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Physics
Letters
B
www.elsevier.com/locate/physletb
Measurement
of
dijet
k
T
in
p–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: Received11March2015
Receivedinrevisedform5May2015 Accepted16May2015
Availableonline19May2015 Editor:L.Rolandi
A measurement of dijet correlations in p–Pb collisions at √sNN=5.02 TeV with the ALICE detector is presented. Jets are reconstructed from charged particles measured in the central tracking detectors and neutral energy deposited in the electromagnetic calorimeter. The transverse momentum of the full jet (clustered from charged and neutral constituents) and charged jet (clustered from charged particles only) is corrected event-by-event for the contribution of the underlying event, while corrections for underlying event fluctuations and finite detector resolution are applied on an inclusive basis. A projection of the dijet transverse momentum, kTy=pTch,jet+nesin(
ϕ
dijet)withϕ
dijetthe azimuthal angle between a full and charged jet and pchT,jet+ne the transverse momentum of the full jet, is used to study nuclear matter effects in p–Pb collisions. This observable is sensitive to the acoplanarity of dijet production and its potential modification in p–Pb collisions with respect to pp collisions. Measurements of the dijet kTyas a function of the transverse momentum of the full and recoil charged jet, and the event multiplicity are presented. No significant modification of kTydue to nuclear matter effects in p–Pb collisions with respect to the event multiplicity or a PYTHIA8 reference is observed.©2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.
1. Introduction
Dijets produced in 2
→
2 leading-order (LO) scattering pro-cessesarebalanced intransversemomentum andback-to-backin azimuth.In proton–protoncollisionsa smallacoplanarityappears duetointrinsic transversemomentumkT frompartonicFermimo-tion[1]andinitialstategluonradiation[2,3].Atlargemomentum transferbetweenthe incomingpartons,the phasespace forhard gluonradiationinthepartonshowerorfromnext-to-leading-order (NLO)processesincreases,resultinginacoplanarityofthedijet sys-tem[4–6].This alsoresultsinan imbalance ofthejet transverse momentaalso referred toasa broadeningofthe dijettransverse momentum.TherelativecontributionofhardQCDradiationtothe dijetkTcanbevariedbyapplyingkinematicandacceptance
selec-tionstothedijetsample.
Inp–Pbcollisions thedijetkinematicsarepotentially modified duetonuclearmattereffectswhichareexpectedtoinducea mo-mentumimbalanceandacoplanarityofdijetpairs withrespectto ppcollisions, so-calledtransversemomentum broadening[7]. For instance,multiplescatteringsinsidethenucleusoftheinitial- and final-statepartonsinhardscatteringscanleadtosuchatransverse momentumbroadening.
In heavy-ion collisions, jets produced in hard scattering pro-cessesareusedto probethepropertiesoftheproduced medium. Highly energetic partons propagate through the medium, which
E-mailaddress:alice-publications@cern.ch.
modifiesthepartonshower resultinginamodified fragmentation pattern of the final hadronization products [8,9]. Heavy-ion jet measurements are compared to measurements in pp collisions to determine the effect of hot nuclear matter on jet observ-ables [10–13]. In the context of such studies, measurements in p–Pbcollisionsserveasabenchmarktostudyhardscattering pro-cessesinanucleartarget.
Measurements presented in [14] of the dijet transverse mo-mentum imbalance and dijetazimuthal angle distributions show resultswhicharecomparabletoresultsobtainedwithppdataand independentoftheeventactivity.Thisletterpresentsa measure-ment ofdijetacoplanarityinp–Pbcollisions at
√
sNN=
5.
02 TeV,recorded with the ALICE detector at the Large Hadron Collider (LHC).Thejetazimuthalcorrelationsaremeasuredatmid-rapidity for jet transverse momentum between 15 and 120 GeV/c. Jets entering inthe acceptanceof electromagnetic calorimeterare re-constructedfromchargedandneutralparticles(fulljet)whilethe recoiljetisreconstructedfromchargedparticlesonly(chargedjet). Measurementsare presentedasafunction ofthefull and associ-ated chargedjet transverse momentumin two eventmultiplicity classes which are correlated to the centrality of the p–Pb colli-sions[15].
2. Experimentalsetupanddatasample
Collisionsofprotonandleadbeamswereprovidedby theLHC inthefirstmonthsof2013.Thebeamenergieswere4TeVforthe
http://dx.doi.org/10.1016/j.physletb.2015.05.033
0370-2693/©2015CERNforthebenefitoftheALICECollaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
protonbeamand1.58TeVper nucleonfortheleadbeam, result-ingincollisionsatacenterofmassenergy
√
sNN=
5.
02 TeV.Thenucleon–nucleoncenter-of-masssystemmovesinrapiditywith re-spectto theALICEreferenceframeby
−
0.
465 inthe directionof theproton beam[16]. Inthefollowingη
refers tothe pseudora-pidityintheALICEreferenceframe.TheV0detectors,twoarraysofscintillatortilescoveringthefull azimuthwithin 2
.
8<
η
<
5.
1 (V0A) and−
3.
7<
η
<
−
1.
7 (V0C), wereusedforonlineminimumbiaseventtriggering,offlineevent selection andcharacterizationofevents indifferentparticle mul-tiplicityclasses.Theminimumbiastrigger requiredasignal from a chargedparticleinboth the V0AandV0C.The totalintegrated luminosityoftheminimumbiaseventsampleis37 μb−1.The electromagnetic calorimeter (EMCal)in ALICE [17] covers 100degrees inazimuth, 1
.
4<
ϕ
<
π
, and|
η
|
<
0.
7. Forthe an-alyzed data set an online jet patch trigger of 32×
32 adjacent towers, corresponding to an area of approximately 0.
2 rad was used.Thisjet patchtrigger firedifan integratedpatchenergy of atleast10 GeV (low-energytrigger)or20 GeV (high-energy trig-ger)was found.The low-energy triggered eventsample provided asignificantoverlapinjetenergybetweentheminimumbiasand high-energytriggereventsamples,allowingassessmentofthe trig-gerbiases.Theeventsampleobtainedwiththelow-energytrigger correspondstoatotalintegratedluminosityof21 μb−1.Theevent samplewiththehighenergythresholdhasatotalintegrated lumi-nosityof1.
6 nb−1.The position of the primary vertex was determined using re-constructedchargedparticletracksintheALICEtrackingsystems, Inner Tracking System (ITS) [18] and Time Projection Chamber (TPC) [19]. The algorithm to reconstruct the primary vertex is fully efficient for events with at least one primary track within
|
η
|
<
1.
4 [20]. Toensure a hightrackingefficiency uniforminη
, eventsareacceptedifthecoordinateofthevertexalongthebeam directioniswithin±
10 cm fromthecenterofthedetector.Thetotal eventsample is dividedintotwo multiplicity classes basedonthetotal chargedepositedinthe V0Adetector[21].For thedatasampleusedinthisanalysis, theV0Adetectorislocated inthedirectionofthePbremnantsandthussensitivetothe frag-mentation of the nucleus limiting a correlation in the definition of the multiplicity class with the dijet measurement at midra-pidity. Two multiplicity classes 0–40% and 40–100% are used in this analysis. The higher multiplicity class (0–40%) corresponds to
dN/
dη
|η|<0.5=
37.
2±
0.
8 and the lower multiplicity class(40–100%) to
dN/
dη
|η|<0.5=
9.
4±
0.
2.3. JetreconstructionanddijetkTy
3.1. Jetreconstruction
Jets are reconstructed with the anti-kT jet algorithm of the
FastJetpackage[22,23]combiningchargedtracksmeasuredinthe centraltrackingdetectors,ITSandTPC,andneutralfragments mea-suredwiththeEMCal[17].TracksfromthecombinedITSandTPC trackreconstruction algorithm are used.Quality criteria fortrack selection follow the same strategy as in [24]. The tracking effi-ciency is 70% for tracks with a transverse momentum pT,track
=
0
.
15 GeV/
c andincreasesto 85%at pT,track=
1 GeV/
c andabove.The pT resolution of tracks is 0.8% (3.8%) for pT,track
=
1 GeV/
c(50 GeV
/
c). EMCal clustersare formed by a clustering algorithm that combines signals from adjacent EMCal towers, with cluster size limited by the requirement that each cluster contains only onelocalenergymaximum.Energydepositedbychargedparticles intheEMCalissubtractedfromthemeasuredenergyintheEMCal clusterswhichpreventscountingthechargedenergytwice[20,25]. ALICEalsoreconstructsjetsfromchargedparticlesonly.Thesejetsare referred to as ‘charged jets’, while jets reconstructed from chargedandneutralfragmentsarecalled‘fulljets’inthisletter.
Inthisanalysis, anti-kT jetsare reconstructedusingthe
boost-invariant pT recombinationschemeandajetresolutionparameter
of R
=
0.
4. A jet is only accepted if it is fully contained in the acceptance in which the constituents are measured: for charged jets in the full azimuth and|
η
chjet
|
<
0.
9−
R while for full jets1
.
4+
R<
ϕ
jetch+ne<
π
−
R and|
η
chjet+ne|
<
0.
7−
R. It was veri-fied that reducing the acceptancewith 0.05 on all edges, inη
jetand
ϕ
jet,hasanegligible effectonthemeasurement.TrackswithpT,track
>
0.
15 GeV/
c and neutralconstituents with ET>
0.
3 GeVare considered. The minimum required area forjets with a res-olution parameter R
=
0.
4 is equal to 0.
3 (≈
60% of the area of a rigid cone with R=
0.
4). This selection does not affect the jet finding efficiency for jets (full and charged) with transverse momentum pT,jet>
15 GeV/
c. Inaddition, jetscontaining atrackwith pT,track
>
100 GeV/
c,forwhichthetrackmomentumresolu-tion exceeds 6.5%, are taggedandrejected. This last requirement has negligible effect in the reported range of jet momenta. The measurementiscorrectedtoparticlelevelaswillbeexplainedin Section3.3.
The measured transversemomentum ofthe anti-kT jet is
cor-rectedforthecontributionoftheunderlyingeventbysubtracting theaveragebackgroundmomentumdensity,
ρ
forfulljetsandρ
chforchargedjets,multipliedbythe areaoftheconsideredjet. The contribution of the underlying event to the charged jets is esti-matedusingclustersreconstructedwiththekTjetalgorithmusing
onlychargedtracks.Thisisachievedbycalculatingevent-by-event themedianchargedbackgrounddensity,
ρ
ch,fromallkTclustersintheeventwithinadditionacorrectionforthesparselypopulated p–Pb events [26,27].The average
ρ
ch in minimum-biasevents isequalto1
.
9 GeV/
c forthe0–40%multiplicityclassand0.
7 GeV/
cforthe40–100%eventmultiplicity classwitha negligible statisti-cal uncertaintyinboth cases.Finally,
ρ
ch is multipliedbya scalefactor toaccount for theneutralenergy toestimate
ρ
. The scale factorisdeterminedbymeasuringtheratiobetweentheenergyof alltheEMCalclustersandthechargedtrackspointingintothe EM-Cal acceptanceinthe minimum-biaseventsample. Theextracted scale factor 1.
28 is independent of eventmultiplicity. The influ-enceofbackgroundfluctuationsisquantifiedandcorrectedforon aninclusivebasis,seeSection3.3.3.2. DijetkTy
Each measured full jet is correlated with the charged jet of highest transverse momentum in the opposite hemisphere. Only pairsforwhichthefulljethasalargertransversemomentumthan theassociatedchargedjetareconsidered.Furthermoreonlydijets pairswith
|
ϕ
dijet−
π
|
<
π/
3,withϕ
dijettheanglebetweenthejet axisofthefullandchargedjet,areconsidered intheanalysis. The selection in
ϕ
dijet rejects 5–8%ofthe dijetpairsdependingon thekinematic selectionof thefull andassociatedcharged jet. The azimuthal acoplanarityofdijets is studied by measuring the transversecomponentofthekTvectorofthedijetsystem,kTy,
de-finedas
kTy
=
pchT,jet+nesin(
ϕ
dijet),
(1)with pchT,jet+ne the transverse momentum of the full jet. It should be noted that this definition differs from the one used in pre-vious publications, for example [28]. Since dN
/
dkTy is asym-metric distribution around zero,
|
kTy|
is reportedthroughout thepaper. For events from the minimum-bias sample full jets with
samples(seeSection 2) onlyjetswith pT,jetch+ne
>
40 GeV/
c forthe low energytrigger and pchT,jet+ne>
60 GeV/
c for the higherenergy triggerareused.Inthesekinematicregimesthe triggersare fully efficient and no fragmentation bias is observed with respect to theminimum-biasjetsample. The|
kTy|
distributionsarereportedat particle level involving a correction for detector effects and
pT-smearingduetotheunderlyingevent,seeSection3.3.
Thedijetsampleisbiasedduetotherequirementthatthefull jethasalargertransversemomentumthantheassociatedcharged jet, while the full jet momentum is used to estimate kTy. In an
unbiasedmeasurement,thefulljetwouldcorrespondtothe lead-ing jet of the eventin only 50% of the cases. A PYTHIA [29,30]
studywasperformedinwhichtheparticle-leveljetwasdefinedas thejet containing all final state particles (no kinematic selection onconstituentsandfullazimuthalacceptance).Applyingthe selec-tionofthisanalysistodetector-levelreconstructed jets,resultsin acorrecttaggingoftheleadingjetin70%ofthedijetevents.This results in a slightly harder
|
kTy|
distribution witha 10% smalleryieldatlow
|
kTy|
and20%higheryieldatlarge|
kTy|
.Theresultsofthep–Pb dataanalysiswill becompared toparticle-levelPYTHIA withthesamedijetselectionincorporatingthementionedbias.
Thedijet acoplanarityismeasured as afunction of the trans-versemomentumofthefulljetwhilethekinematicintervalofthe associatedchargedjetisalsovariedtoexplorekTyformoreorless
balanceddijetsintransversemomentum.InadditionkTy
distribu-tionsarealsopresentedfortwoeventmultiplicityclasses.
3.3.Correctionsandsystematicuncertainties
Themeasureddijet
|
kTy|
distributionsarecorrectedtothepar-ticlelevel, defined asthe dijet
|
kTy|
from jetsclustered from allprompt particles produced in the collision including all decay products, except those from weak decays of light flavor hadrons and muons. Both full and charged jets are accepted at particle level in the full azimuthal acceptance and in the pseudorapid-ityrange of
|
η
jet|
<
0.
5. The correction to particlelevel is basedon a data-driven method to correct for the influence of the un-derlyingeventfluctuationsandonsimulatedPYTHIAevents(tune Perugia-2011[31]) transportedthroughtheALICEdetectorslayout withGEANT3[32].Thecorrectionproceduretakesintoaccountthe
pTandangularresolutionofthemeasureddijets.
Detector-leveljetsaredefinedasjetsreconstructedfrom recon-structedtracksandEMCalclustersaftersubtractionofthecharged energydeposits. The jet energy scale andresolution are affected by unmeasured particles (predominantly K0L and neutrons), fluc-tuationsoftheenergydepositbychargedtracksintheEMCal,the EMCalenergyscaleandthechargedparticletrackingefficiencyand
pT resolution.Aresponsematrixasafunctionof pT,jet ofthefull
andassociatedchargedjet,
ϕ
dijetandkTy iscreatedaftermatch-ingthedetector-leveltotheparticle-leveljetsasdescribedin[33]. The pT-smearing due to fluctuations of the underlying event
isestimated withthe random-cones technique which is also ap-pliedintheanalysisofPb–Pbdata[24].Coneswitharadiusequal to the resolution parameter R are placed in the measured p–Pb eventsat random positions in the
η
–ϕ
plane ensuring the cone isfully contained in thedetector acceptance. The fluctuationsof the background are characterized by the difference between the summedpTofallthetracksandclustersintherandomcone(RC)andtheestimatedbackground:
δ
pT=
iRCpT,i−
A·
ρ
,where A istheareaofthe randomcone( A
=
π
R2)andthesubscript iindi-catesaclusterortrackpointinginsidetherandomcone.Arandom conecanoverlapwithajetbuttoavoidoversamplinginsmall sys-temslikep–Pb,apartialexclusionofoverlapwiththeleading jet intheeventisapplied.Thisisachievedbyexcludingrandomcones
overlappingwithaleadingjetwithagivenprobability,p
=
1/
NcollwhereNcoll isthenumberofbinarycollisions. Ncoll istakenfrom
estimates applying a Glauber fit to the multiplicity measured in the V0A detectorresulting in values between14
.
7 and 1.
52 de-pending ontheeventactivity measuredin theV0Adetector.The widthofthebackgroundfluctuationsforfull (charged)jetsvaries between2.
12 (1.
59)and0.
73 (0.
56)GeV/c dependingonthe mul-tiplicityoftheevent.The influence of background fluctuations is added to the re-sponseextractedfromdetectorsimulation throughaMonteCarlo model assuming that thebackground fluctuation forthe full and associated charged jet are uncorrelatedwithin 20% wide bins of V0Amultiplicityclasses.Withintheseselectedmultiplicity classes thevariationofthebackgroundfluctuationsisnegligible.Sincethe MonteCarlomodeldoesnotgeneratefulleventsandonlyaccounts for the
δ
pT smearing on a jet-by-jet basis, additional jet findinginefficienciesandworseningofangularresolutionduetothe back-ground fluctuation are not taken into account. These effects are negligiblesincethecontributionoftheunderlyingeventtothejets inp–Pbcollisionsissmall.Nocorrectionfortheangularresolution ofthechargedjetduetomissingneutralfragmentsisapplied.This effectincreasesthewidthof
ϕ
dijet by∼
0.
03 andispresent,andof the same magnitude, in the p–Pbdata and the PYTHIA refer-ence.
Themostprobablecorrectiontothejet energy,takinginto ac-countdetectoreffectsandbackgroundfluctuations,forfully recon-structed jets is 28% at pT,jetch+ne
=
20 GeV/
c and decreases to 20% forjetswithpchT,jet+ne>
40 GeV/
c.Theuncertaintyonthejetenergy scaleis evaluatedby changingthe trackingefficiencyindataand full detectorsimulation[34],varying thedoublecounting correc-tion for the hadronic energydeposit in the EMCal andby using differentestimates oftheunderlying-event fluctuations. The final uncertaintyonthejetenergyscaleis4%.Thejetenergyresolution forfulljetsis22%atpT,jetch+ne=
20 GeV/
c anddecreasesgraduallyto 18% atpchT,jet+ne=
120 GeV/
c.The influenceoftheuncertainties on thejetenergyscaleandresolutiononthedijet|
kTy|
measurementarediscussedinthefollowing.
The measured
|
kTy|
distributions are corrected to theparti-cle level by applying bin-by-bin correction factors, which are parametrizedbyalinearfittotheratiobetweentheparticle- and detector-level
|
kTy|
distributionsforagivendijetselection.Thecor-rectionfactorstakeintoaccounttheeffectsoffeed-inandfeed-out oftheselectedkinematicandangularintervalsofthefull and as-sociated charged jets. These effects slightly change the shape of the
|
kTy|
distributions resulting in correction factors which varybetween0
.
9 forsmall|
kTy|
to1.
2 atlarge|
kTy|
.Thecorrection isrelativelysmall,becausewhilefeed-infromlower pchT,jet+ne narrows the
|
kTy|
distribution,feed-infromhigherpchT,jet+ne broadensthedis-tributionresultinginacancellation.Similarlythefeed-outtohigh andlow pchT,jet+ne has a smalleffect onthe observable. By usinga linearfittothecorrectionfactorsthestatisticalfluctuationsofthe detectorsimulation are not propagated to the measurement.The 95%confidencelimit oftheparametrizationusingthelinearfitis included inthe systematic uncertaintyof the measurement. Cor-rectionfactorsareextractedasafunctionofV0Aeventmultiplicity classandkinematicintervalsofthefullandassociatedchargedjet. Thedominantsystematicuncertaintyonthemeasurement orig-inatesfromtheextractionofbin-by-bincorrectionfactors.The un-certaintyoftheparametrizationofthecorrectionfactorsresultsin 10–20%correlatedsystematicuncertaintyonthedijet
|
kTy|
yields.Anadditional2.5%uncertaintyarisesfromthe uncertaintyon the trackingefficiency whichis 4%.Systematicuncertainties originat-ing from the charged hadron energy deposit in EMCal towers,
Fig. 1. Dijet
|
kTy|distributionsinp–Pbcollisionsinthe0–40%V0Amultiplicity eventclassforseveralkinematicintervalsofthe fulljet(pchT,jet+ne).Themeasurementis comparedtoPYTHIA8(tune4C,K=0.7)withandwithoutinitialstateradiation.ThelowerpanelsshowtheratiobetweenthemeasurementandPYTHIA8includinginitial stateradiation.background fluctuations and the average momentum density
ρ
wereevaluatedandfoundtobenegligible.
4. Results
The dijet
|
kTy|
distributions are presented as functionsof thefull-jet transverse momentum, the transverse momentum of the associated charged jet andevent multiplicity classes.The results arecomparedtothepredictionsofthePYTHIA8.176event genera-torwithtune4CandK
=
0.
7[29,30].Thistunehasbeenfoundto giveareasonabledescriptionofjetproductionattheLHC.Thefinal stateparticlesareshiftedinpseudorapiditywithη
shift= −
0.
465 tomimictherapidityshiftofthelaboratoryframeduetotheenergy differenceoftheprotonandPbbeams.
4.1. Evolutionwithfull-jettransversemomentum
Fig. 1 showsthecorrected
|
kTy|
distributions forseveralkine-maticintervalsforthefulljet,frompT,jetch+ne
=
20 GeV/
c topchT,jet+ne=
120 GeV
/
c, in the 0–40% V0A multiplicity class. The associated chargedjet has a minimum transverse momentum, pchT,assoc jet, of
15 GeV/c and isalways oflower transverse momentum thanthe full jet. The mean
|
kTy|
increases with the transversemomen-tum of the full jet. Increasing the transverse momentum of the full jet extends the kinematic reach of
|
kTy|
by opening phase–spaceformoregluon radiation.Thisresultsinaharder
|
kTy|
dis-tribution which drops at large
|
kTy|
because the kinematic limit|
kTy|
max=
pchT,jet,max+ne sin(
2π/
3)
is reached. The p–Pb data pointsandthePYTHIA8calculationshowasimilardependenceonpchT,jet+ne. The lower panels of Fig. 1 show the ratio between data and PYTHIA8, includinginitialstate radiation(ISR), whichisobserved to be consistent with unity for all transverse momentum ranges studied.IntheupperpanelsPYTHIAwithouttheinitialstate radi-ation optionis shownin addition(dashed line).Without ISR the amountof QCD radiation(which includesNLO corrections) is re-duced, resulting in a steeper
|
kTy|
spectrum. The effect is mostpronounced forthe pchT,jet+ne
>
40 GeV/
c wherethep–Pb measure-mentisinagreementwithfullPYTHIAsimulationbutdiffers sig-nificantlyfromPYTHIAwithoutISR.Thisobservationsuggeststhatthedijet
|
kTy|
spectrumforlarge Q2 processesishighly sensitivetotheincreasedavailablephase–spaceofQCDradiationprocesses. Measurementspresentedin[14]ofthedijettransversemomentum imbalanceformoreenergeticjetsthanthemeasurementpresented herealsoshow resultswhicharecomparabletosimulatedpp ref-erenceandindependentoftheforwardtransverseenergy.
4.2. EvolutionwitheventmultiplicityandpchT,assoc jet
In addition tothe measurement of
|
kTy|
inthe highestmulti-plicityp–Pbevents,the
|
kTy|
distributionis alsomeasured inthelower multiplicity V0A eventclass 40–100%.Ifstrong nuclear ef-fects are present they are expected to be stronger in the high multiplicity events due to the larger number of participants in the collision. A comparison is shown in the left panel of Fig. 2. The systematicuncertainties betweenthe twomeasurements are fully correlated since they originate fromthe uncertainty on the jet energyscaleof thefulljet. Theconsistency betweenthe
|
kTy|
distributionsinthehighandlowmultiplicityeventclasswas eval-uatedbytakingtheratioandperformingaconstantfittakinginto account onlythe statistical errors. The fitis within 1
.
2σ
consis-tentwithunity.Thisresultshowsthatinthemeasuredkinematical region, possible nuclear matter effects and/or shadowing in the|
kTy|
distributionsinp–Pbcollisionsarenot observedfordijetsatmidrapidity.
The sensitivity to dijet acoplanarity is enhanced by selecting more pT imbalanced jet pairs. The
|
kTy|
distribution for full jetswith 70
<
pchT,jet+ne<
120 GeV/
c for various pchT,assoc jet ranges is shown in the right panel of Fig. 2. The|
kTy|
distribution tendsto become steeper if jets are more balanced indicating that the influence of QCD radiation decreases. This behavior supports the previous observation (Section 4.1) that the dijet
|
kTy|
observableforhighlyenergeticjetsisoverawiderangeof
|
kTy|
mainlysensi-tivetoQCDradiationprocessesratherthanelasticscatterings.
4.3. Evolutionandcharacterizationvia
|
kTy|
The measured
|
kTy|
distributions are further characterized byFig. 2. Distributions of|kTy|for two V0A event classes (left panel) and three pchT,assoc jetranges (right panel).
Fig. 3. Meanofthe
|
kTy|distributionsasafunctionofthefulljettransversemomentumpchT,jet+ne(left)andtheassociatedchargedjettransversemomentumpchT,assoc jet(right) comparedtoPYTHIA8.extractedmomentisbiasedbystatisticalfluctuationsforlarge val-ues of
|
kTy|
, the distributions are extrapolated using a templategenerated with PYTHIA8 (tune 4C, K
=
0.
7), which agrees well withthep–Pbmeasurement(seeFig. 1
).ThePYTHIA|
kTy|
distribu-tionisnormalizedtominimizethe
χ
2 betweendataandPYTHIA.Thetransition fromthe datato the normalizedtemplate is fixed at60%ofthekinematiclimit
|
kTy|
max.Thetransitionpointisvar-iedtoestimatethesystematicuncertaintyfromthisextrapolation procedure.Inaddition,the normalizationofthePYTHIAtemplate isvaried byonestandarddeviationofthefituncertainty.This re-sultsinan additionalsystematicuncertainty onthe extractionof
|
kTy|
. Forlow pT,jetch+ne theuncertainty on the extractedmean isequal to 2.9% and increases to 8.1% for the highest pchT,jet+ne val-ues.
Theleftpanel of
Fig. 3
showsthemeanofthemeasured|
kTy|
distributionsasafunctionofthefulljettransversemomentumand iscomparedtothePYTHIAvalues.Themeasuredmomentinp–Pb collisionsagreeswithintheuncertaintiesofthemeasurementwith the PYTHIA8 expectation. The mean increases with pchT,jet+ne since the additional kT due to radiative QCD processes increases with
pchT,jet+ne.
The right panel of Fig. 3 shows the evolution of
|
kTy|
as afunction of pchT,assoc jet for 70
<
pT,jetch+ne<
120 GeV/
c. The mean,|
kTy|
, is compared to the earlier presented PYTHIA8 tune inSection 4.1 and is in agreement within the uncertainties of the measurement.The meanfor 60
<
pchT,jet+ne<
80 GeV/
c isreported for two multiplicity event classes in Table 1. No significant dif-ference is observed as a function of the multiplicity measured with V0A.Table 1
Meanofthe
|
kTy|distributionsfor60<pchT,jet+ne<80 GeV/c and15<pchT,assoc jet< pchT,jet+neGeV/c inahigh(0–40%)andlow(40–100%)V0Amultiplicity eventclass. Thefirstquoteduncertaintyisstatisticalwhilethesecondissystematic.Thelast columncorrespondstothe valuesfromthe PYTHIA8calculationat particlelevel withthesamekinematicselection.TheuncertaintyonthePYTHIAcalculationis statistical.0–40% 40–100% PYTHIA8 pp
|kTy|(GeV/c) 14.7±0.8±0.3 13.6±1.1±0.5 15.1±0.1
5. Conclusion
The dijetacoplanarityin p–Pbcollisions was studied by mea-suringdijettransversemomentum
|
kTy|
.Theevolution of|
kTy|
asfunction of the transverse momentum of the full jet, associated chargedjetandeventmultiplicitywaspresented.The
|
kTy|
spectrafordifferentfullandassociatedchargedjettransversemomentum rangesinthe0–40%V0Aeventmultiplicity classwerefound con-sistentwiththePYTHIAprediction.Theobservedincreasewithjet energyfromthemean
|
kTy|
of6.
6±
0.
4 (stat.)±
0.
2 (syst.) GeV/
cto 18
.
8±
1.
3 (stat.)±
1.
5 (syst.) GeV/
c as well as the observed narrowing of|
kTy|
formore balanced jetssuggests that the dijet|
kTy|
spectrum for large Q2 processes is mainly sensitive to theincreasedavailable phase–spaceforQCD radiationprocesses. Fur-thermorethedijetacoplanaritywasfoundtobeconsistent(within 1
.
2σ
) inthetwoeventmultiplicityclassesanalyzedinthisstudy, indicating thatinthe measuredkinematicalregion nostrong nu-clear matter effects in p–Pb collisions are observed. Since these resultsindicatethatnuclearkTeffectsaresmall,thepT imbalanceofjetcorrelationsinPb–Pbresults[35,36]areunlikelytooriginate frommultiplescatteringsinthenucleartarget.
Acknowledgements
The ALICE Collaboration would like to thank all its engineers andtechnicians fortheir invaluablecontributionstothe construc-tionoftheexperimentandtheCERNacceleratorteamsforthe out-standingperformanceoftheLHCcomplex.TheALICECollaboration gratefully acknowledges the resources and support provided by all Gridcenters and theWorldwide LHCComputing Grid(WLCG) Collaboration. The ALICE Collaboration acknowledges the follow-ing funding agencies for their support in building and running theALICEdetector:StateCommitteeofScience,World Federation of Scientists (WFS) andSwiss Fonds Kidagan, Armenia, Conselho Nacionalde DesenvolvimentoCientífico e Tecnológico (CNPq), Fi-nanciadorade Estudose Projetos(FINEP),Fundação de Amparoà Pesquisa do Estado de São Paulo (FAPESP); National Natural Sci-enceFoundation ofChina (NSFC), theChinese Ministryof Educa-tion(CMOE)andtheMinistryofScienceandTechnologyofChina (MSTC); Ministry of Education andYouth of the Czech Republic; Danish Natural Science Research Council, the Carlsberg Founda-tionandthe DanishNationalResearchFoundation;The European ResearchCouncilundertheEuropeanCommunity’sSeventh Frame-work Programme; Helsinki Institute of Physics and the Academy ofFinland;FrenchCNRS-IN2P3, the‘RegionPaysdeLoire’,‘Region Alsace’, ‘Region Auvergne’ and CEA, France; German Bundesmin-isterium fur Bildung, Wissenschaft, Forschung und Technologie (BMBF)andtheHelmholtzAssociation;GeneralSecretariatfor Re-searchandTechnology,MinistryofDevelopment,Greece; Hungar-ianOrszagosTudomanyosKutatasiAlappgrammok(OTKA)and Na-tionalOfficeforResearchandTechnology (NKTH); Departmentof AtomicEnergy andDepartmentofScienceandTechnology ofthe Governmentof India;Istituto Nazionale diFisica Nucleare(INFN) and Centro Fermi – Museo Storico della Fisica e Centro Studi e Ricerche“EnricoFermi”,Italy;MEXTGrant-in-AidforSpecially Pro-motedResearch,Japan;JointInstituteforNuclearResearch,Dubna; National Research Foundation of Korea (NRF); Consejo Nacional de Ciencia y Tecnología (CONACYT), Direccion General de Asun-tos del Personal Academico (DGAPA), México, Amerique Latine Formation academique–European Commission (ALFA–EC) andthe EPLANETProgram(EuropeanParticle PhysicsLatin American Net-work)StichtingvoorFundamenteelOnderzoekder Materie(FOM) andtheNederlandseOrganisatievoorWetenschappelijkOnderzoek (NWO),Netherlands; ResearchCouncil ofNorway (NFR); National ScienceCentre ofPoland;MinistryofNationalEducation/Institute forAtomic PhysicsandConsiliul Na ¸tionalal Cercet˘arii ¸Stiin ¸tifice– Executive AgencyforHigherEducationResearchDevelopmentand Innovation Funding (CNCS–UEFISCDI), Romania; Ministry of Edu-cationandScience oftheRussian Federation,RussianAcademyof Sciences, Russian Federal Agency ofAtomic Energy, Russian Fed-eralAgencyforScience andInnovationsandThe Russian Founda-tionforBasicResearch;MinistryofEducationofSlovakia; Depart-mentofScienceandTechnology,RepublicofSouthAfrica; Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT),E-InfrastructuresharedbetweenEuropeandLatin Amer-ica(EELA), MinisteriodeEconomíayCompetitividad(MINECO)of Spain,XuntadeGalicia(ConselleríadeEducación),Centrode Apli-cacionesTecnológicasyDesarrolloNuclear(CEADEN),Cubaenergía, Cuba, and IAEA (International Atomic Energy Agency); Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW); Ukraine Ministry of Education andScience; United King-domScienceandTechnology FacilitiesCouncil (STFC);TheUnited States Department of Energy, the United States National Science Foundation,the StateofTexas,andtheState ofOhio;Ministry of Science,EducationandSportsofCroatiaandUnitythrough Knowl-edge Fund, Croatia. Council of Scientific and Industrial Research (CSIR),NewDelhi,India.
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ALICECollaboration
