Qq- ->qq- differential cross section and mandelstam variables

[helpcometk]

qq- --->qq- differential cross section and mandelstam variables

Homework Statement

for the problem statement please look the attachement

Homework Equations

The problem is asking to express the mandelstam variables in temrs of the scattering angle θ.I would like to ask the quastion weather there varibles should depend on qq- --->qq- process or they are the same for any process A+B--->A+B ? And then what does it mean massless limit i.e WHICH masses become zero ?
It is not obvious from where i should start to calculate the mandslestam variables in terms of the scaterring angle.

The Attempt at a Solution

 

[andrien]

I think you know about those s,t and u.you may also be aware of fact that only two of it are independent because of the relation
s+t+u=4m²
since everything is asked in cm frame,so it is easy.Because in rest frame as you can show
s=4E²=4(|p|²+m²)
t=-2|p|²(1-cosθ)
u=-2|p|²(1+cosθ),since |p|=|p'|
massless limit is also called high energy limit,in which E>>m,so you can neglect m in comparison to |p|.θ is scattering angle in CM system.

 

[helpcometk]

thanks