Measurement of beam energy dependent nuclear modification factors at STAR
MMeasurement of beam energy dependent nuclearmodification factors at STAR
Stephen P. Horvat (for the STAR Collaboration)
Department of Physics, 217 Prospect Street, New Haven, CT 06511-8499E-mail: [email protected]
Abstract.
The nuclear modification factors R AA and R CP have been used to measure medium-induced suppression in heavy-ion collisions at √ s NN = 200 GeV which was among the earliestevidence for the existence of a strongly interacting medium called a quark-gluon plasma (QGP).Nuclear modification factors for asymmetric collisions ( R dA ) have measured the Cronin Effect,an enhancement of high transverse momentum particle yields in deuteron-gold collisions relativeto proton-proton collisions. A similar enhancement is observed in data presented in theseproceedings and competes with the quenching caused by partonic energy loss in the QGP. Inthese proceedings we will present charged-hadron R CP at mid-rapidity for √ s NN = 7.7 - 62.4GeV as well as identified π + , K + , and p R CP . Comparisons to HIJING motivate possiblemethods for disentangling competing modifications to nuclear transverse momentum spectra.
1. Introduction
The RHIC beam energy scan (BES) is a program to collide Au+Au ions at various collisionenergies in order to explore the QCD phase diagram; searching for a possible critical point andfor a phase boundary marked by the disappearance of key signatures for the formation of a QGP[1]. The nuclear modification factor provides one of these signatures. The ratio of transversemomentum ( p T ) differentiated spectra from central over peripheral collisions and scaled by themean number of binary p + p -like collisions in each event is called the nuclear modification factorand is denoted by R CP . If p + p collisions are used for the reference instead then the nuclearmodification factor is denoted by R AA . In the presence of a QGP, high- p T particles are quenched,transferring energy to lower momentum particles, causing the nuclear modification factor to beless than unity at high p T , or suppressed [2-6]. Quenching competes with any effects that wouldcause enhancement, such as radial boosts or the Cronin Effect [7]. The Cronin Effect has alsobeen observed by STAR as the enhancement of the nuclear modification factor in asymmetric d + Au collisions at √ s NN = 200 GeV [8]. These spectra can be influenced by the spectators,non-interacting nucleons in the collision system, so that a peripheral collision does not equatedirectly to a p + p collision due to cold nuclear matter (CNM) effects. The goal of this analysis isto determine at what beam energy suppression turns off, and to begin disentangling the causesand relative effects of quenching and enhancement.
2. Charged-hadron R CP Au + Au collisions near the center of the detector were analyzed. A particle track was used ifits distance of closest approach ( dca ) was less than one centimeter in order to reduce feed-down a r X i v : . [ nu c l - e x ] M a r igure 1. (Color online) Efficiency corrected charged hadron R CP (left) for RHIC BES energies. p T dependent errors are statistical only. The gray error band corresponds to the p T independentuncertainty in N coll scaling. Charged R CP from HIJING simulated events (right) with jetquenching turned off.contamination. Trigger efficiency, tracking efficiency, and acceptance corrected charged-hadronspectra from mid-rapidity Au + Au collisions at √ s NN = 7.7, 11.5, 19.6, 27, 39, and 62.4 GeV wereproduced for 0-5% central and 60-80% peripheral collisions in the STAR detector. The spectrawere scaled by binary collisions with the scale factors obtained from a Monte Carlo Glaubermodel [9]. Taking the ratio of these scaled spectra for each energy gives the R CP ( √ s NN , p T )shown in Fig. 1 (left) along with STAR’s published 200 GeV result [2]. These spectra were notfeed-down corrected and were taken from -0.5 < η < π ± , K ± , and p ± separately which were than combined and weighted by their relative yields for acharged hadron efficiency. The efficiency correction was extrapolated to the other data sets bymaking the assumption that the efficiency is the same for each particle at the same p T and fromthe same multiplicity bin. This assumption was tested by producing the efficiencies from twodata sets, 39 GeV from 2010 and 27 GeV from 2011, and ensuring that the predicted efficiencyfrom the 39 GeV data set matched the 27 GeV efficiency. Then differences in acceptance dueto detector performance between beam energies were accounted for by using stable portions ofthe detector as a reference. HIJING 1.35 [10] with jet quenching turned off was used to producemore than 10M collisions for each beam energy. The motivation for running the simulatorwith jet quenching turned off was the expectation that at sufficiently low beam energies, wheremedium-induced jet quenching has minimal effect, there would be a quantitative agreementbetween the charged hadron R CP from simulation and data. If the simulation and the dataagreed at low beam energies but deviated at higher beam energies then the beam energies wherethe deviation occurred could be considered candidates for the beam energies where a QGP isformed. Centrality selection was done using the same method as for the data; namely, countingthe number of final state charged hadrons in -0.5 < η < igure 2. (Color online) The datafrom Fig. 1 (left) plotted vs. x T ratherthan p T .central data as being the 5% of events with the highest multiplicity, and so forth for the othercentralities. The result is shown in Fig. 1 on the right. Again, R CP at lower beam energies isenhanced, although we do not see a quantitative agreement with charged hadron R CP from data.We do not see suppression at higher collision energies, as expected since quenching was turnedoff. By running HIJING with jet quenching on and off and comparing with AMPT and othermodels we hope to disentangle the relative contributions of jet quenching, CNM effects, andpossible contributions from radial flow or final state scattering. The results in Fig. 1 (left) areconsistent with suppression for √ s NN ≥
39 GeV. This does not preclude medium induced energylosses at lower energies since other effects could be overwhelming this signature. The advantagefor using charged hadron R CP is that you can measure spectra to higher transverse momentathat you could not reach with particle identification. It was also considered that plotting R CP vs. x T rather than p T might reveal trends in the data that were independent of collision energy. x T is defined as x T = 2 ∗ p T / √ s NN . This sort of scaling was applied to spectra previously [8]where it revealed √ s NN independent trends at high p T . Such a scaling is shown in Fig. 2, usingthe data from Fig. 1, and does not reveal any such trends.
3. Identified hadron R CP Particle yields were extracted from a simultaneous fit to dE/dx distributions measured in theSTAR Time Projection Chamber and time of flight distributions measured in the STAR Timeof Flight detector for each centrality and p T bin at each beam energy. The functions used toextrapolate fit parameters for particle identification were varied in order to obtain the systematicerrors for the high p T bins. Efficiency corrections were obtained through track embedding andhave a 5% systematic error associated with them. The differences between the published 62.4GeV results [11] and those presented here were taken as a point by point systematic error andapplied to all beam energies. The cause of this discrepancy is under investigation. The result(Fig. 3) is qualitatively consistent with published results [11] in that pions are less enhancedthan protons, suggesting that pions may serve as a better gauge for jet quenching within the p T range available through particle identification. Considering 2.5 GeV/c < p T < √ s NN = 27 GeV.
4. Summary
Charged hadron R CP has been measured at mid-rapidity for a range of beam energies in order todetermine at what beam energy the suppression of high p T charged hadrons, a QGP signature, igure 3. (Color online) R CP for identified p , K + , and π + for RHIC BES energies. The boxesare p T dependent systematic uncertainties due to particle identification while the p T independentuncertainty is from N coll scaling.turns off. Suppression is seen to turn off near 39 GeV for unidentified charged hadrons, butdue to unquantified sources of enhancement it is currently unclear where medium-induced jetquenching turns off. Identified R CP is qualitatively similar and promotes pions as a probe thatis less effected by sources of enhancement. For pions, suppression at high p T is seen to turnoff near 27 GeV. A comparison to the HIJING event generator with jet quenching turned offdid not reveal an energy at which the data and the model agree quantitatively, which precludesmeasuring the energy at which data and simulation deviate. This motivates the explorationof additional tunes of HIJING and other models in order to disentangle the competing effectswhich lead to suppression or enhancement. References [1] Mohanty B. (for the STAR Collaboration ) 2011
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