You can calculate the x from the final state, but you can think of the final state as two partons or two jets, and then you can compare the x you get calculating from each final state. The direct process x=1 causes this bulge in parton x--you'll notice we can't reconstruct x that high at the jet level. It's evident that the theta-phi jetfinder is, again, more robust over our kinematic range.
Greg Morrow morrow@physics.rice.edu