Measurement of semi-inclusive jet fragmentation functions in Au+Au collisions at sqrt(s_NN) = 200 GeV in STAR
MMeasurement of semi-inclusive jet fragmentationfunctions in Au+Au collisions at √ s NN = GeV in STAR
Saehanseul Oh (for the STAR Collaboration) a , ∗ a Lawrence Berkeley National Laboratory,1 Cyclotron Rd, Berkeley, CA
E-mail: [email protected]
Jet quenching in relativistic heavy ion collisions can have multiple phenomenological conse-quences: jet energy loss, modification of jet substructure, and induced acoplanarity. In theseproceedings, we report a measurement of the jet fragmentation function, which is one of thejet substructure observables, in peripheral Au+Au collisions at √ s NN =
200 GeV by the STARexperiment at RHIC. In particular, we use a semi-inclusive population of jets recoiling from a hightransverse momentum trigger hadron. The fragmentation function is constructed from the fractionof the transverse momentum of charged particles projected onto the jet axis over the transversemomentum of the jet. In a previous STAR publication of the semi-inclusive charged-jet spectra,the Mixed-Event technique was used along with the semi-inclusive approach to remove the un-correlated background contributions, which enables the measurement of jet distributions at lowtransverse momentum and large jet radius ( R ) in heavy ion collisions. In this analysis we extendthis approach to the measurement of jet fragmentation functions. The reported fragmentationfunctions are corrected for uncorrelated background and instrumental effects via unfolding. Theresults are compared to those in PYTHIA simulations for pp 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 ] A ug easurement of semi-inclusive jet fragmentation functions in Au+Au collisions at √ s NN = GeV in STAR
Saehanseul Oh (for the STAR Collaboration)
1. Introduction
Jet quenching refers to modifications of a jet shower in Quark-Gluon Plasma (QGP) generatedin relativistic heavy-ion collisions, and is one of the key observables used to study the propertiesof QGP [1]. The modification of jet substructure due to jet quenching has been investigated at theLHC, with various observables, such as jet shapes, fragmentation functions, and shared momentumfraction. With increased luminosity from the recent RHIC runs and advanced techniques in handlingbackground jets [2, 3], such measurements have become feasible at RHIC energies with the STARdetector. In these proceedings, we present the measurement of semi-inclusive jet fragmentationfunctions in peripheral Au+Au collisions at √ s NN =
200 GeV.Jet fragmentation functions are defined as, D ( p T , jet , z ) = N jet d N ch ( p T , jet , z ) d z , (1)where z = p T , track cos ( ∆ R )/ p T , jet and ∆ R = (cid:113) ( ϕ jet − ϕ track ) + ( η jet − η track ) , and for tracks satis-fying ∆ R < 0.4. p T , η , and ϕ represent the transverse momentum, pseudo-rapidity, and azimuthalangle, respectively. This corresponds to the distribution of constituent charged particle’s longi-tudinal momentum fraction with respect to the jet momentum normalized per jet. In-mediummodifications of fragmentation functions have been previously reported by LHC collaborations forinclusive jet populations [4, 5]. In the semi-inclusive approach, the fragmentation functions arereported for the population of recoil jets with respect to a high momentum trigger particle.
2. Measurement
For this analysis, Au+Au collisions at √ s NN =
200 GeV, collected in 2014 by the STARexperiment, are used [6]. Charged tracks with 0 . < p T , track < . c and | η track | < . p T , track higher than 30.0 GeV/ c are excluded in the measurement. Jets are reconstructed with thesecharged tracks using the anti- k T sequential jet clustering algorithm from the FastJet package [7]with a resolution parameter R = π / < | ϕ jet − ϕ trig | < π /
4) of a high energyBarrel Electromagnetic Calorimeter (BEMC) trigger tower (9 . < E T < . N jet and those in d N ch / d z are removed independently. For N jet , the number ofjets in each p T , jet bin, the same subtraction procedures as [3] are applied. For d N ch / d z , contributionsfrom 1) uncorrelated jets, and 2) uncorrelated particles in correlated jets are separately evaluatedusing mixed events. For 1), d N MEch / d z is measured with mixed events, and scaled according to theuncorrelated jet fraction in each p T , jet bin. For 2), correlated jets are placed into mixed events,2 easurement of semi-inclusive jet fragmentation functions in Au+Au collisions at √ s NN = GeV in STAR
Saehanseul Oh (for the STAR Collaboration)and combined with mixed-event tracks that satisfy ∆ R < 0.4. These contributions from 1) and 2)are subtracted from d N ch / d z from all recoil jets. After such subtractions, N jet and d N ch / d z areindependently unfolded via 1-dimensional and 2-dimensional Bayesian unfolding [8], respectively,for uncorrelated background effects and instrumental effects in the fragmentation functions.
3. Results
Figure 1 shows jet fragmentation functions for 40-60% Au+Au collisions and three p T , jet ranges,along with PYTHIA 8 (Monash 2013 tune [10]) predictions for pp collisions [9]. Figure 2 shows 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
Figure 1:
Semi-inclusive jet fragmentation functions measured in 40–60% Au+Au collisions for three p T , jet ranges (closed markers), compared to those calculated by PYTHIA 8 for pp collisions (dashed lines).Measured distributions are corrected for detector effects and uncorrelated background effects. the ratios between 40-60% Au+Au collisions and PYTHIA 8 pp estimations. The observed ratiosare consistent with unity within uncertainties over the measured z and p T , jet ranges, indicating nosignificant modification of jet fragmentation functions in 40-60% Au+Au collisions at √ s NN = √ s NN =
200 GeV collisions, or the possibility thatPYTHIA 8 results in Figs. 1 and 2 may not accurately represent the pp events at √ s =
200 GeV, asMonash 2013 tune is based on LHC data. Further studies are needed to validate the above physics 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
Figure 2:
Ratios of jet fragmentation functions measured in 40–60% Au+Au collisions at √ s NN =
200 GeVto those simulated by PYTHIA 8 for pp collisions for three p chT , jet ranges. case, as well as the pp reference. 3 easurement of semi-inclusive jet fragmentation functions in Au+Au collisions at √ s NN = GeV in STAR
Saehanseul Oh (for the STAR Collaboration)
4. Outlook
In these proceedings, semi-inclusive jet fragmentation functions are reported for 40-60%Au+Au collisions. The results in this centrality are observed to be consistent with those of PYTHIA8 predictions for pp collisions. In the future, these measurements will be extended to the mostcentral Au+Au collisions, and compared to the corresponding measurements in pp collisions. Theywill elucidate medium-induced modification of jet substructure at RHIC energies. Acknowledgement
This work was supported by the Director, Office of Science, Office of Basic Energy Sciences,of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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