Modification of jet substructure in heavy ion collisions as a probe of the resolution length of quark-gluon plasma

Abstract

We present an analysis of the role that the quark-gluon plasma (QGP) resolution length, the minimal distance by which two nearby colored charges in a jet must be separated such that they engage with the plasma independently, plays in understanding the modification of jet substructure due to interaction with QGP. The shorter the resolution length of QGP, the better its resolving power. We identify a set of observables that are sensitive to whether jets are quenched as if they are single energetic colored objects or whether the medium that quenches them has the ability to resolve the internal structure of the jet. Using the hybrid strong/weak coupling model, we find that although the ungroomed jet mass is not suitable for this purpose (because it is more sensitive to effects coming from particles reconstructed as a part of a jet that originate from the wake that the jet leaves in the plasma), groomed observables such as the number of Soft Drop splittings n SD , the momentum sharing fraction z g , or the groomed jet mass are particularly well-suited to discriminate the degree to which the QGP medium resolves substructure within a jet. In order to find the optimal grooming strategy, we explore different cuts in the Lund plane that allow for a clear identification of the regions of Soft Drop phase space that enhance the differences in the jet substructure between jets in vacuum and quenched jets. Comparison with present data seems to disfavor an "infinite resolution length", which is to say the hypothesis that the medium interacts with the jet as if it were a single energetic colored object. Our analysis indicates that as the precision of experimental measurements of jet substructure observables and the control over uncertainties in their calculation improves, it will become possible to use comparisons like this to constrain the value of the resolution length of QGP, in addition to seeing how the substructure of jets is modified via their passage through it.

Nuclear modification factors for jet fragmentation

“…This stronger bias towards hardfragmenting jets has been proposed as an explanation for the nuclear enhancement in the fragmentation function observed in the LHC data [1] at large x 0.5. This argument is very general: it applies to a large variety of microscopic pictures for the jet-medium interactions, assuming either weak coupling [5,28], or strong coupling [29,30], or a hybrid scenario [3,7,8]. All these scenarios naturally predict that hard-fragmenting jets lose less energy towards the medium than average jets, for the physical reason that we already mentioned: hard-fragmenting jets contain less partonic sources for in-medium energy loss.…”

“…It is therefore crucial to identify observables which probe different aspects of the inmedium dynamics and can thus be used to test the physical ingredients and assumptions underlying the various theoretical scenarios. In this paper, we focus on one such observable, the nuclear modification of the jet fragmentation function, for which there are interesting data at the LHC [1], but few dedicated conceptual studies (see however [2][3][4][5][6][7][8]). The theoretical framework that we use to address this (and related) observable(s) is the pQCD approach recently developed in refs.…”

“…All these scenarios naturally predict that hard-fragmenting jets lose less energy towards the medium than average jets, for the physical reason that we already mentioned: hard-fragmenting jets contain less partonic sources for in-medium energy loss. This physical argument is manifest in both the pQCD [5,28] and the hybrid approaches [3,7,8], which explicitly include a vacuum-like parton shower. It is also implicit in the strong coupling scenario in [29,30] which is tuned such as to reproduce the angular distribution of jets in p+p collisions at the LHC (itself well described by PYTHIA).…”

Jet Suppression From a Small to Intermediate to Large Radius

“…Jets are reconstructed using FastJet [17] with the anti-k T algorithm for different radius R. In the left panel we show results that do not include the contribution of the wake. Jet suppression increases with R because wider jets contain more energy loss sources, and are more quenched [18][19][20][21]. The middle panel only includes the positive contribution of the wake, the QGP "ridge".…”

The soft drop groomed jet radius at NLL

“…Both on the experimental [5][6][7][8] and the theoretical side [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24], significant progress has been made recently in improving our understanding of soft drop groomed jet observables. In the heavy-ion community, soft drop groomed jet substructure observables have also received increasing attention from both experiment [25][26][27][28] and theory [29][30][31][32][33][34][35][36][37][38][39]. Jet grooming techniques can be used to isolate different aspects of jet quenching and may help to discriminate between different model assumptions [40].…”