Measurement of inclusive jet production in Au+Au collisions at s NN − − − √ = 200 GeV by the STAR experiment
MMeasurement of inclusive jet production in Au+Aucollisions at √ s NN = 200 GeV by the STAR experiment Robert Licenik (for the STAR Collaboration) a , ∗ a Nuclear Physics Institute, Czech Academy of Sciences,250 68, Rez, Czech Republic
E-mail: [email protected]
The STAR Collaboration reports the measurement of inclusive jet production in central (0-10%)and peripheral (60-80%) Au+Au collisions at √ s NN = 200 GeV, using both charged-particleand fully-reconstructed jets. Jet reconstruction is carried out using the anti- k T algorithm withresolution parameters R = 0.2, 0.3 and 0.4. Yield suppression of charged-particle jets is observedfor central Au+Au collisions relative to both peripheral Au+Au collisions and a vacuum baselineutilizing PYTHIA 6 simulations. The magnitude of the suppression is similar to that measuredat the LHC and can be described by theoretical calculations. No evidence of significant medium-induced jet broadening is observed, based on comparison of jet spectra at varying R . The yieldsuppression, when expressed as the jet transverse momentum shift corresponding to energy loss,is consistent in magnitude with coincidence measurements at RHIC based on direct-photon andhadron triggers. There is an indication of larger energy loss observed at the LHC. 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 inclusive jet production in Au+Au collisions at √ s NN = 200 GeV by the STAR experiment Robert Licenik (for the STAR Collaboration)The Quark-Gluon Plasma (QGP) created in high-energy heavy-ion collisions is opaque to jets(jet quenching) [1], a phenomenon that was first observed at RHIC via suppression of high transversemomentum ( p T ) hadron yields and correlations [2]. Such measurements only provide limited insightinto jet quenching mechanisms and dynamics, and more detailed measurements with reconstructedjets are required. While charged-particle and fully-reconstructed inclusive jet productions have beenextensively studied in Pb+Pb collisions at the LHC ([3–5]), these proceedings focus on inclusive jetproduction in heavy-ion collisions at RHIC. We discuss recently reported measurements of charged-particle jets in Au+Au collisions by the STAR Collaboration [6], together with a new analysis tomeasure fully-reconstructed jets which is expected to have greater kinematic reach and improvedsystematic precision. The analysis uses data from the STAR detector [7], a large-acceptance systemutilizing a solenoidal magnetic field, a Time Projection Chamber (TPC) [8] for charged-particletracking and momentum reconstruction, and the Barrel Electromagnetic Calorimeter (BEMC) [9],which measures energy deposited by neutral particles and provides online triggers. STAR offersa full azimuthal coverage within pseudorapidity acceptance | η | <
1. The charged-jet analysisutilizes a dataset for Au+Au collisions at √ s NN = 200 GeV with L int = µ b − , recorded in2011 with a Minimum-Bias trigger. The fully-reconstructed jet analysis uses Au+Au collisions at √ s NN = 200 GeV recorded in 2014 using a High-Tower trigger, which requires at least ∼ . < E T < . p T,lead . However, this cut also biases thefragmentation of the surviving jet population. This bias is measured by varying the p T,lead cut andresults are presented for the unbiased region.Figure 1 shows charged-particle (left) and fully-reconstructed (right) jet distributions as afunction of p recoT,jet ( = p rawT,jet − ρ · A , where A is the jet area and ρ is the median background energydensity, calculated event-wise) for R = 0.4 and various values of the p T,lead cut in central Au+Aucollisions at √ s NN = 200 GeV. It can be seen that the p T,lead cut significantly suppresses thecombinatorial background, especially at low p recoT,jet . The distributions from the fully-reconstructed-jet analysis also indicate its extended kinematic reach, but corrected results are a work in progress.In the following we only show corrected results from the charged-particle jet analysis. Correctionsare applied for the smearing effects of combinatorial background and instrumental effects using theSVD and Bayesian unfolding methods (details in [6]).Figure 2 shows charged-particle jet R CP , the scaled ratio of yields in central to peripheralcollisions, which exhibits a similar level of suppression to charged hadrons at RHIC [10] and LHCenergies [11] and to charged-particle jets at the LHC at higher p T,jet [3], with weak p chT,jet dependence.Figure 3 shows charged-particle jet R PYTHIAAA , the yield suppression for central Au+Au collisionscompared to pp baseline calculated by PYTHIA 6 (Perugia 2012, further tuned by STAR [12]).Calculations based on jet quenching models [13–16], shown in the various colored lines and shadedregions, are consistent with the measured value of R PYTHIAAA .Figure 4 shows the transverse momentum shift − ∆ p T,jet , corresponding to yield suppression2 easurement of inclusive jet production in Au+Au collisions at √ s NN = 200 GeV by the STAR experiment Robert Licenik (for the STAR Collaboration) ) c (GeV/ reco, chT, jet p − − − ) c ( G e V / j e t η d c h T , j e t p N / d d π / e v en t s / N − − − − − − − − − = 200 GeV NN sAu+Au Central (0 10%), R = 0.4 T k anti | < 1 R jet η | = minT,lead p c c c c - - - ] c [GeV/ recoT,jet p - - - - - - - - - ) c ( G e V / h d T dp j e t N d ev t N p c > 0 GeV/ leadT p > 3 leadT p > 5 leadT p > 7 leadT p > 9 leadT p STAR preliminary = 200 GeV, 0-10% NN s Au+Au > 4 GeV
Ttrig
E , R = 0.4 T k Full jets, anti- | < 1 - R h | Figure 1:
Uncorrected distribution of charged-particle [6] (left) and fully-reconstructed (right) jets as afunction of p recoT,jet in 0-10% Au+Au collisions at √ s NN = 200 GeV. Different colors represent different valuesof the p T,lead cut. C P R − , R=0.2 T k anti c = 5 GeV/ minT, lead p = 200 GeV NN sAu+Au ch. jets 0 10% / 60 80% ch. hadrons 0 5% / 60 80% ) c (GeV/ chT p , chT, jet p R=0.3=2.76 TeV NN sPb+Pb ch. jets 0 10% / 50 80% ch. hadrons 0 5% / 60 80% Figure 2: R CP of charged-particle jets reconstructed with R = 0.2 and 0.3 and p T,lead > c (solidstars) [6]. Also shown are similar suppression measurements with jets at the LHC [3] and inclusive chargedhadrons at RHIC [10] and the LHC [11]. [17], from neutral trigger+jet coincidence measurements at RHIC (red and blue points), inclusivejet measurement (green, this analysis) and charged hadron+jet coincidence measurements at RHICand the LHC (black points). Results are consistent between channels at RHIC, and indicate smallerjet energy loss at RHIC than at the LHC (though the relative shift appears larger at RHIC).The ratio of inclusive jet cross-sections at different R and fixed p T,jet measures the jet transverseenergy profile. We do not observe any modification of transverse jet profile compared to pp collisionreference in peripheral or central collisions [6]. Dispersion of the model predictions is larger in thisobservable than in the R PYTHIAAA , which implies strong physical motivation to improve systematicuncertainties and study fully-reconstructed jets.This work is supported by the project LTT18002 of the Ministry of Education, Youth and Sportof the Czech Republic. 3 easurement of inclusive jet production in Au+Au collisions at √ s NN = 200 GeV by the STAR experiment Robert Licenik (for the STAR Collaboration) P y t h i a AA R − = 200 GeV NN sAu+Au Central (0 10%) T k anti R=0.2 STAR charged jets c = 5 GeV/ minT, lead p jet normalization unc.Full jets: SCET NLO pQCD R=0.3 ) c (GeV/ T, jet p , chT, jet p Hybrid model, ch. jets: no medium resp. pos. resp. from wake full medium resp.
R=0.4
Figure 3: R PYTHIAAA as a function p chT,jet of charged-particle jets at STAR reconstructed with R = 0.2, 0.3 and0.4, and p T,lead > c [6]. Bands represent theory predictions. - [ 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:
Transverse momentum shift − ∆ p T,jet from this analysis (middle) compared to various semi-inclusive jet results at RHIC and LHC energies (references in figure).
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