Title:

Differential measurements of jet substructure and partonic energy loss in AuAu at sqrt-sNN=200 GeV

Abstract:

The STAR collaboration presents jet substructure measurements related to both the momentum fraction and the opening angle within jets in \pp and \AuAu collisions at \sqrtsn $= 200$ GeV. The substructure observables include SoftDrop groomed momentum fraction (\zg), groomed jet radius (\rg), and subjet momentum fraction (\zsj) and opening angle (\tsj). The latter observable is introduced for the first time. Fully corrected subjet measurements are presented for \pp collisions and are compared to leading order Monte Carlo models. The subjet \tsj~distributions reflect the jets leading opening angle and are utilized as a proxy for the resolution scale of the medium in \AuAu collisions. We compare data from \AuAu collisions to those from \pp which are embedded in minimum-bias \AuAu events in order to include the effects of detector smearing and the heavy-ion collision underlying event. The subjet observables are shown to be more robust to the background than \zg~and \rg.

We observe no significant modifications of the subjet observables within the two highest-energy, back-to-back jets, resulting in a distribution of opening angles and the splittings that are vacuum-like. We also report measurements of the differential di-jet momentum imbalance ($A_{\rm{J}}$) for jets of varying \tsj. We find no qualitative differences in energy loss signatures for varying angular scales in the range $0.1 < $ \tsj $ < 0.3$, leading to the possible interpretation that energy loss in this population of high momentum di-jet pairs, is due to soft medium-induced gluon radiation from a single color-charge as it traverses the medium.

PA :
Raghav Kunnawalkam Elayavalli (Yale/BN, formerly at WSU where the main analysis was performed)
Joern Putschke (WSU)
Kolja Kauder (BNL)

Target Journal :
Physics Rev C.

GPC :
Chairperson: Rosi Reed
Member(s) at large: Ernst Sichtermann
Member for English/Grammar QA:  Lanny Ray
Member for Code QA:  Tong Liu
PWG representative: Hanseul Oh
PA representative: Raghav Kunnawalkam Elayavalli

PRC referee comments: response document

Paper draft : v1 post PRC comments

Analysis note :  v1

Collaboration Review : Comments and PA responses pdf

Summary:

We present the first differential measurement of partonic energy loss in \AuAu collisions at \sqrtsn $=200$ GeV for jets tagged via their opening angle. The energy loss is quantified with measurements of the momentum asymmetry \Aj~ of specially selected di-jet pairs. The differential nature of these measurement involves identifying and selecting jets of a particular topology or substructure, i.e. those that have narrow vs. wide opening angles. Since we compare \AuAu data to an embedded \ppAA reference, we require the substructure observable to be sensitive to the jet kinematics, while simultaneously being insensitive to the heavy-ion underlying event. For SoftDrop observables with $z_{\rm{cut}} = 0.1$ and $\beta = 0$, we find that the groomed jet radius ($R_{\rm{g}}$) is impacted by the fluctuating underlying event and this results in a significant fraction of jets tagged with a fake splitting as shown in Figure~\ref{fig:zgrgEmbeffect}. This effect arises from background particles which satisfy the grooming criteria, thus complicating the use of $R_{\rm{g}}$, particularly for larger opening angles.

While the fake split fraction might be reduced by limiting the analysis to high \pt jets or by varying the SoftDrop grooming parameters, we instead introduced a new class of subjet observables, \zsj~and \tsj. In the present analysis the subjets are reclustered via the anti-$k_{\rm{T}}$ algorithm with the jet's constituents as input and with a smaller resolution parameter $R_{\rm{SJ}} = 0.1$. We present the fully corrected subjet observables (\tsj~and \zsj) for \pp collisions, both of which display a gradual change in the jet shape from a broad to a narrow distribution as the jet momentum increases. This evolution with jet momentum, as shown in Figure~\ref{fig:pptwosubjet}, indicates a transition from approximately equal sharing of momentum between the two leading subjets to a gradually more asymmetric momentum sharing for higher \pt jets. More importantly for our study, these subjet observables meet the requirement of being both sensitive to the jet kinematics and insensitive to the heavy ion background via an absolute \pt threshold on the subjets as shown in the correlation studies in Figure~\ref{fig:zsjtsjEmbeffect}. This achievement contrasts with the SoftDrop method which uses a fractional momentum cut, thus has a jet \pt dependent bias due to the combination of quenching and surface bias effects on jet selection criteria at RHIC energies.

In comparing the substructure distributions for \AuAu data and the \ppAA reference for both trigger/recoil HardCore/Matched jets, we observe no significant differences in all cases, indicating that the splittings identified in jets via the subjet method are vacuum-like. Measurements of \Aj~for recoil jets of varying \tsj~demonstrate no significant differences in the momentum balance/imbalance of Matched/HardCore di-jet pairs for recoil jets with $0.1 < \tsj < 0.2$ or $0.2 < \tsj < 0.3$. These results support the conclusion that these particular selected di-jets do not undergo significantly different jet-medium interactions as a result of their varying angular scales.

We can now develop a consistent picture of partonic energy loss for specially selected di-jets at RHIC energies based on three significant features that we observe in our data. The first is that these recoil jets are expected to have smaller path-lengths in the medium on average, owing to the restrictive di-jet requirements which favor tangential production vertices, in comparison to an inclusive or semi-inclusive jet population. The second is the observation that the jet substructure distributions are comparable for \AuAu and \ppAA indicating vacuum like splitting. Third, the recovery of the quenched energy for recoil Matched jets is independent of the jet opening angle measured via the \tsj. Thus, we infer that the recoil jet's first hard splitting during jet evolution possibly happens at formation times comparable to the shorter in-medium path length for tangential di-jets, resulting in vacuum like distributions. Given that the HardCore recoil jets do undergo quenching, as shown by an imbalanced \Aj, the medium interaction that these jets undergo happens at earlier times when the hard-scattered parton is traversing the medium.

These three features, together with the surface bias of unmodified trigger jets, lead us to the qualitative conclusion that energy loss in these recoil jets is due to medium induced radiation from a single color charge. Because of the relatively small-scale resolution of the subjet opening angle in this measurement $(\Delta \theta_{\rm{SJ}} = 0.1)$, we were able to observe di-jet balance/imbalance for both narrow and wide jets. From the similarity of the results for narrow and wide jets we conclude that there is no observational evidence of the characteristic signature of coherent or de-coherent energy loss as the range of sampled jet opening angles encompasses the medium coherence length scale. This further implies that we are in fact measuring the energy loss of a single color-charged object as it traverses the medium. The differential measurements presented here can now be utilized in stringent tests of various quenching models to study the impact of soft gluon radiation as in the QCD analog of the LPM effect, which has long been expected to be a significant factor in parton energy loss at RHIC.

Analysis code : 
available here - /star/u/elayavalli/WORK/ANALYSIS/offline/paper/psn0760
Singularity container to read ntuples and for analysis -
$ singularity pull --name star_test.simg docker://rkunnawa/star_user:latest
$ singularity run star_test.simg
inside that you have root6 and fastjet all setup with analysis software to read TStarJetPicos. the code inside psn0760/src essentially does the analysis and the reading ttrees and plotting is done inside /macros/
detailed readme file is forthcoming. the datasets used in this analysis is identical to https://drupal.star.bnl.gov/STAR/blog/nelsey/differential-di-jet-imbalance-paper for the AuAu Run 7 and pp Run 6 and the pp Run12 datasets and analysis is identical to this paper - https://drupal.star.bnl.gov/STAR/blog/elayavalli/run12-pp-jet-sub-structure-measurement-paper-page
For the embedding comparisons, i included a rivet analysis STAR_EMBEDDING_Run7.cc which is in the same code location and it can take input HepMC files and run the fastsim and embed it into minbias AuAu files via trees and produce embedded comparisons.

Presentations to GPC
March 24th - slides, recording 
May 11th - recording
May 25th - recording

Code QA Report from Tong - slides

GPC Comments from Rosi, Lanny and Tong on v5 - responses

GPC Comments on v7 - responses

GPC Comments on v8 - responses

Presentation to PWGC - paper preview slides

Presentation to JetCorr - paper preview slides
Link to analysis details page - https://drupal.star.bnl.gov/STAR/blog/elayavalli/run7-auau-and-run6-pp-analysis

JetCorr PWG Review - pdf

Public Presentations: 
Hard Probes October 2018 -
   Talk : https://indico.cern.ch/event/502239/contributions/2279404/
   Proceedings: https://arxiv.org/abs/1903.12115
High pT Jet Workshop March 2019 -
   Talk : https://indico.cern.ch/event/730616/contributions/3358394/
   Proceedings: https://arxiv.org/abs/1906.05129
Quark Matter November 2019 -
   Talk : https://indico.cern.ch/event/792436/contributions/3537778/
   Proceedings: https://arxiv.org/abs/2002.06386