In deep inelastic scattering, you describe a collision using the variables $Q^2 = -q^2$ (probe virtuality) and $x = Q^2/2p\cdot q$ (Bjorken x, parton momentum fraction). Now, I seem to remember reading somewhere that in the infinite momentum frame, one of these variables primarily depends on momentum in the transverse direction and the other primarily depends on momentum in the longitudinal direction, but I can't remember which, and checking my most likely sources hasn't turned up anything. Is this actually the case, or is my memory playing tricks on me? And if it is true, which is which?
Apologies for asking a vague question, but I'm really drawing a blank on what it is I might have read, and asking here seemed like the quickest way to figure it out ;)