Hi! I'm struggling with this question: A neutrino of energy 10 GeV collides with a proton at rest producing a 9 GeV muon deflected at an angle of 0.01 radians away from the initial neutrino direction. The collision is used to probe the momentum distribution of down quarks in the proton. Show that the requirement that the quark four-momentum be almost zero before and after the collision gives the Bjorken scale variable x. Calculate x for this process. I know that it follows something along the lines of (P'+q)^2=0 P'^2+2P'*q+q^2=2xP*q-Q^2=0 x=Q^2/(2P*q) but i'm a little uncertain about what all the letters represent and how it's all derived... Could anyone lend a hand explaining please?