Charged-particle distributions in $\sqrt{s}=13$ TeV $\mathit{pp}$ interactions measured with the ATLAS detector at the LHC

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Physics Letters B

www.elsevier.com/locate/physletb

Charged-particle distributions in √

s = 13 TeV pp interactions measured with the ATLAS detector at the LHC

.ATLASCollaboration

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received5February2016

Receivedinrevisedform11April2016 Accepted25April2016

Availableonline27April2016 Editor:H.Weerts

Charged-particle distributions aremeasured inproton–proton collisions atacentre-of-mass energyof 13 TeV, usingadata sampleofnearly9 million events,corresponding toan integratedluminosity of 170 μb⁻¹, recorded by the ATLAS detector during a special Large Hadron Collider ﬁll. The charged- particlemultiplicity,itsdependenceontransversemomentumandpseudorapidityandthedependenceof themeantransversemomentumonthecharged-particle multiplicityare presented.Themeasurements are performedwithcharged particleswithtransverse momentumgreaterthan500 MeVand absolute pseudorapidity less than2.5, inevents with at least one charged particlesatisfying thesekinematic requirements.Additionalmeasurementsinareducedphasespacewithabsolutepseudorapiditylessthan 0.8arealsopresented,inordertocomparewithotherexperiments.Theresultsarecorrectedfordetector effects,presented asparticle-leveldistributionsand are comparedtothe predictionsofvarious Monte Carloeventgenerators.

©²⁰¹⁶^CERN^for^the^benefitôf^theÂTLASCollaboration.PublishedbyElsevierB.V.Thisisanopen accessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Charged-particle measurements in proton–proton (pp) colli- sionsprovideinsightintothestronginteractioninthelow-energy, non-perturbativeregionof quantumchromodynamics (QCD).Par- ticle interactions at these energy scales are typically described byQCD-inspired modelsimplementedinMonteCarlo(MC) event generators withfree parameters that canbe constrained by such measurements.Anaccuratedescriptionoflow-energystronginter- actionprocesses isessential forsimulating single pp interactions aswell asthe effects ofmultiple pp interactions athighinstan- taneous luminosity inhadron colliders.Charged-particle distribu- tionshavebeenmeasuredpreviouslyinpp andproton–antiproton collisionsatvariouscentre-of-massenergies[1–7] (andreferences therein).

Thispaperpresentsinclusivemeasurementsofprimarycharged- particledistributionsinpp collisionsatacentre-of-massenergyof

√s=^{13 TeV,} ûsing ^data ^recorded ^by ^the ÂTLAS êxperiment ^[8]

attheLargeHadronCollider(LHC)correspondingtoanintegrated luminosityofapproximately 170 μb⁻¹.Hereinclusivemeans that all processes in pp interactions are included and no attempt to correctfor certain types of process, such as diffraction, is made.

These measurements, together with previous results, shed light ontheevolution ofcharged-particlemultiplicities withcentre-of-

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

mass energy, which is poorly constrained. A strategy similar to that in Ref. [1]is used,where moredetails ofthe analysistech- niquesaregiven.Thedistributionsaremeasuredusingtracksfrom primary chargedparticles, corrected fordetector effects, andare presentedasinclusivedistributionsinawell-definedkinematicre- gion. Primary charged particles are defined as charged particles witha mean lifetime τ>300 ps, eitherdirectly produced in pp interactions or fromsubsequent decaysof directlyproduced par- ticleswith τ<30 ps;particlesproduced fromdecaysofparticles with τ>30 ps,calledsecondaryparticles,areexcluded. Thisdef- inition differsfromearlieranalysesinthat chargedparticleswith ameanlifetime30<τ<300 ps werepreviously included.These are charged strange baryons and have beenremoved due to the low efficiencyofreconstructingthem.¹ Allprimary chargedparticles are required tohave a momentum component transverse to thebeamdirection,² pT,ofatleast500 MeVandabsolutepseudo- rapidity,|η|^,^less^than ^2.5.Êachêventîs^required^to^haveât^least oneprimarychargedparticle.

1 Sincestrangebaryonstendtodecaywithinthedetectorvolume,especiallyif theyhavelowmomentum,theyoftendonotleaveenoughhitstoreconstructa track,leadingtoatrackreconstructioneﬃciencyofapproximately0.3%.

2 ATLASusesaright-handedcoordinatesystemwithitsoriginatthenominalin- teractionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeampipe.

Thex-axispointsfromtheIPtothecentreoftheLHCring,andthey-axispoints upward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbeingthe azimuthalanglearoundthebeampipe.Thepseudorapidityisdeﬁnedintermsof thepolarangleθasη= −ln tan(θ/2).

http://dx.doi.org/10.1016/j.physletb.2016.04.050

0370-2693/©²⁰¹⁶^CERN^for^the^benefitôf^theÂTLASCollaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

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Intheseeventsthefollowingdistributionsaremeasured:

1 N_ev·^dN^ch

dη ^,

1 N_ev · ¹

2πp_T· ^d²^N^ch

dηdp_T, and 1 N_ev·^dN^ev

dn_ch

aswellasthemeanpT (^pT⁾ôfâll^primary^charged^particles^ver- susnch.Herenchisthenumberofprimarychargedparticlesinan event,N_evisthenumberofeventswithn_ch≥^1,ând^Nchistheto- talnumberofprimarychargedparticles inthedatasample.³ The measurementsarealsopresentedinaphasespacethatiscommon to theATLAS, CMS [9]andALICE [10] detectorsin order toease comparisonbetween experiments.Forthis purposean additional requirementof|η_|<0.8 ismadeforallprimarychargedparticles.

TheseresultsarepresentedinAppendix A.Finally,themeannum- berofprimarychargedparticlesfor η=^{0 is}^compared^to^previous measurements atdifferentcentre-of-massenergies. Themeasure- mentsarecomparedtoparticle-levelMCpredictions.

Theremainderofthispaperislaidoutasfollows.Therelevant componentsoftheATLASdetectoraredescribedinSection 2.The MCeventgeneratorsanddetectorsimulationusedintheanalysis are introduced in Section 3. The selection criteriaapplied to the dataandthecontributions frombackgroundeventsarediscussed inSections4and5respectively.Theselectioneﬃciencyandcorre- spondingcorrectionstothedataarediscussedinSections6and7 respectively.Thecorrectedresultsarecomparedtotheoreticalpre- dictionsin Section 8 anda conclusion is givenin Section 9.The measurement of primary charged particles inthe reduced phase spaceof|η|<0.8 ispresentedinAppendix A.

2. ATLASdetector

TheATLASdetectorcoversalmostthewholesolidanglearound the collision point withlayers of tracking detectors,calorimeters andmuon chambers.Forthemeasurements presentedinthispa- per, the trackingdevicesand thetrigger systemare ofparticular importance.

The inner detector(ID) has full coverage in φ andcovers the pseudorapidity range |η|<2.5. It consists of a silicon pixel detector (pixel),a silicon microstrip detector(SCT) anda transition radiation straw-tube tracker (TRT). These detectors span a sen- sitive radial distance from the interaction point of 33–150 mm, 299–560 mmand563–1066 mmrespectively,andaresituatedin- sideasolenoidthatprovidesa2 Taxialmagneticfield.Thebarrel (each end-cap) consists of four (three) pixel layers, four (nine) double-layers of single-sided silicon microstrips witha 40 mrad stereoangle betweentheinner and outer partof adouble-layer, and73(160)layers ofTRTstraws. Theinnermostpixel layer,the insertableB-layer(IBL)[11],wasaddedbetweenRun1andRun2 oftheLHC, arounda newnarrower(radiusof 25 mm)andthin- ner beampipe. It iscomposed of14 lightweight staves arranged in a cylindrical geometry, each made of 12 silicon planar sensors inits central regionand2 × ⁴^3D ^sensors ât^the ênds.^The IBLpixeldimensionsare 50×^{250 μm}² ⁱⁿ^theφ andz directions (comparedwith50×^{400 μm}² ^forôther^pixel^layers).^The^smaller radiusandthereducedpixelsize resultinimprovementsofboth the transverse and longitudinal impact parameter resolutions. In addition,newserviceshavebeenimplementedwhichsignificantly reduce the material atthe boundariesofthe active tracking vol- ume.A trackfromachargedparticletraversingthebarreldetector typically has12silicon measurement points (hits), ofwhichfour arepixelandeightSCT,andmorethan30TRTstrawhits.

3 Thefactor2πpTinthepTspectrumcomesfromtheLorentz-invariantdeﬁnition ofthecrosssectionintermsofd³p.Theresultscouldthusbeinterpretedasthe masslessapproximationtod³p.

The ATLAS detector employs a two-level trigger system: the level-1 hardware stage (L1) and the high-level trigger software stage (HLT). This measurement uses the L1 decision from the minimum-bias trigger scintillators (MBTS), which were replaced between Run 1 andRun 2. The MBTS are mounted at each end of the detector in front of the liquid-argon end-cap calorimeter cryostatsatz= ±³.56 m andsegmentedintotworingsinpseudo- rapidity(2.07<|η|<2.76 and2.76<|η|<3.86).Theinnerringis segmentedintoeightazimuthalsectorswhiletheouterringisseg- mentedintofourazimuthalsectors,givingatotaloftwelvesectors per side. The MBTS trigger selectionused forthis paperrequires one counter abovethresholdfromeithersideofthe detectorand isreferred toasasingle-armtrigger.Theeﬃciencyofthistrigger isstudiedwithanindependentcontroltrigger.Thecontroltrigger selects events randomly at L1 whichare then ﬁltered atHLT by requiringatleastonereconstructedtrackwithpT>200 MeV.

3. MonteCarloeventgeneratorsimulation

The pythia⁸ [12], epos[13] and qgsjet-ii [14] MC generators are used to correct the data fordetector effects andto compare withparticle-levelcorrecteddata.Abriefintroductiontotherele- vantpartsoftheseeventgeneratorsisgivenbelow.

In pythia⁸ inclusivehadron–hadron interactionsaredescribed by a model that splits the total inelastic cross section into non- diffractive(ND)processes,dominatedbyt-channelgluonexchange, and diffractiveprocesses involvinga colour-singlet exchange. The simulation of ND processes includes multiple parton–parton interactions(MPI).The diffractiveprocessesare furtherdividedinto single-diffractivedissociation(SD),whereoneoftheinitialprotons remainsintactandtheotherisdiffractivelyexcitedanddissociates, and double-diffractive dissociation(DD) where both protons dis- sociate. The sample contains approximately22% SD and12% DD processes. Such events tend to have large gaps in particle pro- duction atcentralrapidity.Apomeron-based approachisused to describetheseevents[15].

epos provides an implementation of a parton-based Gribov–

Regge[16]theory,whichisaneffectiveQCD-inspiredﬁeld theory describinghardandsoftscatteringsimultaneously.

qgsjet-ii provides a phenomenological treatment of hadronic and nuclear interactions in the Reggeon ﬁeld theory frame- work [17].The softandsemi-hard parton processesare included inthemodelwithinthe“semi-hardpomeron”approach. epos and qgsjet-iicalculationsdonotrelyonthestandardpartondistribu- tionfunctions(PDFs)asusedingeneratorssuchas pythia⁸.

Different settingsofmodelparametersoptimised toreproduce existing experimental dataare used inthesimulation. Theseset- tings are referred to astunes. For pythia⁸ two tunes are used, a2 [18] and monash [19]; for epos the lhc [20] tune is used.

qgsjet-ii uses the default tune from the generator. Each tune utilises 7 TeV minimum-biasdata andis summarised inTable 1, together with theversion ofeach generator usedto produce the samples.The pythia8 a2sample, combinedwithasingle-particle MC simulation used to populate the high-pT region, is used to derive the detector corrections for these measurements. All the

Table 1

SummaryofMCtunesusedtocomparetothecorrecteddata.Thegeneratorand itsversionaregivenintheﬁrsttwocolumns,thetunenameandthePDFusedare giveninthenexttwocolumns.

Generator Version Tune PDF

pythia 8 8.185 a2 mstw2008lo[21]

pythia 8 8.186 monash nnpdf2.3lo[22]

epos LHCv3400 lhc N/A

qgsjet-ii II-04 default N/A

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eventsare processedthrough theATLAS detectorsimulation program [23], which is based on geant4[24]. Theyare then reconstructed and analysed by the same program chain used for the data.

4. Dataselection

The data were recorded during a period with a special con- ﬁgurationoftheLHC withlowbeamcurrents andreducedbeam focusing,andthusgivingalowexpectedmeannumberofinterac- tionsper bunchcrossing, μ₌0.005.Events wereselected from colliding proton bunches using a trigger which required one or moreMBTScountersabovethresholdoneithersideofthedetec- tor.

Each event is required to contain a primary vertex, recon- structedfromatleasttwotrackswithaminimum p_T of100 MeV, as described in Ref. [25]. To reduce contamination from events withmorethanoneinteractioninabunchcrossing,eventswitha secondvertexcontaining fourormoretracksare removed.Events where the second vertex has fewer than four tracks are not removed.Theseare dominatedby contributionswherea secondary interaction is reconstructed as another primary vertex or where theprimaryvertexissplitintotwovertices,onewithfewtracks.

Thefractionof eventsrejectedby thevetoon additionalvertices dueto splitverticesorsecondaryinteractions isestimatedinthe simulationtobe0.02%,whichisnegligibleandthereforeignored.

Trackcandidatesarereconstructed[26,27]inthesilicondetec- tors and then extrapolated to include measurements in the TRT.

Events are required to contain at least one selected track, pass- ing the following criteria: p_T>500 MeV and |η|<2.5; at least onepixelhitandatleastsixSCThits,withtheadditionalrequire- mentof an innermost-pixel-layerhit ifexpected⁴ (if a hit inthe innermostlayer is not expected, the next-to-innermost hit is re- quiredifexpected); |^d^BL₀ |<1.5 mm,wherethe transverseimpact parameter,d^BL₀ , is calculatedwith respect tothe measured beam lineposition;and|^z^BL0 ·^sinθ|<1.5 mm,wherez^BL₀ isthedifference betweenthelongitudinalpositionofthetrackalongthebeamline atthe pointwhered^BL₀ ismeasured andthelongitudinalposition oftheprimaryvertex,andθisthepolarangleofthetrack.Finally, inordertoremovetrackswithmismeasuredp_Tduetointeractions withthematerialorothereffects,thetrack-ﬁt χ²probabilityisre- quiredtobegreaterthan0.01 fortrackswithpT>10 GeV.There are8.87millioneventsselected,containing atotal of106million selectedtracks.

Theperformance of theID trackreconstruction in the13 TeV dataanditssimulationisstudiedinRef.[28].Overall,goodagree- ment between data and simulation is observed. Fig. 1 shows selected performance plots particularly relevant to this analysis.

Fig. 1(a) shows the average number of siliconhits as a function of η.There isreasonablygoodagreement,although discrepancies ofupto2%(intheend-caps)areseen;however,thesehaveasmall effectonthetrackreconstructioneﬃciency.Thediscrepanciesare duetodifferencesbetweendataandsimulationinthenumberof operationaldetectorelementsandanimperfectdescriptionofthe amount ofdetector material between the pixel detector andthe SCT.Theimpactontheresultsofthesediscrepanciesisdiscussed inSection6.3.Fig. 1(b)showsthe fractionof trackswithagiven numberofIBLhitspertrack.Thereisadifferenceof0.5%between data andsimulation in the fraction of tracks with zero IBL hits, comingpredominantlyfromadifferenceintherateoftracksfrom secondaryparticles,whichisdiscussedinmoredetailinSection5.

4 Ahitisexpectediftheextrapolatedtrackcrossesanactiveregionofapixel modulethathasnotbeendisabled.

Asystematicuncertaintyduetothesmallremaining differencein the eﬃciency of the requirement of at least one IBL hit is dis- cussedinSection6.Figs. 1(c)and 1(d)showthed^BL₀ andz^BL₀ ·^sinθ distributions respectively. In these ﬁgures the fraction of tracks fromsecondaryparticlesinsimulationisscaledtomatchthefrac- tionseenindata,andtheseparatecontributionsfromtracksfrom primary and secondary particles are shown. This,along with the differencesbetweensimulationanddata,whichhaveanegligible impactontheanalysis,arediscussedinSection5.

5. Backgroundcontributionsandnon-primarytracks

The contribution from non-collision background events, such as proton interactions with residual gas molecules in the beam pipe, is estimatedusing eventsthat pass the full eventselection butoccurwhen onlyoneofthetwo beamsispresent. Afternor- malisingtothecontributionexpectedintheselecteddatasample (usingthedifferenceinthetimeoftheMBTS hitsoneachsideof thedetector,whichispossibleasbackgroundeventswithhitson only oneside are negligible) a contributionof lessthan0.01% of eventsisfoundfromthissource,whichisnegligibleandtherefore neglected.Backgroundeventsfromcosmicrays,estimatedbycon- sideringthe expected rateofcosmic-ray events compared to the eventreadout rate, are also found to be negligible andtherefore neglected.

The majorityof eventswithmore thanone interaction inthe samebunchcrossingare removedby therejectionofeventswith morethanone primary vertex.Some eventsmaysurvivebecause theinteractions are veryclosein z andaremergedtogether.The probability to merge vertices isestimated by inspecting the distribution of the difference in the z position of pairs of vertices ( z). This distribution displays a deﬁcit around z=^{0 due} ^to vertexmerging. The magnitudeof thiseffect isused to estimate the probability of merging vertices, which is 3.2%. When this is combinedwiththenumberofexpectedadditionalinteractionsfor

μ₌0.005, the remaining contribution from tracks from additional interactions is found to be less than 0.01%, which is neg- ligibleandtherefore neglected.The additionaltracksin eventsin whichthesecondvertexhasfewerthanfourassociatedtracksare mostly rejectedby the z^BL₀ ·^sinθ requirement, andtheremaining contributionisalsonegligibleandneglected.

Thecontributionfromtracksoriginatingfromsecondaryparti- clesissubtractedfromthenumberofreconstructedtracks before correcting for other detector effects. These particles are due to hadronicinteractions,photonconversionsanddecaysoflong-lived particles. Thereis alsoa contributionoflessthan 0.1%fromfake tracks (those formedby a random combinationofhitsorfroma combination of hits from several particles); these are neglected.

The contribution of tracks from secondary particles is estimated using simulation predictions for the shapes of the d^BL₀ distribu- tionsfortracksfromprimaryandsecondaryparticlessatisfyingall track selection criteria except the one on d^BL₀ . These predictions formtemplatesthatarefittothedatainordertoextracttherel- ativecontributionoftracksfromsecondaryparticles.TheGaussian coreofthe distributionis dominatedby thetracks fromprimary particles,witha widthdetermined bytheir d^BL₀ resolution; tracks fromsecondary particles dominatethe tails.The fit isperformed in the region 4<|^d^BL0 |<9.5 mm,in order to reduce the depen- denceon the descriptionof thed^BL₀ resolution,which affects the coreofthe distribution.From the fit,it was determined thatthe fractionoftracksfromsecondaryparticles insimulationneedsto bescaledbyafactor1.38±⁰.14.Thisindicatesthat(2.3±⁰.6)% of trackssatisfyingthe finaltrackselection criteria(|^d^BL₀|<1.5 mm) originatefromsecondaryparticles,wheresystematicuncertainties are dominant andare discussed below. Of thesetracks 6% come