Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Particle decay and relativity

  1. Sep 8, 2010 #1
    I am answering a question about a particle decaying into two other particles and one needs to find the momentum of the children particles from the rest frame of the parent. I found a very helpful article on Wikipedia under 'Particle Decay', however I am really confused by their mathematical working and wondering if anyone can explain it or agree with me that Wiki has made a mistake :)

    I can't copy all the equations so the link is here: http://en.wikipedia.org/wiki/Particle_decay
    its under the Sub heading Conservation of four momentum.

    I don't understand how they can expand 2(p_M*p_1) into only 2 terms when under normal (a+b)(c+d) expansions you get four. Unless there is some wierd vector multiplication I don't understand?

    Any help GREATLY appreciated!
  2. jcsd
  3. Sep 9, 2010 #2


    User Avatar
    Science Advisor

    That easy. Each momentum p in these equations is a four-momentum, so for p this reads p = (E, p). The multiplication is just the vector multiplication, where the first term involves the E's, whereas the second term involves the three-momentum p. The minus sign appears due to the law how to multiply co- and contra-variant four-vectors.
  4. Sep 11, 2010 #3
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook