Jet substructure in p + p and p +Au collisions at s NN − − − √ =200 GeV at STAR
JJet substructure in p + p and p +Au collisions at √ s NN = GeV at STAR
Isaac Mooney for the STAR Collaboration a , ∗ a Wayne State University,666 W. Hancock St., Detroit, USA
E-mail: [email protected]
In order to attribute the partonic energy loss experienced by jets (jet quenching) observed in A+Acollisions to the traversal of partons through the hot QCD medium, it is necessary to examine thecold nuclear matter (CNM) effects on the corresponding jets. Such an examination has historicallybeen done using p +A collisions. We present fully corrected measurements of the jet mass andSoftDrop groomed jet mass in p + p and p +Au collisions at STAR at √ s NN =
200 GeV as afunction of the event activity (EA) to increase or decrease the magnitude of CNM effects. EA isdetermined in the backward (Au-going) rapidity ( − . < η < − .
3) by the STAR Beam-BeamCounter to minimize auto-correlation with jets measured at mid-rapidity. Comparison of the jetmass distribution in p +Au collisions to that in p + p collisions allows for isolation of CNM effectsin anticipation of an upcoming jet mass measurement in Au+Au collisions. HardProbes20201-6 June 2020Austin, Texas ∗ Speaker © Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ nu c l - e x ] S e p . Introduction As hard partons from high- Q scatterings evolve in vacuum, they radiate stochastically andfragment into hadrons which lend the resulting jet a unique substructure. Studying jet substructuregives insight into various aspects of QCD, from the initial hard scattering, to the parton shower,and eventual hadronization. Jet mass, M , is one such substructure observable, belonging to ageneric class of angularity observables [1], defined as the magnitude of the four-momentum sum ofconstituents ( M = | (cid:205) i ∈ J p i | = (cid:112) E − p ). The mass of a reconstructed jet is a proxy for the initialparton’s virtuality [2]. Measurements of the jet mass are expected to provide vital inputs to MonteCarlo (MC) models’ implementations of parton shower and hadronization algorithms. In addition,to focus on the perturbative parton shower, we suppress wide-angle non-perturbative (soft) radiationwith SoftDrop grooming [7] and report both groomed and ungroomed jet mass.In 2016, the PHENIX collaboration reported an unexpected jet production enhancement (sup-pression) in peripheral (central) p +Au collisions compared to p + p collisions [3]. Studying jetsubstructure in p +Au collisions will help determine whether this effect is due to jet modification ina cold nuclear medium. Addressing this question is necessary for interpreting measurements of jetmass in a hot nuclear medium.
2. Measurement
This study utilizes STAR data for proton-proton collisions at √ s =
200 GeV from 2012 andproton-gold collisions at √ s NN =
200 GeV from 2015. For both collision systems, we require a jetpatch trigger (see [4]) and reconstruct jets from charged tracks in the time projection chamber (TPC)and energy deposits in the barrel electromagnetic calorimeter (BEMC) using the anti- k T algorithmwith a jet resolution parameter R = .
4. Event, track, tower, and jet selections are the same asin [4] but for one additional selection: we require jet mass above 1 GeV / c , due to poor detectorresolution below this. We report jets with transverse momentum ( p T ) between 20 and 45 GeV / c .To correct for detector effects such as tracking efficiency and momentum resolution, we performa two-dimensional ( M , p T ) iterative Bayesian unfolding implemented in the RooUnfold package [5].In the case of p + p collisions, we construct a response matrix with particle-level events simulatedby PYTHIA-6.4.28 Perugia 2012 (a STAR tune) [6] and detector-level events simulated by thePYTHIA events run through a GEANT-3 STAR detector simulation, and embedded in p + p zero-bias events as an estimate of background. In p +Au collisions, we use the same particle-level events,but embed these detector-level events further into a p +Au minimum-bias background.Systematic uncertainties are made up of four main components: a tracking efficiency uncer-tainty of 4%; a tower gain uncertainty of 3.8%; a hadronic correction variation from the nominal100% subtraction of matched tracks’ momenta from tower energy to 50%; and uncertainties on theunfolding procedure such as variation in iteration parameter and the shape of the priors. Uncertaintyin background estimation for p +Au unfolding is also considered.For p +Au collisions, the event activity (EA) is determined by deposited energy in STAR’sbackward ( − < η < − .
4) inner Beam-Beam Counter (iBBC) on the Au-going side of the detector.In this work, we compare p+Au events with 0-50% EA (high-EA) and 50-100% EA (low-EA).These wide ranges will be refined in future work.1 et substructure at STAR
Isaac Mooney for the STAR Collaboration ] M [GeV/c / G e V ] / d M [ c j e t d N j e t / N STAR < 30 GeV/c T
20 < p = 200 GeVsp+p | < 1-R jet h , R = 0.4, | T anti-k ] M [GeV/c M C / da t a ] M [GeV/c / G e V ] / d M [ c j e t d N j e t / N PYTHIA-6 Perugia 2012HERWIG-7 EE4CPYTHIA-8 Monash < 45 GeV/c T
30 < pSTAR Preliminary ] M [GeV/c
Figure 1:
Measurement of the ungroomed jet mass, M , of anti- k T jets in p + p collisions at √ s =
200 GeVfor two jet p T ranges: 20 < p T <
30 GeV / c (left), and 30 < p T <
45 GeV / c (right). The fully corrected data(with red shaded band denoting systematic uncertainties) are shown in solid red star markers. We comparevia ratio to PYTHIA-6 (Perugia 2012 Tune, solid blue line), PYTHIA-8 (Monash Tune, solid black line),and HERWIG-7 (EE4C Tune, solid magenta line) predictions. In the lower panels the relative systematicuncertainty is drawn. Statistical uncertainties are smaller than the size of the marker in all figures.
3. Results
The fully corrected jet mass is shown for p + p collisions in Fig. 1 for R = . < p T <
30 GeV / c (left) and 30 < p T <
45 GeV / c (right). As the jet p T increases, we observe anincrease in the mean jet mass, as expected from pQCD, as well as a broadening of the distributiondue to the increase in the available phase space. We also compare the results to three leading-order (LO) MC models: PYTHIA-6 with Perugia 2012 tune, PYTHIA-8 with Monash tune, andHERWIG-7 with EE4C tune, where the latter two are tuned to LHC data. Relevant differencesbetween PYTHIA and HERWIG lie in the shower and hadronization mechanisms, with PYTHIAusing a p T -ordered shower and string fragmentation, while HERWIG utilizes an angular-orderedshower and cluster hadronization. We note that PYTHIA-6 describes the data well within systematicuncertainties, while the HERWIG-7 and PYTHIA-8 prefer lower and higher mass jets, respectively.Next, in order to remove jet constituents arising from soft radiation, we apply the SoftDropgrooming algorithm in tagging mode with z cut = . β =
0. We report the groomed mass ( M g ) inFig. 2 for ranges of the corresponding ungroomed jet p T to allow direct comparison to the ungroomedjet mass. Note that (cid:10) M g (cid:11) is reduced ( cf . Fig. 1) due to the suppression of non-perturbative effects.As before, we compare data (red star markers) to the three LO MC models (solid lines) mentionedabove, in the ratio panel. Here we see a reduced systematic uncertainty on the groomed jet mass.The trends are similar to the ungroomed jet mass although disagreement between data and the twoLHC-tuned MC models (HERWIG-7 and PYTHIA-8) is reduced.Figure 3 shows the comparison of the fully corrected jet mass (left) and groomed jet mass(right) in low-EA p +Au collisions (blue stars) to p + p collisions (black stars) for jets with 20 < p T <
30 GeV / c . We observe no significant difference between them, which is expected.In Fig. 4, we compare the fully corrected jet mass in low-EA p +Au collisions (blue stars) to high-2 et substructure at STAR Isaac Mooney for the STAR Collaboration ] [GeV/c g M / G e V ] [ c g / d M j e t d N j e t / N STAR < 30 GeV/c T
20 < p = 200 GeVsp+p | < 1-R jet h , R = 0.4, | T anti-k ] [GeV/c g M M C / da t a ] [GeV/c g M / G e V ] [ c g / d M j e t d N j e t / N PYTHIA-6 Perugia 2012HERWIG-7 EE4CPYTHIA-8 Monash < 45 GeV/c T
30 < pSTAR Preliminary = 0 b = 0.1, cut SoftDrop z ] [GeV/c g M Figure 2:
Measurement of the groomed mass, M g , of anti- k T jets in p + p collisions at √ s =
200 GeV. SeeFig. 1 for a description of the curves. ] M [GeV/c / G e V ] [ c ( g ) / d M j e t d N j e t / N STAR p+p50-100% EA p+Au < 30 GeV/c
T,jet
20 < p = 200 GeV NN sp+Au, p+p | < 1-R jet h , R = 0.4, | T anti-k ] [GeV/c g M / G e V ] / d M [ c j e t d N j e t / N STAR Preliminary = 0 b = 0.1, cut SoftDrop z detector uncertaintyemb.+bkg. uncertainty
Figure 3:
Measurements of jet mass, M (left), and groomed jet mass, M g (right), in low event activity p +Aucollisions (blue stars), compared to those in p + p collisions (black stars) as shown in Fig. 1 for a single jet p T selection, 20 < p T <
30 GeV / c . See § p +Au and p + p analyses, while the boxes denote the additionalembedding and background uncertainty assessed for the p +Au data. EA p +Au collisions (red stars). The jet mass between the two is consistent within uncertainties,suggesting that the jet structure is unmodified by cold nuclear matter effects in high-EA p +Aucollisions. Additionally, the groomed jet mass exhibits similar behavior, indicating that the core ofthe jets is unmodified as well.
4. Conclusions
We have presented the first fully corrected inclusive jet mass measurements in p + p and p +Aucollisions at STAR. The p + p jet mass measurements present an opportunity for further Monte Carlotuning, while the p +Au jet mass measurement indicates that jet substructure is not significantly3 et substructure at STAR Isaac Mooney for the STAR Collaboration ] M [GeV/c / G e V ] [ c ( g ) / d M j e t d N j e t / N < 30 GeV/c T,jet
20 < p = 200 GeV NN sp+Au | < 1-R jet h , R = 0.4, | T anti-k ] M [GeV/c l o w / h i gh EA ] M [GeV/c / G e V ] / d M [ c j e t d N j e t / N STAR Preliminary = 0 b = 0.1, cut SoftDrop z detector uncertaintyemb.+bkg. uncertainty ] [GeV/c g M Figure 4:
Measurements of ungroomed jet mass, M (left), and groomed jet mass, M g (right), in high-EA p +Au collisions (red stars), compared to low-EA p +Au collisions (blue stars) as shown in Fig. 3 for a singlejet p T selection, 20 < p T <
30 GeV / c . See § affected by CNM effects. It is possible that competing effects on the angular and momentum scalesof the jet are cancelled in the mass, so we will investigate the groomed jet momentum fraction, z g , and radius, R g , for a full suite of jet substructure observables in p +Au collisions. Additionally,jet resolution parameter dependence will be investigated, and event activity selections narrowed toenhance potential cold nuclear matter effects on the jet mass. Finally, we will use this measurementas a baseline for a similar measurement in the hot nuclear environment of Au+Au collisions. References [1] Z.-B. Kang, K. Lee, F. Ringer,
Jet angularity measurements for single inclusive jet production ,JHEP 04 (2018)[2] A. Majumder, J. Putschke,
Mass depletion : a new parameter for quantitative jet modification ,PRC 93 5[3] A. Adare et al., Centrality-dependent modification of jet-production rates in deuteron-goldcollisions at √ s NN =
200 GeV, PRL 116 12[4] J. Adam et al.,
Measurement of groomed jet substructure observables in p + p collisions at √ s NN =
200 GeV with STAR , arXiv:2003.02114[5] T. Adye et al., hepunx.rl.ac.uk/~adye/software/unfold/RooUnfold.html[6] J. Adam et al.,
Longitudinal double-spin asymmetry for inclusive jet and dijet production in ppcollisions at √ s =
510 GeV, PRD 100 5[7] A. J. Larkoski, S. Marzani, G. Soyez, J. Thaler,