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Particle momenta and cross section

  1. Oct 4, 2015 #1
    Will someone explain this step to me?
     

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  2. jcsd
  3. Oct 5, 2015 #2

    DrDu

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    Some more background would be helpful. I suppose this is for massless particles where p^2=0?
     
  4. Oct 5, 2015 #3
    Yes. It's for electron muon scattering
     
  5. Oct 5, 2015 #4

    DrDu

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    But neither electrons nor myons are massless. Or is this some approximation?
     
  6. Oct 5, 2015 #5
    The event is viewed in a CM frame.
     
  7. Oct 5, 2015 #6

    ChrisVer

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    it has nothing to do with CM frame ([itex]p^2= m^2[/itex] in any frame -frame independent minkowski product).
    Probably they consider electron/muon with high enough momenta/Energy so that they can be considered massless (i.e. the last [itex]=[/itex] in 2nd line should be [itex]\approx[/itex] instead)
     
  8. Oct 5, 2015 #7

    mfb

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    They neglect the particle masses, right.
     
  9. Oct 8, 2015 #8
    I was in a rush. CM has nothing to do with it.

    Indeed, the particles considered are massless.

    I figured it out.
     
    Last edited: Oct 8, 2015
  10. Oct 8, 2015 #9
    Why can we set E/c = p?
     
  11. Oct 8, 2015 #10

    mfb

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    It is the same approximation of massless particles.
     
  12. Oct 9, 2015 #11

    ChrisVer

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    [itex] E = \sqrt{p^2 + m^2} \Rightarrow E = p + \mathcal{O}\big(\frac{m^2}{2p}\big)[/itex]
    (taylor expanding the square root for m<<p)
     
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