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Proton-Antiproton collision

  1. May 13, 2004 #1


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    in a particle accelerator a proton and an antiproton, travelling at the same speed, undergo a head on collision and produce sub atomic particles

    a) the total kinetic energy of the two particles just before collision is 3.2 x 10^-10J

    I) what happens to the proton and anti proton during collision?

    my answer----Two pions are formed from an up anti down quark pair and an anti up down quark pair, leaving a down and an anti down quark which will annihilate to produce photon(s)
    is this right? it is just a guess.

    II) state why the total energy after the collision is more than 3.2 x 10^-10J

    i don't know. only thing i can think of is because some mass had been converted to energy, but i don't see how because when the photons are produced they are then travelling at the speed of light even if they have virtually 0 mass so by K.E=0.5mv^2 will K.E not be conserved? someone explain!

    (b) in another experiment the total K.E of the colliding proton and anti proton is greater than 3.2 x 10^-10J

    state two possible differences this could make to the subatomic particles produced.

    ths only thing i can think of is higher frequency photons but photons aren't subatomic particles so i am sunk=/. i don't know.

    Just so you know, this is a revision question i found myself, as opposed to homework ( i have exams in two weeks :surprise: ) so i am not just trying to find a cheap way of getting out of homework! i have looked in text books and notes but can't find anything useful and i'm worried!
  2. jcsd
  3. May 14, 2004 #2
    I think your choices for what is produced in the annihilation is a little broader - any reaction not prohibited has a chance of occurring. So other pairs of lighter particles could be produced, as well as a pair of photons.

    The significance of the 3.2e-10J is that's about the rest energy of a proton-antiproton pair. It's possible for the collision with KE exceeding that to produce two sets of proton-antiproton pairs.
  4. May 14, 2004 #3


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    This is a question for school, rather than university isn't it? (thatt's what it looks like to me anyway), so I think you're complicating the matters too much PJC and not giving them what they want.

    I)I think all they're looking for in the first one is "they anihilate", you could speculate on what's produced if you knew the energies of the proton and anti-proton, but you don't.

    II) What they're looking for is total energy before the collsion = KE + 2mpc2, as mp is most defintely greater than zero and energy is conserved, the total energy always be greater than the KE of the proton and antiproton.

    b) Again keep it simple, the two ways that this could affect the subatomic particles produced is: 1. they could have a larger total mass than in a) 2. they could have the same total mass but a greater total KE than in a)
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