Calculating Invariant pT in 2->2 Processes

[JoePhysicsNut]

What's the definition of invariant pT in a 2->2 process?

I know how to calculate the invariant mass in this case, but I am not sure about pT.

 

[mfb]

I guess pT means transverse momentum here - the two momentum components perpendicular to some axis (the beam axis in collider experiments). The whole 4-momentum of the system is conserved, conserved energy is just one of those 4 components.