Disentangling jet modification in jet simulations and in Z+jet data

Abstract

The selection of jets in heavy-ion collisions based on their pT after jet quenching is known to bias towards jets that lost little energy in the quark-gluon plasma. In this work, we study and quantify the impact of this selection bias on jet substructure observables so as to isolate effects caused by the modification of the substructure of jets by quenching. We do so at first in a simplified Monte Carlo study in which it is possible to identify the same jet before and after quenching. We show explicitly that jets selected based on their quenched (i.e. observable) pT have substantially smaller fractional energy loss than those selected based on the pT that they would have had in the absence of any quenching. This selection bias has a large impact on jet structure and substructure observables. As an example, we consider the angular separation ∆R of the hardest splitting in each jet, and find that the ∆R distribution of the (biased) sample of jets selected based upon their quenched pT is almost unmodified by quenching. In contrast, quenching causes dramatic modifications to the ∆R distribution of a sample of jets selected based upon their unquenched pT, with a significant enhancement at larger ∆R coming from the soft particles originating from the wake of the jet in the quark-gluon plasma. The jets which contribute to this enhancement are those which have lost the most energy and which were, therefore, left out of the sample selected after quenching. In a more realistic study, we then show that the same qualitative effects can all be observed in Z+jet events. Selecting jets in such events based on either the jet pT or the Z-boson pT provides an experimentally accessible way to quantify the effects of selection biases in jet observables and separate them from the modification of jet substructure caused by quenching. Selecting Z+jet events based upon the jet pT yields a ∆R distribution that appears almost unmodified whereas selecting Z+jet events based upon the Z-boson pT reveals a significant modification to the ∆R-distribution caused by quenching, once again arising from the wakes of those jets that lose more energy.