Impact of Pile-up on Jet Constituent Multiplicity in ATLAS

One of the biggest challenges facing jet constituent multiplicity measurements is contamination from extraneous sources of soft radiation, such as pile-up. This note studies the impact of pile-up on various definitions of multiplicity. In particular, the impact of selected pile-up suppression techniques is evaluated: soft-drop declustering and constituent subtraction. Additionally, a first study of iterative soft-drop (ISD) multiplicity in ATLAS is presented. It is found that the application of pile-up suppression counteracts the increase in the measured multiplicity values due to pile-up, as expected. As constituent multiplicity is the single most important observable for distinguishing quark-initiated from gluon-initiated jets, these studies are an important input for developing a powerful and robust quark-versus-gluon jet tagger in the future.