- #1

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##s=(p_{1}+p_{2})^{2}=(p_{3}+p_{4})^{2}##

##t=(p_{1}-p_{3})^{2}=(p_{2}-p_{4})^{2}##

##u=(p_{1}-p_{4})^{2}=(p_{2}-p_{3})^{2}##

where ##p_1## and ##p_2## are the four-momenta of the incoming particles and ##p_3## and ##p_4## are the four-momenta of the outgoing particles.

How is ##s## is the square of the center-of-mass energy?

How is ##t## the square of the four-momentum transfer?

What is the physical interpretation of ##u##?

Are ##s##, ##t## and ##u## related to the ##s##-channel, ##t##-channel and ##u##-channel respectively?