Jet shapes and fragmentation functions in Au+Au collisions at sqrt(sNN) = 200 GeV in STAR
NNuclear Physics A 00 (2020) 1–4
NuclearPhysics A / locate / procedia XXVIIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions(Quark Matter 2019)
Jet shapes and fragmentation functions in Au + Au collisions at √ s NN =
200 GeV in STAR
Saehanseul Oh (for the STAR Collaboration)
Yale University, New Haven, Connecticut 06520
Abstract
The STAR Collaboration reports measurements of di ff erential jet shapes and semi-inclusive jet fragmentation functionsin Au + Au collisions at √ s NN =
200 GeV with the STAR detector at RHIC. Jet shapes, which represent the radialdistribution of momentum carried by constituents, are measured di ff erentially for (1) the charged particles transversemomentum and (2) the jet azimuthal angle relative to the second-order event plane. Based on the semi-inclusive popu-lation of jets recoiling from a high transverse momentum trigger hadron, jet fragmentation functions in 40-60% centralheavy-ion collisions are measured, and compared to those in PYTHIA simulations for pp collisions. Keywords: jet, jet shape, jet fragmentation function, STAR
1. Introduction
Jet quenching, which refers to the interaction of a jet shower with Quark-Gluon Plasma (QGP) generatedin relativistic heavy-ion collisions, is one of the key signals to probe the existence and properties of QGP (arecent review of jet studies in relativistic heavy ion physics is given in [1]). In particular, the modificationof jet substructure in heavy-ion collisions with respect to the vacuum reference has been investigated atthe LHC with various observables, such as jet shapes, fragmentation functions, and shared momentumfraction. With increased data sample sizes during the recent RHIC runs and advanced techniques in handlingbackground jets [2, 3], such measurements have become feasible at RHIC energies with the STAR detector.In these proceedings, we present two jet substructure measurements in heavy-ion collisions in STAR: (1)di ff erential jet shapes and (2) semi-inclusive jet fragmentation functions.Both measurements commonly utilize Au + Au collisions at √ s NN =
200 GeV, collected in 2014 by theSTAR experiment [4]. Events containing a high energy Barrel Electromagnetic Calorimeter (BEMC) tower( E T > ff ects of back-ground via a mixed-event technique [3]. Jets are reconstructed with the anti- k T sequential jet clusteringalgorithm from the FastJet package [5] with a radius parameter R = a r X i v : . [ nu c l - e x ] M a r / Nuclear Physics A 00 (2020) 1–4 all angles: background subt. dep. Y mixed events include Leading Jets All angles combined
STAR Preliminary = 200 GeV, 0-10% NN s Au-Au >2.0 GeV towT E , c chT p c = 20-40 GeV/ ch+neT unc, jet p full jets, R=0.4 T k Anti- r) D ( r - - - -
10 110 c < 1.5 GeV/ assocT p r) D ( r - - - -
10 110 c < 2.0 GeV/ assocT p c < 3.0 GeV/ assocT p c < 4.0 GeV/ assocT p r D r) D ( r - - - -
10 110 c < 6.0 GeV/ assocT p r D c > 6.0 GeV/ assocT p r D c > 1.0 GeV/ assocT p Total
Fig. 1. Di ff erential jet shapes as a function of r for 20-40 GeV / c and R = . + Au collisions. Backgroundcontributions are estimated with the mixed-event technique and subtracted in each p assocT range. in the BEMC towers as neutral jet constituents. Mixed events are formulated for distinguished classes ofevents based on their multiplicity, primary vertex position along the beam direction, and the second-orderevent-plane angle ( Ψ EP ), and used in both measurements to subtract background contributions.
2. Di ff erential jet shapes The jet shape function, ρ ( ∆ r ), provides information about the radial distribution of the momentum car-ried by the jet constituents [6]. The jet shape function is defined as ρ ( ∆ r ) = δ r N jet (cid:88) jet (cid:80) track ∈ ( r a , r b ) p T , track p T , jet , (1)where ∆ r = (cid:113) ( ϕ track − ϕ jet ) + ( η track − η jet ) , r a = ∆ r − δ r / r b = ∆ r + δ r /
2, and δ r is the radial annulusbin size. In order to suppress the e ff ects of background fluctuations and combinatorial jets not originatingfrom an initial hard scatter, jets are first reconstructed with tracks and BEMC towers with p T > . / c and E T > . p T , jet calculation [2]. Then ρ ( ∆ r ) is estimated byassociating these jets with charged tracks with 1 . < p T , track < . / c and r < R . For these proceedings,results with only the highest- p T jets in each event (leading jets) are presented.Di ff erential jet shape functions for 20-40 GeV / c and R = . + Au collisionsare shown in Fig. 1, where di ff erent panels represent di ff erent p T , track (i.e., p assocT ) ranges. Statistical uncer-tainties and systematic uncertainties are represented with lines and colored boxes, respectively. Backgroundcontributions are estimated by associating measured jets with tracks in mixed events of the same eventclass, and subtracted accordingly. These results are corrected for tracking e ffi ciency e ff ects. While high- p T tracks are more collimated to the jet axis (in part from the “hard-core” jet selection and anti- k T jet findingalgorithm) compared to low- p T tracks, the di ff erential jet shape over all constituent momenta (bottom rightpanel) at this collision energy is observed to be broader than those at the LHC energies [6] with variationsin kinematics and jet selection.In addition, the event-plane dependence of jet shapes is investigated for the first time in relativistic heavy-ion collisions. Jets are classified based on the azimuthal angle with respect to Ψ EP , measured following the Nuclear Physics A 00 (2020) 1–4 Leading JetsEP resolution corr. c < 1.5 GeV/ assocT p c < 2.0 GeV/ assocT p c < 3.0 GeV/ assocT p c < 4.0 GeV/ assocT p c < 6.0 GeV/ assocT p c > 6.0+ GeV/ assocT p c > 1.0+ GeV/ assocT p Total: r D r) D ( r - - -
10 110
In-plane NN s Au+Au: r D Mid-plane full jets, R=0.4 T k Anti- c = 10-15 GeV/ ch+neT unc, jet p >2.0 GeV clusT E , c chT p r D r) D ( r - - - Out-of-plane
STAR Preliminary
Fig. 2. Event-plane dependent di ff erential jet shapes for 10-15 GeV / c and R = . + Au collisions.Jets are classified based on the azimuthal angle with respect to the second-order event plane: in-plane (left), mid-plane (mid), andout-of-plane (right). Di ff erent colored boxes represent di ff erent p assocT ranges. same procedure in [7], into in-plane (0 < | ϕ jet − Ψ EP | < π/ π/ < | ϕ jet − Ψ EP | < π/ π/ < | ϕ jet − Ψ EP | < π/
2) jets. Such classification may shed light on in-medium path-lengthdependent e ff ects originating from the initial collision geometry. Due to the almond shape of the initialgeometry in 20-50% central collisions, out-of-plane jets are expected to have a longer path length throughthe medium on average than in-plane jets. Figure 2 shows event-plane dependent di ff erential jet shape for10-15 GeV / c and R = . + Au collisions as an example. While high- p T tracksare less sensitive to the azimuthal angle of jets with respect to Ψ EP , low- p T tracks show a hint of dependencein their yields, which seem to be pushed toward farther distances in the out-of-plane direction relative tothe in-plane direction. The overall particle yield increases for out-of-plane jets in comparison to in-planejets, particularly at larger distances from the jet axis. These observations may indicate larger in-mediumpath-length dependent e ff ects in out-of-plane jets relative to in-plane jets.
3. Semi-inclusive jet fragmentation functions
Jet fragmentation functions, 1 / N jet d N ch / d z , correspond to the distribution of constituent charged parti-cle longitudinal momentum fraction with respect to the jet momentum normalized per jet, and have beenpreviously reported by LHC collaborations [8, 9]. In order to have a proper handle on background jets,a semi-inclusive approach is used following similar procedures in [3]. Charged jets in the recoil region(3 π/ < | ϕ jet − ϕ trig | < π/
4) of a high momentum BEMC tower (9 . < E T < . z -
10 1 d N / d z j e t / N - - - = 200 GeV, 40-60% NN sAu+Au, T > 0.35, R = 0.4, anti-k jet A < 30.0 GeV
T,trig
STAR Preliminary < 20 GeV/c chT,jet p £ -1 < 25 GeV/c, x10 chT,jet p £ -2 < 30 GeV/c, x10 chT,jet p £
25 PYTHIA 8, stat. uncertainty only
Fig. 3. Semi-inclusive jet fragmentation functions in 40–60%central Au + Au collisions for three p T , jet ranges (closed markers),and PYTHIA 8 estimations for pp collisions for the correspond-ing p chT , jet ranges (dashed lines). Data results are fully unfoldedfor detector e ff ects and uncorrelated background e ff ects. in the measurement, and z ≡ p T , track cos( r ) / p T , jet of charged tracks with 0 . < p T , track < . / c and r < R is calculated by associating thosecharged tracks with the corresponding jet.Uncorrelated components with respect to thetrigger particle are removed independently in N jet and d N ch / d z via a mixed-event technique. For N jet , the number of jets in each p T , jet bin, the samesubtraction procedures as [3] are applied. Ford N ch / d z , contributions from uncorrelated jets anduncorrelated particles in correlated jets are inde-pendently evaluated using mixed events, and sub-tracted accordingly. After such subtractions, N jet and d N ch / d z are independently unfolded via 1-dimensional and 2-dimensional Bayesian unfold-ing [10], respectively, for remaining uncorrelatedbackground e ff ects and instrumental e ff ects in thefragmentation functions.Figure 3 shows jet fragmentation functions for40-60% central Au + Au collisions and three p T , jet ranges, along with PYTHIA 8 estimations for pp collisions / Nuclear Physics A 00 (2020) 1–4 [11], which was used in the recent STAR publication, for the corresponding p T , jet ranges. Also, the ratiosbetween 40-60% central Au + Au collisions and PYTHIA 8 pp estimations are shown in Fig. 4. It shouldbe noted that only statistical uncertainties are included in PYTHIA 8 results. The ratios are observed to beconsistent with unity within uncertainties throughout z and p T , jet ranges, which correspond to no significantmodifications of jet fragmentation functions in 40-60% central heavy-ion collisions at √ s NN =
200 GeV.These results can be connected to various physics scenarios, for example, 1) tangential jet selection witha high- p T trigger particle and recoil jet configuration, which causes no significant in-medium path lengthof the jet, 2) no significant jet-medium interactions in 40-60% central heavy-ion collisions at √ s NN = ff erentiate them. z -
10 1 R a t i o , ( - % ) / ( PY T H I A ) = 200 GeV NN sAu+Au, T > 0.35, R = 0.4, anti-k jet A < 30.0 GeV
T,trig < 20 GeV/c chT,jet p £ z -
10 1 R a t i o , ( - % ) / ( PY T H I A ) (40-60%)/(PYTHIA) < 25 GeV/c chT,jet p £ z -
10 1 R a t i o , ( - % ) / ( PY T H I A ) < 30 GeV/c chT,jet p £ STAR Preliminary
Fig. 4. Ratios of jet fragmentation functions between 40–60% central Au + Au collisions and PYTHIA 8 estimations for pp collisionsfor three p chT , jet ranges.
4. Outlook
In these proceedings, preliminary di ff erential jet shapes with the STAR experiment are reported. Thefull measurements include jet shapes with various jet finding parameter ( R ), those in central to peripheralcollisions in addition to the pp collisions, and a comparison among leading, sub-leading, and inclusive jets.Meanwhile, semi-inclusive jet fragmentation functions are shown for 40-60% central Au + Au collisions.Such measurements will be extended to the most central Au + Au collisions, and compared to those in pp collisions as a vacuum reference. These results will be reported in the forthcoming publications, and shouldelucidate medium-induced modification of jet substructure at RHIC energies. Acknowledgement
This work is supported by the US Department of Energy under award number DE-SC004168.