Measurements of hard probes of the quark-gluon plasma with the ATLAS experiment at the LHC

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Nuclear Physics B Proceedings Supplement 00 (2014) 1–7

Supplement

Measurements of hard probes of the quark-gluon plasma with the ATLAS experiment at the LHC

Barbara Wosiek for the ATLAS Collaboration¹

H. Niewodnicza´nski Institute of Nuclear Physics, Krak´ow, Poland

Abstract

ATLAS results on the production of high-transverse momentum probes in Pb+Pb andp+Pb collisions at the LHC are presented. The focus is on the jet measurements, which provide a useful tool to study the hot, dense and coloured matter created in ultra-relativistic heavy ion collisions. The ATLAS experiment has measured inclusive jet yields inpp and Pb+Pb collisions at √

s_NN =2.76 TeV and in p+Pb collisions at √

s_NN =5.02 TeV. The jet nuclear modification factor,RAA, is shown as a function of jet pT, rapidity and collision centrality. TheRAAweakly increases with pT, shows no dependence on rapidity and smoothly decreases with the collision centrality. In 10% of the most central collisions, jet production is suppressed by a factor of two relative toppyields scaled by the number of binary nucleon- nucleon collisions. Charged-particle fragmentation functions of jets are also measured and ratios of fragmentation functions between different centrality intervals show a centrality-dependent modification. The jet measurements in p+Pb collisions show jet enhancement in peripheral collisions and suppression in central collisions as compared to the scaledppjet yields. These modifications are most pronounced at forward rapidities and at large jet transverse momenta. Jet production at forward rapidities shows a scaling in the total jet energy, suggesting that the modification of jet production inp+Pb collisions may depend on the kinematics of the initial hard parton-parton scattering.

Keywords: heavy-ion collisions, quark-gluon plasma, jet quenching

1. Introduction

In ultra-relativistic heavy ion collisions, a medium of the dense matter, composed of deconfined quarks and gluons, is produced. This medium is transparent to colour-neutral probes, such as electroweak bosons, but is opaque to coloured probes, such as high transverse momentum partons produced in hard scattering processes. Partons, propagating through the medium, lose energy either via the gluon radiation or by collid- ing with medium constituents. The resulting scattered partons fragment into streams of angularly correlated large–momentum particles, referred to as jets. The parton energy loss in the dense medium results in the suppression of jet yields in nuclear collisions as compared

1The list of members of the ATLAS Collaboration and acknowl- edgements can be found at the end of this issue.

to the jet spectrum measured in ppcollisions, the phenomenon known as the jet quenching. It also may lead to modifications of the jet fragmentation with respect to the fragmentation in the vacuum fragmentation.

A key observable in studying the medium modification of the high–pT probe, such as a jet, is the nuclear modification factor,RAA, defined as the ratio of the jet yield in Pb+Pb collisions to the yield measured in pp as:

R_AA=

1 hT_AAi

d²N_jet dp_Tdy|_cent

d²σ^pp_jet dpTdy

=

1 hN_colli

d²N_jet dp_Tdy|_cent

d²N_jet^pp dpTdy

, (1)

where the numerator denotes the per-event jet yield in a given centrality interval scaled by the average flux of nucleons for that centrality bin, hTAAi, and the de- nominator is the jet cross-section measured in ppcolli-

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sions at the same energy and jet pT. The mean number of nucleon-nucleon collisions, hNcolli, is equal to hT_AAi ×σ^NN_inelcalculated with a Glauber model [1]. The jet production modification can also be studied using the central to peripheral ratio of jet yields,R_CP:

RCP= hN_coll^P i hN_coll^C i

d²N^C_jet dp_Tdy d²N^P_jet dp_Tdy

, (2)

where indexes C and P denote central and peripheral nucleus-nucleus collisions, respectively. Both,RAAand RCPmeasure deviations of jet yields from a simple hypothesis of incoherent superposition of binary nucleon- nucleon collisions, commonly referred to as factoriza- tion or geometric scaling, under which R_AA = 1 and R_CP=1.

The geometric scaling hypothesis can be tested with measurements of probes insensitive to medium proper- ties. Such a test provide electroweak bosons decaying into colour neutral particles. Indeed, the yields of pho- tons as well asW andZ bosons decaying leptonically and measured in Pb+Pb collisions at the LHC scale, as expected, with the number of binary nucleon-nucleon collisions [2, 3, 4]. This confirms that the geometric enhancement in the yield of the high-pT probe in more central collisions, which is due to the larger overlap, is correctly accounted for by scaling the per-event yield in a given centrality interval by the average per-collision flux of nucleons,hTAAi, or equivalently, the mean number of nucleon-nucleon collisions,hNcolli.

In these proceedings we report on the recent AT- LAS measurement of inclusive jet production in Pb+Pb, p+Pb, and pp collisions. The ATLAS detector [5]

is well designed to measure jets in the dense heavy- ion environment thanks to its high-resolution calorimeters covering the pseudorapidity interval|η| < 5. The data sets used in this analysis were recorded during the 2011 LHC Pb+Pb run at a collision energy of

√s_NN = 2.76 TeV, the 2013 proton-proton run at the same centre-of-mass energy and the 2013 p+Pb run at

√s_NN = 5.02 TeV. The integrated luminosities of the analyzed data samples are of approximately 0.14 nb⁻¹, 4 pb⁻¹and 28 nb⁻¹for Pb+Pb,ppandp+Pb collisions, respectively. For data-taking, ATLAS used either a min- imum bias or the jet trigger [6], providing high statistics samples of jets.

2. Inclusive jet spectra

The jets were reconstructed using the electromagnetic and hadronic calorimeters [7] with the anti-ktalgorithm

[8] with jet radius parameter R = 0.4. The contribu- tion of the underlying event to each jet was subtracted on a jet-by-jet basis. The jet spectra were unfolded to account for the detector response and corrected for the trigger and reconstruction efficiency. PYTHIA dijets events were overlaid onto real data to evaluate the detector response and perform the unfolding.

The unfolded and efficiency corrected double differential cross-sections measured inppcollisions at √

s= 2.76 TeV are shown in Fig. 1 as a function of jet p_T in five different ranges in jet rapidity [9]. Theppjet cross- sections are used to calculate nuclear modification factors for jets measured in Pb+Pb and p+Pb collisions.

[GeV]

pT

[ nb/GeV ] yd Tpdσ2d

10-5 10-4 10-3 10-2 10-1 1 10 102 103 104 105 106 107 108 109 1010 1011

40 60 100 200 400

8 )

× 10

| < 2.1 ( y

|

6 )

× 10

| < 0.3 ( y

|

4 )

× 10 | < 0.8 ( y

≤ | 0.3

2 )

× 10 | < 1.2 ( y

≤ | 0.8

0 )

× 10 | < 2.1 ( y

≤ | 1.2

Preliminary ATLAS

= 2.76 TeV s

=0.4,

tR

k anti-

= 4.0 pb-1 Lint data, pp 2013

Figure 1: The double differential jet cross-section inppcollisions as a function of pTfor different rapidity ranges [9]. Error bars and shaded bands denote statistical and systematic uncertainties respectively. Each spectrum is scaled by an overall multiplicative factor for presentation purposes.

For Pb+Pb collisions the jet spectra are measured over the kinematic range 32<pT <500 GeV,|y|<2.1 and as a function of the collision centrality [9]. The centrality of Pb+Pb collisions is characterized byΣE^FCal_T , the total transverse energy measured in the ATLAS forward calorimeters covering 3.2 <|η| <4.9. The standard technique [10] is used to define centrality classes in terms of percentiles of the ΣE_T^FCal distribution. A Glauber Monte Carlo model [1] is employed to calcu- latehTAAi,hNcolliand the average number of participat- ing nucleons,hNparti, for each centrality class. Figure 2 shows the jet yields scaled byhTAAiin different central-

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[GeV]

pT

[ nb/GeV ] yd Tpd

jetN2d 〉 AAT 〈evtN1

10-10

10-8

10-6

10-4

10-2

1 102

104

106

108

1010

1012

1014

1016

40 60 100 200 400

8 )

× 10 0 - 10 % (

6 )

× 10 10 - 20 % (

4 )

× 10 20 - 30 % (

2 )

× 10 30 - 40 % (

0 )

× 10 40 - 50 % (

-2 )

× 10 50 - 60 % (

-4 )

× 10 60 - 70 % (

-6 )

× 10 70 - 80 % (

Preliminary ATLAS

= 2.76 TeV sNN = 0.4 jets,

tR

k anti-

= 0.14 nb-1 Lint 2011 Pb+Pb data,

| < 2.1 y

|

Figure 2: The jet yield in Pb+Pb collisions, scaled byhT_AAias a function ofp_Tfor different centrality bins [9]. Error bars and shaded bands denote statistical and systematic uncertainties respectively. The dashed lines represent theppjet cross-section scaled by an overall multiplicative factor, the same as used for Pb+Pb jet yields.

ity classes measured over the range|η|<2.1. For comparison, theppjet cross-section measured in the same rapidity interval is also shown. It can be seen that the scaled jet yields in Pb+Pb collisions are suppressed as compared to theppreference.

Forp+Pb collisions the jets are measured over a wide range inpT: 25–1000 GeV and over more than 6 units in the centre-of-mass rapidity:−2.1 <y^∗<4.4 [11]. The jet rapidity,y^∗is defined in the nucleon-nucleon centre- of-mass frame, withy^∗>0 in the direction of the proton beam. The total transverse energy measured in forward calorimeters on the Pb-going side (−4.9 < η < −3.2), ΣE_T^Pb, is used to characterize the centrality ofp+Pb collision events. TheΣE_T^Pbdistribution is divided into centrality intervals, each representing a percentile fraction of all events. For each centrality interval, a standard Glauber model is used to estimatehT_pPbi, hN_colli and hNparti. Figure 3 shows the inclusive jet yields, corrected for all detector effects, for the full accessible range of the collision centrality, 0–90% and different ranges in

[GeV]

pT

]-1 *) [GeVyd TpN/d2)(d evt(1/N

10-12

10-10

10-8

10-6

10-4

10-2

1 102

104

106

108

×1 +3.6 < y* < +4.4,

101

× +2.8 < y* < +3.6,

102

× +2.1 < y* < +2.8,

103

× +1.2 < y* < +2.1,

104

× +0.8 < y* < +1.2,

105

× +0.3 < y* < +0.8,

106

× -0.3 < y* < +0.3,

107

× -0.8 < y* < -0.3,

108

× -1.2 < y* < -0.8,

109

× -2.1 < y* < -1.2,

= 5.02 TeV sNN

+Pb p

=0.4 R

t, k anti-

∫

Ldt = 27.8 nb^-1 0-90%

ATLAS

Preliminary

20 100 1000

Figure 3: The jet yield inp+Pb collisions as a function ofpTfor different intervals iny^∗ and averaged over the centrality range 0–90%

[11]. Error bars and boxes denote statistical and systematic uncertainties respectively.

y^∗. The wide coverage in the jet kinematics is seen.

3. Jet quenching in Pb+Pb collisions

Previous observations [12, 13] of the energy imbal- ance of dijets produced in central Pb+Pb collisions can be attributed to the jet quenching phenomenon. In addi- tion, the measuredRCPratios indicate also that jet yields are suppressed in central relative to peripheral collisions [7]. The comparison of jet yields in Pb+Pb collisions to ppcollisions using the jetRAA (see Eq. 1) can provide better understanding of the jet quenching effect. Fig- ure 4 shows the transverse momentum dependence of the jet RAA measured in three representative centrality bins and three ranges in rapidity [9]. The jet yields are suppressed by a factor of two in the most central 10% of collisions. In central collisions, a modest but significant increase ofRAA with pT is observed. No dependence ofRAAon rapidity is seen, as shown in the top panel of

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AAR

0 0.5 1

| < 2.1 y

| Preliminary

ATLAS anti-k_tR = 0.4 jets = 2.76 TeV sNN

= 0.14 nb-1 Lint 2011 Pb+Pb data,

AAR

0 0.5 1

| < 0.8 y 0.3 < |

0 - 10 % 30 - 40 % 60 - 80 %

[GeV]

pT

AAR

0 0.5 1

40 60 100 200 400

| < 2.1 y 1.2 < |

0 - 10 % 30 - 40 % 60 - 80 %

Figure 4: The jetRAAas a function ofpTfor three different centrality bins and three intervals iny[9]. Error bars and boxes denote statistical and partially correlated inp_T systematic uncertainties respectively.

The uncertainties fully correlated inp_Tare indicated by shaded bands.

The uncertainties on the luminosity andhT_AAiare indicated by shaded boxes centered at one.

| y

|

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

AAR

0 0.5 1

0 - 10 % 30 - 40 % 60 - 80 %

< 100 GeV pT

80 < ATLAS Preliminary

part〉

〈N

0 50 100 150 200 250 300 350 400

AAR

0 0.5 1

< 100 GeV pT

80 < | y | < 2.1

= 0.4 jets

tR

k anti-

= 2.76 TeV sNN = 0.14 nb-1

Lint 2011 Pb+Pb data,

Figure 5: The jetR_AAas a function of rapidity for three different centrality bins in the selected range inpT(top) and as a function ofhN_parti measured over|y|<2.1 (bottom) [11]. Error bars denote statistical uncertainties while fully correlated and partially correlated systematic uncertainties are indicated in the top plot by boxes and shaded bands respectively. In the bottom plot the black lines indicate the point-to- point correlatedhT_AAiuncertainties.

Fig. 5. The bottom panel of Fig. 5 shows thatRAAsys- tematically decreases with centrality, characterised by hNparti, reaching a minimal value of 0.4 in 1% of the

z

10-1 1

D(z)

10-2

10-1

1 10 102

103

104

26

× 0-10%

25

× 10-20%

24

× 20-30%

23

× 30-40%

22

× 40-50%

21

× 50-60%

60-80%

Systematic Uncertainty

ATLAS

=2.76 TeV sNN

Pb+Pb

0.14 nb-1

R=0.4 anti-kT

> 100 GeV

jet

pT

Figure 6: Unfolded fragmentation function,D(z), measured in different centrality classes [14]. The statistical uncertainties are smaller than the points, the systematic uncertainties are shown as shaded boxes. The lines connecting the central values of the measurements are shown to guide the eye.

most central collisions.

4. Fragmentation of the quenched jets

The study of the internal structure of jets produced in ultra-relativistic heavy ion collisions provides addi- tional information on the modification of parton show- ers in the created hot and dense medium. Here we report on the measurements of distributions of z ≡

~p^ch_T ·~p^jet_T /|~p^jet_T|², the longitudinal momentum fraction of charged jet constituents [14]. Figure 6 shows the unfolded distribution of the longitudinal momentum fraction, D(z), measured in different centrality classes in Pb+Pb collisions at √

s_NN = 2.76 TeV. A steep fall at z →1, characteristic of the jet fragmentation function, can be noted. To investigate the centrality dependence of the fragmentation function, the ratioRD(z)of theD(z) distribution measured in different centrality classes to the same distribution measured in the peripheral cen-

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trality class, 60–80%, is investigated. Figure 7 shows RD(z) in six 10% centrality bins spanning the range 0–

60%. For all collision centralities, an enhancement in the yield of low–zfragments and depletion at intermediatezvalues is observed. These modifications are the strongest in 10% of the most central collisions and are gradually reduced with decreasing centrality. At largez values, close to unity, the large statistical and systematic uncertainties preclude a definite statement. However, R(D(z)|_z≈1 exceeds one with a significance of about 1σ, in contrast to predictions of strong reduction in the yield of large-zfragments from radiative energy loss models [15, 16].

[GeV]

pT

10 10²

)TD(pR

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

0-10%/60-80%

>100 GeV jet pT

R=0.4 anti-kT

ATLAS

=2.76 TeV sNN Pb+Pb 0.14 nb-1

[GeV]

pT

10 10²

)TD(pR

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

30-40%/60-80%

ATLAS

[GeV]

pT

10 10²

)TD(pR

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

Data Systematic Uncertainty 10-20%/60-80%

ATLAS

[GeV]

pT

10 10²

)TD(pR

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

40-50%/60-80%

ATLAS

[GeV]

pT

10 10²

)TD(pR

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

20-30%/60-80%

ATLAS

[GeV]

pT

10 10²

)TD(pR

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

50-60%/60-80%

ATLAS

Figure 7: Ratios of theD(z) distributions measured in six centrality intervals to the distribution measured in peripheral 60–80% centrality class [14]. Statistical and systematic uncertainties are shown by error bars and shaded boxes respectively.

5. Jet production modification inp+Pb collisions Inp+Pb collisions no significant jet quenching is expected owing to the absence of the hot and dense par- tonic medium. However, studies of the nucelar modification factor in p+Pb collisions are important to estab- lish a baseline reference for the interpretation of Pb+Pb results. Theppdata at the same energy as that ofp+Pb collisions, √

s=5.02 TeV, are not available. Therefore, the referenceppjet production cross-section at this energy is constructed based on the interpolation between the 2.76 TeV and 7 TeV pp cross-sections using the xT-scaling [17, 11]. Due to the large uncertainties in the energy scale andxTinterpolation for theppdata at large rapidities, the reference 5.02 TeV ppspectra are derived only for−2.1 <y^∗ <2.8 [11]. In this rapidity range, the jetRpPbcan be calculated. Figure 8 shows the transverse momentum dependence of the nuclear modification factor in eight rapidity bins for jets produced

in 0–90%p+Pb collisions. In the covered rapidity bins, the inclusive jetRpPbis consistent with unity, within the systematic uncertainties of the measurement, or shows a marginal enhancement of the order of 10%. The data are compared in Fig. 8 with the NLO pQCD calculation of RpPbobtained using the EPS09 parameterisation of nuclear parton distribution functions [18]. The measurements are systematically slightly higher than the calculations, although, within the systematic uncertainties, the general consistency of data and calculations can be concluded. It follows from the above that the p+Pb jet production, averaged over all accessible collision centralities, shows no significant modification relative topp data.

[GeV]

T p pPb R +2.1 < y* < +2.8

ATLAS Preliminary

[GeV]

T p

pPbR

+0.8 < y* < +1.2

[GeV]

pT

pPbR

-0.3 < y* < +0.3

[GeV]

pT

pPbR

-1.2 < y* < -0.8

= 5.02 TeV sNN

+Pb p

=0.4 R

t, k anti-

[GeV]

T p pPbR +1.2 < y* < +2.1

+Pb, 0-90%

p

EPS09 calculation

[GeV]

T p

pPbR

+0.3 < y* < +0.8

[GeV]

pT

pPbR

-0.8 < y* < -0.3

[GeV]

pT

pPbR

-2.1 < y* < -1.2

∫

L^pPbdt = 27.8 nb^-1

∫

L^ppdt = 4.0 pb^-1 1.6

1.0 0.4

1.6

1.0 0.4

1.6

1.0

0.4

1.6 1.0

0.4

pPbR

40 100 1000 40 100 1000

[GeV]

pT

Figure 8: Transverse momentum dependence of the jetR_pPbmeasured in different rapidity intervals in √

s_NN =5.02 TeVp+Pb collisions averaged over all centralities [11]. Statistical and systematic uncertainties are shown by error bars and boxes respectively. Shaded bands represent the NLO pQCD calculations using the modified parton distribution functions [18].

Centrality dependence of the jet production inp+Pb collisions is studied using RpPb and RCP evaluated in different classes of the collision centrality, which is de-

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fined with the help ofΣE^Pb_T . Figure 9 shows theRpPbin central (0–10%), mid-central (20–30%) and peripheral (60–90%) collisions in all accessible rapidity bins. Jet yields are suppressed in central collisions and enhanced in peripheral collisions as compared toppdata. The observation of the near-idealNcollscaling of theRpPb averaged over all collision centralities (see Fig. 8) results from this variation ofRpPb with centrality. The deviations from the geometric scaling (i.e. expected scaling withNcoll) increase withpTand are dominant at forward rapidities. Aty^∗<0 and lowpT, the geometric scaling is restored.

[GeV]

T p pPb R +2.1 < y* < +2.8

[GeV]

T p pPb R +0.8 < y* < +1.2

[GeV]

pT pPb R -0.3 < y* < +0.3

ATLAS

Preliminary

[GeV]

pT pPb R -1.2 < y* < -0.8

= 5.02 TeV sNN

+Pb p

=0.4 R

t, k anti-

[GeV]

T p pPb R +1.2 < y* < +2.1

0-10%

20-30%

60-90%

[GeV]

T p pPb R +0.3 < y* < +0.8

[GeV]

pT pPb R -0.8 < y* < -0.3

[GeV]

pT pPb R -2.1 < y* < -1.2

∫

L^pPbdt = 27.8 nb^-1

∫

L^ppdt = 4.0 pb^-1 1.6

1.0 0.4

1.6

1.0 0.4

1.6

1.0

0.4

1.6 1.0

0.4

pPbR

40 100 1000 40 100 1000

[GeV]

pT

Figure 9: Transverse momentum dependence of the jetR_pPb measured in different rapidity intervals and in three centrality classes in

√s_NN =5.02 TeVp+Pb collisions [11]. Statistical and systematic uncertainties are shown by error bars and boxes respectively. The uncertainties on theppluminosity andhT_pPbiare indicated by shaded boxes centered at one.

The central-to-peripheral ratio,R_CP, is evaluated in different centrality intervals using the 60–90% bin as a reference. The strong modifications ofRCPare observed and are consistent with those shown by theRpPb.

A gradually increasing suppression of jets withy^∗in

the forward rapidity hemisphere may result from the jet kinematics. To test this hypothesis, the RpPb in each forward rapidity bin is replotted using the quantity p_T×cosh (hy^∗i), wherehy^∗iis the centre of the rapidity bin. This quantity approximates the total energy of the jet. Figure 10 shows the RpPb as a function of pT×cosh (hy^∗i) for 0–10% of the most central collisions (left plot) and peripheral 60–90% of collisions (right plot) measured in four bins of forward rapidities. The approximate rapidity-independent trends can be seen, although in central collisions the RpPb decreases with pT×cosh (hy^∗i) while an increase is observed in peripheral collisions.

∫Ldt = 27.8 nb^-1 = 5.02 TeV sNN +Pb p

0-10%

=0.4 R t, k anti-

ATLASPreliminary

+2.1 < y* < +2.8 +1.2 < y* < +2.1

+0.8 < y* < +1.2 +0.3 < y* < +0.8 60-90%

1.6

1.0

0.4

pPbR

40 100 1000 40 100 1000

cosh(<y*>) [GeV]

T× p

Figure 10: JetRpPbas a function ofpT×cosh (hy^∗i) measured in four forward rapidity intervals in 10% of the most central collisions (left) and in the 60–90% peripheral centrality class [11]. Statistical and systematic uncertainties are shown by error bars and boxes respectively.

The uncertainties on the ppluminosity andhT_pPbiare indicated by shaded boxes centered at one.

∫Ldt = 27.8 nb^-1 = 5.02 TeV sNN +Pb p

0-10%

=0.4 R t, k anti- ATLAS Preliminary

+3.6 < y* < +4.4 +2.8 < y* < +3.6 +2.1 < y* < +2.8 +1.2 < y* < +2.1 +0.8 < y* < +1.2

+0.3 < y* < +0.8 -0.3 < y* < +0.3 -0.8 < y* < -0.3 -1.2 < y* < -0.8 -2.1 < y* < -1.2 1.4

1.0

0.4

CPR

40 100 1000 40 100 1000

cosh(<y*>) [GeV]

T× p

Figure 11: JetR_CPas a function ofp_T×cosh (hy^∗i) measured in different rapidity intervals and in 10% of the most central collisions with the 60–90% centrality class used as a reference [11]. Statistical and systematic uncertainties are shown by error bars and boxes respectively. The uncertainties on ratios ofhN_colliare indicated by shaded boxes centered at one.

TheRCPfor 0–10% of the most central collisions is plotted against pT×cosh (hy^∗i) for all studied rapidity ranges in Fig. 11. Interestingly, at forward and mid- rapidities (y^∗>−0.3) theR_CPmeasured in different rapidity bins follows the same, almost linear in logarithm of pT×cosh (hy^∗i), dependence on the jet total energy.

This single trend exhibited byRCPis a consequence of

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opposite trends seen inRpPbmeasured in central and peripheral collisions. At backward rapidities (y^∗ <−0.3) this trend is not observed. For other collision centralities, the same trends are observed with a centrality- dependent slope inR_CPversus lnp_T×cosh (hy^∗i).

6. Summary and discussion

These proceedings present the ATLAS measurements of the inclusive jet production in the LHC √

s_NN = 2.76 TeV Pb+Pb and √

s_NN = 5.02 TeV p+Pb collisions. Unfolded, double differential jet spectra are measured over a broad kinematic range in the jet transverse momentum and rapidity. Centrality dependence of jet yields is studied using the reference ppjet cross- sections and the central-to-peripheral ratio of the jet yields. The modifications of the jet yields with respect to the geometric scaling with the number of binary nucleon-nucleon collisions are observed.

The jet nuclear modification factor, R_AA, measured in Pb+Pb collisions shows suppression of jet yields as compared to the ppreference data. This suppression monotonically increases with increasing centrality and shows a weak dependence on the jetpT, consistent with the theoretical prediction from Ref. [19]. TheRAA is found to be independent of jet rapidity, over the range

|y| <2.1, for all collision centralities and jet transverse momenta. This may indicate that the two effects, the steeper pT spectrum at forward rapidities and poten- tially smaller energy loss of quark jets, dominating the forwardyregion, compensate each other.

The measurements of charged-particle fragmentation functions of the quenched jets in Pb+Pb collisions show medium-dependent modifications, which smoothly evolve with the collision centrality. The central-to-peripheral ratio of distributions of the longitudinal momentum fraction shows an enhancement in the yield of low–zfragments, suppression of fragments with the intermediatezvalues and a weak enhancement of large–z fragments. The latter observation, which seems to contradict the theoretical expectations, may be partially attributed to the fact, that in this analysis the quenched jets are used as a reference while model predictions refer to the suppression of large–z fragments with respect to the vacuum-produced jets.

Forp+Pb collisions, the measured jet yields are consistent with the expected geometric scaling, but only for collisions averaged over all centralities. When studied as a function of the collision centrality, the jet yields are strongly modified, showing an enhancement in peripheral collisions and suppression in central collisions.

These modifications are amplified at high transverse

momenta and in the forward rapidity regions. In ad- dition, at forward rapidities, the jet nuclear modification factors are found to scale with the total jet energy. These observations may imply either a large change in the im- pact parameter-dependent number of the hard-scattered partons or a correlation between the kinematics of the hard scattering process and the soft interactions which are the basis for the centrality determination, or the combination of both.

The presented results provide valuable constrains on the theoretical description of jet quenching in ultra- relativistic nuclear collisions.

This work was in part supported in Poland by the Na- tional Science Centre grants DEC-2011/03/B/ST2/0261 and DEC-2013/08/M/ST2/00320, and by PL-Grid In- frastructure.

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