Abstract
Measurements of jet substructure in heavy-ion collisions may provide key insight to the nature of jet quenching in the quark-gluon plasma. Jet grooming techniques from high-energy physics have been applied to heavy-ion collisions in order to isolate theoretically controlled jet observables and explore possible modification to the hard substructure of jets. However, the grooming algorithms used have not been tailored to the unique considerations of heavy-ion collisions, in particular to the experimental challenge of reconstructing jets in the presence of a large underlying event. We report a set of simple studies illustrating the impact of the underlying event on identifying groomed jet splittings in heavy-ion collisions, and on associated groomed jet observables. We illustrate the importance of the selection of grooming algorithm, as certain groomers are more robust to these effects, while others, including those commonly used in heavy-ion collisions, are susceptible to large background effects -- which, when uncontrolled, can mimic a jet quenching signal. These experimental considerations, along with appropriate theoretical motivation, provide input to the choice of grooming algorithms employed in heavy-ion collisions.
Highlights
Jet grooming techniques were developed in the highenergy physics community to mitigate pileup contamination and improve the theoretical calculability of jet observables in pp collisions
Grooming techniques have recently been applied to heavy-ion collisions in order to establish whether jet quenching in the quark-gluon plasma modifies the hard substructure of jets, such as the splitting function, and to elucidate whether jets lose energy coherently, as a single color charge, or incoherently, as multiple independent substructures [4,5,6,7,8,9,10,11,12]
Monte Carlo (MC) event generators suggest that jet splittings identified by grooming algorithms are correlated to parton shower splittings, raising the possibility that identifying groomed jet splittings in heavy-ion collisions may provide a handle on the space-time evolution of jet propagation through the hot QCD medium
Summary
Jet grooming techniques were developed in the highenergy physics community to mitigate pileup contamination and improve the theoretical calculability of jet observables in pp collisions. Local background fluctuations in a heavy-ion environment can result in an incorrect splitting (unrelated to the jet) being identified by the grooming algorithm. This problem is analogous to the well-known experimental problem of “combinatorial” jets in heavy-ion collisions, which is typically treated by either (1) reporting jet measurements in the background-free region of phase space, namely, at sufficiently large pT and/or small R, or (2) subtracting the combinatorial jet distribution on an ensemble basis. The presence of background contamination in groomed jet observables has been recognized to some extent since the first measurements in heavy-ion collisions; the magnitude of the effect has not been quantified, nor has its qualitative impact been understood. We discuss implications on the interpretation of previous measurements
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