The fragmentation properties of inclusive jets produced in PbPb collisions at a center ... âlostâ momentum reappears in low-momentum particles emitted at large ...
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Nuclear Physics A 904–905 (2013) 1007c–1010c www.elsevier.com/locate/nuclphysa
Measurements of jet shape in 2.76 TeV PbPb collisions with CMS Yaxian MAO (for the CMS Collaboration)1 Vanderbilt University, Nashville, TN, USA
Abstract The fragmentation properties of inclusive jets produced in PbPb collisions at a center of mass √ energy of sNN =2.76 TeV are characterized by measuring differential jet shapes. The jet shape is defined as the fractional jet transverse momentum carried by charged particles emitted at a distance r from the jet axis, with jets reconstructed using the anti-kT clustering algorithm. The jet shapes are measured for reconstructed jet pT,jet > 100 GeV/c and charged hardons with pT > 1 GeV/c. Different background subtraction methods are employed to remove the contribution from the underlying events. The results obtained for PbPb collisions as a function of collision centrality are compared to reference distributions from pp data collected at the same collision energy. For the most central collisions, the jet shapes are found to be modified at large distance from the jet axis. 1. Introduction High transverse momentum partons produced from hard scatterings lose energy as they traverse the hot and dense medium created in heavy-ion collisions at ultra-relativistic energies [1]. Suppression of high-pT particles (as first seen at RHIC and more recently at the LHC, including CMS [2], ATLAS [3] and ALICE [4]) was taken as an evidence for this effect. CMS data have been used to directly demonstrate the parton energy loss using the difference in momentum between back-to-back emitted pairs of jets [5] and photon tagged jets [6]. It is observed that the “lost” momentum reappears in low-momentum particles emitted at large angles with respect to the jet axis [5]. Taking advantage of the high statistics data collected in 2011 (used 129μb−1 of PbPb collisions at center-of-mass energy of 2.76 TeV), the present analysis allows us to further study the parton energy energy loss in the hot QCD medium. The jet shapes, describing how the transverse momentum of the particles comprising the jet is distributed radially with respect to the jet axis, are studied to gain more insight on the energy loss mechanism of parton-medium interactions [7]. 2. Jet Shape Observable The jet shape is defined as the average fractionof the jet transverse momentum carried by partcles emitted in a cone of a given radius r = (ηi − η j )2 + (φi − φ j )2 around the jet axis , 1A
list of members of the CMS Collaboration and acknowledgements can be found at the end of this issue. © CERN for the benefit of the CMS Collaboration.
0375-9474/ © 2013 CERN Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nuclphysa.2013.02.185
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where i refers to the particle and j to the jet. In this analysis, only charged particles are considered for the construction of the jet shape variables. The differential jet shape is defined as the fraction of the transverse momentum carried by particles in a concentric ring around the jet axis between inner radius r − δr/2 and outer radius r + δr/2 in the η − φ plane: ρ(r) =
1 1 1 pT (r − δr/2, r + δr/2) , jet fch δr Njet jets p
(1)
T
where fch =
1 pT (0, R) jet Njet jets pT
(2)
is the fraction of the jet transverse momentum carried by charged particles. The integral of ρ(r)dr in the limits 0 ≤ r ≤ R is normalized to unity. 3. Analysis and Results The analysis is based on jets reconstructed using the anti-kT jet algorithm, with a radius parameter R = 0.3, combining tracking and calorimetric information from CMS particle-flow (PF) objects [8, 9]. The contribution of the underlying heavy-ion event is removed using an iterative pileup subtraction method [10]. For the present analysis, the events with jets above pT > 100 GeV/c within 0.3 < |η| < 2 region are selected, where the jet trigger efficiency is fully efficient. The central pseudorapidity region of |η| < 0.3 is excluded in order to avoid jet cones overlaping with the η-reflected cone used for the background shape estimation. The minimum transverse momentum for tracks entering the analysis is set to 1 GeV/c. Jet shapes in a cone of R = 0.3 are measured as a function of collision centrality for four centrality bins, 50-100%, 30-50%, 10-30% and 0-10%. The underlying event contribution to the charged particle distribution in the cone is subtracted using an event-by-event “η-reflected cone” method [11]. The systematic uncertainties on the jet shape measurement arise from several sources: background subtraction uncertainty (a major uncertainty source for the measurement, estimated by event mixing background technique), uncertainty in the tracking efficiency, and jet energy scale uncertainties. The combined systematic uncertainties are calculated by adding different sources in quadrature [12]. The jet shapes for pTjet > 100 GeV/c with track pT > 1 GeV/c in PbPb collisions are shown on the top panel of Fig. 1 for different centrality bins, ranging from mid-peripheral 30–50% collisions to the most central 0–10% collisions. The bottom panel shows the ratio of the jet shapes for the various centrality bins to the most peripheral 50–100% one, together with the systematic uncertainties. A deviation from unity may indicate a modification of the hard scattered parton shower by the medium produced in central collisions. For mid-central 10–30% and 0– 10% central collisions, a moderate rise in the central/peripheral ratio of jet shapes is observed, reaching values of 1.18 ± 0.07 (syst.) and 1.24 ± 0.08 (syst.), respectively, for the largest radius. For a direct comparison between pp and PbPb collisions, the worsening of the jet momentum resolution in PbPb events need to be taken into account. For this purpose, the reconstructed pT of every jet in the pp data is smeared by the quadratic difference of the jet energy resolution in PbPb and pp. In order to keep the same jet energy scale in pp and PbPb, the reweighting factor, calculated from the ratio of the PbPb to pp smeared distribution, is applied to the pp jet pT after smearing. The smeared and reweighted pp jet pT distribution matches with PbPb jet pT spectra
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Figure 1: Differential jet shapes for pT > 100 GeV/c with track pT > 1 GeV/c in PbPb collisions (top panels) and the ratio of the jet shape from different centrality bins to the most peripheral one (50%-100%).
in each centrality bin of this analysis, which allow us to directly compare pp jet shapes to PbPb jet shapes. In the Fig. 2, the measured jet shapes from PbPb collisions are compared with the shapes obtained for the pp-based reference , after resolution-smearing and reweighting. The bottom panel presents the ratio of the jet shape between PbPb and pp for different centralities. The ratios are close to unity for mid-peripheral and peripheral collisions (30-50% and 50-100%), and show a rising trend towards large radius r for the mid-central and most central collisions (10–30% and 0–10%). For r > 0.25, a PbPb/pp jet shape ratio of 1.38 ± 0.10 (syst.) is observed. The results indicate a modification of the jet structure in the most central collisions. 4. Conclusion √ The correlations of inclusive jets in PbPb collisions at sNN = 2.76 TeV together with their charged fragmentation products have been characterized for the first time by using jet shapes. The results of the jet shapea in PbPb collisions are compared to those obtained from pp collisions at the same collision energy. For the 50–100% and 30–50% peripheral collisions, the differential jet shapes ρ(r) are similar in PbPb and in pp collisions. In more central collisions, indications for a modification of the jet shapes at large radii are observed. For the 10% most central collisions, the ratio of the differential jet shape in PbPb compared to the pp reference is consistent with 1 for r < 0.2, but then rises to a value of 1.38 ± 0.10(syst.) for 0.25 < r < 0.3. A similar trend in observed in the central/peripheral jet shape ratios measured in PbPb collisions (0-10%/50-100%) reaching a value of 1.24 ± 0.08 (syst.) for the largest measured radius. In summary, the first detailed measurements of the redistribution of fragmentation products for inclusive jets with pT, jet > 100 GeV/c in PbPb have been presented. The jet shape ratio for
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Figure 2: Differential jet shapes in PbPb and pp collisions for pT > 100 GeV/c with track pT > 1 GeV/c (top panels) and the ratio of the PbPb and pp jet shapes (bottom pannels).
central PbPb/pp collisions indicate a transport of energy to larger radii and from intermediate to low pT , respectively. Such an observation is consistent with the jet quenching picture [7]. References [1] Bjorken, J. D., ”Energy loss of energetic partons in QGP: possible extinction of high pT jets in hadron-hadron collisions”, (1982) FERMILAB-PUB-82-059-THY. √ [2] CMS Collaboration, ”Study of high-pT charged particle suppression in PbPb compared to pp collisions at sNN = 2.76 TeV”, Eur. Phys. J. C 72 (2012) 1945. √ [3] ATLAS Collaboration, ”Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at sNN = 2.76 TeV with the ATLAS Detector at the LHC”, Phys. Rev. Lett. 105 (2010) 252303. [4] ALICE Collaboration, ”Suppression of charged particle production at large transverse momentum in central PbPb √ collisions at sNN = 2.76 TeV”, Phys. Lett. B 696 (2011) 30. √ [5] CMS Collaboration, ”Observation and studies of jet quenching in PbPb collisions at sNN =2.76 TeV”, Phys. Rev. C 84 (2011) 024906. [6] CMS Collaboration, ”Studies of jet quenching using isolated-photon+jet correlations in PbPb and pp collisions at √ sNN = 2.76 TeV”, arXiv:1205.0206. [7] Vitev, I., et. al., ”A theory of jet shapes and cross sections: from hadrons to nuclei”, arXiv:0810.2807. [8] CMS Collaboration, ”Particle-Flow Event Reconstruction in CMS and Performance for Jets, Taus, and ETmiss ”, (2009) CMS-PAS-PFT-09-00. [9] CMS Collaboration, ”Commissioning of the Particle-Flow Reconstruction in Minimum-Bias and Jet Events from pp Collisions at 7 TeV”, (2010) CMS-PAS-PFT-10-002. [10] Kodolova, O. et. al., ”The performance of the jet identification and reconstruction in heavy ions collisions with CMS detector”, Eur. Phys. J. C 50 (2007) 117. [11] CMS Collaboration, ”Measurement of jet fragmentation into charged particles in pp and PbPb collisions at √ sNN =2.76 TeV”, arXiv:1205.5872. [12] CMS Collaboration, ”Detailed Characterization of Jets in Heavy Ion Collisions Using Jet Shapes and Jet Fragmentation Functions”, (2012) CMS-PAS-HIN-12-013.
