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Particle physics - invariant mass question

  1. Oct 20, 2009 #1
    1. The problem statement, all variables and given/known data

    At HERA 30 GeV electrons collided head on with 820 GeV protons. Calculate the invariant mass of ep collisions.
    (masses: e=0.0005GeV, p=0.938GeV)

    2. Relevant equations

    M^2 = (E1 + E2)^2 - (p1 + p2)^2 ?

    3. The attempt at a solution

    I know the numerical answer to this question is 314GeV, but so far I haven't been able to get it. I've tried plugging in the known values to the above equation, using E^2 = p^2 + m^2 to find momentum, but obviously didn't get the right answer.

    What should I be doing?

    Thanks!
     
  2. jcsd
  3. Oct 20, 2009 #2
    Hint: Consider direction of their momentum.
     
  4. Oct 20, 2009 #3
    Wow. That was really stupid of me! Thanks for the hint! Got it now :)
     
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