Measurement of Z-tagged charged-particle yields in 5.02 TeV Pb+Pb and p p collisions with the ATLAS detector

ATLAS-CONF-2019-052 04November2019

ATLAS CONF Note

ATLAS-CONF-2019-052

31st October 2019

Measurement of Z -tagged charged-particle yields in 5.02 TeV Pb+Pb and p p collisions with the ATLAS

detector

The ATLAS Collaboration

The yields of charged particles azimuthally balanced by a high-transverse-momentum (p_T)Z boson are measured inppand Pb+Pb collision data recorded by the ATLAS detector at the Large Hadron Collider. The measurement is performed using 260 pb⁻¹ofppcollisions and up to 1.7 nb⁻¹of Pb+Pb collisions, recorded at a center of mass energy of 5.02 TeV per nucleon pair in 2017 and 2018, respectively. The charged-particle yield perZboson is measured as a function of charged-hadronp_T and the hadron-to-bosonp_Tratiox_hZ. The per-Zyields and their ratio between Pb+Pb andppcollisions,I_AA, are reported in differentZ bosonp_Tranges and Pb+Pb centrality selections. TheI_AAis observed to be significantly suppressed at large hadronp_Torx_hZ, presumably due to interaction of the hard-scattered partons with a hot and dense quark-gluon plasma. TheI_AAresults are compared to the predictions of jet modification in several Monte Carlo generators and theoretical calculations.

© 2019 CERN for the benefit of the ATLAS Collaboration.

Reproduction of this article or parts of it is allowed as specified in the CC-BY-4.0 license.

1 Introduction

Collisions of heavy nuclei at ultra-relativistic energies at the Large Hadron Collider (LHC) or the Relativistic Heavy Ion Collider (RHIC) are understood to produce an extended region of hot and dense matter where partons exist in a deconfined state known as the quark-gluon plasma (QGP). The high density of unscreened color charges in the QGP causes the showers of hard-scattered partons with large transverse momentum (p_T) to be modified as they traverse the medium. These modifications are apparent in measurements of dijet and photon–jet momentum imbalance [1–4], and in jet fragmentation functions [5,6].

The larger integrated luminosity of Pb+Pb collisions delivered during the LHC Run 2 has enabled new measurements of jets produced in association with a high-p_T Z boson. At leading order, theZ and leading jet are produced back-to-back in the azimuthal plane, with equalp_T. SinceZ bosons, or similarly, photons, do not participate in the strong interaction and are not modified by the QGP [7,8], they provide an estimate of thep_Tand transverse direction of a hard-scattered parton before the developing shower becomes modified through interactions with the QGP [9,10]. Measurements of photon-tagged fragmentation functions at the LHC [11,12] and photon–hadron correlations at RHIC [13,14] used this important feature to perform detailed studies of jet quenching. However, the use of isolated photons at low photon-p_Tis considerably more difficult due to the large hadron-decay background, motivating the use of Z bosons. These have a lower production rate, but also have much lower backgrounds at lowp_T. An initial measurement of Z+jet production with p^Z

T > 60 GeV by CMS demonstrates that the totalp_Tcarried inside the jet cone is decreased in Pb+Pb events compared to that inppevents [15]. However, the modification of the jet’s constituent particlep_Tdistributions has not yet been studied.

This note presents a measurement of the yield of charged particles produced opposite in azimuth to a Z boson with p^Z

T > 30 GeV in Pb+Pb and pp collisions at a nucleon–nucleon centre-of-mass energy

√s_NN =5.02 TeV with the ATLAS detector at the LHC. The Pb+Pb andppdata were recorded in 2018 and 2017, respectively, and correspond to respective integrated luminosities of up to 1.7 nb⁻¹and 260 pb⁻¹. This measurement explores similar physics phenomena as previous measurements of the photon-tagged jet fragmentation function [11]. However, since there is no explicit requirement on the presence of a reconstructed jet, it allows additional insight into energy loss at low-Q²values where reconstructed jet measurements are infeasible. Events containing aZboson withp^Z

T >30 GeV are studied, and measured charged hadrons must havep^ch

T >1 GeV and be approximately back-to-back with theZ in the transverse plane,|∆φ| =

φ^Z−φ^ch

>3π/4.¹ In Monte Carlo simulations ofppcollisions, charged hadrons meeting these criteria come primarily from the fragmentation of the leading jet azimuthally opposite to theZboson.

The per-Z yields are reported as a function of p^ch

T, (1/N_Z) d²N_ch/dp^ch

Td∆φ

, and as a function of the hadron-to-bosonp_Tratiox_hZ ≡p^ch

T/p^Z

T,(1/N_Z) d²N_ch/dx_hZd∆φ .

Z bosons are reconstructed through their dielectron and dimuon decays, in a manner similar to previous measurements performed in 2015 Pb+Pb andppdata at the same collision energy [8,16]. The charged particle yields are then corrected using a data-driven technique to establish the background contribution of charged hadrons from the underlying event (UE). To quantify the modification which results from the parton’s propagation through the QGP medium, the ratio of per-Z hadron yields between Pb+Pb collisions andppcollisions,I_AA, is reported. TheI_AAvalues are compared to similar measurements of per-photon hadron yields and to expectations from theoretical calculations.

1ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point in the centre of the detector and thez-axis along the beam pipe. Cylindrical coordinates(r, φ)are used in the transverse plane,φbeing the azimuthal angle around thez-axis. The pseudorapidity is defined in terms of the polar angleθasη=−ln tan(θ/2).

2 Experimental setup

The ATLAS experiment [17] at the LHC is a multi-purpose particle detector with a forward-backward symmetric cylindrical geometry and a near 4πcoverage in solid angle. It consists of an inner tracking detector surrounded by a thin superconducting solenoid providing a 2 T axial magnetic field, electromagnetic and hadron calorimeters, and a muon spectrometer.

The inner tracking detector (ID) covers the pseudorapidity range|η| <2.5. It consists of silicon pixel, silicon microstrip, and transition radiation tracking detectors. Lead/liquid-argon (LAr) sampling calorimeters provide electromagnetic (EM) energy measurements with high granularity. A hadron (steel/scintillator-tile) calorimeter covers the central pseudorapidity range (|η| < 1.7). The end-cap and forward regions are instrumented with LAr calorimeters for both EM and hadronic energy measurements up to|η|=3.2. The forward calorimeter (FCal) is a liquid-argon sampling calorimeter located on either side of the interaction point and covering 3.1< |η| <4.9. It is used to characterise the centrality of Pb+Pb collisions as described below. The muon spectrometer surrounds the calorimeters up to|η| < 2.7 and is based on three large air-core toroidal superconducting magnets with eight coils each. The field integral of the toroids ranges between 2.0 and 6.0 T m across most of the detector. The muon spectrometer includes a system of precision tracking chambers and fast detectors for triggering. Finally, zero-degree calorimeters (ZDC) are situated at large pseudorapidity,|η| >8.3, and are primarily sensitive to spectator neutrons.

A two-level trigger system described in Ref. [18] is used to select events. Data for this analysis were collected primarily using single-lepton triggers which require an electron or muon candidate with ap_T threshold of 15 and 14 GeV, respectively. In addition to the leptonp_Trequirement, a loose likelihood-based identification requirement, optimized separately forppand Pb+Pb data-taking, was applied in the electron trigger [19]. Pb+Pb events were also recorded with a combination of minimum-bias (MB) triggers; these events are used to estimate the combinatoric contribution from UE particles to the charged-hadron yields.

The Pb+Pb MB triggers are based on the presence of a minimum amount of transverse energy in the calorimeter system or, for events that do not meet this condition, on substantial energy deposits in both ZDC detectors and an ID track identified by the high-level trigger system. In ppdata-taking, the MB trigger required only an ID track in a filled bunch crossing.

3 Data selection and simulation samples

Events with a single high-p_Telectron or muon are initially selected for analysis by the high-level triggers described above. Electron events are sampled from 1.7 nb⁻¹of Pb+Pb collisions while muon events are sampled from a subset of the data-taking period when the ATLAS toroid was enabled, corresponding to an integrated luminosity of 1.4 nb⁻¹. The centrality of Pb+Pb events is defined as in other 5.02 TeV measurements [3,20] using the total transverse energy measured in the FCal, evaluated at the electromagnetic scale and denoted byΣE^Pb

T . Pb+Pb events are divided into three categories which correspond to the 0–10%, 10–30%, and 30–80% centrality intervals. The mean number of participating nucleons in MB Pb+Pb collisions in these selections is estimated using a Glauber model simulation, and ranges from N_part

=359±2 in 0–10% events (largestΣE^Pb

T values and degree of nuclear overlap) to N_part

=64±1 in 30–80% events (smallestΣE^Pb

T values and degree of nuclear overlap). An additional requirement in Pb+Pb collisions, based on the correlation of the signals in the ZDC and the FCal, is used to reject a small number of recorded events consistent with two Pb+Pb interactions in the same bunch crossing (pile-up) [21].

Charged-particle tracks and collision vertices are reconstructed in the ID using the algorithms described in Ref. [22]. During theppdata-taking, the average number of interactions per bunch crossing typically ranged from 2–4, so no pile-up rejection is applied inppcollisions. Instead, all charged-particle tracks and decay leptons are required to come from the primary reconstructed vertex, defined as the vertex with the maximum value ofΣp^ch

T, where the sum is performed over all tracks coming from that vertex.

Monte Carlo simulations of

√s = 5.02 TeV ppcollisions with Z bosons decaying in the electron and muon channels are used to understand the detector performance and correct the data for bin migration and reconstruction inefficiencies. For all the samples described below, the generated events were passed through a full Geant4 simulation [23, 24] of the ATLAS detector under the same conditions present during data-taking and were digitized and reconstructed in the same way as the data. The processes of interest were generated with the Powheg-Box 2 program [25] interfaced to the Pythia 8.186 parton shower model [26]. The CT10 parton distribution function (PDF) set [27] was used in the matrix element, while the CTEQ6L1 PDF set [28] and AZNLO tune [29] were used to describe the initial-state parton shower.

Four million total events were generated to serve as the simulation sample forppcollisions. To simulate Pb+Pb events, five million events were generated composed of four nucleon–nucleon combinations (pp,pn, np, andnn) corresponding to their relative abundance in the colliding lead nuclei; after generation, these events were overlaid at the detector-hit level with MB Pb+Pb events. The relative contribution of events in this “data-overlay” sample were reweighted on an event-by-event basis to match theΣE^Pb

T distribution observed in theZ events in Pb+Pb data selected for analysis. Thus, the Pb+Pb simulation samples contain UE activity levels identical to those in data.

4 Analysis

Z bosons inppand Pb+Pb events are reconstructed in their dielectron and dimuon decay channels through procedures closely following those described in Refs. [8,16].

Reconstructed electrons are required to have a transverse momentump^e

T >20 GeV, to lie within the fiducial pseudorapidity acceptance of the EM barrel (|η^e|< 1.37) or endcap (1.56< |η^e| <2.47) detectors, and to pass likelihood-based identification criteria, which are determined separately forppand Pb+Pb events [30].

Reconstructed muons are required to have a transverse momentump^µ

T > 20 GeV, to lie within the fiducial pseudorapidity acceptance of the muon spectrometer (|η^µ| < 2.5), and to pass the “medium” selection requirements described in Ref. [31]. For both channels, only opposite-sign lepton pairs are considered.

The invariant mass of theZ candidate is required to be within the range 76< m_ll <106 GeV. This is a narrower mass selection than that used in previous measurements in order to reduce the contribution from Drell-Yan and QCD backgrounds, the former of which was estimated in simulation to be smaller than 1%, and the latter is determined in Ref. [8] to be 2% for electron and 0.5% for muon channels. The effects of background event contamination are neglected in this analysis. These criteria select approximately 1,300 and 9,000Z →eeevents in Pb+Pb andppdata, respectively that contain aZ withp^Z

T >30 GeV. In the muon channel, approximately 1,500 and 11,000Z →µµevents withp^Z

T >30 GeV are selected in Pb+Pb andppcollisions, respectively.

In the analysis, eachZ event is assigned a weight to account for the finite trigger, reconstruction and selection efficiencies of the decay leptons. Lepton trigger efficiencies,^l

trig, are determined directly inpp and Pb+Pb data using tag-and-probe techniques [18,19] in which the tag lepton is required to fire the trigger and the probe lepton paired with it gives an invariant mass in the required range. Typical ranges of the

trigger efficiency are 0.70–0.80 for muons and 0.75–0.95 for electrons, depending on the collision system and pseudorapidity. The reconstruction and selection efficiencies,_rec^l , are determined using simulation and have typical ranges of 0.65–0.80 for muons and 0.65–0.95 for electrons. The weight for each Zevent is assigned as 1/(^Z

trig_rec^Z), where^Z

trig=1− (1−^l1

trig)(1−^l2

trig)due to the single lepton trigger requirement, and_rec^Z = _rec^l1_rec^l2. All efficiencies were observed to have a modest centrality dependence, except the muon trigger efficiency which is found to be centrality-independent. Although the efficiencies may vary substantially with the individual leptonp_T,ηandφ, the resulting dependence on Z p_Tis very weak due to the broad relationship between boson and decay lepton kinematics. Thus, the overall impact of this correction on the per-Z charged-hadron yields is small.

Charged-particle tracks are reconstructed from hits in the inner detector using an algorithm which, in Pb+Pb collisions, is optimized for the high-occupancy conditions [32,33]. They are required to meet several criteria including a minimum number of hits, the presence of hits expected by the algorithm, and a small distance-of-closest approach to the vertex. For the charged-hadron yield measurement, all reconstructed tracks with p_T > 1 GeV, |η| < 2.5 and |∆φ| =

φ^trk−φ^Z

> 3π/4 are considered. The charged-particle yield is corrected for the finite reconstruction and selection efficiency on a per-track basis using a simulation-derived efficiency at the givenp_Tandη. The efficiency varies from 60% to 80%

depending on occupancy and track kinematics.

The contribution to the yield from UE particles is estimated using MB Pb+Pb data and is statistically subtracted from the measured yields. For each signalZ event in data, forty unique MB events are used to estimate the per-event background contribution. The MB events are required to have a centrality which is within 2%, 1%, and 0.5% of the signal event centrality, for signal events in the 30–80%, 10–30%, and 0–10% centrality ranges, respectively. The MB events are further required to have been recorded during a similar data-taking period. The charged-hadron background yield is estimated in the same absolute azimuthal region of the detector as the signal event. The resulting signal-to-background ratio varies strongly with charged-hadronp_Tand Pb+Pb centrality, with a minimum of≈1% at the lowestp^ch

T and most central events. The UE subtraction is also applied to theppdata using MBppevents, where it results in a 20–30% change in the yield forp^ch

T =1–2 GeV, and has a decreasing impact at higherp^ch

T. An additional, multiplicative correction is applied to the subtracted yield to account for the contribution of reconstructed tracks which do not arise from primary particles. This correction is derived in simulation and typically corrects the yields downwards by 1–2%.

The data are further corrected for bin migration resulting from the finite resolution in theZ p_Tmeasurement.

The purity of theZboson selection, defined as the fraction ofZ bosons in the reconstructedp_Tbin which originated from the same bin at the generator level, is evaluated in simulation. The purity is found to be greater than 90% for all cases reported here, due to the small lepton kinematic resolution compared to the size of thep^Z

T bins. The impact of the finite p_T resolution is evaluated by comparing the per-Z charged-hadron yields, where the Z selection is made at the generator level, compared to that at the reconstructed level. The ratio of the distributions is used to determine a multiplicative bin-by-bin correction factor as a function ofp^ch

T orx_hZ, and is evaluated separately for each event centrality interval,p^Z

Tselection, and decay channel. This correction is applied to the data and is typically within 2–3% of unity, though it increases withp^ch

T orx_hZto as much as 5%.

Finally, an internal consistency check was performed by comparing the per-Zyields between the electron and muon decay channels. This check was performed by examining the distribution of pull values, defined as the difference in the yield between the channels divided by its statistical uncertainty. While no systematic difference between the channels or in any specific kinematic region was observed, the width of the

distribution over allp^Z

T,p^ch

T and centrality selections was larger than expected given the measured statistical uncertainties. To account for this difference, the statistical uncertainties were increased by approximately 50% so that the new pull distribution had a width of unity.

5 Systematic uncertainties

The primary sources of systematic uncertainty can be grouped into those affecting theZboson reconstruction, those affecting the charged hadron yield measurement, and those affecting the UE background estimation and subtraction.

The results are sensitive to the reconstruction of electrons and muons originating from aZ decay and the measurement of their kinematics. The impact on the measured quantities is evaluated using a common set of uncertainties for electron [34] and muon [31] energy scale measurements performed by ATLAS.

Several sources of tracking-related uncertainty are also investigated. Since the efficiency depends on the charged hadron species, the evaluated reconstruction efficiency is sensitive to the particle composition of charged hadrons in simulation. This is evaluated by assuming all charged particles are charged pions.

Other uncertainties which are subdominant include an uncertainty in the rate of tracks not arising from primary particles [35], and the sensitivity of the corrected yield in data to the track selection criteria. These are described in more detail in previous measurements of charged-particle fragmentation functions, such as those described in Ref [5].

Two uncertainties related to the determination of the UE charged hadron background yield were evaluated.

The first arises from the statistical uncertainty in the background yields from the finite number of MB events used to determine them. The second is a systematic uncertainty arising from event properties to which the UE background yields may be sensitive, but which are not controlled for in the mixed-event selection. For example, different distributions of the reaction-plane angle or the absolute amplitude of the flow modulation between Z events in data and mixed events, may affect the estimated UE yield.

Based on previous studies [36], the upper bound on the uncertainty on the total background yield is estimated to be 0.5%, which propagates to the subtracted yield in a way that is inversely proportional to the signal-to-background ratio.

At lowp^ch

T and in central events, the uncertainties in the per-Zyield from the background determination are dominant due to the small signal-to-background ratio and can reach up to 30%. At highp^ch

T and in lower-multiplicity events, the uncertainties in the per-Zyield associated with the tracking efficiency are typically dominant, reaching up to 5%. Where possible, the correlation of the uncertainties between Pb+Pb events andppevents has been evaluated and properly accounted for in theI_AAratio. However, both the background normalization and particle-composition uncertainties are treated as uncorrelated between Pb+Pb andppcollisions.

6 Results

This section presents the measurement of the per-Zcharged-hadron yields, their ratio between Pb+Pb and ppevents,I_AA, and the comparison of theI_AAto previous measurements and to theoretical predictions.

[GeV]

ch

pT

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10

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10 1 10 ]-1 ) [GeVφ∆ d Tp / dchN2 ) (d Z(1/N

1 2 3 4 5 6 7 10 20 30 60

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pp

Pb+Pb 30-80%

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]-1 ) [GeVφ∆ d Tp / dchN2 ) (d Z(1/N

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pT

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pT

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pp

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xhZ

)φ∆ dx / dchN2 ) (d Z(1/N

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= 5.02 TeV, 260 pb-1

s , pp

= 5.02 TeV, 1.4-1.7 nb-1

sNN

Pb+Pb,

> 60 GeV

Z

pT

Figure 1: Per-Zcharged hadron yield reported as a function of charged-hadronp^ch

T (top panels) and the hadron-to-boson p_Tratiox_hZ(bottom panels). Results are reported forp^Z

T=30–60 GeV (left panels) andp^Z

T>60 GeV (right panels).

Each panel shows the yield inppevents and the three categories of Pb+Pb events. The vertical bars and boxes correspond to the statistical and total systematic uncertainties in the data. Points have been displaced horizontally for visibility.

Figure 1 presents the measured per-Z charged hadron yield, in ppevents and three Pb+Pb centrality selections. Per-Zyields are reported for twop^Z

Tselections as a function of bothp^ch

T and the hadron-to-boson p_Tratio,x_hZ= p^ch

T/p^Z

T. The yields in Pb+Pb collisions are observed to be modified with respect to that in ppcollisions.

To better reveal the modification, Figure2presents theI_AAvalues, the ratio of the per-Z charged-hadron yields in Pb+Pb events to those inppevents. TheI_AAvalues are observed to show suppression at large charged-particlep_Torx_hZ, with a systematically larger suppression in more central events and for lower p^Z

Tselections. At low charged-particle p_T / 2–3 GeV (depending on the p^Z

T range) orx_hZ / 0.05, the I_AA values are typically enhanced above unity, although the magnitude of the uncertainties limit the precision with which this enhancement can be measured. The suppression over a wide range ofp^ch

T andx_hZ values, and the monotonic increase with decreasingp^ch

T and x_hZ, leading to an enhancement at low values, are qualitatively similar to those observed in the ratios of jet fragmentation functions in photon-tagged

[GeV]

ch

pT

0 0.5 1 1.5

2 2.5 IAA 3

1 2 3 4 5 6 7 10 20 30

Preliminary ATLAS

= 5.02 TeV, 260 pb-1

s , pp

= 5.02, 1.4-1.7 nb-1

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xhZ 10-2

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< 60 GeV

Z

pT

30 < 30-80%

10-30%

0-10%

[GeV]

ch

pT

0 0.5 1 1.5

2 2.5 3 IAA 3.5

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= 5.02, 1.4-1.7 nb-1

sNN

Pb+Pb,

xhZ 10-2

×

2 10^-1 2×10^-1 1

> 60 GeV

Z

pT 30-80%

10-30%

0-10%

Figure 2: Ratio (I_AA) of the per-Zcharged hadron yield in Pb+Pb collisions to those inppcollisions. I_AAratios are reported as a function of charged-hadronp^ch

T (left panels) and the hadron-to-bosonp_Tratiox_hZ(right panels), for 30<p_Z<60 GeV(top panels) andp_Z>60 GeV (bottom panels). Each panel shows theI_AAin the three categories of Pb+Pb collisions. The vertical bars and boxes correspond to the statistical and total systematic uncertainties in the data. Points have been displaced horizontally for visibility.

events [11].

Figures3and4compares theI_AAmeasurement presented here to the results of two theoretical predictions.

The first is a perturbative calculation within the framework of soft-collinear effective field theory with Glauber gluons (SCET_G) in the soft-gluon-emission (energy-loss) limit, with jet-medium coupling g=2.0±0.2 [37,38] (labeled “Li & Vitev” in Fig.3). The second is the Hybrid Strong/Weak Coupling model [39], which combines initial production using Pythia8 with a parameterization of energy loss derived from holographic methods, including back-reaction effects. The SCETGand the Hybrid models both quantitatively reproduce the degree of suppression at largep^ch

T orx_hZ, and the Hybrid model qualitatively captures the increase at lowp^ch

T or x_hZ. In Figure4, the Hybrid model also captures the relative difference in theI_AAbetween the twop^Z

Tselections.

Figure5compares theI_AAmeasurement in 0–10% events to that measured using photon–hadron correlations at the LHC and in Au+Au collisions at 200 GeV at RHIC. In the left panel, the results are compared to a

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0 0.5 1 1.5 2

IAA 2.5 ATLAS Preliminary

= 5.02 TeV, 260 pb-1

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= 5.02 TeV, 1.4-1.7 nb-1

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< 60 GeV

Z

pT

30 <

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×10

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xhZ

AAI [GeV]^Z_Tp60+30-60

ATLAS 0-10% Pb+Pb Hybrid Model Li & Vitev

> 60 GeV

Z

pT

Figure 3: Ratio (I_AA) of the per-Z charged hadron yield in 0–10% Pb+Pb events to those in pp events, for 30<p^Z

T<60 GeV (left) andp^Z

T>60 GeV (right). The data are compared to theoretical calculations (see text). The vertical bars and boxes around the data correspond to the statistical and total systematic uncertainties. The shaded bands around the theoretical predictions represent the theoretical uncertainty.

1 2 3 4 5 6 7 10 20 30 40 [GeV]

ch

pT

0 0.5 1 1.5 2 2.5 3

IAA 3.5 ATLAS Preliminary

= 5.02 TeV, 260 pb-1

s , pp

= 5.02 TeV, 1.4-1.7 nb-1

sNN

Pb+Pb,

30-60 60+ ^Z [GeV]

pT

ATLAS 0-10% Pb+Pb Hybrid Model

Figure 4: Ratio (I_AA) of the per-Zcharged hadron yield in 0–10% Pb+Pb events to those inppevents, forp^Z

T=30–60 GeV (blue squares) andp^Z

T>60 GeV (red circles). The data are compared to calculations provided in the Hybrid model framework (see text). The vertical bars and boxes around the data correspond to the statistical and total systematic uncertainties. Points have been displaced horizontally for visibility. The shaded bands around the theoretical predictions represent the theoretical uncertainty.

photon-hadron measurement withp^γ

T >60 GeV in Pb+Pb events performed by the CMS Collaboration in the 0–10% centrality interval [12]. The CMSI_AA data are systematically higher than those presented here forx_hZ > 0.5. However, the CMS measurement requires a reconstructed jet withp_T >30 GeV threshold on the away-side, which thus in principle selects events with a smaller degree of jet modification. In the

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pT ch / pT /Z =

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jet

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> 60 GeV,

γ

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CMS

Preliminary ATLAS

= 5.02 TeV, 260 pb-1

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= 5.02 TeV, 1.4-1.7 nb-1

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pT ch / pT /Z =

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AAI > 60 GeV, 0-10% Pb+Pb^Z

pT

ATLAS

< 9 GeV, 0-40% Au+Au

γ

pT

PHENIX 5 <

< 20 GeV, 0-12% Au+Au

γ

pT

STAR 12 <

Preliminary ATLAS

= 5.02 TeV, 260 pb-1

s , pp

= 5.02 TeV, 1.4-1.7 nb-1

sNN

Pb+Pb,

Figure 5: Ratio (I_AA) of the per-Zcharged hadron yield in 0–10% Pb+Pb events to those inppevents, forp^Z

T>60 GeV, as a function of the hadron-to-bosonp_Tratiox_hZ. The data are compared to measurements of theI_AAfor photon–

hadron correlations at the LHC (left: ATLAS and CMS) and RHIC (right: ATLAS, PHENIX, and STAR, see text).

In all cases, the vertical bars and boxes around the data points correspond to the statistical and total systematic uncertainties, respectively. In the case of photon-tagged yield measurements, the x-axis variable is calculated with the photon momentum instead of the Z boson momentum.

right panel, the results are compared to measurements by the PHENIX Collaboration withp^γ

T=5–9 GeV in 0–40% Au+Au events [13] and by the STAR Collaboration withp^γ

T =12–20 GeV in 0–12% Au+Au events [14]. TheI_AA data at RHIC show a qualitatively similar suppression at largex_hZ > 0.1 to that measured here, but the extended bosonp_Trange accessible at the LHC also allows the measurement to extend to lowerx_hZ.

7 Conclusion

This note presents a measurement of charged-particle yields produced in the azimuthal direction opposite to aZboson withp_T > 30 GeV. The measurement is performed using 260 pb⁻¹ofppand up to 1.7 nb⁻¹ of Pb+Pb collision data at 5.02 TeV with the ATLAS detector at the Large Hadron Collider. The per-Z charged-particle yields are observed to be systematically modified in Pb+Pb collisions compared topp collisions presumably due to the interactions between the parton shower and the hot and dense QGP medium. The charged-particle p_T distribution in Pb+Pb collisions is softer than that in ppcollisions, with a suppression at highp^ch

T orx_hZand an enhancement at lowp^ch

T orx_hZ. The degree of modification varies with Pb+Pb event centrality, consistent with a larger and hotter QGP being created in central events compared to peripheral events. The modification pattern is qualitatively similar to that observed in measurements of photon-tagged jet fragmentation functions. The data presented in this note demonstrates the feasibility of measuring energy loss in a kinematic region difficult to access in photon–jet or inclusive hadron measurements and without the requirement of a reconstructed jet. Such measurements have the potential to significantly improve the understanding of energy loss in the strongly-coupled QGP medium.

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