to the spin-averaged pdf of the antiup-quark in antiproton. And neither of them is small.

    This quote "the quark antiquark collision is a very rare event because the antiquark has a small parton distribution function" might make sense in the context of trying to observe Z on a proton-proton collider. It is indeed rather unlikely to find an energetic antiquark inside a proton.
    Last edited: May 10, 2009
  10. May 10, 2009 #9
    If I CPT an up quark, I get an antiup, not a down.
  11. May 10, 2009 #10
    grr yes that's what I meant.
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