Multiplicity dependence of charged pion, kaon, and (anti)proton production at large transverse momentum in p–Pb collisions at √sNN=5.02 TeV

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Physics Letters B

www.elsevier.com/locate/physletb

Multiplicity dependence of charged pion, kaon, and (anti)proton production at large transverse momentum 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 ra c t

Articlehistory:

Received16January2016

Receivedinrevisedform13July2016 Accepted20July2016

Availableonline22July2016 Editor:L.Rolandi

The productionofchargedpions,kaonsand(anti)protons hasbeenmeasuredatmid-rapidity(−⁰.5<

y<0) inp–Pbcollisions at√_s

NN=⁵.02 TeV usingthe ALICEdetector atthe LHC. Exploitingparticle identification capabilitiesathightransversemomentum(pT),thepreviouslypublished pTspectrahave been extended toinclude measurements upto 20 GeV/cfor seven event multiplicity classes.The pT spectraforppcollisionsat√

s=^{7 TeV,neededto}interpolateappreferencespectrum,havealsobeen extendedupto20 GeV/ctomeasurethenuclearmodificationfactor(R_pPb)innon-singlediffractivep–Pb collisions.

At intermediate transverse momentum (2<pT<10 GeV/c) the proton-to-pion ratio increases with multiplicityinp–Pbcollisions,asimilareffectisnotpresentinthekaon-to-pionratio.ThepTdependent structure ofsuchincreaseisqualitatively similar tothoseobserved inppandheavy-ion collisions.At high pT(>10 GeV/c),theparticleratiosareconsistentwiththosereportedforppandPb–Pbcollisions attheLHCenergies.

AtintermediatepTthe(anti)protonRpPbshowsaCronin-likeenhancement,whilepionsandkaonsshow littleornonuclearmodification.AthighpTthechargedpion,kaonand(anti)protonRpPb areconsistent withunitywithinstatisticalandsystematicuncertainties.

©^{2016TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

In heavy-ion collisions at ultra-relativistic energies, it is well establishedthatastronglycoupledQuark–Gluon-Plasma(sQGP)is formed[1–5].Someofthecharacteristicfeatures ofthesQGPare strongcollectiveflowandopacitytojets.Thecollectivebehavioris observedbothasanazimuthalanisotropyofproducedparticles[6], wherethemagnitudeisdescribed byalmostideal(reversible)hy- drodynamics,andasahardeningofp_Tspectraforheavierhadrons, such as protons,by radial flow [7].Jet quenchingis observed as a reduction of both high pT particles [8,9] and also fully recon- structedjets[10].TheinterpretationofthesesQGP propertiesre- quirescomparisonswithreferencemeasurementslikeppandp–A collisions. Recentmeasurements in highmultiplicity pp,p–A and d–Acollisions atdifferentenergies haverevealedstrongflow-like effects even in these small systems [11–20]. The origin of these phenomenaisdebated[21–29]andthedatareportedhereprovide furtherinputstothisdiscussion.

E-mailaddress:alice-publications@cern.ch.

In a previous work, we reported the evidence ofradial flow- like patternsin p–Pbcollisions [30].Thiseffect was foundto in- crease with increasing event multiplicity and to be qualitatively consistent with calculationswhich incorporatethe hydrodynami- cal evolution of the system. It was also discussed that in small systems, mechanisms like color-reconnection mayproduce radial flow-like effects. Thepresent paperreportscomplementary mea- surements coveringtheintermediate p_T region(2–10 GeV/c)and thehigh-pTregion(10–20 GeV/c)exploitingthecapabilitiesofthe High MomentumParticle IdentificationDetector(HMPID)andthe Time ProjectionChamber (TPC). Inthis way,highprecision mea- surementsare achievedintheintermediate p_T regionwherecold nuclear matter effects like the Cronin enhancement [31,32] have beenreportedbypreviousexperiments[33,34],andwherethepar- ticle ratios, e.g., the proton(kaon) production relative to that of pions,areaffectedbylargefinalstateeffectsincentralPb–Pbcol- lisions[35].Particleratiosareexpectedtobemodifiedbyflow,but hydrodynamicsistypicallyexpectedtobeapplicableonlyup toa few GeV/c [36].Athigher pT,ideas suchaspartonrecombination havebeenproposed leadingtobaryon–meson effects[37].Inthis waythenewdatasetcomplementsthelower pT results.

http://dx.doi.org/10.1016/j.physletb.2016.07.050

0370-2693/©^{2016TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

Inaddition,particleidentificationatlargetransversemomenta inp–Pbcollisions providesnewconstraintsonthenuclearparton distributionfunctions(nPDF) whicharekeyinputsininterpreting a large amount ofexperimental data like d–Au anddeep inelas- ticscattering [38]. Finally,the measurement is alsoimportant to studytheparticlespeciesdependencyofthenuclearmodification factor(R_pPb),tobetterunderstandpartonenergylossmechanisms inheavy-ioncollisions.

In this paper, the charged pion, kaon and (anti)proton RpPb are reportedfor non-singlediffractive (NSD) p–Pb collisions. The pp reference spectra for this measurement were obtained us- ing interpolations of data at different collision energies. The al- ready published p_T spectra for inelastic (INEL) pp collisions at

√s=^{7 TeV [39] were extended up to 20 GeV/c andthe results} are presented here for the first time. These measurements to- gether with the results for INEL pp collisions at √

2=2.76 TeV (pT<20 GeV/c)[35]wereusedtodetermineppreferencespectra at√

s=⁵.02 TeV usingtheinterpolationmethoddescribedin[40].

Thepaperisorganized asfollows.InSec.2theALICEdetector aswellastheeventandtrackselectionsarediscussed.Theanaly- sisprocedures forparticleidentificationusingtheHMPIDandTPC detectorsareoutlinedinSec. 3andSec. 4,respectively.Section5 presentstheresultsanddiscussions.Finally,Sec.6summarizesthe mainresults.

2. Datasample,eventandtrackselection

The results are obtained using data collected with the ALICE detectorduring the 2013 p–Pbrun at √

s_NN=⁵.02 TeV. The de- taileddescription oftheALICEdetectorcanbe foundin [41] and the performance during run 1 (2009–2013) is described in [42].

Becauseofthe LHC 2-in-1 magnetdesign,it isimpossibleto ad- justtheenergy ofthe protonandlead-ionbeams independently.

Theyare 4 TeV per Z which gives different energies due to the differentZ/A ofthecollidingprotonsandleadions.Thenucleon–

nucleoncenter-of-masssystemismoving inthelaboratory frame with a rapidity of yNN= −⁰.465 in the direction of the proton beamrapidity.Inthefollowing, ylab(

η

lab)areusedtoindicatethe (pseudo)rapidityinthelaboratoryreferenceframe,whereas y (

η

) denotesthe (pseudo)rapidityin thecenter-of-massreferencesys- temwherethePbbeamisassignedpositiverapidity.

In the analysis of the p–Pb data, the event selection follows that used in the analysis of inclusive charged particle produc- tion[43].The minimumbias(MB)triggersignal wasprovided by theV0 counters[44],which contain two arraysof32 scintillator tileseach coveringthefullazimuthwithin 2.8<

η

lab<5.1 (V0A) and −³.7<

η

_lab<−¹.7 (V0C). The signal amplitude and arrival timecollectedineachtilewererecorded.Acoincidenceofsignals inboth V0Aand V0Cdetectors was required to remove contam- inationfrom single diffractiveand electromagneticevents.In the offlineanalysis,backgroundeventswerefurthersuppressedbyre- quiring the arrival time of signals on the neutron Zero Degree CalorimeterA,whichispositionedinthePb-goingdirection,tobe compatiblewith a nominal p–Pbcollision occurringclose to the nominalinteractionpoint.Theestimatedmeannumberofinterac- tionsper bunch crossingwas below1% inthe sample chosenfor thisanalysis.Duetotheweakcorrelationbetweencollisiongeom- etryandmultiplicity,theparticleproductioninp–Pbcollisions is studiedineventmultiplicityclassesinsteadofcentralities[45].The multiplicity classes are defined using the total charge deposited in the V0A detector as in [30], where V0A is positioned in the Pb-going direction. The MB results have been normalized to the totalnumberofNSDeventsusingatriggerandvertexreconstruc- tionefficiencycorrection whichamountsto3.6%±³.1% [46].The multiplicitydependentresultshavebeennormalizedtothevisible

Table 1

Transverse momentum ranges(GeV/c)covered bythe individual and combined analysesforppcollisionsat√

s=7 TeV andp–Pbcollisionsat√

sNN=5.02 TeV.

Analysis π⁺+π⁻ K⁺+K⁻ p+ ¯p

pp Published[39]^a 0.1–3.0 0.2–6.0 0.3–6.0

TPC dE/dxrel. rise 2–20 3–20 3–20

p–Pb Published[30]^b 0.1–3.0 0.2–2.5 0.3–4.0

HMPID 1.5–4.0 1.5–4.0 1.5–6.0

TPC dE/dxrel. rise 2–20 3–20 3–20

a Includeddetectors:ITS,TPC,Time-of-Flight(TOF),HMPID.Theresultsalsoin- cludethekink-topologyidentificationoftheweakdecaysofchargedkaons.

b Includeddetectors:ITS,TPC,TOF.

(triggered) cross-section correcting for the vertex reconstruction efficiency (this was not done in [30]). This correction is of the order of 5% for the lowest V0A multiplicity class (80–100%) and negligiblefortheothermultiplicityclasses(<1%).

Inthe√

s=^{7 TeV ppanalysistheMB triggerrequireda hitin} the two innermostlayers of the Inner TrackingSystem(ITS), the SiliconPixelDetector(SPD),orinatleastoneoftheV0scintilla- torarraysincoincidence withthe arrivalofprotonbunchesfrom both directions.The offline analysisto eliminate backgroundwas done usingthetime informationprovidedby theV0 detectorsin correlationwiththenumberofclustersandtracklets¹intheSPD.

TracksarerequiredtobereconstructedinboththeITSandthe TPC.Additionaltrackselectioncriteriaarethesameasin[47]and basedonthe numberofspacepoints,thequality ofthetrackfit, and the distance of closest approach to the reconstructed colli- sionvertex. Chargedtracks wheretheidentityofthe particlehas changeddueto aweakdecay, e.g.,K⁻→

μ

⁻_{+ ¯}

ν

μ,areidentified bythetrackingalgorithmduetotheirdistinctkinktopologies[48]

andrejectedinthisanalysis. Theremainingcontaminationisneg- ligible (^{1%). In order to have the same kinematic coverage as} used in the p–Pb low pT analysis [30], the tracks were selected in the pseudorapidity interval −⁰.5<

η

<0. In addition, for the HMPID analysisit isrequired that thetracks are propagated and matchedtoaprimaryionizationclusterintheMulti-WirePropor- tionalChamber(MWPC)gapoftheHMPIDdetector[39,47].

The published results of charged pion,kaon and(anti)proton production at low pT for pp [39] and p–Pb [30] collisions at

√s=^{7 TeV and}√

sNN=⁵.02 TeV,respectively,useddifferentPar- ticleIDentification (PID) detectorsandtechniques.A summary of the p_T rangescoveredbythepublishedanalysesandtheanalyses presentedinthispapercanbefoundinTable 1.

In the following, the analysis techniques used to obtain the identified particle p_T spectra in the intermediate and high-p_T rangesusingHMPIDandTPCwillbediscussed.

3. HMPIDanalysis

TheHMPID detector[49]islocated about5 mfromthebeam axis, covering a limited acceptance of |

η

_lab|<0.5 and 1.2^◦<

φ <58.5^◦, that corresponds to ∼^{5% of the TPC} geometrical ac- ceptance (2

π

in azimuthal angle and the pseudo-rapidity inter- val |

η

|<0.9 [50]) for high pT tracks. The HMPID analysis uses

∼⁹×¹⁰⁷ minimum-bias p–Pb events at √

sNN=⁵.02 TeV. The eventandtrackselectionandtheanalysistechniquearesimilarto thosedescribedin[39,47].Itisrequiredthattracksarepropagated andmatchedtoaprimaryionizationclusterintheMulti-WirePro- portionalChamber(MWPC)gapoftheHMPIDdetector.ThePIDin theHMPIDisdonebymeasuringtheCherenkovangle,θCh[49]:

1 TrackletsarepairsofhitsfromthetwolayersoftheSPDwhichmakealine pointingbacktothecollisionvertex.

Fig. 1.(Coloronline.)CherenkovanglemeasuredintheHMPIDasafunctionofthe trackmomentuminp–Pbcollisionsat√

sNN=5.02 TeV forthe0–5%V0Amultiplic- ityclass(seethetextforfurtherdetails).Thedashedlinesrepresenttheexpected curvescalculatedusingEq.(1)foreachparticlespecies.

cos

θ

_Ch

=

¹

n

β =⇒ θ

_Ch

=

^arccos

p²

+

^m2 np

,

(1)

wheren istherefractive indexofthe radiatorused(liquid C₆F₁₄ withn=¹.29 atEph=⁶.68 eV andtemperatureT=^{20◦C), pand} m are the momentum and the mass of the given particle, re- spectively. The measurement of the single photon θCh angle in theHMPIDrequirestheknowledgeofthetrackparameters,which areestimatedbythetrackextrapolationfromthecentral tracking detectors up to the radiator volume, where the Cherenkov pho- tons are emitted. Only one charged particle cluster is associated to each extrapolatedtrack, selected as the closest cluster to the extrapolatedtrackpoint on thecathode plane. Toreject thefake cluster-matchassociations inthe detector,a selectionon thedis- tanced_(track-MIP)computedonthecathodeplanebetweenthetrack extrapolationpointandthereconstructedcharged-particle cluster position is applied. The distance has to be less than 5 cm, in- dependent oftrack momentum. Starting fromthe photon cluster coordinates on the photocathode, a back-tracking algorithm cal- culatesthe correspondingemissionangle.TheCherenkovphotons areselectedbytheHoughTransformMethod(HTM)[51]thatdis- criminatesthe signal fromthe background.Foragiventrack, the Cherenkovangle θCh is then computed asthe weighted mean of the single photon angles selected by the HTM. Fig. 1 showsthe θ_Ch asa function ofthe trackmomentum.The reconstructed an- gledistributionforagivenmomentuminterval isfittedbya sum ofthreeGaussiandistributions, correspondingto thesignalsfrom pions,kaons,andprotons.The fittingisdone intwosteps.Inthe firststeptheinitialvaluesoffitparametersaresettotheexpected values.Themeanvalues,θCh i,are obtainedfromEq.(1),tuning therefractiveindextomatchtheobservedCherenkovangles,and theresolutionvaluesare takenfromaMonteCarlosimulation of thedetectorresponse.Afterthisfirststep,the pT dependencesof themeanandwidtharefittedwiththefunctiongivenby Eq.(1) anda polynomialone,respectively. Inthesecondstep,the fitting isrepeatedwiththeyieldsastheonlyfreeparameters,constrain- ingthemeanandresolutionvaluestothefittedvalue.Thesecond iteration is particularly important at high p_T where the separa- tionbetweendifferentspeciesisreduced.Fig. 2givesexamplesof fitstothereconstructedCherenkovangledistributionsintwonar- rowp_Tintervalsforthe0–5%multiplicityclass.Therawyieldsare then correctedby thetotal reconstructionefficiency givenbythe convolution of the tracking,PID efficiency, anddistance cut cor- rection. The tracking efficiency, convoluted with the geometrical

Fig. 2. (Color online.) Distributions of the Cherenkov angle measured in the HMPID for positivetracks having pT between2.5–2.6 GeV/c (left)and between 3.8–4.0 GeV/c(right),inp–Pbcollisionsat√

sNN=5.02 TeV forthe0–5%V0Amul- tiplicityclass(seethetextforfurtherdetails).

acceptanceofthedetector,hasbeenevaluatedusingMonteCarlo simulations. Forall threeparticle speciesthisefficiencyincreases from ∼^{5% at1.5 GeV/c upto} ∼^{6%at high p}T.The PIDefficiency isdetermined bytheCherenkovanglereconstruction efficiency.It has been computedby means of Monte-Carlo simulations and it reaches∼^{90%forparticleswithvelocity}β∼^{1,withnosignificant} difference betweenpositive andnegative tracks.The distancecut correction,definedastheratiobetweenthenumberofthetracks that pass thecut ond_(track-MIP) andall thetracksin thedetector acceptance, has been evaluated from data. It is momentum de- pendent,andit is equalto ∼^{53% at1.5GeV/c, reaches}∼^70%for particles withvelocityβ∼^{1.Asmalldifference betweenpositive} and negative tracks is present; negative tracks having a distance correction ∼^{2%lower thanthepositiveones.Thiseffectiscaused} by aradialresidualmisalignmentoftheHMPIDchambersandan imperfect estimation ofthe energyloss inthe materialtraversed by the track. Tracking efficiency, PID efficiency and distance cut correctiondonotshowvariationwiththeeventtrackmultiplicity.

3.1. Systematicuncertainties

ThesystematicuncertaintyontheresultsoftheHMPIDanaly- sishascontributionsfromtracking,PIDandtracksassociation[39, 47].Theuncertaintiesrelatedtothetrackinghavebeenestimated by changingthe trackselectioncutsindividually,e.g. thenumber of crossed readout rowsin the TPC and the value of the track’s

χ

² normalizedtothenumberofTPCclusters.ToestimatethePID contribution,theparameters(meanandresolution)ofthefitfunc- tion used to extract the raw yields were varied by a reasonable quantity,leavingthemfreeinagivenrange;therangechosenfor the mean values is [θ_Ch −

σ

, θ_Ch +

σ

] andfor the resolution [

σ

−0.1

σ

,

σ

+0.1

σ

].A variationof10% oftheresolutioncorre- sponds to its maximum expected variation when takinginto ac- countthedifferentrunningconditionsofthedetectorduringdata takingwhichhaveanimpactonitsperformance.Whenthemeans are changed,the resolution values are fixed to the defaultvalue andvice versa.The variation ofparameters isdone forthe three Gaussians(correspondingtothethreeparticlespecies)simultane- ously.Inaddition,theuncertaintyoftheassociationofthetrackto thechargedparticlesignalisobtainedbyvaryingthedefaultvalue ofthe distancecutrequiredforthematchby ±^{1 cm.Thesecon-} tributions do notvary withthecollision multiplicity.A summary ofthedifferentcontributionstothesystematicuncertaintyforthe HMPIDp–PbanalysisisshowninTable 2.

4. TPCdE/dxrelativisticriseanalysis

The relativistic rise regime of the specific energyloss, dE/dx, measuredbytheTPCallowsidentificationofchargedpions,kaons,

Table 2

MainsourcesofsystematicuncertaintiesfortheHMPIDp–Pbanalysis.

Effect π⁺+π⁻ K⁺+^{K− p}+ ¯^p

pTvalue (GeV/c) 2.5 4 2.5 4 2.5 4

PID 6% 12% 6% 12% 4% 5%

Tracking efficiency 6% 6% 7%

Distance cut correction 6% 2% 6% 2% 4% 2%

Fig. 3.(Coloronline.)Specificenergyloss,dE/dx,asafunctionofmomentumpin thepseudorapidityrange−⁰.5<η<−⁰.375 forminimumbiasp–Pbcollisions.In eachmomentumbinthedE/dxspectrahavebeennormalizedtohaveunitintegrals andonlybinswithmorethan2%ofthecountsareshown(makingelectronsnot visibleinthefigure,exceptatverylowmomentum).ThecurvesshowthedE/dx responseforpions,kaons,protonsandelectrons.

and(anti)protons up to pT=^{20 GeV}/c. The results presentedin this paper were obtained using the method detailed in [47]. In thisanalysis, around 8×^{107 (4}.7×^{107) p–Pb(pp) MB triggered} eventswere used.Theeventandtrackselection hasalreadybeen discussedinSection2.

Asdiscussedin[47],thedE/dxiscalibratedtakingintoaccount chambergainvariations,trackcurvatureanddiffusion,toobtaina response that essentially only dependson β

γ

. Inherently, tracks atforwardrapiditywillhavebetterresolutionduetolongerinte- gratedtrack-lengths,soinordertoanalyzehomogeneoussamples theanalysisisperformedinfour

η

intervals.Samplesoftopolog- icallyidentified pions (fromK⁰_S decays),protons (from decays) andelectrons (from

γ

conversions)wereusedtoparametrizethe Bethe–Blochresponse, ^dE/dx (β

γ

), andthe relative resolution,

σ

dE/dx (^dE/dx) [47]. For the p–Pb data, these response func- tionsarefoundtobeslightlymultiplicitydependent(the ^dE/dx changes by ∼^{0.4% and the sigma by} ∼^{2.0%). However, a single} setof functionsisused forall multiplicity intervals, andthe de- pendenceisincludedinthesystematicuncertainties.Fig. 3shows dE/dx asafunction ofmomentumforp–Pb events.The charac-

teristic separation power between particle species in number of standard deviations (Sσ) as a function of p, is shown in Fig. 4 forminimumbiasp–Pbcollisions. Forexample, Sσ forpionsand kaonsiscalculatedas:

S_σ

=

dE

dx

π⁺+π⁻

−

dE dx

K⁺+^K− 0

.

5

σ

_π++π⁻

+ σ

_K++^K−

.

(2) Theseparationinnumberofstandarddeviationsisthe largest (smallest) betweenpions andprotons(kaonsandprotons)andit isnearlyconstantatlargemomenta.

Themainpartofthisanalysisisthedeterminationoftherela- tive particleabundances,hereafter calledparticlefractions,which are definedasthe

π

⁺+

π

⁻, K⁺+^K−,p+ ¯^{p and e+}+^{e− yields} normalized to that for inclusive charged particles. Since the TPC dE/dxsignalisGaussiandistributedasillustrated in[47],particle fractions are obtainedusingfour-Gaussian fits to dE/dx distribu- tions in

η

and p intervals. The parameters (mean and width) of thefits arefixed usingtheparametrizedBethe–Blochandresolu- tioncurvesmentioned earlier.Examplesofthesefitscan beseen in Fig. 5 for two momentum intervals, 3.4<p<3.6 GeV/c and 8<p<9 GeV/c.Theparticlefractionsina p_Trange,areobtained astheweightedaverageofthecontributingp intervals.Since the particlefractionsasafunctionof pT arefoundtobe independent of

η

,they are averaged. The particlefractions measured in p–Pb and pp collisions are corrected forrelative efficiency differences usingDPMJET[52] andPHOJET[53]MonteCarlo(MC) generators, respectively.Furthermore,therelativepionandprotonabundances werecorrectedforthecontaminationofsecondaryparticles(feed- down),moredetailsofthemethodcanbefoundin[47].

Theinvariantyields,1/(2

π

pT)d²N/dydpT,areconstructedus- ingtwo components:thecorrectedparticlefractionsandthecor- rected invariant charged particle yields. For the pp analysis at

√s=^{7 TeV,thelattercomponentwastakendirectlyfromthepub-} lished results forinclusive chargedparticles [40].However, anal- ogous resultsfor p–Pb data are neither available for neither the kinematic range −⁰.5<y<0 nor for the different multiplicity classes [54], they were therefore measured here and the results usedtodeterminetheinvariantyields.

4.1. Systematicuncertainties

Thesystematicuncertaintiesmainlyconsistoftwocomponents:

thefirstisduetotheeventandtrackselection,andthesecondone isduetothePID.Thefirstcomponentwasobtainedfromtheanal- ysisofinclusivechargedparticles[40,54].ForINELppcollisionsat 7TeV,thesystematicuncertaintieshavebeentakenfrom[40].For p–Pb collisions, thereare no measurements in the

η

interval re- portedhere(−⁰.5<

η

<0);however,it hasbeenshownthat the systematicuncertaintyexhibitsa negligibledependenceon

η

and multiplicity [45].Therefore, thesystematic uncertainties reported

Fig. 4.Separationinnumberofstandarddeviationsbetween:pionsandprotons(leftpanel),pionsandkaons(middlepanel),andkaonsandprotons(rightpanel).Resultsfor minimumbiasp–Pbdataandfortwospecificpseudorapidityintervalsareshown.Moredetailscanbefoundin[47].

Fig. 5.(Coloronline.)Four-Gaussianfits(lines)tothedE/dxspectra(markers)fortrackshavingmomentum3.4<p<3.6 GeV/c(toprow)and8.0<p<9.0 GeV/c(bottom row)within−0.125<η<0.Allofthespectraarenormalizedtohaveunitintegrals.ColumnsrefertodifferentV0Amultiplicityclasses.Individualsignalsofchargedpions, kaons,and(anti)protonsareshownasred,green,andbluedashedareas,respectively.Thecontributionofelectronsisnotvisibleandisnegligible(<1%).

Table 3

Summaryofthesystematicuncertaintiesforthechargedpion,kaon,and(anti)protonspectraandfor theparticle ratios.NotethatK/π=(K⁺+K⁻)/(π⁺+π⁻)and p/π=(p+ ¯^p)/(π⁺+π⁻).

pT(GeV/c) π⁺+π⁻ K⁺+K⁻ p+ ¯p K/π p/π

2.0 10 3.0 10 3.0 10 3.0 10 3.0 10

pp collisions Uncertainty

Eventandtrackselection^a 7.3% 7.3% 7.3% – –

Feed-downcorrection 0.2% – 1.2% 0.2% 1.2%

Efficiencycorrection 3.2% 3.2% 3.2% 4.5% 4.5%

Correctionformuons 0.3% 0.5% – – 0.3% 0.5% 0.3% 0.5%

ParametrizationofBethe–Bloch andresolutioncurves

1.8% 1.9% 20% 6.9% 24% 15% 17% 9.0% 17% 19%

p–Pb collisions Uncertainty

Eventandtrackselection^a 3.3% 3.6% 3.3% 3.6% 3.3% 3.6% – –

Feed-downcorrection ≤^{0.2% – 2.6% 0.7%} ≤^{0.2% 2.6% 0.7%}

Efficiencycorrection 3.2% 3.2% 3.2% 4.5% 4.5%

Correctionformuons^b 0.3% 0.4% – – 0.3% 0.4% 0.3% 0.4%

ParametrizationofBethe–Bloch andresolutioncurves Multiplicity

classes

0–5% 1.7% 1.9% 17% 8.0% 15% 13% 16% 10.4% 12% 11%

5–10% 1.7% 2.0% 17% 5.6% 16% 12% 18% 7.2% 14% 24%

10–20% 1.6% 1.9% 16% 7.2% 16% 12% 18% 9.5% 16% 15%

20–40% 1.6% 2.0% 16% 6.7% 17% 15% 18% 8.0% 17% 18%

40–60% 1.5% 1.9% 15% 6.5% 17% 12% 18% 8.3% 18% 13%

60–80% 1.6% 1.8% 16% 6.3% 20% 13% 21% 8.3% 22% 18%

80–100% 1.4% 1.5% 13% 5.9% 20% 13% 16% 7.3% 23% 21%

a Commontoallspecies,valuestakenfrom[40,54].

b Foundtobemultiplicityindependent.

in [54] have been assigned to the identified charged hadron pT spectraforalltheV0Amultiplicityclasses.

Thesecond componentwas measuredfollowingtheprocedure described in [47], where the largestcontribution is attributedto theuncertainties in theparameterization ofthe Bethe–Blochand resolutioncurvesusedtoconstrainthefits.Theuncertaintyiscal- culatedby varyingthe^dE/dx and

σ

dE/dx intheparticlefraction fits(Fig. 5)withintheprecisionofthedE/dxresponsecalibration,

∼^{1% and5%fordE}/dx and

σ

dE/dx,respectively.Asmallfraction ofthisuncertaintywasfoundtobemultiplicitydependent,itwas estimatedasdoneinthepreviousALICEpublication[30].

A summary of the main systematic uncertainties on the pT spectraand theparticle ratios forp–Pb andpp collisions can be

found inTable 3fortwo pT intervals. Forpions,themaincontri- bution is related toevent andtrackselection andthe associated commoncorrections. Inthe caseofkaonsandprotonsthelargest uncertainty isattributedto the parametrizationof thedE/dx re- sponse.Forkaons,theuncertaintydecreaseswithpT andincreases withmultiplicitywhileforprotonsthemultiplicitydependenceis opposite.Thisvariationmainlyreflectsthechangesintheparticle ratioswithpT andmultiplicity.

5. Resultsanddiscussions

The totalsystematicuncertaintyforallthe spectraforagiven particlespeciesisfactorizedforeach p_T interval intoamultiplic-

Fig. 6.(Coloronline.)TheratioofindividualspectratothecombinedspectrumasafunctionofpTforpions(left),kaons(middle),andprotons(right).Fromtop-to-bottom therowsshowtheV0Amultiplicityclass0–5%,20–40%and60–80%.Statisticalanduncorrelatedsystematicuncertaintiesareshownasverticalerrorbarsanderrorbands, respectively.OnlythepTrangeswhereindividualanalysisoverlapareshown.Seethetextforfurtherdetails.

Fig. 7.(Coloronline.) Transversemomentumspectraofchargedpions(left),kaons(middle),and(anti)protons(right)measuredinp–Pbcollisionsat√_s

NN=⁵.02 TeV.

Statisticalandsystematicuncertaintiesareplottedasverticalerrorbarsandboxes,respectively.Thespectra(measuredforNSDeventsandfordifferentV0Amultiplicity classes)havebeenscaledbytheindicatedfactorsinthelegendforbettervisibility.

ityindependentandmultiplicitydependentsystematicuncertainty.

Thetransversemomentum distributionsobtainedfromthediffer- ent analyses are combined in the overlapping pT region using a weightedaverage.The weightforthe combinationswasdone ac- cordingtothetotalsystematicuncertaintytoobtainthebestover- allprecision.Sincethesystematicuncertaintiesduetonormaliza- tionandtrackingarecommontoalltheanalyses,theywereadded directly to the final combined results. The statistical uncertain-

tiesaremuchsmallerandthereforeneglectedinthecombination weights.Themultiplicitydependentsystematicuncertaintyforthe combinedspectrais alsopropagated usingthesameweights. For theresults showninthispaperthe fullsystematicuncertainty is always used,butthemultiplicitycorrelated anduncorrelatedsys- tematicuncertaintiesaremadeavailableatHepData.Fig. 6shows examples of the comparisons amongthe individual analyses and the combined p_T spectra, focusingon the overlapping p_T region.

Fig. 8.(Coloronline.)Transversemomentumspectraofchargedpions(left),kaons(middle),and(anti)protons(right)measuredinINELppcollisionsat√

s=2.76 TeV andat

√s=7 TeV.Statisticalandsystematicuncertaintiesareplottedasverticalerrorbarsandboxes,respectively.Thespectrumat√

s=5.02 TeV representsthereferenceinINEL ppcollisions,constructedfrommeasuredspectraat√

s=².76 TeV andat√

s=^{7 TeV.Seethetextforfurtherdetails.Panelsonthebottomshowtheratioofthemeasured} yieldstotheinterpolatedspectra.Onlyuncertaintiesoftheinterpolatedspectraareshown.

Withinsystematicandstatisticaluncertaintiesthenewhigh-pT re- sults, measured with HMPID and TPC, agree with the published results [30]. Similar agreement is obtained forthe pT spectra in INELppcollisionsat7 TeV[39].

5.1. Transversemomentumspectraandnuclearmodificationfactor

Thecombined chargedpion,kaonand(anti)proton p_T spectra inp–Pbcollisions fordifferentV0Amultiplicity classesare shown inFig. 7. Asreported in [30], for pT below2–3 GeV/c the spec- tra behave like in Pb–Pb collisions, i.e., the pT distributions be- comeharder asthemultiplicity increasesandthechangeismost pronounced forprotons andlambdas. Inheavy-ion collisions this effectiscommonlyattributedtoradial flow. Forlargermomenta, thespectrafollowapower-lawshapeasexpectedfromperturba- tiveQCD.

In order to quantify any particle species dependence of the nuclear effects in p–Pb collisions, comparisons to reference pT spectrain pp collisions areneeded. In theabsence ofpp data at

√s=⁵.02 TeV,thereferencespectraareobtainedbyinterpolating datameasuredat√

s=².76 TeV andat√

s=^{7 TeV.Theinvariant} crosssectionforidentifiedhadronproductioninINELppcollisions, 1/(2

π

p_T) d²

σ

_pp^INEL/dydp_T, isinterpolated ineach p_T interval, as- sumingapowerlawdependenceasafunctionof√

s.Themethod was cross-checked using events simulated by Pythia 8.201 [55], wherethedifference betweentheinterpolated andthesimulated referencewas found tobe negligible.The maximum relativesys- tematicuncertaintyofthespectraat √

s=².76 TeV and at√ s= 7 TeV hasbeenassignedasa systematicuncertainty totherefer- ence.Inthetransversemomentuminterval3<pT<10 GeV/c,the totalsystematicuncertainties forpions andkaons arebelow8.6%

and10%,respectively.Whilefor(anti)protonsitis7.7%and18%at 3 GeV/c and10 GeV/c,respectively.Theinvariant yieldsareshown inFig. 8,wheretheinterpolated pTspectraarecomparedtothose measuredinINELppcollisionsat2.76TeVand7TeV.

Thenuclearmodificationfactoristhenconstructedas:

R_pPb

=

^d

2NpPb

/

dydpT

T_pPb

d²

σ

_pp^INEL

/

dydpT

(3)

Fig. 9.(Coloronline.)Thenuclearmodificationfactor RpPbasafunctionoftrans- verse momentum pT for differentparticle species.Thestatisticalandsystematic uncertainties areshownas verticalerror barsandboxes, respectively. Thetotal normalization uncertainty is indicated bya verticalscale ofthe empty boxat pT=0 GeV/c and RpPb=1.Theresultforinclusivechargedhadrons[54]isalso shown.

where, for minimum bias (NSD) p–Pb collisions the average nu- clear overlap function, TpPb

, is 0.0983±⁰.0035 mb⁻¹ [43]. In absenceofnucleareffectstheRpPb isexpectedtobeone.

Fig. 9 showsthe identified hadron R_pPb compared to that for inclusivechargedparticles (h^±)[54] inNSDp–Pbevents.Athigh pT (>10 GeV/c), all nuclear modification factors are consistent withunity withinsystematicandstatisticaluncertainties. Around 4 GeV/c,where aprominent Croninenhancement hasbeen seen atlowerenergies[33,34],theunidentifiedchargedhadron RpPb is above unity, albeitbarely significantwithin systematicuncertain- ties [54]. Remarkably, the (anti)proton enhancement is ∼^{3 times} largerthanthatforchargedparticles,whileforchargedpionsand kaons the enhancement is below that of charged particles. The STAR andPHENIXCollaborations have observeda similar pattern at RHIC,where thenuclear modification factorforMB d–Au col-

Fig. 10.(Coloronline.) Kaon-to-pion(upperpanel)andproton-to-pion(bottompanel)ratiosasafunctionofpT fordifferentV0Amultiplicityclasses.Resultsfor p–Pb collisions(fullmarkers)arecomparedtotheratiosmeasuredinINELpp collisionsat2.76TeV[35](emptycircles)andat 7TeV[39](fullcircles).Thestatisticaland systematicuncertaintiesareplottedasverticalerrorbarsandboxes,respectively.

lisions, RdAu, in the range 2<pT<5 GeV/c, is 1.24±⁰.13 and 1.49±⁰.17 forchargedpionsand(anti)protons,respectively[20].

Anenhancement of protons inthe same p_T rangeis alsoob- served in heavy-ion collisions [35,47], whereit commonlyis in- terpretedasradial-flowandhasastrongcentralitydependence.In thenextsection, westudythemultiplicitydependenceofthein- variantyield ratiostoseewhetherprotonsaremoreenhancedas afunctionofmultiplicitythanpions.

5.2.Transversemomentumandmultiplicitydependenceofparticle ratios

The kaon-to-pion andthe proton-to-pion ratios asa function of p_T fordifferent V0Amultiplicity classes are shownin Fig. 10.

The results for p–Pb collisions are compared to those measured forINELpp collisions at2.76 TeV [35] andat7TeV [39]. Within systematicandstatisticaluncertainties,thepTdifferentialkaon-to- pionratiosdonot show anymultiplicitydependence.In fact,the

resultsaresimilartothoseforINELppcollisionsatbothenergies.

The pT differentialproton-to-pionratiosshow aclearmultiplicity evolution at low and intermediate p_T (<10 GeV/c). This multi- plicityevolutionisqualitativelysimilartothe centralityevolution observedinPb–Pbcollisions[35,47].

Itisworthnotingthattheaveragemultiplicitiesatmid-rapidity for peripheral Pb–Pb collisions (60–80%) and high multiplicity p–Pb collisions (0–5% V0A multiplicity class) are very similar, ^dNch/d

η

∼^{50.Evenifthephysicalmechanismsforparticlepro-} duction could bedifferent, it seems interesting tocompare these systemswithsimilarunderlyingactivityasdoneinFig. 11,where INEL √

s=^{7 TeV ppresultsareincludedasan}approximatebase- line. Withinsystematic andstatistical uncertainties, the kaon-to- pion ratiosare the same forall systems. Onthe other hand,the proton-to-pionratiosexhibitsimilarflow-likefeaturesforthep–Pb andPb–Pb systems,namely,the ratiosare belowthepp baseline for pT<1 GeV/c andabove for pT>1.5 GeV/c.Quantitative dif- ferences areobserved betweenp–Pb andPb–Pb results,butthey

Fig. 11.(Coloronline.)Particleratiosasafunctionoftransversemomentum.Three differentcollidingsystemsarecompared,pp(opensquares),0–5%p–Pb(opencir- cles)and60–80%Pb–Pb(fulldiamonds)collisionsat√_s

NN=^{7 TeV,5.02TeVand} 2.76TeV,respectively.

Fig. 12.(Coloronline.)ParticleratiosasafunctionofdNch/dη ineachV0Amul- tiplicityclass(see[30]formoredetails).Threedifferentcollidingsystemsarecom- pared:pp,p–PbandperipheralPb–Pbcollisions.

canbeattributedtothedifferencesintheinitialstateoverlapge- ometryandthebeamenergy.

Theresultsfortheparticleratiossuggestthat themodification ofthe(anti)protonspectralshapegoingfrompptop–Pbcollisions couldplaythedominantroleintheCroninenhancementobserved forinclusivechargedparticleR_pPbatLHCenergies.Toconfirmthis picture one would have to studythe nuclear modificationfactor asa functionof multiplicity aswe didin [45], where,thepossi- blebiasesintheevaluationofthemultiplicity-dependentaverage nuclearoverlap function T_pPb

werediscussed. Theseresultswill becomeavailableinthefuture.

InFig. 12wecomparetheparticleratiosathighpT(10<pT<

20 GeV/c) measured inINEL√

s=^{7 TeV pp} collisions,peripheral Pb–Pb collisions and the multiplicity dependent results in p–Pb collisions.Withinstatisticalandsystematicuncertainties,theratios donotshowanyevolutionwithmultiplicity.Moreover,sinceithas beenalreadyreportedthat inPb–Pb collisionsthey are centrality independent[47]we concludethatthey aresystem-sizeindepen- dent.

The strongsimilarity ofparticle ratios asa function of multi- plicityinp–PbandcentralityinPb–Pbcollisionsinthelow,inter- mediate,andhigh-pTregionsisstriking.Ingeneral,theresultsfor p–Pbcollisions appeartoraise questionsaboutthelong standing ideas ofspecificphysics modelsforsmallandlarge systems[56].

For example, in the low p_T publication [30], hydrodynamic in- spired fits gave higher transverse expansion velocities (βT ) for p–PbthanforPb–Pbcollisions.Hydrodynamics,whichsuccessfully describesmanyfeatures ofheavy-ioncollisions, hasbeenapplied tosmall systemsandcan explain thiseffect [21],but careneeds to be taken since its applicability to small systemsis still under debate [56]. On the other hand, models like color reconnection, wherethesoftandhardcomponentsareallowed tointeract, pro-

duce this kindof effectsin pp collisions [29,57].Evenmore, the hard collisions which could be enhanced via the multiplicity se- lection insmallsystems, alsocontribute to increase βT [58].In general,colorreconnectioneffectsinp–PbandPb–Pbcollisionsare underinvestigationandmodelsfortheeffectofstrongcolorfields in small systems are in general underdevelopment [59]. Finally, ithasbeenproposedthatind–Aucollisions therecombinationof soft and shower partons in the final state could explain the be- havior ofthenuclear modificationfactoratintermediate pT [32].

The CMSCollaboration hasfound that thesecond-order (v₂) and thethird-order(v3)anisotropyharmonicsmeasuredforK⁰_S and showconstituentquarkscalinginp–Pbcollisions [60].

6. Conclusions

We havereportedon thechargedpion, kaonand(anti)proton productionuptolargetransversemomenta(pT≤^{20 GeV}/c)inp–

Pb collisions at √

sNN=⁵.02 TeV. The pT spectra in √

s=^{7 TeV} ppcollisionswerealsomeasuredupto20 GeV/ctoallowthede- terminationofthe√

s=⁵.02 TeV ppreferencecrosssectionusing theexistingdataat2.76 TeVandat7 TeV.

At intermediate pT (2<pT<10 GeV/c), the(anti)proton RpPb for non-singlediffractivep–Pb collisions was found tobe signifi- cantlylarger than thoseforpions andkaons, inparticularin the region where the Cronin peak was observed by experiments at lowerenergies.Hence,themodestenhancementwhichwealready reportedforunidentifiedchargedparticlescanbeattributedtothe modification ofthe protonspectral shape going fromppto p–Pb collisions. Athigh p_T thenuclearmodificationfactorsforcharged pions, kaons and (anti)protons are consistent with unity within systematicandstatisticaluncertainties.

Theenhancement ofprotonswithrespecttopionsatinterme- diate p_T showsastrongmultiplicitydependence.Thisbehavioris not observed for the kaon-to-pion ratio. At high transverse mo- menta (10<pT<20 GeV/c) the pT integratedparticleratios are system-size independent for pp, p–Pb and Pb–Pb collisions. For a similar multiplicity at mid-rapidity, the pT-differential particle ratios are alike for p–Pb andPb–Pb collisions over the broad pT rangereportedinthispaper.

Acknowledgements

The ALICECollaboration would like to thank all its engineers andtechniciansfortheirinvaluablecontributionstotheconstruc- tion ofperformanceofthe LHCcomplex.TheALICECollaboration gratefully acknowledges the resources and support provided by all Grid centresandthe WorldwideLHC ComputingGrid (WLCG) collaboration. The ALICE Collaboration acknowledges the follow- ing funding agencies for their support in building and running the ALICEdetector:State CommitteeofScience,WorldFederation of Scientists (WFS)and Swiss Fonds Kidagan, Armenia; Conselho Nacional de Desenvolvimento Científico e Tecnológico(CNPq), Fi- nanciadora de Estudose Projetos(FINEP),Fundaçãode Amparoà Pesquisa do Estado de São Paulo (FAPESP); National Natural Sci- ence Foundation of China (NSFC), the Ministry of Education of the People’s Republic of China (CMOE) and the Ministry of Sci- ence and Technology of the People’s Republic of China (MSTC);

Ministry of Education, Youth and Sports of the Czech Republic;

Danish Natural Science Research Council, the Carlsberg Founda- tion andtheDanish NationalResearchFoundation;The European ResearchCouncilundertheEuropeanCommunity’sSeventhFrame- work Programme; Helsinki Institute of Physics and the Academy of Finland; French CNRS–IN2P3, the ‘Region Pays de Loire’, ‘Re- gion Alsace’, ‘Region Auvergne’ and CEA, France; German Bun- desministerium fur Bildung, Wissenschaft, Forschung und Tech-