Limits based on Mandelstam Variables

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Limits based on Mandelstam Variables

Since the Mandelstam variables are the scalar product of 4-momenta, which are invariants, they are invariants as well. The sum of these invariant variables must also be invariant as well. Find the sum of the 3 Mandelstam variables when the two particles have equal mass in the center of mass frame gives:

[math]s+t+u=(4(m^2+\vec p \ ^{*2}))+(-2 p \ _1^{*2}(1-cos\ \theta))+(-2 p \ _1^{*2}(1+cos\ \theta))[/math]

[math]s+t+u \equiv 4m^2[/math]