Jet vetoes versus giant K-factors in the exclusive Z+1-jet cross section

The ATLAS measurement of the exclusive Z+1-jet cross section shows a surprising agreement with fixed-order predictions in the kinematic region expected to be dominated by large jet-veto logarithms. We identify the explanation for this effect: the jet-isolation criterion implemented by ATLAS allows dijet events where an energetic jet is collinear to a final-state lepton. This process contains a giant $K$-factor arising from the collinear emission of a Z-boson from the dijet configuration which overwhelms the effect of the jet-veto logarithms. We provide numerical results for $7~TeV$, $8~TeV$ and $14~TeV$ LHC collisions that demonstrate the interplay between the jet-veto logarithms and the giant $K$-factor in the theoretical prediction. We suggest an alternate isolation criterion that removes the giant $K$-factor and allows for a direct test of the jet-veto resummation framework in the Z+1-jet process.