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Understanding a p-p collision qualitatively

  1. Jan 9, 2013 #1
    If two protons collide at the LHC and a quark from each undergo a weak interaction together to form a W/Z boson, then what happens to the remnants of the protons?

    I think that if the weakly interacted quark was a sea quark, then the proton can continue being itself, but may lead to jets. If, however, the weakly interacted quark was a valence quark, then the remnant of the proton must lead to jets since a valence u or d is missing. Ultimately, the final state will still contain two protons to conserve baryon number.

    Is this correct?

    Also, are the two quarks that form the W/Z "ripped out" of the proton or rather one proton enters the other so that the interaction could occur? That is to say, a volume of space normally occupied by one proton is temporarily occupied by two protons and also the newly created W/Z boson?
  2. jcsd
  3. Jan 9, 2013 #2


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    There are many ways this can happen, so let's describe just one. A W boson couples to the weak current, which for baryons derives from quark-quark bar. Each proton contains three quarks: up, up and down. So say one of the up quarks emits a gluon, which turns into down and down-bar. At this point we have a total of four ups, three downs and a down-bar.

    The down-bar combines with one of the up quarks from the other proton, emitting a W+. What we now have left is three ups and three downs. Minimally this could result in a proton and a neutron.

    In reality there will be many other processes going on, and numerous other particles produced.
  4. Jan 9, 2013 #3


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    You cannot point to a quark and say "this is a valence quark!". If you take any quark and let it interact with some parton in the other proton, you always change the quark content, and hadronization has to care about that afterwards.

    There are elastic collisions - collisions where one or both protons stay protons. But those correspond to a small momentum exchange, I would be surprised to see a heavy boson produced in an elastic collision. Usually, they form jets.
  5. Jan 9, 2013 #4

    Vanadium 50

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    The proton remnants go down the beam pipe. They form hadrons, and as you guessed, global baryon number is conserved.
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