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Relativistic kinematics

  1. May 14, 2008 #1
    Hi all,

    I would be very grateful if anyone would be willing to check my understanding of this stuff as it has been several years since I used it in undergrad calculations.

    If a proton and a photon collide head-on with known energies, then the energy in the center of mass frame will be given by the invariant mass [itex]E_\mathrm{com} = W[/itex]. Thus [itex]E_\mathrm{com} = \sqrt{(E_\gamma+ E_p)^2-(p_\gamma + p_p)^2} = \sqrt{(E_\gamma+ E_p)^2-\left(E_\gamma + \sqrt{E_p^2 - m_p^2}\right)^2} [/itex].

    Does this sound reasonable?
  2. jcsd
  3. May 15, 2008 #2


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    sure why not, remember that you should have vector p's in the first expression:

    [tex]E_\mathrm{com} = \sqrt{(E_\gamma+ E_p)^2-(\vec{p}_\gamma + \vec{p}_p)^2}[/tex]

    Just for completeness, then use the fact that they are 'head on'
  4. May 15, 2008 #3


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    That means you need a minus sign in your last term.
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