Measurement of γ +jet and π 0 +jet in central Au+Au collisions at s NN − − − √ = 200 GeV with the STAR experiment
MMeasurement of γ +jet and π +jet in central Au+Aucollisions at √ s NN = 200 GeV with the STAR experiment Nihar Ranjan Sahoo (for the STAR Collaboration) a , ∗ a Shandong University,Institute of Frontier and Interdisciplinary ScienceQingdao, China
E-mail: [email protected], [email protected]
We present the semi-inclusive measurement of charged jets recoiling from direct-photon and π triggers in central Au+Au collisions at √ s NN = 200 GeV, using a dataset with integrated luminosity13 nb − recorded by the STAR experiment in 2014. The photon and π triggers are selected withintransverse energy ( E trigT ) between 9 GeV and 20 GeV. Charged jets are reconstructed with the anti- k T algorithm with resolution parameters R = 0.2 and 0.5. A Mixed-Event technique developedpreviously by STAR is used to correct the recoil jet yield for uncorrelated background, enablingrecoil jet measurements over a broad p T , jet range. We report fully corrected charged-jet yieldsrecoiling from direct-photon and π triggers for the above two jet radii and also discuss the jet R dependence of in-medium parton energy loss at the top RHIC energy. 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 γ +jet and π +jet in central Au+Au collisions at √ s NN = 200 GeV with the STAR experiment Nihar Ranjan Sahoo (for the STAR Collaboration)Jet quenching arises from partonic interactions in the Quark-Gluon Plasma (QGP) formedin heavy-ion collisions [1]. A valuable observable to probe the QGP is the coincidence of areconstructed jet recoiling from a high transverse energy (high E trigT ) direct photon ( γ dir ) [2], since γ dir does not interact strongly with the medium. A comparison of γ dir +jet and π +jet measurementsmay elucidate the color factor and path-length dependence of jet quenching [3]. In addition, acomparison of recoil jet distributions with different cone radii provides a probe of in-medium jetbroadening.In these proceedings, we present the analysis of fully-corrected semi-inclusive distributionsof charged jets recoiling from high- E trigT γ dir and π triggers in central Au+Au collisions at √ s NN = 200 GeV. The data were recorded during the 2014 RHIC run with a trigger requiring anenergy deposition greater than 5.6 GeV in a tower of the STAR Barrel Electromagnetic Calorimeter(BEMC), corresponding to an integrated luminosity of 13 nb − . We compare the measured recoiljet yield in Au+Au collisions to a pp reference via PYTHIA simulation and corresponding yieldsuppression is then further compared with theoretical calculations. We express the suppression interms of jet energy loss and compare to other in-medium jet measurements at RHIC and the LHC. - - - - - - - ) [ G e V / c ] j e t h d c h T , j e t / ( dp j e t s d N t r i g / N =0.2 jet +jet, R dir g (a) STAR Preliminary
PYTHIA-8PYTHIA-6, STAR tune < 11 GeV trigT trigT
11 < E < 20 GeV trigT
15 < E =0.5 jet +jet R dir g (b) chT,jet p - - - - - ) [ G e V / c ] j e t h d c h T , j e t / ( dp j e t s d N t r i g / N Au+Au, 0-15%=0.2 jet +jet R p T anti-k (c) chT,jet p =0.5 jet +jet R p PYTHIA-8 < 11 GeV trigT trigT
11 < E (d)
Figure 1:
Semi-inclusive distributions of charged jetsrecoiling from γ dir (upper) and π (lower) triggers.Light and dark bands represent systematic and statis-tical uncertainties, respectively. Broken and dottedlines represent calculations based on PYTHIA-8 andPYTHIA-6 STAR tune. The offline analysis selects events corre-sponding to the 0-15% most central Au+Au col-lisions, based on uncorrected charged-particlemultiplicity within | η | < 1. The BEMC ShowerMax Detector (BSMD) was used offline to se-lect clusters in the range 9 < E trigT < 20 GeVthat have an enhanced population of direct pho-tons ( γ rich ) or π ( π ). A Transverse ShowerProfile (TSP) method is used to discriminatebetween π and γ rich triggers [3]. The purityof direct photons in the γ rich sample is 65–85%in the range 9 < E trigT < 20 GeV. The final cor-rections are applied on both γ rich and π toget the fully corrected recoil jet yields. Chargedjets are reconstructed with the anti- k T algorithm[4, 5] for R = 0.2 and 0.5, using charged particletracks measured in the Time Projection Cham-ber (TPC) with 0 . < p T <
30 GeV/ c and | η | <1. The jet acceptance is | η jet | < 1- R .Recoil jets are selected with a ∆ φ ∈[ π / , π / ] , where ∆ φ is the azimuthal an-gle between the trigger cluster and the jet axis.The semi-inclusive distribution is defined as theyield of recoil jets in a bin of transverse mo-mentum ( p chT , jet ) normalized by the number oftriggers. The uncorrelated background jet yield in this distribution is corrected using the Mixed-Event (ME) technique developed in [6]. Corrections to the recoil jet distributions for instrumental2 easurement of γ +jet and π +jet in central Au+Au collisions at √ s NN = 200 GeV with the STAR experiment Nihar Ranjan Sahoo (for the STAR Collaboration)effects and residual p chT , jet fluctuations due to background are carried out using unfolding methods.The main systematic uncertainties arise from unfolding, ME normalization, and γ dir purity.Due to limited trigger statistics in the current analysis of STAR pp data, the reference dis-tribution from pp collisions is calculated using the PYTHIA event generators. For γ dir -triggereddistributions, both PYTHIA-8 [7] and PYTHIA-6 STAR tune [8] events are used, whereas for π -triggered distributions only PYTHIA-8 is used. -
10 1 PY T H I A - AA I Au+Au 200 GeV, 0-15% R = 0.2 T anti-k < 15 GeV/c const.T < 11 GeV, p trigT [GeV/c] chT,jet p -
10 1 PY T H I A - AA I +jet p +jet dir g < 15 GeV trigT
11 < E STAR Preliminary -
10 1 PY T H I A - AA I Au+Au 200 GeV, 0-15% = 0.5 jet R T anti-k STAR Preliminary < 11 GeV trigT [GeV/c] chT,jet p -
10 1 PY T H I A - AA I < 15 GeV trigT
11 < E +jet p +jet dir g Figure 2: I PYTHIA − vs. p chT , jet for γ dir triggers (red) and π triggers (blue) with 9 < E trigT <11 GeV (upper)and 11 < E trigT < 15 GeV (lower) and for jets with R = 0.2 (left) and 0.5 (right). Light and dark bands representsystematic and statistical uncertainties. [GeV/c] chT,jet p PY T H I A - AA I < 20 GeV trigT
15 < E
R=0.2 T +jet, Au+Au 200 GeV, 0-15%, anti-k dir g chT,jet p PY T H I A - AA I < 20 GeV trigT
15 < E
Jet-fluidLBTSCET
R=0.5
STAR Preliminary [GeV/c] chT,jet p PY T H I A - AA I < 20 GeV trigT
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R=0.2 T +jet, Au+Au 200 GeV, 0-15%, anti-k dir g chT,jet p PY T H I A - AA I < 20 GeV trigT
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Figure 3: γ dir +jet: I PYTHIA − (upper) and I PYTHIA − (lower) vs. p chT , jet for 15 < E trigT < 20 GeV and jets with R = 0.2 (left) and 0.5 (right). Light and dark bands represent systematic and statistical uncertainties. Theorycalculations: Jet-fluid [9], LBT [10], and SCET [11]. Figure 1 shows fully corrected charged-jet p T spectra for R = 0.2 and 0.5 recoiling from γ dir in three E trigT bins, and π in two E trigT bins, measured in central Au+Au collisions and comparedto those calculated by PYTHIA for pp collisions. The two PYTHIA versions exhibit negligibledifference for R = 0.2 and up to 40% difference for R = 0.5. The ratio of recoil jet yield measured inAu+Au collisions to PYTHIA calculations for pp collisions are denoted as I PYTHIA − and I PYTHIA − for the two versions of PYTHIA used.Figure 2 shows I PYTHIA − for γ dir and π triggers in 9 < E trigT < 15 GeV for R = 0.2 and 0.5.The recoil jet yields show similar suppression for both triggers for R = 0.2, with no significant E trigT dependence. Smaller suppression is observed for R = 0.5 for both triggers compared to R = 0.2.3 easurement of γ +jet and π +jet in central Au+Au collisions at √ s NN = 200 GeV with the STAR experiment Nihar Ranjan Sahoo (for the STAR Collaboration) R = . Au+Au 200 GeV, 0-15%| < 1-R jet h , | T anti-k < 11 GeV trigT +jet: 9 < E p [PRC 96, 924905 (2017)] < 30 GeV/c trigT h+jet: 9 < p STAR Preliminary [GeV/c] chT,jet p R = . / < 20 GeV trigT +jet: 15 < E dir g PYTHIA-8, pp 200 GeVPYTHIA-6 STAR tune, pp 200 GeV - [ G e V / c ] T , j e t p D - +jet dir g +jet p Inclusive jet h+jet h+jetRHIC-STAR LHC-ALICE
Au+Au 200 GeV Pb+Pb 2.76 TeV :10-20 GeV/c chT,jet p :10-20 GeV/c chT,jet p :15-25 GeV/c chT,jet p :10-20 GeV/c chT,jet p :60-100 GeV/c chT,jet p STAR Preliminary arXiv:2006.00582 PRC 96 (2017) 024905 JHEP09 (2015) 170
R=0.2R=0.4R=0.5
PYTHIA-6 STAR tune pp referencePYTHIA-8 pp reference
Figure 4:
Left panel: Ratio of recoil jet yields for R = 0.2 and 0.5 as a function p chT , jet . Upper: h+jet and π +jet. Lower: γ dir +jet. Right panel: The p chT , jet shift (- ∆ p chT , jet ) for γ dir +jet, π +jet, inclusive jet, h+jetmeasurements at RHIC, and h+jet at the LHC. Note the different p chT , jet ranges. Figure 3 compares I PYTHIA − and I PYTHIA − for γ dir triggers with 15 < E trigT < 20 GeV. Compar-ison is also made to theoretical model calculations [9–11], which predict different p T dependenceto those observed in data.Figure 4, left panel, shows the ratio of recoil jet yields for R = 0.2 and 0.5 measured incentral Au+Au collisions with both γ dir and π triggers. This ratio is sensitive to the jet transverseprofile [6, 12]. The γ dir -triggered ratio is consistent with a calculation based on the PYTHIA-6 STARtune, indicating no significant in-medium broadening of recoil jets whereas a notable quantitativedifference is observed between Au+Au and PYTHIA-8. The ratios for π and charged-hadrontriggers measured in central Au+Au collisions are consistent within uncertainties.Jet quenching is commonly measured by yield suppression at fixed p T ( R AA and I AA ). However,these ratio observables convolute the effect of energy loss with the shape of the spectrum. Toisolate the effect of energy loss alone we convert the suppression to a p T -shift, -∆ p chT , jet , enablingquantitative comparison of jet quenching measurements with different observables, and comparisonof jet quenching at RHIC and the LHC. Figure 4, right panel, shows -∆ p chT , jet from this measurement,compared to those of inclusive jets and h+jet at RHIC, and h+jet at the LHC [6, 12–14]. The energyloss from the RHIC measurements is largely consistent for the different observables, with someindication of smaller energy loss for R = 0.5 than for R = 0.2 considering PYTHIA-8 for the vacuumexpectation. In addition, the results from R = 0.2 measurements at RHIC are comparable to thosefrom inclusive π [15]. An indication of smaller in-medium energy loss is observed at RHIC thanat the LHC.In summary, we have presented the analysis of semi-inclusive charged-jet distributions recoilingfrom γ dir and π triggers in central Au+Au collisions at √ s NN = 200 GeV. Significant yieldsuppression is observed for recoil jets with R = 0.2, and a less suppression is seen for R = 0.5using PYTHIA-8 as pp reference. However, the difference between PYTHIA-8 and PYTHIA-6precludes quantitative conclusions. On the other hand, a definitive conclusion on in-medium jetbroadening from the ratio of recoil jet yields at different R can be drawn when the vacuum referencewill be resolved by the same measurements in pp collisions at 200 GeV, currently in progress.Theoretical calculations of jet quenching predict a different p T -dependence of the suppression than4 easurement of γ +jet and π +jet in central Au+Au collisions at √ s NN = 200 GeV with the STAR experiment Nihar Ranjan Sahoo (for the STAR Collaboration)that observed in data. Conversion of the measured suppression to a p T -shift reveals similar energyloss due to the quenching of various jet measurements at RHIC and an indication of smaller energyloss at RHIC than at the LHC. Acknowledgments:
This work was supported by the Fundamental Research Funds of ShandongUniversity and DOE DE-SC0015636.
References [1] X. N. Wang, M. Gyulassy and M. Plumer, Phys. Rev. D , 3436-3446 (1995)doi:10.1103/PhysRevD.51.3436 [arXiv:hep-ph/9408344 [hep-ph]].[2] X. N. Wang, Z. Huang and I. Sarcevic, Phys. Rev. Lett. , 231-234 (1996)doi:10.1103/PhysRevLett.77.231 [arXiv:hep-ph/9605213 [hep-ph]].[3] L. Adamczyk et al. [STAR], Phys. Lett. B , 689-696 (2016)doi:10.1016/j.physletb.2016.07.046 [arXiv:1604.01117 [nucl-ex]].[4] M. Cacciari, G. P. Salam and G. Soyez, JHEP , 063 (2008) doi:10.1088/1126-6708/2008/04/063 [arXiv:0802.1189 [hep-ph]].[5] M. Cacciari, G. P. Salam and G. Soyez, Eur. Phys. J. C , 1896 (2012)doi:10.1140/epjc/s10052-012-1896-2 [arXiv:1111.6097 [hep-ph]].[6] L. Adamczyk et al. [STAR], Phys. Rev. C , no.2, 024905 (2017)doi:10.1103/PhysRevC.96.024905 [arXiv:1702.01108 [nucl-ex]].[7] T. Sjostrand, S. Mrenna and P. Z. Skands, Comput. Phys. Commun. , 852-867 (2008)doi:10.1016/j.cpc.2008.01.036 [arXiv:0710.3820 [hep-ph]].[8] J. Adam et al. [STAR], Phys. Rev. D , no.5, 052005 (2019)doi:10.1103/PhysRevD.100.052005 [arXiv:1906.02740 [hep-ex]].[9] N. B. Chang and G. Y. Qin, Phys. Rev. C , no.2, 024902 (2016)doi:10.1103/PhysRevC.94.024902 [arXiv:1603.01920 [hep-ph]].[10] T. Luo, S. Cao, Y. He and X. N. Wang, Phys. Lett. B , 707-716 (2018)doi:10.1016/j.physletb.2018.06.025 [arXiv:1803.06785 [hep-ph]].[11] M. D. Sievert, I. Vitev and B. Yoon, Phys. Lett. B , 502-510 (2019)doi:10.1016/j.physletb.2019.06.019 [arXiv:1903.06170 [hep-ph]].[12] J. Adam et al. [ALICE], JHEP , 170 (2015) doi:10.1007/JHEP09(2015)170[arXiv:1506.03984 [nucl-ex]].[13] J. Adam et al. [STAR], [arXiv:2006.00582 [nucl-ex]].[14] R. Licenik, Hard Probes-2020 proceedings.[15] A. Adare et al. [PHENIX], Phys. Rev. C87