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Physics
Letters
B
www.elsevier.com/locate/physletb
Measurement
of
Z
0
-boson
production
at
large
rapidities
in
Pb–Pb
collisions
at
√
s
NN
=
5
.
02 TeV
.
ALICE
Collaboration
a
r
t
i
c
l
e
i
n
f
o
a
b
s
t
r
a
c
t
Articlehistory:
Received7December2017
Receivedinrevisedform21February2018 Accepted2March2018
Availableonline6March2018 Editor: L.Rolandi
The productionof Z0 bosonsat large rapidities in Pb–Pb collisions at√sNN=5.02 TeV is reported.
Z0candidatesarereconstructedinthedimuondecaychannel(Z0→
μ
+μ
−),basedonmuonsselectedwith pseudo-rapidity −4.0<
η
<−2.5 and pT>20 GeV/c. The invariant yield and the nuclearmodificationfactor,RAA,arepresentedasafunctionofrapidityandcollisioncentrality.ThevalueofRAA
for the0–20%central Pb–Pbcollisions is0.67±0.11(stat.)±0.03(syst.)±0.06(corr. syst.), exhibiting
a deviation of 2.6
σ
from unity. The results are well-described by calculations that include nuclearmodifications of the parton distribution functions, while the predictions using vacuum PDFs deviate
fromdataby2.3
σ
inthe0–90%centralityclassandby3σ
inthe0–20%centralcollisions.©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense
(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
Z0 bosons are weakly interacting probes formed early in the evolution of hadronic collisions (tf
∼
1/
M0.
01 fm/c), witha typical decay time td
∼
0.
1 fm/c. Their leptonic decays are ofparticularinterestinheavy-ioncollisions,sinceleptons donot in-teractstronglyandtheirin-mediumenergylossbybremsstrahlung isnegligible [1]. Z0-boson productionrates in hadroniccollisions are well-understood, and their measurement via leptonic decays therefore serves as a valuable medium-blind reference for hard processesinheavy-ioncollisions [2,3].
Z0-boson properties have been extensively studied at LEP (CERN),SLC(SLAC),Tevatron(FNAL)andLHC(CERN)ine+e−,pp and pp collisions [4–15]. Z0-boson production in hadronic colli-sions is well-described by perturbative Quantum Chromodynam-ics (pQCD) calculations at next-to-next-to-leading order (NNLO) [16,17], and their comparison with data provides constraints on Parton Distribution Functions (PDFs) [18,19]. In heavy-ion colli-sions,Z0-bosonproductioncan beaffectedby initial-stateeffects. Asaresultofthedifferentbalanceofthenumberofuandd va-lence quarksin protons andin leadnuclei (isospin),the yield of Z0 bosons in Pb–Pb collisions at
√
sNN=
5.
02 TeV is expectedto increase relative to that inpp collisions by 5–8% atlarge rapidi-ties,anddecreaseby3% atcentralrapidities [20].Modificationsof thePDFsinnuclei(nPDFs) [21–27] introducearapidity-dependent changeinyield,withadecreaseinyieldrelativetothatinpp col-lisionsof8–15%atlargerapidities,correspondingtotheBjorken-xE-mailaddress:alice-publications@cern.ch.
ranges x1
10−1 and x210−3, andan increase by 3% at cen-tralrapidity,correspondingtox1,2∼
10−2[20,21].Theyieldcould alsodependuponeffectssuchasmultiplescatteringand medium-inducedbremsstrahlungoftheinitialpartonsinlargenuclei [28].The ATLAS,ALICE, CMSandLHCbcollaborationshavereported measurementsofW±- andZ0-bosonproductioninp–Pbcollisions at
√
sNN=
5.
02TeV [29–33], withcomplementaryrapidity cover-age.Thesemeasurementsarewell-describedbynext-to-leading or-der (NLO)pQCDcalculations [20] andbyNNLOcalculationsusing theFullyExclusiveWandZProductioncode(FEWZ) [34],utilising both nPDFs [32] and vacuumPDFs.Theforward–backward asym-metry ofW±-bosonproduction suggeststhe presence of nuclear modification ofPDFs [31].This sensitivitytonuclear effects indi-catestheneedtoincludethesedatainthefuturenPDFfits.In Pb–Pb collisions, W±- and Z0-boson measurements at
√
sNN
=
2.
76 TeV have been carried out at central rapidity by the ATLAS andCMScollaborations [35–38]. PreliminaryZ0-boson measurements in Pb–Pb collisions at√
sNN=
5.
02 TeV at cen-tral rapidityhave alsobeenreportedrecentlyby ATLAS [39]. The W±- and Z0-boson nuclear modification factor, RAA, defined as the ratio of the yields in Pb–Pb collisions and the cross-section inppcollisions normalisedbythenuclearoverlapfunction TAA, whichrepresentstheeffectiveoverlaparea ofthe twointeracting nuclei [40],ismeasuredtobeconsistentwithunitywithin uncer-tainties,withnocentralitydependence [37–39].Measurementsathighcollision energyandlargerapidities are sensitive tolow Bjorken-x processes,andare thereforeimportant to furtherconstrain theinitial-stateeffects onelectroweak boson production andtoestablisha referenceformedium-sensitive ob-servables.
https://doi.org/10.1016/j.physletb.2018.03.010
0370-2693/©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
This paper presents the first measurement of Z0-boson pro-duction in Pb–Pb collisions at
√
sNN=
5.
02 TeV at large rapidi-ties. Opposite-signmuon pairs from Z0-boson decayswith 2.
5<
y
<
4.
01 are measured with the ALICE detector. The yield ofμ
+μ
− pairsincludescontributionsfromvirtual-photonprocesses andfromtheir interferenceeffects. Thismeasurementprobes the nPDFsoflarge-x valencequarks(x110−1)andlow-x seaquarks (x210−3) at Q2∼
MZ2. The invariant yields and RAA are re-portedasafunctionofrapidityandcollisioncentrality.Theresults arecompared tomodel calculationsincludingnPDFs.These mea-surements complement the measurements in p–Pb collisions at√
sNN
=
5.
02 TeV at large rapidities [32,33], providing increased precision andnew informationon rapidity andcentrality depen-dence.ThecombinationoftheseresultswithfutureW± measure-ments ina similar kinematic interval will provideconstraints on the flavor dependence of nPDFs, in particular the strange quark contribution [21].Thisletterisorganisedasfollows:theexperimentalsetup and data sample are described in Sect. 2; the analysis procedure is presented inSect. 3; the results are presentedin Sect. 4; and a summaryisgiveninSect.5.
2. Experimentalsetupanddataset
TheALICEdetectorisdescribedindetailinRef. [41].Z0 bosons are reconstructed via their muonic decay with the ALICE muon spectrometer,which provides muontrigger, trackingand identifi-cationin the pseudo-rapidity range
−
4.
0<
η
<
−
2.
5. The muon spectrometer,asseenfromtheinteractionpoint,consistsofafront absorberof10 interactionlengths(λ
int) thickness, whichreduces thecontaminationofhadronsandmuonsfromthedecayoflight particles; five tracking stations; an iron absorber with thickness 7.2λ
int; and two trigger stations. Each tracking station is com-posed of two planes of multi-wire proportional chambers with cathode-planereadout, whileeach trigger stationconsistsof two planesofresistiveplatechambers.Thethirdtrackingstationis lo-catedinsidethegap ofadipole magnet, whichprovides a3 T·
m magneticfield integral.Themuonspectrometeriscompletedbya beamshieldsurroundingthebeampipethatprotectsthe appara-tusfromsecondaryparticlesproducedintheinteractionoflarge-η
primaryparticleswiththepipeitself.
Theinteractionvertexis reconstructedusingthetwo cylindri-callayersoftheSiliconPixelDetector,locatedataradialdistance of3.9 and7.6 cm from the beamaxis andcovering
|
η
|
<
2 and|
η
|
<
1.
4,respectively.TheV0detector,consistingoftwoarraysof scintillatorcounterscovering2.
8<
η
<
5.
1 and−
3.
7<
η
<
−
1.
8, isusedfortriggeringandevaluationofcollisioncentrality.Finally, theZeroDegreeCalorimeter,placedat112.5 mfromthe interac-tionpoint along the beamline, is used toreject electromagnetic interactions [42].The dataset used in this analysis consists of Pb–Pb events at
√
sNN
=
5.
02 TeV selected with a dimuon trigger that requires the coincidence of a minimum-bias (MB) trigger and a pair of tracks with opposite sign in the muon spectrometer, each withpT
1 GeV/c. The MB trigger is defined by the coincidence of thesignalsfrombotharraysoftheV0.TheMBtriggerisfully ef-ficientforevents within the 0–90% centralityinterval, which are usedinthisanalysis.Themuontriggerefficiencyhasa plateauof about98%for muonswith pT>
5 GeV/c.The resulting efficiency forpairs ofopposite-sign muons, withmuon pT>
20 GeV/c and1 IntheALICEreferenceframethemuonspectrometercoversanegativeηrange and,consequently,anegativey range.However,sincethePb–Pbsystemis symmet-ricinrapidity,apositivey notationisusedtopresenttheresults.
−
4.
0<
η
<
−
2.
5,is95%.Afteralleventselectioncuts,thedataset correspondstoanintegratedluminosityofabout225 μb−1.3. Analysisprocedure
The procedure for Z0-boson signal extraction in this analysis is the same as that used in the analysis of p–Pb collisions at
√
sNN
=
5.
02 TeV [32].Tracksarereconstructedinthemuon spec-trometer using the algorithm described in Ref. [43]. Tracks are selectedforanalysisiftheyhavepseudorapidity−
4.
0<
η
<
−
2.
5 and polarangle 170◦< θ
abs<
178◦, measured atthe end of the frontabsorber.Thisselectionrejectsparticles thatcrossthe high-densityregionofthefrontabsorber andundergosignificant mul-tiple scattering. Tracks reconstructed in the tracking stations are identified asmuonsifthey matcha tracksegment inthetrigger stations,placeddownstreamtheironwall.Thecontaminationfrom backgroundtracksthatdonotpointtotheinteractionvertexis re-ducedbyutilisingtheproductofthemomentumandthedistance ofclosestapproachtotheinteractionvertex.Thiscutremoves88% ofall tracksforevents inthe0–90% centrality interval,while re-taining all signal candidates with negligible residual background contribution.Only muons with pT
>
20 GeV/c are used in this analysis. Thisselection reduces thecontribution ofmuonsfromthe decay ofcharm,beautyandlow-mass resonances(seebelow). Z0-boson candidatesareformedbycombiningpairsofopposite-signmuons. The candidatesare further selectedby requiringthat their rapid-ity,calculatedusingthemeasuredinvariantmass,isintheinterval 2.
5<
y<
4.
0.Fig.1presentstheμ
+μ
− invariant mass distribu-tioninthecentralityintervals0–90% inFig.1(a),0–20%inFig.1(b), and20–90%inFig. 1(c).The distributionforthe0–90% centrality interval is comparedwiththe resultof aMonte Carlo(MC) sim-ulation obtained using the POWHEG [44] event generator paired withPYTHIA 6.4.25 [45] forthepartonshower.Thepropagationof the particles throughthe detectoris simulatedwith theGEANT3 code [46]. The isospin of the Pb-nucleus is accounted for by a weightedaverageofneutronandprotoninteractions,butno mod-ification of the nucleon PDF was applied to account fornuclear effects.The simulations account forvariations inthe detector re-sponsewithtimeandin-situalignmenteffects.Adata-driven de-scriptionofthemuonmomentumresolution isalsoimplemented (seeRef. [32] fordetails).Theshapeoftheμ
+μ
−invariant mass distribution, which is mainly affected by the momentum resolu-tion,issimilarindataandMC.Various background sources contribute to the
μ
+μ
− invari-antmassdistribution.Contaminationfromthedecayoftt (tt−→
μ
+μ
−X )andτ
(Z0−→
τ τ
−→
μ
+μ
−X )pairsisestimatedwith POWHEGsimulations [10,44,47] andfoundtobesmallerthan0.5% ofthesignalyield,whichisconsideredasasystematicuncertainty. Thecontributionofopposite-signmuonpairsfromthedecayofcc (cc−→
μ
+μ
−X )andbb (bb−→
μ
+μ
−X ) pairswas studied in p–Pb collisions [32] andfound to be smaller thanthat of tt andτ
pairs.InPb–Pb collisions,the presenceofhigh-pT muonsfrom thedecayofheavy-flavourpairsisexpectedtobefurtherreduced duetothein-mediumenergylossofheavy quarks.This contribu-tion was therefore neglected. Finally,the combinatorial contribu-tionfromtherandom pairingofmuonsintheeventisevaluated vialike-signmuonpairs(μ
±μ
±).Thiscombinatorialcontribution isfoundtobesmall(onecandidateinthe20–90% centrality inter-val)andissubtractedfromthesignalestimate.ThenumberofZ0 candidatesisestimatedusingtheprocedure described in Ref. [32], by countingthe entries in the
μ
+μ
− in-variant mass interval 60<
Mμμ<
120 GeV/c2 after subtracting the contribution from like-sign pairs for each centrality and ra-pidity interval. A total of 64 candidates is found in the 0–90%Fig. 1. Invariantmassdistributionofμ+μ−pairsforPb–Pbcollisionsat√sNN=5.02 TeV,reconstructedusingmuonswith−4.0<η<−2.5 andpT>20 GeV/c (black points).Thepanelspresentthedistributionindifferentcentralityintervals.Theerrorbarsareofstatisticaloriginonly.Theinvariantmassdistributionoflike-signmuonpairs isalsoshown(redopenpoints).Onlyonelike-signcandidateisfoundinthe20–90%centralityinterval.ThesolidbluelinedrawninFig.1(a)representsthedistribution fromaPOWHEGsimulationforPb–PbcollisionswithoutnuclearmodificationofPDFs(seetextfordetails).(Forinterpretationofthecoloursinthefigure(s),thereaderis referredtothewebversionofthisarticle.)
centrality bin, of which 37 are in the 0–20% bin and 27 in the 20–90% bin.Asafunctionofrapidity,33candidatesareinthe in-terval2
.
5<
y<
3.
0,and31areintheinterval 3.
0<
y<
4.
0.The raw yields are corrected for the detector acceptance and for re-construction andselection efficiency( A·
ε
). The value of A·
ε
is 75% for events in the 0–90% centrality interval, estimated using the POWHEG [44] simulations described previously. The depen-denceof theefficiency on thedetector occupancy was evaluated byembeddingthegeneratedZ0 signalinrealMBPb–Pbdata.TheA
·
ε
termis constantas afunction of centralityfromperipheral tosemi-centraleventsanddecreasesinthemostcentralcollisions. Thevalueof A·
ε
is78%inthe20–90%centralityintervaland74% inthe0–20%interval, withcentrality-independent systematic un-certaintyof5%,asdiscussedbelow.To evaluate the invariant yields (dN
/
d y), the raw dimuon-triggered mass distribution must be normalised by the factorFi
μ-trig/MB, which is the inverse of the probability to observe a dimuon pair in a MB event for the centrality class i. The value of Fμi-trig/MB iscalculated in two different ways, by applyingthe dimuonselectioncriteriontoMBevents,andbytherelative count-ing rate of the two triggers [48]. The variation in Fi
μ-trig/MB de-termined by these two methods is 0.5% and contributes to the systematicuncertainty.
ThenuclearmodificationfactorRAArequiresthedetermination ofthecollisioncentrality,whichistypicallyquantifiedbythe aver-agenumberofnucleonsparticipatingintheinteractionforagiven
Table 1
Valuesoftheaveragenuclearoverlapfunction,TAA,thenumberofparticipating nucleons,Npart,andthenumberofbinarynucleon–nucleoncollisions,Ncoll,for eachcentralityinterval.Theaveragenumberofparticipantsasweightedbythe av-eragenumberofcollisions,NpartNcoll,isalsoreported.
Centrality TAA (mb−1) Npart Ncoll NpartNcoll
0–90% 6.2±0.2 126±2 435±41 263±3 0–20% 18.8±0.6 311±3 1318±130 322±3 20–90% 2.61±0.09 73±1 183±15 141±2
centralitybin,Npart.However,therateofhardprocessesisknown to scale with the average number of nucleon–nucleon collisions
Ncoll. The average centrality for hard processes is therefore pre-sentedastheaveragenumberofparticipantnucleonsweightedby thenumberofcollisions
NpartN
coll.Table1showstheestimatesof theaveragenuclearoverlapfunctionTAA,thenumberof partici-patingnucleonsNpartandthenumberofbinarynucleon–nucleon collisions Ncoll, which are obtained via a Glauber model fit of thesignal amplitudeinthetwoarraysoftheV0detector [49,50]. The resulting NpartN
coll is also shown. The classification of the events in givencentrality intervalshas an associated uncertainty of1.5–2.3%(centralitydependent),thatwasestimatedby compar-ing the number of candidates selected by varying the centrality rangesby±
0.
5%,toaccountforthecentralityresolution [49,50].The sources of systematicuncertainties in the yields and RAA aresummarisedinTable2.Thesystematicuncertaintyinthe
track-Table 2
RelativesystematicuncertaintiesintheyieldsandRAA.TherangesquotedforTAA andthecentralitylimits, representtheuncertaintyvariationwithcentrality.The centrality-dependentcorrelateduncertaintiesaremarkedbythesymbol(),while theuncertaintysourcesthatarecorrelatedasafunctionofrapidityareindicated by ( ).
Source Relative systematic uncertainty Background contamination <1.0% Tracking efficiency 3.0% () Trigger efficiency 1.5% () Tracker/trigger matching 1.0% () Alignment 3.5% () Fμ-trig/MB 0.5% ( ) σpp 4.5% () TAA 3.2–3.5% ( ) Centrality limits 1.5–2.3% ( )
ingefficiencyis3%,obtainedfromthecomparisonoftheefficiency estimated in data and MC by exploiting the redundancy of the trackingchamber information [51]. The systematicuncertaintyof thedimuontriggerefficiencyis1
.
5%,evaluatedbypropagatingthe uncertaintyof the efficiency of the detection elements, which is estimated from data using the redundancy of the trigger cham-ber information. In addition, the choice of theχ
2 cut used to match the tracker and trigger tracks introduces 1% uncertainty, obtainedfromthe difference betweendata andsimulation when applyingdifferentχ
2 cuts. The uncertainties inthe track resolu-tion andalignment are estimated by comparing the A·
ε
values obtainedwithtwodifferentsimulations.Inthefullsimulation,the alignmentismeasuredusingtheMILLEPEDE [52] packageandthe residualmisalignmentistakenintoaccount.Inthefastsimulation, thetracker response is basedon a parameterisation of the mea-suredresolution ofthe clustersassociatedwith a track [32]. The resultingsystematicuncertaintyis3.5%.Thetotal systematicuncertainty in theyield and RAA are de-termined by summing in quadrature the uncertainty from each source,listedinTable2.Alluncertaintiesexceptthosedueto
TAA andthecentralitybinboundariesareindependentofcollision cen-trality.Correlationsincentralityorrapidityoftheuncertaintiesof differentsources areindicated inTable 2.The relative systematic uncertaintyintheproton–protonreferenceσ
pp,whichaffectstheRAA,correspondsto4.5%andisestimatedbyvaryingthe factoriza-tionandrenormalisationscales andaccountingforthe uncertain-tiesinthePDFs [20].
4. Results
Theinvariantyieldof
μ
+μ
− fromZ0 bosons in2.
5<
y<
4.
0, divided by TAA, is 6.
11±
0.
76(
stat.)
±
0.
38(
syst.)
pb for the 0–90% centralityinterval. The comparisonwith theoretical calcu-lationsatNLOisshowninFig.2.TheCT14 [53] predictionutilises free proton and neutron PDFs, with relative weights to account forthe isospin of the Pbnucleus. The uncertaintyon the model includetheuncertainty onthe NLO calculationsandofthe mea-surementsconsideredinthePDF fit.Themeasuredinvariantyield deviatesfromthelower limit of thispredictionby 2.
3σ
. Forthe description of nuclear PDFs, two different approaches were con-sidered. The standard approach evaluates the nPDF as the free PDFmultipliedbyaparameterisationofnuclearmodifications.The calculations obtained with the EPS09 [54] and the more recent EPPS16 [22] parameterisations are shown. Inthe other approach, thenPDFsareobtainedbyfittingthenucleardatainasimilarway asdoneforfreeprotondata,butusingaparameterisationthat de-pends on the atomic mass of the nucleus. The results obtained with the nCTEQ15 nuclear PDFs [21,55] are also presented. TheFig. 2. Invariantyieldofμ+μ−fromZ0productionin2.5<y<4.0 dividedbythe averagenuclearoverlapfunctioninthe0–90%centralityclass,consideringmuons with−4.0<η<−2.5 andpT>20 GeV/c.Thehorizontalsolidlinerepresentsthe statisticaluncertaintyofthemeasurementwhiletheyellowfilledbandshowsthe systematicuncertainty.Theresultiscomparedtotheoreticalcalculationswithand withoutnuclearmodificationofthePDFs [21,22,53–55].Allmodelcalculations in-corporatePDFsornPDFsdeterminedbyconsideringtheisospinofthePb-nucleus.
nPDFsetsare characterisedby theirdifferentapproximationsand by differentinput dataincluded inthecalculations(see Ref. [22] and referencestherein for details).Only the mostrecent EPPS16 parameterisation includesLHCjet, W± andZ0 data,although the W± and Z0 dataprovide only weak constraints on nPDFs at the currentperturbativeorderofthecalculation(NLO) [21,56].In gen-eral,thenPDFshavelargeruncertaintiescomparedtothefree pro-ton PDFs,sincethey are lessconstrainedfromdata.CT14
+
EPS09 andCT14+
EPPS16 estimatescombineCT14andEPS09orEPPS16 uncertainties,whereasnCTEQ15doesaglobalstudyoftheproton andnuclearmeasurementuncertaintiesincludedinthefit.EPPS16 allows much morefreedom for theflavour dependenceofnPDFs than othercurrentanalyses, whichresults inlargeruncertainties. AllpQCDcalculationsshowninFig.2thatusenPDFsdescribethe measurementwell.The rapidity dependence of the Z0-boson invariant yields di-videdby
TAAisshownin Fig.3(a).Theresultsarecompared to pQCD calculations usingthe CT14 [53] PDFset both with(green filled box) and without (blue hatched box) the EPPS16 [22] pa-rameterisation of the nPDFs. In both cases, the Pb-isospin effect ismodelled bycombiningtheprotonandneutronPDFsornPDFs. EPPS16 decreasestheyields butdoesnothaveastrong influence ontherapiditydependenceofthecalculation.Thecalculationsthat utilise vacuumPDFs overestimate data in the two rapidity inter-vals,whereasthosethatutilisenPDFsareingoodagreementwith data.In this analysis, the ratio RAA utilises a theoretically calcu-latedreferencecrosssectionforppcollisions [20],whichis
σ
pp=
11
.
92±
0.
43 pb. The value of RAA forthe 0–90% centralityclass isdetermined tobe 0.
77±
0.
10(
stat.)
±
0.
06(
syst.)
,deviatingby 2.
1σ
fromunity.The pQCD calculation usingCT14 [53] and con-sideringonlytheisospineffects,finds RCT14AA=
1.
052±
0.
038.The modification of the PDFs in nuclei results in a net reduction of theyields,andconsequently inRAA valueslowerthanunity,withrapid-Fig. 3. Invariantyieldofμ+μ−fromZ0in2.5<y<4.0 dividedbyT
AA(a)andnuclearmodificationfactor(b)asafunctionofrapidityforPb–Pbcollisionsat√sNN= 5.02 TeV,consideringmuonswith−4.0<η<−2.5 andpT>20 GeV/c.Theverticalerrorbarsarestatisticalonly.Thehorizontalerrorbarsdisplaythemeasurementbin width,whiletheboxesrepresentthesystematicuncertainties.Thefilledblackboxinpanel(b),locatedat RAA=1,showsthenormalisationuncertainty.Theresultsare comparedtotheoreticalcalculationswithandwithoutnuclearmodificationofthePDFs.ThefilledblueboxesshowthecalculationusingtheCT14PDF,whilethegreen stippledboxesshowthecalculationusingCT14PDFwithEPPS16nPDF [22,53].AllmodelcalculationsincorporatePDFsornPDFsthataccountfortheisospinofthePb nucleus.
Fig. 4. Invariantyieldofμ+μ−fromZ0in2.5<y<4.0 dividedbyT
AA(a)andnuclearmodificationfactor(b)asafunctionofcentrality(representedbyNpartNcoll)for
Pb–Pbcollisionsat√sNN=5.02 TeV,consideringmuonswith−4.0<η<−2.5 andpT>20 GeV/c.Theverticalerrorbarsarestatisticalonly,whiletheboxesrepresentthe systematicuncertainties.Thefilledblackboxinpanel(b),locatedatRAA=1,showsthenormalisationuncertainty.Theresultsarecomparedtotheoreticalcalculationswith centrality-dependentnPDFsthataccountfortheisospinofthePbnucleus [53,54,57].
ity dependence of RAA is presented in Fig. 3(b). The values are smallerthanunity,withaslightrapiditydependence.Thedataare well-describedbycalculationsincludingnPDFs(greenfilledboxes), whilethecalculationsincludingonlyisospineffects(bluehatched boxes)tendtooverestimatethemeasuredvalues.
Z0-boson production is studied as a function of the collision centrality,expressedintermsof
NpartN
collasshowninFig.4.The valueofRAAiscompatiblewithunityinperipheralcollisions,withRAA(20–90%)
=
0.
96±
0.
19(stat.)±
0.
04(syst.)±
0.
06(corr. syst.), whileit is2.
6σ
smallerthan unityinthecentral collisions, withRAA (0–20%)
=
0.
67±
0.
11(stat.)±
0.
03(syst.)±
0.
04(corr. syst.). The value for 0–20% central collisions deviatesfrom the predic-tionsusingvacuumPDFs(RCT14AA )by3
σ
.Thedataarecomparedto calculationsincludingacentrality-dependentnuclearmodification ofthePDFs [57],whichdescribethedatawithinuncertainties.5. Conclusion
We havereportedthe first measurement ofZ0-boson produc-tionatforwardrapiditiesinPb–Pbcollisions at
√
sNN=
5.
02TeV. Theinvariant yields divided by theaveragenuclear overlap func-tionareevaluatedasafunctionofrapidityandaveragenumberof participantnucleons weightedby thenumberofbinary nucleon–nucleon collisions. Thecorresponding valuesof thenuclear mod-ification factor are estimated by dividing the measured yields in Pb–Pbcollisionsbytheexpectedcross-sectioninppcollisions esti-matedwithNLOpQCDcalculations.ThevalueofRAAiscompatible with unityin the 20–90% centralityclass (withinlarge statistical uncertainty), whereas it is smaller than unity by 2.6 times the quadraticsumofthestatisticalandsystematicuncertaintiesinthe 0–20% most central collisions. The results are well-described by the calculationsthat include modifications of thePDFs innuclei. In contrast, the calculations with vacuumPDFs overestimate the centrality-integrated RAAby2
.
3σ
andRAA inthe0–20% most cen-tralcollisionsby3σ
.Acknowledgements
The ALICE Collaboration would like to thank H. Paukkunen forproviding thepQCD calculationswithEPS09 andEPPS16,and F. LyonnetandA. KusinaforprovidingthepQCDcalculationswith nCTEQ15.
The ALICECollaboration would like to thank all its engineers andtechniciansfortheirinvaluablecontributionstothe construc-tion of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE
Collab-oration gratefully acknowledges the resources and support pro-videdbyallGridcentresandtheWorldwideLHCComputingGrid (WLCG) collaboration. The ALICECollaboration acknowledges the followingfundingagenciesfortheir support inbuildingand run-ningtheALICEdetector:A.I. Alikhanyan NationalScience Labora-tory(YerevanPhysicsInstitute)Foundation (ANSL),State Commit-teeofScienceandWorldFederationofScientists(WFS), Armenia; AustrianAcademy ofSciences andNationalstiftung fürForschung, Technologie und Entwicklung, Austria; Ministry of Communica-tions and High Technologies, National Nuclear Research Center, Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Universidade Federal do Rio Grande do Sul (UFRGS), Financiadora de Estudos e Projetos (Finep) and Fun-dação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil; Ministry of Science & Technology of China (MSTC), Na-tional Natural Science Foundation of China (NSFC) and Ministry of Education of China (MOEC), China; Ministry of Science, Edu-cationandSports andCroatian Science Foundation,Croatia; Min-istryofEducation,YouthandSportsoftheCzechRepublic, Czech Republic; The Danish Council for Independent Research – Natu-ral Sciences, the Carlsberg Foundation and Danish National Re-search Foundation (DNRF), Denmark;Helsinki Institute ofPhysics (HIP),Finland;Commissariatàl’EnergieAtomique(CEA)and Insti-tut Nationalde Physique Nucléaire etde Physique des Particules (IN2P3)andCentre National de laRecherche Scientifique (CNRS), France; Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF) and GSI Helmholtzzentrum für Schwe-rionenforschungGmbH,Germany;GeneralSecretariatforResearch and Technology, Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office, Hungary; Department of Atomic Energy, Government of India (DAE),DepartmentofScienceandTechnology,GovernmentofIndia (DST),University Grants Commission, Governmentof India(UGC) andCouncil ofScientificandIndustrialResearch(CSIR), India; In-donesian Institute of Science, Indonesia; Centro Fermi – Museo StoricodellaFisicaeCentroStudieRicercheEnricoFermiand Isti-tutoNazionalediFisicaNucleare(INFN),Italy;Institutefor Innova-tiveScienceandTechnology,NagasakiInstituteofAppliedScience (IIST),Japan Societyforthe PromotionofScience(JSPS)KAKENHI andJapanese Ministry of Education, Culture, Sports, Science and Technology(MEXT),Japan;ConsejoNacionaldeCiencia(CONACYT) yTecnología,throughFondodeCooperaciónInternacionalen Cien-cia y Tecnología (FONCICYT) and Dirección General de Asuntos delPersonalAcademico(DGAPA),Mexico;NederlandseOrganisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Re-search Council of Norway, Norway; Commission on Science and TechnologyforSustainableDevelopmentintheSouth(COMSATS), Pakistan;PontificiaUniversidadCatólicadelPerú,Peru;Ministryof ScienceandHigherEducationandNationalScienceCentre,Poland; KoreaInstituteofScienceandTechnologyInformationandNational ResearchFoundationofKorea(NRF),RepublicofKorea;Ministryof EducationandScientificResearch,Institute ofAtomicPhysicsand RomanianNationalAgencyforScience,TechnologyandInnovation, Romania; Joint Institute for Nuclear Research (JINR), Ministry of EducationandScienceoftheRussianFederationandNational Re-search Centre Kurchatov Institute, Russia; Ministry of Education, Science,Researchand SportoftheSlovak Republic, Slovakia; Na-tionalResearchFoundationofSouthAfrica,SouthAfrica;Centrode AplicacionesTecnológicasyDesarrolloNuclear(CEADEN), Cubaen-ergía,CubaandCentrodeInvestigacionesEnergéticas, Medioambi-entalesyTecnológicas(CIEMAT),Spain;SwedishResearchCouncil (VR)andKnut&AliceWallenbergFoundation(KAW),Sweden; Eu-ropean Organization for Nuclear Research, Switzerland; National Scienceand Technology DevelopmentAgency (NSDTA), Suranaree University of Technology (SUT) and Office of the Higher
Educa-tionCommissionunderNRUprojectofThailand,Thailand;Turkish Atomic Energy Agency (TAEK), Turkey; National Academy of Sci-encesofUkraine,Ukraine;ScienceandTechnologyFacilities Coun-cil (STFC), United Kingdom; National Science Foundation of the United Statesof America (NSF) andUnited StatesDepartment of Energy,OfficeofNuclearPhysics(DOENP),UnitedStatesof Amer-ica.
References
[1]S. Peigne, A. Peshier,Collisionalenergy lossof afastmuon inahot QED plasma,Phys.Rev.D77(2008)014015,arXiv:0710.1266 [hep-ph].
[2]V.Kartvelishvili, R.Kvatadze, R.Shanidze,OnZ andZ + jetproduction in heavyioncollisions,Phys.Lett.B356(1995)589–594,arXiv:hep-ph/9505418. [3]Z.ConesadelValle,Vectorbosonsinheavy-ioncollisionsattheLHC,Eur.Phys.
J.C61(2009)729–733,arXiv:0903.1432 [hep-ex].
[4]UA1Collaboration,C.Albajar,etal.,IntermediatevectorBosoncross-sectionsat theCERNsuperprotonsynchrotroncolliderandthenumberofneutrinotypes, Phys.Lett.B198(1987)271.
[5]UA2Collaboration,J.Alitti,etal.,AMeasurementoftheW andZ production
cross-sectionsandadeterminationofW attheCERNp p collider,¯ Phys.Lett.
B276(1992)365–374.
[6]CDF Collaboration,F. Abe,et al.,Measurement ofσ·B(W →eν) and σ·
B(Z0→e+e−)inpp collisions¯ at√s=1.8TeV,Phys. Rev.Lett. 76(1996)
3070–3075,arXiv:hep-ex/9509010.
[7]D0Collaboration,B.Abbott,etal.,ExtractionofthewidthoftheW bosonfrom measurementsofσ(pp¯→W+X)×B(W→eν)andσ(pp¯→Z+X)×B(Z→
ee)andtheirratio,Phys.Rev.D61(2000)072001,arXiv:hep-ex/9906025. [8]CDF Collaboration, A.Abulencia, et al., Measurements ofinclusive W and
Z cross sections in p p collisions at √s=1.96 TeV, J. Phys. G34 (2007) 2457–2544,arXiv:hep-ex/0508029.
[9]ATLASCollaboration,G.Aad,etal.,MeasurementoftheW→ νandZ/γ∗→ productioncrosssectionsinproton–protoncollisionsat√s=7TeV with theATLASdetector,J.HighEnergyPhys.12(2010)060,arXiv:1010.2130 [hep -ex].
[10]ATLAS Collaboration, G.Aad, et al.,Measurement oftheinclusive W± and
Z/γ∗ crosssectionsintheelectron andmuondecay channelsin pp
colli-sionsat√s=7 TeVwiththeATLASdetector,Phys.Rev.D85(2012)072004, arXiv:1109.5141 [hep-ex].
[11]ATLASCollaboration,G.Aad,etal.,MeasurementofW±andZ -boson produc-tioncrosssectionsinpp collisionsat√s=13 TeVwiththeATLASdetector, Phys.Lett.B759(2016)601–621,arXiv:1603.09222 [hep-ex].
[12]CMSCollaboration,S.Chatrchyan,etal.,MeasurementoftheinclusiveW and Z productioncrosssectionsinpp collisionsat√s=7TeV,J.HighEnergyPhys. 10(2011)132,arXiv:1107.4789 [hep-ex].
[13]CMSCollaboration,S.Chatrchyan,etal.,MeasurementofinclusiveWandZ bosonproductioncrosssectionsinppcollisionsat√s=8TeV,Phys.Rev.Lett. 112(2014)191802,arXiv:1402.0923 [hep-ex].
[14]LHCbCollaboration,R.Aaij,etal.,MeasurementoftheforwardZ boson pro-ductioncross-sectioninpp collisionsat√s=7TeV,J.HighEnergyPhys.08 (2015)039,arXiv:1505.07024 [hep-ex].
[15]LHCb Collaboration,R.Aaij,etal., MeasurementofforwardWandZboson productioninpp collisionsat√s=8TeV,J.HighEnergyPhys.01(2016)155, arXiv:1511.08039 [hep-ex].
[16]A.D.Martin,R.G.Roberts,W.J.Stirling,R.S.Thorne,Partondistributionsandthe LHC:W and Z production,Eur.Phys.J.C14(2000)133–145,arXiv:hep-ph/ 9907231.
[17]U.Baur,S.Keller,D.Wackeroth,ElectroweakradiativecorrectionstoW boson
productioninhadroniccollisions,Phys.Rev.D59(1999)013002,arXiv:hep -ph/9807417.
[18]J.Rojo,etal.,ThePDF4LHCreportonPDFsandLHCdata:resultsfromRun I andpreparationforRun II,J.Phys.G42(2015)103103,arXiv:1507.00556 [hep -ph].
[19]J.Butterworth,etal.,PDF4LHCrecommendationsforLHCRunII,J.Phys.G43 (2016)023001,arXiv:1510.03865 [hep-ph].
[20]H. Paukkunen,C.A.Salgado,Constraintsfor thenuclearparton distributions fromZandWproductionattheLHC,J.HighEnergyPhys.1103(2011)071, arXiv:1010.5392 [hep-ph].
[21]A.Kusina,F.Lyonnet,D.B. Clark,E.Godat,T. Jezo,K. Kovarik,F.I.Olness, I. Schienbein,J.Y.Yu,Vectorbosonproductioninproton–leadandlead–lead colli-sionsattheLHCanditsimpactonnCTEQ15PDFs,arXiv:1610.02925 [nucl-th]. [22]K.J.Eskola,P.Paakkinen,H.Paukkunen,C.A.Salgado,EPPS16:nuclearparton distributionswithLHCdata,Eur.Phys.J.C77(2017)163,arXiv:1612.05741 [hep-ph].
[23]D.deFlorian,R.Sassot,Nuclearpartondistributionsatnext-to-leadingorder, Phys.Rev.D69(2004)074028,arXiv:hep-ph/0311227.
[24]M. Hirai, S. Kumano,T.H. Nagai, Determination ofnuclear parton distribu-tionfunctionsandtheiruncertaintiesinnext-to-leadingorder,Phys.Rev.C76 (2007)065207,arXiv:0709.3038 [hep-ph].
[25]S. AtashbarTehrani,Nuclear partondensitiesand their uncertaintiesatthe next-to-leadingorder,Phys.Rev.C86(2012)064301.
[26]H.Khanpour,S.AtashbarTehrani,Globalanalysisofnuclearpartondistribution functionsandtheiruncertaintiesatnext-to-next-to-leadingorder,Phys.Rev.D 93(2016)014026,arXiv:1601.00939 [hep-ph].
[27]R.Vogt,Shadowingeffectsonvectorbosonproduction,Phys.Rev.C64(2001) 044901,arXiv:hep-ph/0011242.
[28]R.B. Neufeld, I. Vitev, B.-W. Zhang, A possibledetermination of the quark radiation lengthin coldnuclearmatter, Phys. Lett. B704(2011) 590–595, arXiv:1010.3708 [hep-ph].
[29]ATLAS√ Collaboration,G.Aad,etal.,Z bosonproductioninp+Pb collisionsat
sNN=5.02TeV measuredwiththeATLASdetector,Phys.Rev.C92(2015) 044915,arXiv:1507.06232 [hep-ex].
[30]CMS Collaboration, V.Khachatryan, et al., Study ofZ boson production in pPb collisionsat √sNN=5.02TeV, Phys. Lett. B 759(2016) 36–57,arXiv: 1512.06461 [hep-ex].
[31]CMSCollaboration,V.Khachatryan,etal.,StudyofWbosonproductioninpPb collisionsat√sNN=5.02TeV,Phys.Lett.B750(2015)565–586,arXiv:1503. 05825 [nucl-ex].
[32]ALICECollaboration,J.Adam,etal.,WandZbosonproductioninp-Pb colli-sionsat√sNN=5.02TeV,J.HighEnergyPhys.02(2017)077,arXiv:1611.03002 [nucl-ex].
[33]LHCbCollaboration,R.Aaij,etal.,ObservationofZ productioninproton–lead collisionsatLHCb,J.HighEnergyPhys.1409(2014)030,arXiv:1406.2885 [hep -ex].
[34]R. Gavin,Y. Li,F.Petriello,S. Quackenbush,FEWZ2.0:acodefor hadronic Z production at next-to-next-to-leadingorder, Comput.Phys. Commun.182 (2011)2388–2403,arXiv:1011.3540 [hep-ph].
[35]ATLASCollaboration,G.Aad,etal.,Measurementoftheproductionandlepton chargeasymmetryofW bosonsinPb+Pbcollisionsat√sNN=2.76TeV with theATLASdetector,Eur.Phys.J.C75(2015)23,arXiv:1408.4674 [hep-ex]. [36]ATLAS Collaboration,G.Aad, etal., Measurementof Z bosonProductionin
Pb+PbCollisionsat√sNN=2.76TeV withtheATLASDetector,Phys.Rev.Lett. 110(2013)022301,arXiv:1210.6486 [hep-ex].
[37]CMSCollaboration,S.Chatrchyan,etal.,StudyofZproductioninPbPbandpp collisionsat√sNN=2.76TeV inthedimuonanddielectrondecaychannels,J. HighEnergyPhys.1503(2015)022,arXiv:1410.4825 [nucl-ex].
[38]CMSCollaboration,S.Chatrchyan,etal.,StudyofW bosonproductioninPbPb and pp collisionsat√sNN=2.76TeV,Phys.Lett.B715(2012)66–87,arXiv: 1205.6334 [nucl-ex].
[39] ATLAS Collaboration, Z boson production in Pb+Pb collisions at √sNN= 5.02TeV withtheATLASdetectorattheLHC,ATLAS-CONF-2017-010,2017. [40]M.L.Miller,K.Reygers,S.J.Sanders,P.Steinberg,Glaubermodelinginhigh
en-ergynuclearcollisions,Annu.Rev.Nucl.Part.Sci.57(2007)205–243,arXiv: nucl-ex/0701025.
[41]ALICECollaboration,K.Aamodt,etal.,TheALICEexperimentattheCERNLHC, J.Instrum.3(2008)S08002.
[42]ALICECollaboration,B.Abelev,etal.,Measurementofthecrosssectionfor elec-tromagneticdissociationwithneutronemissioninPb–Pbcollisionsat√sN N=
2.76TeV,Phys.Rev.Lett.109(2012)252302,arXiv:1203.2436 [nucl-ex]. [43]ALICECollaboration,K.Aamodt,etal.,Rapidityandtransversemomentum
de-pendenceofinclusiveJ/ψ production inpp collisions at√s=7TeV, Phys. Lett.B704(2011)442–455,arXiv:1105.0380 [hep-ex],Erratum:Phys.Lett.B7 (2012)18,692.
[44]S.Alioli,P.Nason,C.Oleari,E.Re,NLOvector-bosonproductionmatchedwith showerinPOWHEG,J.HighEnergyPhys.07(2008)060,arXiv:0805.4802 [hep -ph].
[45]T.Sjostrand, S.Mrenna, P.Z.Skands,PYTHIA6.4physicsandmanual,J.High EnergyPhys.05(2006)026,arXiv:hep-ph/0603175.
[46] R.Brun,F.Carminati,S.Giani,GEANTdetectordescriptionandsimulationtool, CERN-W-5013,1994.
[47]CMSCollaboration, S. Chatrchyan,et al., Measurementof the rapidity and transversemomentumdistributionsofZ bosonsinpp collisionsat√s=7TeV, Phys.Rev.D85(2012)032002,arXiv:1110.4973 [hep-ex].
[48]ALICECollaboration,J.Adam,etal.,J/ψsuppressionatforwardrapidityinPb– Pbcollisionsat √sNN=5.02TeV, Phys. Lett.B766(2017) 212–224,arXiv: 1606.08197 [nucl-ex].
[49]ALICECollaboration,J.Adam,etal.,Centralitydependenceofparticle produc-tioninp–Pbcollisionsat√sNN=5.02TeV,Phys. Rev.C91(2015)064905, arXiv:1412.6828 [nucl-ex].
[50]ALICECollaboration, J. Adam, et al.,Centrality dependence ofthe charged-particle multiplicity density at midrapidity inPb–Pb collisions at √sNN= 5.02TeV,Phys.Rev.Lett.116(2016)222302,arXiv:1512.06104 [nucl-ex]. [51]ALICECollaboration,B.B.Abelev,etal.,PerformanceoftheALICEexperimentat
theCERNLHC,Int.J.Mod.Phys.A29(2014)1430044,arXiv:1402.4476 [nucl -ex].
[52] V. Blobel, C. Kleinwort, A new method for the high precision align-ment of track detectors, in: Advanced Statistical Techniques in Particle Physics. Proceedings, Conference, Durham, UK, March 18–22, 2002, 2002, pp. URL–STR(9),arXiv:hep-ex/0208021,http://www.ippp.dur.ac.uk/Workshops/ 02/statistics/proceedings//blobel1.pdf,2002.
[53]S. Dulat, T.-J. Hou, J. Gao, M. Guzzi, J. Huston,P. Nadolsky, J. Pumplin, C. Schmidt,D.Stump,C.P.Yuan,Newpartondistributionfunctionsfromaglobal analysisofquantumchromodynamics,Phys.Rev.D93(2016)033006,arXiv: 1506.07443 [hep-ph].
[54]K.J.Eskola,H.Paukkunen,C.A.Salgado,EPS09:anewgenerationofNLOand LOnuclearpartondistributionfunctions,J.HighEnergyPhys.04(2009)065, arXiv:0902.4154 [hep-ph].
[55]K.Kovarik, etal., nCTEQ15–globalanalysisofnuclearpartondistributions withuncertaintiesinthe CTEQ framework,Phys. Rev.D93(2016)085037, arXiv:1509.00792 [hep-ph].
[56]N.Armesto,H.Paukkunen,J.M.Penín,C.A.Salgado,P.Zurita,Ananalysisofthe impactofLHCRunIproton–leaddataonnuclearpartondensities,Eur.Phys.J. C76(2016)218,arXiv:1512.01528 [hep-ph].
[57]I.Helenius,K.J.Eskola,H.Honkanen,C.A.Salgado,Impact-parameter depen-dentnuclearpartondistributionfunctions:EPS09sandEKS98sandtheir ap-plications innuclear hard processes, J. High Energy Phys. 07 (2012) 073, arXiv:1205.5359 [hep-ph].
TheALICECollaboration
