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ATLAS CONF NoteATLAS-CONF-2017-009

Study of inclusive jet yields in Pb+Pb collisions at√sNN = 5.02 TeV

The ATLAS Collaboration

5th February 2017

Measurements of the yield and nuclear modification factor, RAA, for inclusive jets are per-formed using 25 pb−1 of pp data at

√s = 5.02 TeV and 0.49 nb−1 of Pb+Pb data at

√sNN = 5.02 TeV. The jets are reconstructed with the anti-kt algorithm, with R = 0.4,

and are measured over the transverse momentum range of 100–1000 GeV in six rapidity in-tervals covering |y | < 2.8. The magnitude of the RAA increases with increasing pT and withdecreasing centrality of the Pb+Pb collision. The RAA is independent of rapidity at low pTand it decreases with increasing rapidity at high pT.

© 2017 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

Heavy-ion collisions at ultra-relativistic energies produce a hot, dense medium of strongly interactingnuclear matter understood to be composed of deconfined color charges that is commonly called a quark-gluon plasma (QGP) [1–4]. Products of hard scattering of quarks and gluons occurring in these collisionsevolve as parton showers that propagate through the heavy ion medium. Full parton shower or its con-stituents emit medium-induced gluon radiation and as a consequence the resulting jet loses energy. Thisphenomenon is termed “jet quenching” [5–7]. It can be observed as the suppression of the jet produc-tion relative to a pp reference and modification of the jet internal structure. Generally, the energy lossof partons traversing QGP will result in a reduction in the jet yield at fixed transverse momentum (pT).Thus, the jets are observed to be suppressed in central collisions relative to peripheral or pp collisions.The inclusive jet suppression has previously been measured at the LHC in

√sNN = 2.76 TeV Pb+Pb

collisions in the hard scattering rate [8–11].

The centrality dependence of measured hard scattering rates must be normalized by the nuclear thicknessfunction, 〈TAA〉, that accounts for the geometric enhancement of per-collision nucleon-nucleon luminosityto allow for a proper assessment of the quenching effects. Such an assessment is quantified by the nuclearmodification factor

RAA =

1N tot

evt

d2Njet

dpTdy

������cent

〈TAA〉d2σjet

dpTdy

������pp

, (1)

where N totevt is the total number of Pb+Pb collisions within a chosen centrality interval. A value of RAA ≈

0.5 in central collisions was reported in Pb+Pb measurements at√

sNN = 2.76 TeV [9, 10]. This impliesa suppression of jet yields by roughly a factor of two in central collisions with respect to expectationsfrom scaled pp collisions at the same center-of-mass energy. Two interesting features were revealed bythose studies, RAA increases only very slowly with increasing jet pT; RAA exhibits no dependence on therapidity1 of the jet.

This note describes new measurements of jet yields performed with 0.49 nb−1 of Pb+Pb data collectedin 2015 at

√sNN = 5.02 TeV and 25 pb−1 of pp data collected at

√s = 5.02 TeV in the same year.

This new study extends the original measurement to higher transverse momentum and rapidity. It shouldprovide needed inputs for a more detailed understanding of jet suppression, especially its dependence onthe collision energy.

2 Experimental setup

The measurements presented in this note were performed using the ATLAS calorimeter, trigger and dataacquisition systems [12]. The calorimeter system consists of a sampling liquid argon (LAr) electromag-netic (EM) calorimeter covering |η | < 3.2, a steel–scintillator sampling hadronic calorimeter covering

1 ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the center of the detectorand the z-axis along the beam pipe. The x-axis points from the IP to the centre of the LHC ring, and the y-axis points upward.Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the beam pipe. Thepseudorapidity is defined in terms of the polar angle θ as η = − ln tan(θ/2). Rapidity y is defined as y = 0.5 ln E+pz

E−pzwhere

E and pz are the energy and the component of the momentum along the beam direction.

2

|η | < 1.7, a LAr hadronic calorimeter covering 1.5 < |η | < 3.2, and two LAr forward calorimet-ers (FCal) covering 3.1 < |η | < 4.9. The hadronic calorimeter has three sampling layers longitudinalin shower depth. The EM calorimeters are segmented longitudinally in shower depth into three layerswith an additional pre-sampler layer. The EM calorimeters have a granularity that varies with layer andpseudorapidity, but which is generally much finer than that of the hadronic calorimeter. The minimum-bias trigger scintillators (MBTS) [12] detect charged particles over 2.1 < |η | < 3.9 using two segmentedcounters each segment placed at z = ±3.6 m. Each counter provides measurements of both the pulseheights and arrival times of ionization energy deposits.

A multi-level trigger system was used to select the Pb+Pb and pp collisions analyzed here. The first,hardware based trigger stage Level-1, is implemented with custom electronics. The next level is the soft-ware based High Level Trigger (HLT). The jets were selected by the HLT seeded by Level-1 jet, minimumbias, or total energy triggers. The total energy trigger required a total transverse energy measured in thecalorimeter system to be greater than 5 GeV in pp interactions and 50 GeV in Pb+Pb interactions [13].The HLT jet trigger used a jet reconstruction algorithm similar to that applied in the offline analysis andselected events containing jets with a range of transverse energy thresholds up to 100 GeV in Pb+Pbcollisions and up to 85 GeV in pp collisions. In both pp and Pb+Pb collisions, the highest threshold jettrigger sampled the full delivered luminosity while all lower thresholds were prescaled. Jets that are usedin this analysis were selected from jet triggers in the region of jet pT for which the trigger efficiencies aregreater than 99%.

3 Collision data, MC sample and event selection

The collision data used in this analysis were recorded during the LHC Run 2 in 2015. The data consist of25 pb−1 of pp data at

√s = 5.02 TeV and 0.49 nb−1 of Pb+Pb data at

√sNN = 5.02 TeV.

The impact of the detector effects on the measurement was determined using a simulated detector re-sponse evaluated by running Monte Carlo (MC) samples through a Geant4-based detector simulationpackage [14, 15]. Two MC samples were used in this study. The first one was Powheg+Pythia 8 [16,17] dijet sample at

√s = 5.02 TeV with the A14 ATLAS tune [18] and the CT10 NLO (PDFs) [19]. In

total, 29 × 106 dijet events, spanning a range of the jet transverse momentum 20–1300 GeV, were simu-lated. The second MC sample consisted of the same signal dijet events as those used in the first samplebut embedded into real minimum-bias heavy ion events. This data was combined with the signal fromPowheg+Pythia 8 simulation at the digitization stage, and then reconstructed as a combined event. ThisMC sample provided the same kinematic coverage as the first sample and the same number of events.

The level of overall event activity or “centrality” in heavy ion collisions was characterized using thesum of the total transverse energy in the forward calorimeter, ΣEFCal

T , at the electromagnetic scale. TheΣEFCal

T distribution was divided into percentiles of the total inelastic cross-section for Pb+Pb collisions.The minimum bias trigger and event selection were estimated to sample 84.5% of the total inelastic cross-section, with an uncertainty of 1%. For this analysis the data has been divided into eight centrality classes,0-10%, 10-20%, 20-30%, 30-40%, 40-50%, 50-60%, 60-70%, and 70-80%, where 0-10% are the mostcentral collisions, with the highest ΣEFCal

T .

The geometric parameters of the collisions were quantified using the Glauber model [20]. The MCimplementation of this model was used to determine the mean number of nucleons participating in thecollision, 〈Npart〉, the nuclear thickness function, 〈TAA〉, and their uncertainties (including the uncertainties

3

Centrality range 〈Npart〉 〈TAA〉 [1/mb]70-80% 15.4 ± 1.0 0.22 ± 0.0260-70% 30.6 ± 1.6 0.57 ± 0.0450-60% 53.9 ± 2.0 1.28 ± 0.0740-50% 87.0 ± 2.3 2.63 ± 0.1130-40% 131.4 ± 2.6 4.94 ± 0.1520-30% 189.2 ± 2.8 8.64 ± 0.1710-20% 264.1 ± 2.8 14.3 ± 0.180-10% 358.8 ± 2.2 23.4 ± 0.20

Table 1: The mean number of participants, 〈Npart〉 and the nuclear thickness function, 〈TAA〉 and their uncertaintiesfor different centrality intervals.

on the nuclear density, the nucleon-nucleon cross-section, and the estimated sampling efficiency) for thecentrality classes used here. These are shown in Table 1.

4 Jet reconstruction and analysis procedure

The jet reconstruction in pp and Pb+Pb collisions closely follows the procedures described in Refs. [8,21] including the underlying event (UE) subtraction procedure which is applied in Pb+Pb collisions. Abrief summary is given here. First, the jets were reconstructed using the anti-kt algorithm [22], which isimplemented in the FastJet software package [23] with R = 0.2. The jets were formed by combining∆η × ∆φ = 0.1 × π

32 “towers” that were constructed using energy deposits in nearby calorimeter cells.A background subtraction procedure was applied that uses the UE average transverse energy density, ρ,and the magnitude (vn) and phases (Ψn) of the modulation due to harmonic flow. The background ismodulated in this way because global azimuthal correlations in the particle production arise from thehydrodynamical evolution of the medium as defined by Eq. 2 where φ is the azimuthal angle of the jet.

dET

dηdφ≈

dET

dη(1 +∑n

vn cos (n(φ − Ψn ))) (2)

The modulation is dominated by the lowest order harmonics, v2 and v3 [24]. In the reconstruction usedfor Run 1 results, the background subtraction only accounted for the contributions of elliptical flow (v2)to the UE [8]. In Run 2 results, the third and fourth harmonics were also used to further improve theUE estimation. It was found that the inclusion of v3 and v4 (especially v3) substantially improved the jetenergy resolution. Once each of the phases and magnitudes for n = 2 − 4, where n indicates the order ofthe flow harmonic, were determined using the entire calorimeter, the subtraction was applied cell-by-cellwithin each jet. An iterative procedure was used to remove the effects of jets on the ρ and the estimation ofthe magnitude of the flow harmonics. First the average UE density and the flow harmonics were estimatedfrom the transverse energy of cells within |η | < 3.2. The background was subtracted cell-by-cell withinthe jet to obtain the subtracted jet kinematics. Then the ρ and vn were recalculated by excluding anycells within ∆R < 0.4 of seed jets that were defined to be the jets with transverse energy, ET > 30 GeV.These were then used to evaluate a new subtracted energy by applying these to the original cells and thencalculating new jet kinematics. In this final step, R = 0.4 jets were also reconstructed which are used inthis analysis.

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Figure 1: The left panel shows the JES as a function of ptruthT and the right panel shows the JER as a function of

ptruthT . Both are for jets inclusive in |η | < 2.8. The curves are for pp (black), and Pb+Pb with different centralities;

0-10% (red), 10-20% (blue), 20-30% (green), 30-40% (purple), 40-50% (orange), 50-60% (teal), 60-70% (brown),and 70-80% (yellow).

The performance of the jet reconstruction was characterized by evaluating the jet energy scale (JES) andresolution (JER), which are the mean and width of the jet response (preco

T /ptruthT ) in the MC simulations.

Hereafter “truth” denotes MC generator-level jets reconstructed from final-state particles using techniquesdescribed in Ref. [25]. The response was generated by matching the truth to reconstructed jets in the MCwithin a cone of ∆R =

√∆η2 + ∆φ2 < 0.2. The ratio of the reconstructed pT to the truth pT was evaluated

as a function of the truth pT. The performance of the jet reconstruction is summarized in Figure 1, wherethe left and right panels show the JES and JER, respectively. The ratio, or closure, between the truthand reconstructed pT in the MC is demonstrated by the value of the JES in the left panel of Figure 1 andis shown to be about 1% at high pT. This also has a small centrality dependence at high pT which isexpected. The JER has been parameterized as

σ =a√ptruth

T

⊗b

ptruthT⊗ c, (3)

where a, b, and c are free parameters. The first and last terms in the right-hand side of Eq. 3 are sensitiveto aspects of the detector response and are expected to be independent of centrality, while the second (b)term is driven by fluctuations uncorrelated with the jet pT. This “noise” term is often understood in termsof electronic or pileup noise, but in all but the most peripheral HI collisions, both of these contributionsare small compared to the magnitude of the UE fluctuations. The JER for different centrality intervals andfor pp collisions is shown on the right in Figure 1. Fits using Eq. 3 are indicated with dashed lines. TheJER is largest in the more central collisions, as expected from the larger magnitudes of the total transverseenergy in the UE. The JER is ≈ 16% at 100 GeV in central collisions and decreases with increasing pT toa constant at about 5-6% for jets with pT greater than 500 GeV.

The effect of the subtraction procedure on JES relative to the n = 2 and n = 3 phase n|Ψn − φ|, whereΨn is the phase of the harmonic modulation due to flow as described above and shown in Eq. 2 and φ

is the azimuthal angle of the jet, is shown in the left and right panel of Figure 2, respectively. The openblack points on each figure show the angular dependence without including harmonic flow in the UEsubtraction. The filled points show the impact of including the harmonic flow correction, resulting in onlya small residual dependence on event plane angle, substantially improving the jet energy resolution.

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Simulation PreliminaryATLASPowheg + Pythia 8 inclusive jetsPb+Pb data overlay

= 5.02 TeVNN

s = 0.4 jets, R tkanti-

| < 1.0η| < 200 GeV truth

Tp100 <

Figure 2: The jet energy scale as a function of 2|Ψ2 − φtruth | (left) and 3|Ψ3 − φ

truth | (right) for jets with a truth pTbetween 100-200 GeV. The black unfilled points are for centrality of 0-10% without the harmonic flow subtraction.The filled points show the JES with the harmonic flow subtraction applied for different centralities, namely 0-10%(black circles), 20-30% (red squares), 40-50% (blue diamonds), and 60-70% (green crosses).

The jet cross-section in pp collisions, jet yields and RAA in Pb+Pb collisions were measured for jets withpT > 100 GeV in the following jet rapidity intervals: |y | < 0.3, 0.3 < |y | < 0.8, 0.8 < |y | < 1.2,1.2 < |y | < 1.6, 1.6 < |y | < 2.1, 2.1 < |y | < 2.8. The individual uncorrected jet spectra werecorrected using the iterative Bayesian unfolding method [26, 27], which accounts for the bin migrationdue to the finite jet energy resolution and for other detector effects. The response matrices used as theinput to the unfolding were built from truth level jets that were matched to reconstructed jets in the MC.The unmatched truth jets were used as part of the prior and are incorporated as an inefficiency after theunfolding. The truth jets and reconstructed jets were selected to have pT > 40 GeV and pT > 80 GeV,respectively. The response matrices were generated separately for pp and Pb+Pb and for each rapidityand centrality interval. The responses were reweighted along the truth jet axis to better represent thedata by a data-to-MC ratio of the reconstructed jet pT spectra. The number of iterations in the unfoldingwas chosen based on the stability of the result with respect to changing the number of iterations. It wasfound that three iterations are sufficient for all the centrality and rapidity intervals both in Pb+Pb and ppcollisions.

The unfolding procedure was verified by evaluating a closure test in the MC, defined as the ratio betweenthe unfolded and original distributions, before simulating the detector response. This ratio is consistentwith unity for all the rapidity and centrality intervals in the kinematic selection considered for the analysis.The unfolding procedure was further tested by performing a ‘refolding’, where the unfolded results issmeared using the response matrix, and compared with the uncorrected simulated distributions. Thestability of the unfolding was confirmed by observing no large deviations between the uncorrected andrefolded distributions.

6

5 Systematic uncertainties

The following sources of systematic uncertainties were identified for this analysis: jet energy scale, jetenergy resolution, uncertainty due to the unfolding procedure, uncertainty on the determination of the〈TAA〉 values and the uncertainty on the pp luminosity.

Uncertainties on the RAA can be categorized into two classes: uncertainties that are common for the nu-merator and denominator of the RAA and uncertainties that differ between the numerator and denominatorof the RAA. For uncertainties common to both the numerator and denominator of the RAA, the uncertain-ties were taken as correlated. For uncorrelated uncertainties, the uncertainty was added in quadrature.

The strategy for determining the JES and its uncertainty for heavy ion jets is discussed in detail inRef. [28]. The JES for heavy ion (HI) jets was established by calibrating them with respect to stand-ard “EM+JES” jets used in pp collisions [29]. This calibration was performed using 13 TeV pp datawhere the jet energy scale and its uncertainty had previously been evaluated [30, 31]. This JES was thenadapted to 5.02 TeV. Several components of the JES uncertainty were considered. The main componentwas described by a set of eleven independent nuisance parameters which include effects from uncer-tainties derived through in-situ calibration and which were derived for EM+JES jets [29]. The flavorcomponents include the uncertainty in flavor composition of jets and the uncertainty due the sensitivityof the response to jets of different flavors. The components were derived using the same procedure as forthe EM+JES jets, but the flavor fractions were evaluated at 5.02 TeV center-of-mass energy. The cross-calibration serves as an alternative derivation of the in-situ calibration of HI jets while also accounting forany residual differences arising from the difference in the reconstruction procedures themselves. Thesecomponents of the JES uncertainty were taken to be correlated between Pb+Pb and pp collisions.

The quenching component of the JES reflects a modification of parton showers by the Pb+Pb environ-ment. The resulting jets may have different flavor compositions, or more generally different particlecontent. The impact of the quenching on the measured jet pT was evaluated by defining the pT ratio ofcalorimeter jets and corresponding track jets, which match the calorimeter jets within ∆R < 0.4. This ra-tio is called rtrk . The data-to-MC ratio of rtrk was evaluated and then compared between pp and Pb+Pb,which showed a small shift. This shift represents the typical difference in the JES in Pb+Pb collisionsand pp collisions. It is 0.5% in the most central and linearly decreases to 0% at 60% centrality. Thisdifference constitutes a heavy ion specific component of the JES uncertainty.

For each component of the JES uncertainty a new response matrix was obtained by shifting the recon-structed jet pT. These response matrices were then used to unfold the data. A difference between thedata unfolded with the new response matrix and the original response matrix was used to determine thesystematic uncertainty.

Similarly to the JES uncertainty, the systematic uncertainty due to the jet energy resolution was alsoobtained by performing the unfolding with modified response matrices. The modified response matriceswere generated for both pp and Pb+Pb collisions with the JER uncertainty which was quantified in ppcollisions using data-driven techniques [30]. An additional uncertainty which is specific for the heavy ionenvironment was used, which is the uncertainty related to the impact of fluctuations in the UE to the JER.Both these components were used to smear the reconstructed jet momentum in the MC and regeneratethe response matrices.

The results presented in this note were obtained using the unfolding procedure with response matriceswhich were reweighted to better characterize the data as described in Section 4. To assess the sensitivity

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Figure 3: Typical systematic uncertainties, for pp jet cross-section (left), Pb+Pb jet yields (middle) and jet RAA(right).

to the input MC distributions the unfolding was also run without the reweighting. The difference betweenthe nominal results and results obtained with response matrices without the reweighting was used tocalculate the uncertainty due to the unfolding procedure. The response without reweighting, which has asmooth simple slope with respect to the data, provides a reasonable variation of how the prior could bedifferent.

The uncertainty on the 〈TAA〉 arises from geometric modeling uncertainties (e.g. nucleon-nucleon inelasticcross-section, Woods-Saxon parameterization of the nucleon positions) and the uncertainty on the fractionof selected inelastic Pb+Pb collisions (the “efficiency” uncertainty). The values of these uncertaintiesalong with 〈TAA〉 are tabulated in Tab. 1.

The integrated luminosity determined for 2015 pp data was calibrated based on data from dedicatedbeam-separation scans, known as Van der Meer scans. Determination of systematic uncertainty followedthe procedure described in Ref. [32], leading to a relative uncertainty of 5.4%.

Typical uncertainties are summarized in Figure 3 for pp jet cross-section on the left, Pb+Pb jet yieldsin the middle and the RAA on the right. In the pp cross-section the largest uncertainty is from the JESbetween 7-10% depending on the pT of the jet. The JES is also the largest contribution to the uncertaintyin central Pb+Pb where it is also between 7-10%. The uncertainties on the RAA are smaller than those onthe cross-sections and yields because the correlated systematics uncertainties that are common for pp andPb+Pb mostly cancel in the ratio. The largest contribution to the uncertainty on the RAA is the heavy ioncomponent of the JES uncertainty which is approximately 3%.

6 Results

The inclusive jet cross-section obtained from pp collision data is shown in the left panel of Figure 4.The cross-section is reported for six intervals of rapidity. The error bars in the figure represent statisticaluncertainties while the shaded boxes represent systematic uncertainties. Systematic uncertainties alsoinclude the uncertainty due to the luminosity which is correlated for all the data points.

The right panel of Figure 4 shows the Pb+Pb jet yields scaled by 〈TAA〉, which are shown for four selectedcentrality intervals for jets with |y | < 2.8. For a direct comparison with the jet production in pp collisions,

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Figure 4: Left: Inclusive jet cross-section in pp data evaluated as a function of jet pT scaled by successive powers of102. Right: Per event jet yield in Pb+Pb collisions, multiplied by 〈TAA〉, as a function of jet pT scaled by successivepowers of 102. The solid lines represent the pp cross-section for the same rapidity selection scaled by the samefactor to allow for a comparison with the Pb+Pb data at different centralities. The error bars represent statisticaluncertainties, shaded boxes represent systematic uncertainties including uncertainties on 〈TAA〉 and luminosity.

included in the figure are the values of pp cross-section which are shown without systematic uncertaintiesfor readability.

The nuclear modification factor evaluated as a function of jet pT is shown in the upper panel of Figure 5for four centrality selections. The RAA is evaluated for jets with pT in the interval of 100–1000 GeV and|y | < 2.8. The higher pT intervals are combined in the cross section and yields before evaluating theRAA because of the large statistical uncertainties at high pT. A clear suppression of the jet productionin central Pb+Pb collisions with respect to pp collisions can be seen. In 0-10% central collisions theRAA is approximately 0.45 near pT = 100 GeV. The RAA is observed to grow slowly with increasingjet momentum reaching a value of approximately 0.6 for jets with pT around 800 GeV. The error barsin the figure represent the statistical uncertainties. Shaded boxes represent fully correlated systematicuncertainties for which all the data-points can move upward or downward for a given change in theuncertainty. The open boxes represent uncorrelated systematic uncertainties for which individual datapoints can vary independently.

The RAA evaluated for jets with |y | < 2.1 can be compared with a previous measurement performed at√

sNN = 2.76 TeV [9]. This is shown for the centrality selection of 0-10% in the bottom panel of Figure 5.The two measurements are observed to agree within their uncertainties.

The 〈Npart〉 dependence of the RAA is shown in Figure 6 for jets with |y | < 2.8 and 100 < pT < 125 GeV.A smooth evolution of the RAA is seen with the largest values in the most peripheral collision and thesmallest values in the most central collisions. The error band here represents the correlated systematic

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Figure 5: Upper panel: The RAA as a function of jet pT for jets with |y | < 2.8 for three centrality bins. Bottompanel: The RAA as a function of jet pT for jets with |y | < 2.1 in 0-10% central collisions compared to the samequantity measured in

√sNN = 2.76 Pb+Pb collisions published in Ref. [9]. The error bars represent statistical

uncertainties, the shaded boxes around the data points represent correlated systematic uncertainties, open boxesrepresent uncorrelated systematic uncertainties. In the upper panel, the colored shaded boxes at unity represent〈TAA〉 uncertainties and the gray shaded box represents the uncertainty on pp luminosity. The horizontal width onthe shaded boxes represent the width of the pT interval and the horizontal width on the open boxes are arbitrary forbetter visibility. In the bottom panel, the colored shaded boxes at unity represent the combined 〈TAA〉 uncertaintieswith the uncertainties on pp luminosity. error bars on the

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Figure 6: The RAA for jets with pT = 100−125 GeV and |y | < 2.8 evaluated as a function of 〈Npart〉. The black andred open boxes represent correlated and uncorrelated systematic uncertainties, respectively. The horizontal size oferror boxes represents the uncertainty in the determination of 〈Npart〉. The statistical uncertainties are smaller thanthe data-points. Gray shaded box represents the uncertainty on pp luminosity.

uncertainties which include also the uncertainty on 〈TAA〉. The open boxes represent uncorrelated sys-tematic uncertainties. The statistical uncertainties are smaller than the data points for all RAA values.

The rapidity dependence of the RAA is shown in Figure 7 by evaluating the ratio of the RAA as a functionof rapidity to the RAA at |y | < 0.3. This representation was chosen because all systematic uncertaintieslargely cancel in the ratio. This is shown in intervals of increasing values of pT in the four panels. Therapidity dependence is shown to be flat with rapidity at lower pT. As the pT is increased the RAA starts todecrease with rapidity and this decrease is the most significant in the highest pT interval. This is the firsttime a significant rapidity dependence has been observed.

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|y|

|<0.

3)y(|

AA

R|)

/y(|

AA

R

0.6

0.8

1

1.2 < 200 GeV

Tp158 <

|y|

0.6

0.8

1

1.2 < 251 GeV

Tp200 <

|y|

0.6

0.8

1

1.2 < 316 GeV

Tp251 <

|y|0 0.5 1 1.5 2 2.5

0.6

0.8

1

1.2 < 562 GeV

Tp316 <

PreliminaryATLAS = 5.02 TeVNNs = 0.4 jets, R tkanti-

0 - 10 %

-12015 Pb+Pb data, 0.49 nb-1 data, 25 pbpp2015

Figure 7: The ratio of the RAA as a function of |y | to the RAA at |y | < 0.3 for jets with centrality of 0-10% in thefollowing pT bins on each panel: 158 < pT < 200 GeV (red squares), 200 < pT < 251 GeV (blue diamonds), 251< pT < 316 GeV (green crosses), and 316 < pT < 562 GeV (purple stars). The error bars represent statisticaluncertainties, the shaded boxes around the data points represent correlated systematic uncertainties.

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7 Summary

In this note measurements of jet cross-sections in pp collisions, inclusive jet yields in Pb+Pb collisionsand the jet RAA are presented. The measurements were performed using pp and Pb+Pb collisions at thenucleon-nucleon (pp) center-of-mass energy of

√sNN = 5.02 TeV collected by the ATLAS detector at

the LHC. Jets reconstructed using the anti-kt algorithm, with R = 0.4, were measured over the transversemomenta range of 100–1000 GeV in six rapidity intervals with |y | < 2.8. The jet yields measured in heavyion collisions are scaled by the nuclear thickness function and are observed to be suppressed with respectto the jet cross-section measured in pp collisions. The magnitude of the RAA monotonically decreasesmoving from peripheral to central collisions. The RAA is flat with rapidity at low pT and then decreaseswith rapidity at high pT. The rapidity dependence at high pT is observed for the first time as a consequenceof the availability of a high-statistics data set while the previous measurement was statistically limited athigh pT and forward rapidity. The magnitude of the suppression as well as its evolution with jet pT andrapidity are consistent with those reported in the similar measurement performed in Pb+Pb collisions at√

sNN = 2.76 TeV in the kinematic region where the two measurements overlap.

The new results presented in this note extend the previous measurements to significantly higher transversemomenta and larger rapidities of jets as well as improve on the precision of the measurement. This allowsfor precise and detailed comparisons of the data to theoretical models of the jet quenching. The newresults can also be used as additional needed input to understand the center-of-mass dependence of jetsuppression.

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