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Srednicki says in Ch20, we must remember that the mandelstam variable s is positive. However s is defined as [tex]s=-(k_1+k_2)^2=(E_1+E_2)^2-(\vec{k_1+k_2}).(\vec{k_1+k_2}) [/tex]. Using metric (-+++). I can't quite see why this must be always positive? or why for that matter t and u are negative?

Also he says we get [tex] V_3(s) [/tex], from the general [tex] V_3(k_1,k_2, k_3) [/tex] by setting two of the three k's to -m^2, and the remaining one to -s. Does anyone know why these values? I would have imagined one would be [tex] sqrt(-s)=k_1+k_2 [/tex] and the other two were just left as [tex] k_1[/tex] and [tex] k_2 [/tex] for scattering in s channel kind of way.

Thanks again

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# Srednicki CH20

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