Jet broadening in the opacity and twist expansions

We compute the in-medium jet broadening ⟨ p ⊥ 2 ⟩ to leading order in energy in the opacity expansion. At leading order in α s the elastic energy loss gives a jet broadening that grows with ln E . The next-to-leading order in α s result is a jet narrowing, due to destructive LPM interference effects, that grows with ln 2 E . We find that in the opacity expansion the jet broadening asymptotics are – unlike for the mean energy loss – extremely sensitive to the correct treatment of the finite kinematics of the problem; integrating over all emitted gluon transverse momenta leads to a prediction of jet broadening rather than narrowing. We compare the asymptotics from the opacity expansion to a recent twist-4 derivation of ⟨ p ⊥ 2 ⟩ and find a qualitative disagreement: the twist-4 derivation predicts a jet broadening rather than a narrowing. Comparison with current jet measurements cannot distinguish between the broadening or narrowing predictions. We comment on the origin of the difference between the opacity expansion and twist-4 results.