Anomalous Evolution of the Near-Side Jet Peak Shape in Pb-Pb Collisions at √sNN = 2.76 TeV

This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail. Author(s): Title: Year: Version: Please cite the original version: All material supplied via JYX is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Anomalous Evolution of the Near-Side Jet Peak Shape in Pb-Pb Collisions at √sNN = 2.76 TeV ALICE Collaboration ALICE Collaboration. (2017). Anomalous Evolution of the Near-Side Jet Peak Shape in Pb-Pb Collisions at √sNN = 2.76 TeV. Physical Review Letters, 119(10), Article 102301. https://doi.org/10.1103/PhysRevLett.119.102301 2017 Anomalous Evolution of the Near-Side Jet Peak Shape in Pb-Pb Collisions at ffiffiffiffiffiffiffiffi sNN p=2.76 TeV J. Adam et al.* (ALICE Collaboration) (Received 28 September 2016; published 8 September 2017) The measurement of two-particle angular correlations is a powerful tool to study jet quenching in a pT region inaccessible by direct jet identification. In these measurements pseudorapidity (Δη) and azimuthal (Δφ) differences are used to extract the shape of the near-side peak formed by particles associated with a higher pTtrigger particle (1<p T;trig <8GeV=c). A combined fit of the near-side peak and long-range correlations is applied to the data allowing the extraction of the centrality evolution of the peak shape in Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV. A significant broadening of the peak in the Δηdirection at low pTis found from peripheral to central collisions, which vanishes above 4GeV=c, while in the Δφdirection the peak is almost independent of centrality. For the 10% most central collisions and 1<p T;assoc <2GeV=c, 1<p T;trig <3GeV=c a novel feature is observed: a depletion develops around the center of the peak. The results are compared to pp collisions at the same center of mass energy and AMPT model simulations. The comparison to the investigated models suggests that the broadening and the development of the depletion is connected to the strength of radial and longitudinal flow. DOI: 10.1103/PhysRevLett.119.102301 In elementary interactions with large momentum transfer (Q2≫Λ2 QCD), partons with high transverse momentum (pT) are produced. They evolve from high to low virtuality producing parton showers and eventually hadronizing into a spray of collimated hadrons called jets. In interactions between heavy ions, such high-pTpartons are produced at the early stages of the collisions. They propagate through the dense and hot medium created in these collisions and are expected to lose energy due to medium-induced gluon radiation and elastic scatterings, a process commonly referred to as jet quenching. Correspondingly, an inclusive jet suppression has been observed at the LHC [1–3] together with a large dijet energy asymmetry [4,5], while studies of the momentum and angular distributions of jet fragments show only a small modification of the jet core [6–8], and an excess of soft particles radiated to large angles from the jet axis [9]. Semi-inclusive hadron-jet correlations show a suppression of recoil jet yield, with no in-medium modification of transverse jet structure observed [10]. Dihadron angular correlations represent a powerful complementary tool to study jet modifications on a statistical basis in an energy region where jets cannot be identified event by event over the fluctuating background. Such studies involve measuring the distributions of the relative azimuthal angle Δφand pseudorapidity Δη between particle pairs consisting of a trigger particle in a certain transverse momentum pT;trig interval and an associated particle in a pT;assoc interval. In these correlations, jet production manifests itself as a peak centered around ðΔφ¼0;Δη¼0Þ(near-side peak) and a structure elongated in Δηat Δφ¼π(the away side or recoil region). At low pT, resonance decays as well as femtoscopic correlations also contribute to the near-side peak. The advantage of using dihadron correlations is that an event-averaged subtraction of the background from particles uncorrelated to the jet can be performed. This advantage is shared with the analysis of hadron-jet correlations recently reported in Refs. [9,10]. At RHIC, the near-side particle yield and peak shape of dihadron correlations have been studied for different systems and collision energies [11–13]. Small modifications of the yields with respect to a pp reference from PYTHIA are observed and there is remarkably little dependence on the collision system at the center-of-mass energies of ffiffiffiffiffiffiffiffi sNN p¼62.4and 200 GeV. An exception is the measurement in central Au-Au collisions at ffiffiffiffiffiffiffiffi sNN p¼ 200 GeV where the jetlike correlation is substantially broader and the momentum spectrum softer than in peripheral collisions and than those in collisions of other systems in this kinematic regime. In Ref. [12], the broadening observed in central Au-Au collisions at ffiffiffiffiffiffiffiffi sNN p¼ 200 GeV is seen as an indication of a modified jet fragmentation function. At the LHC, the measurement of the yield of particles associated with a high-pTtrigger particle (8–15 GeV=c) in central Pb-Pb collisions relative *Full author list given at the end of the article. Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. PRL 119, 102301 (2017) PHYSICAL REVIEW LETTERS week ending 8 SEPTEMBER 2017 0031-9007=17=119(10)=102301(13) 102301-1 © 2017 CERN, for the ALICE Collaboration to the pp reference at pT;assoc >3GeV=c shows a suppression on the away side and a moderate enhancement on the near side, indicating that medium-induced modifications can also be expected on the near side [14]. Much stronger modification is observed for lower trigger and associated particle pT(3<p T;trig <3.5GeV=c and 1<pT;assoc <1.5GeV=c)[15,16]. In the most central Pb-Pb collisions, the near-side yield is enhanced by a factor of 1.7. The present Letter expands these studies at the LHC to the characterization of the angular distribution of the associated particles with respect to the trigger particle. The angular distribution is sensitive to the broadening of the jet due to its energy loss and the distribution of radiated energy. Moreover, possible interactions of the parton shower with the collective longitudinal expansion [17–19] or with turbulent color fields [20] in the medium would result in near-side peak shapes that are broader in the Δηthan in the Δφdirection. Results from the study of the near-side peak shape of charged particles as a function of centrality and for different combinations of trigger and associated particle pT are discussed. The data presented in this Letter were taken by the ALICE detector, of which a detailed description can be found in Ref. [21]. The main subsystems used in the present analysis are the Inner Tracking System (ITS), and the Time Projection Chamber (TPC). These have a common acceptance of jηj<0.9. The ITS consists of six layers of silicon detectors for vertex finding, tracking, and triggering. The TPC is the main tracking detector measuring up to 159 space points per track. The V0 detector, two arrays of 32 scintillator tiles each, covering 2.8<η<5.1(V0-A) and −3.7<η<−1.7(V0-C), was used for triggering and centrality determination. All these detector systems have full azimuthal coverage. Data from the 2010 and 2011 Pb-Pb runs of the LHC at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV are combined in the present analysis and compared with the 2011 pp run at the same energy. In total, about 39 million Pb-Pb and 30 million pp events are used. Details about the trigger and event selection in Pb-Pb (pp) collisions can be found in Ref. [22] (Ref. [23]), while the centrality determination is described in Ref. [24]. The collision-vertex position is determined with tracks reconstructed in the ITS and TPC [25], and its value in the beam direction (zvtx) is required to be within 7 cm of the detector center. The Pb-Pb analysis is performed in the centrality classes 0%–10% (most central), 10%–20%, 20%–30%, 30%–50%, and 50%–80%. The analysis uses tracks reconstructed in the ITS and TPC with 1<p T< 8GeV=c and in a fiducial region of jηj<0.8. The track selection is described in Refs. [26,27]. The efficiency and purity of the primary charged-particle selection are estimated from a Monte Carlo (MC) simulation using the HIJING 1.383 event generator [28] (for Pb-Pb) and the PYTHIA 6.4 event generator [29] with the tune Perugia-0 [30] (for pp) with particle transport through the detector using GEANT 3[31]. The combined efficiency and acceptance for the track reconstruction in jηj<0.8is about 82%–85% at pT¼1GeV=c, and decreases to about 76%–80% at pT¼ 8GeV=c depending on collision system, data sample, and event centrality. The contamination originating from secondary particles from weak decays and interactions in the detector material decreases from 2.5%–4.5% to 0.5%–1% in the pTrange from 1 to 8GeV=c. The contribution from fake tracks is negligible. From these quantities a correction factor is computed as a function of η,pT,zvtx, and event centrality, which is applied as a weight for each trigger particle and particle pair in the analysis. The correlation between two charged particles (denoted trigger and associated particle) is measured as a function of Δφ(defined within −π=2and 3π=2) and Δη[32]. The correlation is expressed in terms of the associated yield per trigger particle for intervals of pT;trig and pT;assoc, measured as 1 Ntrig d2Nassoc dΔηdΔφ¼SðΔη;ΔφÞ BðΔη;ΔφÞ;ð1Þ where Ntrig is the total number of trigger particles in the centrality class and the pT;trig interval, ranging from 0.18 to 36 per event. The signal distribution SðΔη;ΔφÞ¼ 1=Ntrigd2Nsame=dΔηdΔφis the associated yield per trigger particle for particle pairs from the same event. The background distribution BðΔη;ΔφÞ¼αd2Nmixed=dΔηdΔφ accounts for the acceptance and efficiency of pair reconstruction. It is constructed by correlating the trigger particles in one event with the associated particles from other events. The background distribution is scaled by a factor αwhich is chosen such that Bð0;0Þis unity for pairs where both particles travel in approximately the same direction (i.e., Δφ≈0,Δη≈0), and thus the efficiency and acceptance for the two particles are identical by construction. A selection on the opening angle of the particle pairs is applied to both signal and background to avoid a bias due to the reduced efficiency for pairs with small opening angles. Furthermore, correlations induced by secondary particles from long-lived neutral-particle decays (K0 sand Λ) and γ conversions are suppressed by rejecting pairs in the corresponding invariant mass region. A correction is performed for a mild Δηdependence of the structures in the two-particle correlation, which is due to a minor dependence of particle production and anisotropic flow on pseudorapidity. For further details on the analysis procedure, see Ref. [33]. In order to characterize the near-side peak shape, a simultaneous fit of the peak, the combinatorial background, and the long-range correlation background stemming from collective effects is performed. This exploits that in twoparticle correlations the near-side peak is centered around Δφ¼0,Δη¼0, while long-range correlation structures PRL 119, 102301 (2017) PHYSICAL REVIEW LETTERS week ending 8 SEPTEMBER 2017 102301-2 are mostly independent of Δη[34]. The away-side peak; however, is elongated in Δη; therefore, this strategy cannot be applied to studying the away side. The fit function used is a combination of a constant, a generalized twodimensional Gaussian function and cosðnΔφÞterms for n¼2,3,4. FðΔφ;ΔηÞ¼C1þX 4 n¼2 2VnΔcosðnΔφÞ þC2GγΔφ;wΔφðΔφÞGγΔη;wΔηðΔηÞ;ð2Þ Gγx;wxðxÞ¼ γx 2wxΓð1=γxÞexp −jxj wxγx:ð3Þ Thus, in Pb-Pb collisions, the background is characterized by four parameters (C1,VnΔ), where VnΔare the Fourier components of the long-range correlations [35], and it should be noted that the inclusion of orders higher than four does not significantly change the fit results. In pp collisions, the background consists effectively only of the pedestal C1. The peak magnitude is characterized by C2, and the shape, which is the focus of the present analysis, by four parameters (γΔφ,wΔφ,γΔη,wΔη). The aim of using this fit function is to allow for a compact description of the data rather than attempting to give a physical meaning to each parameter. Therefore, the variance of Gis calculated, which reduces the description of the peak shape to two parameters (σΔφand σΔη). To describe the evolution of the peak shape from peripheral to central collisions the ratios of the widths in the central bin (0%–10%) and the peripheral bin (50%– 80%), denoted by σCP Δφand σCP Δη, are also calculated. In the data a depletion around Δφ¼0,Δη¼0is observed at low pT, however, the fit function does not include such a depletion. To avoid a bias on the extracted peak width, some bins in the central region are excluded from the fit. The size of the excluded region varies with pT and collision centrality (from no exclusion to 0.3). Thus, by definition, the peak width describes the shape of the peak outside of the central region. The depletion in the central region is quantified below by computing the difference between the fit and the per-trigger yield within the exclusion region. In Pb-Pb collisions, the obtained χ2=ndf values of the fits are in the range 1.0–2.5; most are around 1.5. In the highest two pTbins (i.e., in 3<p T;assoc <8GeV=c and 4<p T;trig <8GeV=c) the values increase up to about 2.5 showing that at high pTthe peak shape starts to depart from the generalized Gaussian description. In pp collisions, the χ2=ndf values are in the range 1.3–2.0. Systematic uncertainties connected to the measurement are determined by modifying the event and track selections. In addition, uncertainties related to the cut on pairs with small opening angles and neutral-particle decays, as well as the sensitivity to the pseudorapidity range are considered. The difference in the extracted parameters is studied as a function of pT, centrality, and collision system, but these dependencies are rather weak and in most cases one uncertainty value can be quoted for each type of systematic uncertainty. Finally, the different sources of systematic uncertainties are added in quadrature. The extracted peak widths are rather insensitive to changes in the selections (total uncertainty of about 2%–4.5%), while the near-side depletion yield is more sensitive (about 24%–45% uncertainty). The contribution from resonance decays was studied by performing the analysis separately for like and unlike sign pairs, and a significant influence on the results presented below was not found. Figure 1shows the near-side peak in 1<p T;trig < 2GeV=c and 1<p T;assoc <2GeV=c for the 10% most central collisions. In addition to the two-dimensional representation, projections are shown where the background estimated with Eq. (2) has been subtracted. The near-side peak is asymmetric, i.e., wider in Δηthan in Δφ. It is also broader than in peripheral Pb-Pb and pp collisions, where it is mostly symmetric in Δφand Δη (not shown, see Ref. [33]). Furthermore, a depletion around Δφ¼0,Δη¼0develops which will be discussed in more detail below. Also at higher pT, the near-side peak is broader in central collisions than in peripheral or pp collisions. This broadening is less pronounced at high pTthan at low pT, and the asymmetry between Δφand Δη disappears at the two highest pTbins; see Ref. [33]. The extracted shape parameters σΔφand σΔηare presented in Fig. 2.Inpp collisions, the σvalues range from about 0.14 to about 0.43 showing a pTdependence qualitatively expected due to the boost of the evolving parton shower: at larger pTthe peak is narrower. In the Δφ direction (left panel) the values obtained in pp collisions are consistent with those in peripheral Pb-Pb collisions. The peak width increases towards central events, which is most pronounced in the lowest pTbin (20% increase). In the higher pTbins no significant width increase can be observed. In the Δηdirection (right panel) a much larger broadening is found towards central collisions. Already in (rad) ϕ Δ 1 0 1 η Δ 1 0 1 ) -1 (rad dϕΔηΔd assoc N 2 d trig N 1 0 0.1 c < 2 GeV/ T,trig p1 < c < 2 GeV/ T,assoc p 1 < ALICE, Pb-Pb = 2.76 TeV NN s 0-10% ηΔ (rad) or ϕΔ 101 ) -1 (rad ϕΔd assoc Nd or ηΔ d assoc Nd trig N 1 0 0.1 0.2 0.3 0-10% proj. proj. fit fit ϕΔ ηΔ ϕΔ ηΔ = 2.76 TeV NN sALICE, Pb-Pb, c < 2 GeV/ T,trig p1 < c < 2 GeV/ T,assoc p 1 < /2π ϕΔ ηΔ < < 1.6 4% scale uncertainty FIG. 1. Left panel: associated yield per trigger particle as a function of Δφand Δη. The background obtained from the fit function has been subtracted in order to emphasize the near-side peak. Right panel: projections to the Δφand Δηaxes overlaid with the peak part of the fit function. PRL 119, 102301 (2017) PHYSICAL REVIEW LETTERS week ending 8 SEPTEMBER 2017 102301-3 peripheral collisions the width is larger than in pp collisions. From peripheral to central collisions the width increases further up to σΔη¼0.67 in the lowest pTbin and the largest relative increase of about 85% is observed for 2<p T;trig <3GeV=c and 2<p T;assoc <3GeV=c.For all but the two largest pTbins a significant broadening can be observed. This increase is quantified for all pTbins in Fig. 3by σCP Δφand σCP Δη. The increase is quantified with respect to peripheral Pb-Pb instead of pp collisions to facilitate the MC comparisons discussed below. In pp collisions, the peak shows circular symmetry in the Δη–Δφplane for all pTvalues. In Pb-Pb collisions, the peak becomes asymmetric towards central collisions for all but the two highest pTbins. The magnitude of this asymmetry depends on pTand is largest with about 70% (σΔη>σΔφ) in the range 2<p T;assoc <3GeV=c and 2<p T;trig <3GeV=c. These results are compatible with a similar study by the STAR Collaboration at ffiffiffiffiffiffiffiffi sNN p¼ 200 GeV [12], which is detailed in the companion paper [33]. In Ref. [17] it was suggested that the interplay of longitudinal flow with a fragmenting high pTparton can lead to the observed asymmetric peak shape. The authors argue that hard partons interact with a medium which shows collective behavior, contrary to the simpler picture where the parton propagates through an isotropic medium with respect to the parton direction. In their calculation the scattering centers are Lorentz boosted by applying a momentum shift depending on the collective component transverse to the parton-propagation direction. The calculation in Ref. [17] for Au-Au collisions at ffiffiffiffiffiffiffiffi sNN p¼ 200 GeV predicts a 20% increase from peripheral to central events in the Δφdirection and a 60% increase in the Δη direction, which is in good agreement with the measurements by the STAR Collaboration. Despite the different center of mass energy and collision systems, the calculation is in quantitative agreement with the results presented in this Letter as well. In order to study further the possibility that an interplay of flow and jets can cause the observation, the data is compared to results from a multiphase transport model (AMPT) [36,37]. Two mechanisms in AMPT produce collective effects: partonic and hadronic rescattering. Before partonic rescattering, the initially produced strings may be broken into smaller pieces by the so-called string melting. Three different AMPT settings are considered which have either string melting [configuration (a)] or hadronic rescattering (b) or both activated (c) [38]. The peak widths are extracted from particle-level AMPT simulations in the same way as for the data. None of the AMPT settings provides an accurate description of the measured absolute widths. Further discussion and the corresponding figure can be found in Ref. [33]. In order to provide, nevertheless, a meaningful comparison of the relative increase, σCP Δφand σCP Δηfrom the models are shown together with the data in Fig. 3. In the Δφdirection, the setting with string melting deactivated and hadronic rescattering active follows the trend of the data closest. The two other settings show a more uniform distribution across pTand only differ in the two lowest pTbins. In the Δηdirection, the setting with string melting deactivated and hadronic rescattering active quite remarkably follows the trend of the data including the large increase for intermediate pT. The two other settings show qualitatively a similar trend but miss the data quantitatively. The presented results have focused up to now on the overall shape of the near-side peak. In addition to the broadening, a distinct feature is observed, a depletion around Δφ¼0,Δη¼0(see Fig. 1). An extensive set of studies was carried out to exclude that this depletion could arise from a detector effect. A similar structure is found in AMPT simulations with hadronic rescattering regardless of the string melting setting [33]. In order to quantify this depletion, the difference is computed between the fit (where the depletion region has been excluded, see above) and the per-trigger yield for each pTbin. This is normalized by the total peak yield and it is referred to as depletion yield in the following. The region, where effects are expected from the limited two-track reconstruction efficiency (jΔφj<0.04 and jΔηj<0.05, which corresponds to 0.5%–6% of the integrated region), is excluded from this calculation. Figure 4presents the depletion yield as a function of centrality for the pTbins, where it is different from 0. It can be seen that ð2.20.5Þ% Centrality (%) 020406080 pp σ (rad) or ηΔϕΔ σ 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 (rad) ϕΔ σ ALICE = 2.76 TeV NN sPb-Pb = 2.76 TeVspp Centrality (%) 0 20406080pp ηΔ σ 1-2 : 1-2 2-3 : 1-2 2-3 : 2-3 3-4 : 1-2 3-4 : 2-3 3-4 : 3-4 4-8 : 1-2 4-8 : 2-3 4-8 : 3-4 4-8 : 4-8 )c (GeV/ T,assoc p) : c (GeV/ T,trig p FIG. 2. Shape parameters σΔφ(left panel) and σΔη(right panel) as a function of centrality in different pTranges. Lines indicate statistical uncertainties (mostly smaller than the marker size), while boxes denote systematic uncertainties. The markers are placed at the center of the centrality bins. PRL 119, 102301 (2017) PHYSICAL REVIEW LETTERS week ending 8 SEPTEMBER 2017 102301-4 of the yield is missing in the lowest pTbin and in the 10% most central events. This value decreases gradually with centrality and with pT. No significant depletion yield is observed for 50%–80% (30%–80%) centrality or pp collisions for the lowest (second lowest) pTrange. The depletion observed in the AMPT events is present only in the lowest pTbin, where its value is compatible with the data for both settings where hadronic rescattering is switched on. For larger pTbins and for the configuration without hadronic rescattering the depletion yield is consistent with 0 in AMPT. The reported results can be interpreted in the context of radial and anisotropic flow by calculating the radial-flow expansion velocity βTand the elliptic flow coefficient v2f2gfrom the 10% most central events from data and from the AMPT samples. The expansion velocity βTis extracted from a blast-wave fit to the pTspectra of π,K, and pin the rapidity range of jyj<0.5[39]. The v2f2gis extracted from two-particle correlations within jηj<0.8 and 0.2<p T<5GeV=c [40]. The depletion (Fig. 4) occurs in the two AMPT configurations (b) and (c) where the βTis large, while the configuration (a) without the depletion has the smallest βT. The coefficient v2f2ghas significantly different values in the two configurations (b) and (c) with depletion, and the relative increase of the peak width (Fig. 3) is best described by the AMPT configuration with the largest βT(b). Based on these studies, it seems that the depletion and the broadening observed in the data are more likely accompanied by radial flow than elliptic flow. In summary, we have presented a detailed characterization of the flow-subtracted near-side peak in two-particle correlations in Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV together with a measurement in pp collisions at the same energy. The near-side peak shows several untypical characteristics in Pb-Pb collisions: the peak gets broader and more asymmetric from peripheral towards central collisions over a wide pTrange, and an unexpected depletion develops in central collisions at low pT. The broadening is present both in the Δφand the Δηdirections, but it is significantly stronger in the Δηdirection, leading to the asymmetric shape of the peak. The near-side peak also shows a characteristic pTdependence in both Pb-Pb and pp collisions. AMPT simulations show also an asymmetric broadening, and the depletion is present when hadronic rescattering is included. The AMPT configuration with hadronic rescattering and without string melting reproduces quantitatively the relative peak broadening as well as the size of the depletion, underlining the importance of the hadronic phase in heavy-ion collisions. The extraction of the radialflow expansion velocity suggests that the stronger the radial flow, the stronger the observed effects are. In addition, earlier theoretical and phenomenological work connected the longitudinal broadening of the near-side jetlike peak to strong longitudinal flow in AMPT [41], as well as to an interplay of partons traversing the longitudinally expanding medium [17]. Thus, a possible scenario is that the presented observations are caused by the interplay of the jet with the collective expansion. The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centers and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their )c (GeV/ T,assoc p 1-2 1-2 2-3 1-2 2-3 3-4 1-2 2-3 3-4 4-8 CP σ 1 1.2 1.4 1.6 1.8 (50-80%) ηΔ σ (0-10%) ηΔ σ = ηΔ CP σ CP ηΔ ϕΔ σ 1 1.2 1.4 1.6 1.8 1-2 2-3 3-4 4-8 )c (GeV/ T,trig p (50-80%) ϕΔ σ (0-10%) ϕΔ σ = ϕΔ CP σ = 2.76 TeV NN sPb-Pb AMPT, String melting on, rescattering on AMPT, String melting on, rescattering off AMPT, String melting off, rescattering on ALICE FIG. 3. Ratio of the peak widths in Δφ(top) and Δη(bottom) observed in central (0%–10%) and peripheral (50%–80%) collisions as a function of pT;trig and pT;assoc ranges. The data are compared with different AMPT settings. Note that the xaxis combines the pT;assoc and pT;trig axis, and therefore, a uniform trend of the values is not expected. Centrality (%) 10 20 30 40 50 60 Depletion yield (%) 0 1 2 3 4c < 2 GeV/ T,assoc p < 2 : 1 < T,trig p1 < c < 2 GeV/ T,assoc p < 3 : 1 < T,trig p2 < c < 3 GeV/ T,assoc p 2 < c < 2 GeV/ T,assoc p < 2 : 1 < T,trig pAMPT: 1 < String melting on, rescattering on String melting off, rescattering on ALICE, PbPb = 2.76 TeV NN s FIG. 4. Missing yield in the depletion region relative to the overall peak yield extracted from the fit. The arrows indicate the upper limit in case the uncertainty bands touch 0. For comparison, the nonzero values from two AMPT simulations are shown as lines. PRL 119, 102301 (2017) PHYSICAL REVIEW LETTERS week ending 8 SEPTEMBER 2017 102301-5 support in building and running the ALICE detector: A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL), State Committee of Science and World Federation of Scientists (WFS), Armenia; Austrian Academy of Sciences and Nationalstiftung für Forschung, Technologie und Entwicklung, Austria; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Financiadora de Estudos e Projetos (Finep), and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil; Ministry of Education of China (MOE of China), Ministry of Science & Technology of China (MOST of China), and National Natural Science Foundation of China (NSFC), China; Ministry of Science, Education and Sport and Croatian Science Foundation, Croatia; Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Cuba; Ministry of Education, Youth and Sports of the Czech Republic, Czech Republic; Danish National Research Foundation (DNRF), The Carlsberg Foundation, and The Danish Council for Independent Research—Natural Sciences, Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l’Energie Atomique (CEA) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3), and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF), and GSI Helmholtzzentrum für Schwerionenforschung GmbH, Germany; Ministry of Education, Research and Religious Affairs, Greece; National Research, Development and Innovation Office, Hungary; Department of Atomic Energy Government of India (DAE), India; Indonesian Institute of Science, Indonesia; Centro Fermi—Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi, and Istituto Nazionale di Fisica Nucleare (INFN), Italy; Institute for Innovative Science and Technology, Nagasaki Institute of Applied Science (IIST), Japan Society for the Promotion of Science (JSPS) KAKENHI and Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; Consejo Nacional de Ciencia (CONACYT) y Tecnología, through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT), and Dirección General de Asuntos del Personal Academico (DGAPA), Mexico; Nationaal instituut voor subatomaire fysica (Nikhef), Netherlands; The Research Council of Norway, Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú, Peru; Ministry of Science and Higher Education and National Science Centre, Poland; Ministry of Education and Scientific Research, Institute of Atomic Physics and Romanian National Agency for Science, Technology and Innovation, Romania; Joint Institute for Nuclear Research (JINR), Ministry of Education and Science of the Russian Federation and National Research Centre Kurchatov Institute, Russia; Ministry of Education, Science, Research and Sport of the Slovak Republic, Slovakia; National Research Foundation of South Africa, South Africa; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), South Korea; Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT) and Ministerio de Ciencia e Innovacion, Spain; Knut & Alice Wallenberg Foundation (KAW) and Swedish Research Council (VR), Sweden; European Organization for Nuclear Research, Switzerland; National Science and Technology Development Agency (NSDTA), Office of the Higher Education Commission under NRU project of Thailand and Suranaree University of Technology (SUT), Thailand; Turkish Atomic Energy Agency (TAEK), Turkey; National Academy of Sciences of Ukraine, Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the U.S. (NSF) and United States Department of Energy, Office of Nuclear Physics (DOE NP), United States. [1] G. Aad et al. (ATLAS Collaboration), Measurement of the jet radius and transverse momentum dependence of inclusive jet suppression in lead-lead collisions at ffiffiffiffiffiffiffiffi sNN p¼ 2.76 TeV with the ATLAS detector, Phys. Lett. B 719, 220 (2013). [2] B. Abelev et al. (ALICE Collaboration), Measurement of charged jet suppression in Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, J. High Energy Phys. 03 (2014) 013. [3] J. Adam et al. (ALICE Collaboration), Measurement of jet suppression in central Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, Phys. Lett. B 746, 1 (2015). [4] G. Aad et al. (ATLAS Collaboration), Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV with the ATLAS Detector at the LHC, Phys. Rev. Lett. 105, 252303 (2010). [5] S. Chatrchyan et al. (CMS Collaboration), Observation and studies of jet quenching in Pb-Pb collisions at nucleonnucleon center-of-mass energy ¼2.76 TeV, Phys. Rev. C 84, 024906 (2011). [6] S. Chatrchyan et al. (CMS Collaboration), Measurement of jet fragmentation into charged particles in pp and Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, J. High Energy Phys. 10 (2012) 087. [7] S. Chatrchyan et al. (CMS Collaboration), Measurement of jet fragmentation in Pb-Pb and pp collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, Phys. Rev. C 90, 024908 (2014). [8] G. Aad et al. (ATLAS Collaboration), Measurement of inclusive jet charged-particle fragmentation functions in Pb+Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV with the ATLAS detector, Phys. Lett. B 739, 320 (2014). [9] V. Khachatryan et al. (CMS Collaboration), Correlations between jets and charged particles in Pb-Pb and pp collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, J. High Energy Phys. 02 (2016) 156. PRL 119, 102301 (2017) PHYSICAL REVIEW LETTERS week ending 8 SEPTEMBER 2017 102301-6 [10] J. Adam et al. (ALICE Collaboration), Measurement of jet quenching with semi-inclusive hadron-jet distributions in central Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, J. High Energy Phys. 09 (2015) 170. [11] B. I. Abelev et al. (STAR Collaboration), Long range rapidity correlations and jet production in high energy nuclear collisions, Phys. Rev. C 80, 064912 (2009). [12] G. Agakishiev et al. (STAR Collaboration), System size and energy dependence of near-side di-hadron correlations, Phys. Rev. C 85, 014903 (2012). [13] L. Adamczyk et al. (STAR Collaboration), Jet-Hadron Correlations in ffiffiffiffiffiffiffiffi sNN p¼200 GeV pþpand Central Au þ Au Collisions, Phys. Rev. Lett. 112, 122301 (2014). [14] K. Aamodt et al. (ALICE Collaboration), Particle-Yield Modification in Jetlike Azimuthal Dihadron Correlations in Pb-Pb Collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, Phys. Rev. Lett. 108, 092301 (2012). [15] S. Chatrchyan et al. (CMS Collaboration), Centrality dependence of dihadron correlations and azimuthal anisotropy harmonics in Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, Eur. Phys. J. C 72, 2012 (2012). [16] J. Adam et al. (ALICE Collaboration), Jet-like correlations with neutral pion triggers in pp and central Pb-Pb collisions at 2.76 TeV, Phys. Lett. B 763, 238 (2016). [17] N. Armesto, C. A. Salgado, and U. A. Wiedemann, Measuring the Collective Flow with Jets, Phys. Rev. Lett. 93, 242301 (2004). [18] N. Armesto, C. A. Salgado, and U. A. Wiedemann, Low-p(T) collective flow induces high-p(T) jet quenching, Phys. Rev. C 72, 064910 (2005). [19] P. Romatschke, Momentum broadening in an anisotropic plasma, Phys. Rev. C 75, 014901 (2007). [20] A. Majumder, B. Muller, and S. A. Bass, Longitudinal Broadening of Quenched Jets in Turbulent Color Fields, Phys. Rev. Lett. 99, 042301 (2007). [21] K. Aamodt et al. (ALICE Collaboration), The ALICE experiment at the CERN LHC, J. Instrum. 3, S08002 (2008). [22] K. Aamodt et al. (ALICE Collaboration), Centrality Dependence of the Charged-Particle Multiplicity Density at Midrapidity in Pb-Pb Collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, Phys. Rev. Lett. 106, 032301 (2011). [23] B. Abelev et al. (ALICE Collaboration), Measurement of inelastic, single- and double-diffraction cross sections in proton–proton collisions at the LHC with ALICE, Eur. Phys. J. C 73, 2456 (2013). [24] B. Abelev et al. (ALICE Collaboration), Centrality determination of Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV with ALICE, Phys. Rev. C 88, 044909 (2013). [25] B. Abelev et al. (ALICE Collaboration), Centrality Dependence of charged particle production at large transverse momentum in Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, Phys. Lett. B 720, 52 (2013). [26] B. Abelev et al. (ALICE Collaboration), Measurement of event background fluctuations for charged particle jet reconstruction in Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, J. High Energy Phys. 03 (2012) 053. [27] B. Abelev et al. (ALICE Collaboration), Underlying event measurements in pp collisions at ffiffiffi s p¼0.9and 7 TeV with the ALICE experiment at the LHC, J. High Energy Phys. 07 (2012) 116. [28] X.-N. Wang and M. Gyulassy, HIJING: A Monte Carlo model for multiple jet production in pp, pA and AA collisions, Phys. Rev. D 44, 3501 (1991). [29] T. Sjostrand, S. Mrenna, and P. Z. Skands, PYTHIA 6.4 physics and manual, J. High Energy Phys. 05 (2006) 026. [30] P. Z. Skands, Tuning Monte Carlo generators: The Perugia tunes, Phys. Rev. D 82, 074018 (2010). [31] R. Brun et al., Geant detector description and simulation tool, CERN Program Library Long Write-up, W5013 (1994). [32] B. Abelev et al. (ALICE Collaboration), Long-range angular correlations on the near and away side in p-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼5.02 TeV, Phys. Lett. B 719,29 (2013). [33] J. Adam et al. (ALICE Collaboration), Evolution of the longitudinal and azimuthal structure of the near-side jet peak in Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, Phys. Rev. C 96, 034904 (2017). [34] G. Aad et al. (ATLAS Collaboration), Measurement of the centrality and pseudorapidity dependence of the integrated elliptic flow in lead-lead collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV with the ATLAS detector, Eur. Phys. J. C 74, 2982 (2014). [35] K. Aamodt et al. (ALICE Collaboration), Harmonic decomposition of two-particle angular correlations in Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, Phys. Lett. B 708, 249 (2012). [36] Z.-W. Lin, C. M. Ko, B.-A. Li, B. Zhang, and S. Pal, A Multi-phase transport model for relativistic heavy ion collisions, Phys. Rev. C 72, 064901 (2005). [37] J. Xu and C. M. Ko, Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV in a multiphase transport model, Phys. Rev. C 83, 034904 (2011). [38] J. Adam et al. (ALICE Collaboration), Higher harmonic flow coefficients of identified hadrons in Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, J. High Energy Phys. 09 (2016) 164. [39] B. Abelev et al. (ALICE Collaboration), Centrality dependence of π,K,pproduction in Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, Phys. Rev. C 88, 044910 (2013). [40] K. Aamodt et al. (ALICE Collaboration), Elliptic Flow of Charged Particles in Pb-Pb Collisions at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV, Phys. Rev. Lett. 105, 252302 (2010). [41] G. L. Ma, S. Zhang, Y. G. Ma, X. Z. Cai, J. H. Chen, and C. Zhong, Longitudinal broadening of near side jets due to parton cascade, Eur. Phys. J. C 57, 589 (2008). PRL 119, 102301 (2017) PHYSICAL REVIEW LETTERS week ending 8 SEPTEMBER 2017 102301-7 J. Adam,39 D. Adamová,86 M. M. Aggarwal,90 G. Aglieri Rinella,35 M. Agnello,113,31 N. Agrawal,48 Z. Ahammed,137 S. Ahmad,18 S. U. Ahn,70 S. Aiola,141 A. Akindinov,55 S. N. Alam,137 D. S. D. Albuquerque,124 D. Aleksandrov,82 B. Alessandro,113 D. Alexandre,104 R. Alfaro Molina,65 A. Alici,107,12 A. Alkin,3J. Alme,22,37 T. Alt,42 S. Altinpinar,22 I. Altsybeev,136 C. Alves Garcia Prado,123 M. An,7C. Andrei,80 H. A. Andrews,104 A. Andronic,100 V. Anguelov,96 C. Anson,89 T. Antičić,101 F. Antinori,110 P. Antonioli,107 R. Anwar,126 L. Aphecetche,116 H. Appelshäuser,61 S. Arcelli,27 R. Arnaldi,113 O. W. Arnold,97,36 I. C. Arsene,21 M. Arslandok,61 B. Audurier,116 A. Augustinus,35 R. Averbeck,100 M. D. Azmi,18 A. Badalà,109 Y. W. Baek,69 S. Bagnasco,113 R. Bailhache,61 R. Bala,93 S. Balasubramanian,141 A. Baldisseri,15 R. C. Baral,58 A. M. Barbano,26 R. Barbera,28 F. Barile,33 G. G. Barnaföldi,140 L. S. Barnby,104,35 V. Barret,72 P. Bartalini,7K. Barth,35 J. Bartke,120,†E. Bartsch,61 M. Basile,27 N. Bastid,72 S. Basu,137 B. Bathen,62 G. Batigne,116 A. Batista Camejo,72 B. Batyunya,68 P. C. Batzing,21 I. G. Bearden,83 H. Beck,96 C. Bedda,31 N. K. Behera,51 I. Belikov,66 F. Bellini,27 H. Bello Martinez,2R. Bellwied,126 L. G. E. Beltran,122 V. Belyaev,77 G. Bencedi,140 S. Beole,26 A. Bercuci,80 Y. Berdnikov,88 D. Berenyi,140 R. A. Bertens,54,129 D. Berzano,35 L. Betev,35 A. Bhasin,93 I. R. Bhat,93 A. K. Bhati,90 B. Bhattacharjee,44 J. Bhom,120 L. Bianchi,126 N. Bianchi,74 C. Bianchin,139 J. Bielčík,39 J. Bielčíková,86 A. Bilandzic,36,97 G. Biro,140 R. Biswas,4S. Biswas,81,4 S. Bjelogrlic,54 J. T. Blair,121 D. Blau,82 C. Blume,61 F. Bock,96,76 A. Bogdanov,77 L. Boldizsár,140 M. Bombara,40 M. Bonora,35 J. Book,61 H. Borel,15 A. Borissov,99 M. Borri,128 E. Botta,26 C. Bourjau,83 P. Braun-Munzinger,100 M. Bregant,123 T. A. Broker,61 T. A. Browning,98 M. Broz,39 E. J. Brucken,46 E. Bruna,113 G. E. Bruno,33 D. Budnikov,102 H. Buesching,61 S. Bufalino,26,31 P. Buhler,115 S. A. I. Buitron,63 P. Buncic,35 O. Busch,132 Z. Buthelezi,67 J. B. Butt,16 J. T. Buxton,19 J. Cabala,118 D. Caffarri,35 H. Caines,141 A. Caliva,54 E. Calvo Villar,105 P. Camerini,25 F. Carena,35 W. Carena,35 F. Carnesecchi,27,12 J. Castillo Castellanos,15 A. J. Castro,129 E. A. R. Casula,24 C. Ceballos Sanchez,9J. Cepila,39 P. Cerello,113 J. Cerkala,118 B. Chang,127 S. Chapeland,35 M. Chartier,128 J. L. Charvet,15 S. Chattopadhyay,137 S. Chattopadhyay,103 A. Chauvin,36,97 V. Chelnokov,3M. Cherney,89 C. Cheshkov,134 B. Cheynis,134 V. Chibante Barroso,35 D. D. Chinellato,124 S. Cho,51 P. Chochula,35 K. Choi,99 M. Chojnacki,83 S. Choudhury,137 P. Christakoglou,84 C. H. Christensen,83 P. Christiansen,34 T. Chujo,132 S. U. Chung,99 C. Cicalo,108 L. Cifarelli,12,27 F. Cindolo,107 J. Cleymans,92 F. Colamaria,33 D. Colella,35,56 A. Collu,76 M. Colocci,27 G. Conesa Balbastre,73 Z. Conesa del Valle,52 M. E. Connors,141,‡J. G. Contreras,39 T. M. Cormier,87 Y. Corrales Morales,113 I. Cortés Maldonado,2 P. Cortese,32 M. R. Cosentino,125,123 F. Costa,35 J. Crkovská,52 P. Crochet,72 R. Cruz Albino,11 E. Cuautle,63 L. Cunqueiro,35,62 T. Dahms,36,97 A. Dainese,110 M. C. Danisch,96 A. Danu,59 D. Das,103 I. Das,103 S. Das,4A. Dash,81 S. Dash,48 S. De,123,49 A. De Caro,30 G. de Cataldo,106 C. de Conti,123 J. de Cuveland,42 A. De Falco,24 D. De Gruttola,30,12 N. De Marco,113 S. De Pasquale,30 R. D. De Souza,124 A. Deisting,100,96 A. Deloff,79 C. Deplano,84 P. Dhankher,48 D. Di Bari,33 A. Di Mauro,35 P. Di Nezza,74 B. Di Ruzza,110 M. A. Diaz Corchero,10 T. Dietel,92 P. Dillenseger,61 R. Divià,35 Ø. Djuvsland,22 A. Dobrin,84,35 D. Domenicis Gimenez,123 B. Dönigus,61 O. Dordic,21 T. Drozhzhova,61 A. K. Dubey,137 A. Dubla,100 L. Ducroux,134 A. K. Duggal,90 P. Dupieux,72 R. J. Ehlers,141 D. Elia,106 E. Endress,105 H. Engel,60 E. Epple,141 B. Erazmus,116 F. Erhardt,133 B. Espagnon,52 S. Esumi,132 G. Eulisse,35 J. Eum,99 D. Evans,104 S. Evdokimov,114 G. Eyyubova,39 L. Fabbietti,36,97 D. Fabris,110 J. Faivre,73 A. Fantoni,74 M. Fasel,76,87 L. Feldkamp,62 A. Feliciello,113 G. Feofilov,136 J. Ferencei,86 A. Fernández Téllez,2E. G. Ferreiro,17 A. Ferretti,26 A. Festanti,29 V. J. G. Feuillard,72,15 J. Figiel,120 M. A. S. Figueredo,123 S. Filchagin,102 D. Finogeev,53 F. M. Fionda,24 E. M. Fiore,33 M. Floris,35 S. Foertsch,67 P. Foka,100 S. Fokin,82 E. Fragiacomo,112 A. Francescon,35 A. Francisco,116 U. Frankenfeld,100 G. G. Fronze,26 U. Fuchs,35 C. Furget,73 A. Furs,53 M. Fusco Girard,30 J. J. Gaardhøje,83 M. Gagliardi,26 A. M. Gago,105 K. Gajdosova,83 M. Gallio,26 C. D. Galvan,122 D. R. Gangadharan,76 P. Ganoti,35,91 C. Gao,7C. Garabatos,100 E. Garcia-Solis,13 K. Garg,28 P. Garg,49 C. Gargiulo,35 P. Gasik,36,97 E. F. Gauger,121 M. B. Gay Ducati,64 M. Germain,116 P. Ghosh,137 S. K. Ghosh,4P. Gianotti,74 P. Giubellino,113,35 P. Giubilato,29 E. Gladysz-Dziadus,120 P. Glässel,96 D. M. Goméz Coral,65 A. Gomez Ramirez,60 A. S. Gonzalez,35 V. Gonzalez,10 P. González-Zamora,10 S. Gorbunov,42 L. Görlich,120 S. Gotovac,119 V. Grabski,65 L. K. Graczykowski,138 K. L. Graham,104 L. Greiner,76 A. Grelli,54 C. Grigoras,35 V. Grigoriev,77 A. Grigoryan,1 S. Grigoryan,68 N. Grion,112 J. M. Gronefeld,100 J. F. Grosse-Oetringhaus,35 R. Grosso,100 L. Gruber,115 F. Guber,53 R. Guernane,35,73 B. Guerzoni,27 K. Gulbrandsen,83 T. Gunji,131 A. Gupta,93 R. Gupta,93 I. B. Guzman,2R. Haake,35,62 C. Hadjidakis,52 H. Hamagaki,131,78 G. Hamar,140 J. C. Hamon,66 J. W. Harris,141 A. Harton,13 D. Hatzifotiadou,107 S. Hayashi,131 S. T. Heckel,61 E. Hellbär,61 H. Helstrup,37 A. Herghelegiu,80 G. Herrera Corral,11 F. Herrmann,62 B. A. Hess,95 K. F. Hetland,37 H. Hillemanns,35 B. Hippolyte,66 J. Hladky,57 D. Horak,39 R. Hosokawa,132 P. Hristov,35 C. Hughes,129 T. J. Humanic,19 N. Hussain,44 T. Hussain,18 D. Hutter,42 D. S. Hwang,20 R. Ilkaev,102 M. Inaba,132 PRL 119, 102301 (2017) PHYSICAL REVIEW LETTERS week ending 8 SEPTEMBER 2017 102301-8