Transverse momentum, rapidity, and centrality dependence of inclusive charged-particle production in $\sqrt{^{S}NN}=5.02$ TeV p + Pb collisions measured by the ATLAS experiment

(1)

Physics Letters B 763 (2016) 313–336

Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Transverse momentum, rapidity, and centrality dependence of inclusive charged-particle production in √

s

_NN

= ⁵ . 02 TeV p + ^Pb collisions measured by the ATLAS experiment

.TheATLASCollaboration

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

Articlehistory:

Received23May2016

Receivedinrevisedform14August2016 Accepted24October2016

Availableonline29October2016 Editor:D.F.Geesaman

Measurements ofthe per-eventcharged-particle yieldas afunctionofthe charged-particletransverse momentumandrapidityareperformedusing p+Pb collisiondatacollectedbythe ATLASexperiment at the LHC ata centre-of-massenergy of √_s

NN=⁵.02TeV.Charged particlesare reconstructedover pseudorapidity |η_|<2.3 and transverse momentum between 0.1 GeV and 22 GeV in a dataset correspondingtoanintegratedluminosityof1 μb⁻¹.Theresultsarepresentedintheformofcharged- particle nuclear modiﬁcation factors, where the p+Pb charged-particle multiplicities are compared betweencentralandperipheral p+Pb collisionsaswellastocharged-particlecrosssectionsmeasured

inpp collisions.The p+Pb collisioncentralityischaracterizedbythetotaltransverseenergymeasured

in−⁴.9<η<−³.1,whichisinthedirectionoftheoutgoingleadbeam.Threedifferentestimationsof the numberofnucleonsparticipatinginthe p+Pb collisionare carriedoutusingtheGlaubermodel and twoGlauber–Gribovcolour-fluctuationextensionstotheGlaubermodel.Thevaluesofthenuclear modificationfactorsarefoundtovarysignificantlyasafunctionofrapidityandtransversemomentum.

A broadpeakisobservedforallcentralitiesandrapiditiesinthenuclearmodiﬁcationfactorsforcharged- particle transverse momentum values around 3 GeV. The magnitude ofthe peakincreases for more centralcollisionsaswellasrapidityrangesclosertothedirectionoftheoutgoingleadnucleus.

©²⁰¹⁶^TheÂuthor(s).^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Proton–nucleuscollisionsatultrarelativisticenergiesprovidean opportunity to understand the role of the nuclear environment inmodifying hardscatteringrates. Severalphysicseffects areex- pectedtoinducedeviationsfromasimpleproportionalitybetween the scattering rate and the number of binary nucleon–nucleon collisions[1].First,nuclear shadowingeffectshavelongbeenob- servedindeep-inelasticscatteringonnuclei,aswellasinproton–

nucleuscollisions,indicatingthatnucleonsembeddedinanucleus have a modified structure. This modification tends to suppress hadron production at low to moderate momentum, and is ad- dressedbyavarietyoftheoreticalapproaches[2,3].Someofthese approachesdescribe hadronproductioncrosssectionsintermsof a universal set of nuclear parton distribution functions (nPDF), whichareparameterizedasmodificationstothefreenucleonPDFs [4–12]. Second, energy loss in “cold nuclear matter” is expected to modify hadron production rates at high transverse momen- tum (p_T) [13–16]. Third, a relative enhancement of hadron pro-

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

ductionratesatmoderatemomentaisobservedinproton–nucleus collisions [17], which can be attributedto initial-state scattering oftheincomingnucleon[18,19]orradialﬂoweffects[20].Finally, the appearance of “ridge-like” structures in high-multiplicity pp and p+^{Pb events} ^[21–25] ^suggests ^that ^small ^collision ^systems havethesamehydrodynamicoriginasPb+^{Pb events}^[26],^or^that therearealreadystrongcorrelationsintheinitialstatefromgluon saturation [27]. All these effects can be explored experimentally by the measurement ofcharged-hadron productionasa function oftransversemomentum.

For proton-lead (p+^Pb) collisions, assuming that the initial parton densities are the incoherent superposition of the nucle- onic partondensities, theper-event particleproduction yieldcan be estimated by the product σNN× ^TPb^.^Here σNN is the cross section for the analogous nucleon–nucleon collision process and

^TPb îs^the âverage ^value ôf^the ^nuclear ^thickness^function ôver adistributionoftheimpactparametersofprotonsincidentonthe nuclear target. It canbe thought ofas aper-collision luminosity.

Thenuclearmodificationfactor, RpPb,isdefinedastheratioofthe measured charged-particle production yield in p+Pb collisions, normalized by ^TPb^, ^to ^the^cross ^section ôf^the charged-particle productionyieldinpp collisions:

http://dx.doi.org/10.1016/j.physletb.2016.10.053

0370-2693/©²⁰¹⁶^TheÂuthor(s).^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

(2)

R_pPb(p_T,y^∗)= ¹

^TPb

1/N_evtd²N_pPb/d y^∗dp_T d²σpp/d y^∗dpT

, (1)

where N_evt is the number of p+^{Pb events,} ^d²^NpPb/d y^∗dp_T is thedifferentialyield ofchargedparticlesin p+Pb collisionsand d²σpp/d y^∗dpT isthedifferentialcharged-particleproductioncross sectioninpp collisions.Bothnumeratoranddenominatorarepre- sentedintermsofy^∗,therapidityinthenucleon–nucleoncentre- of-massframe. Inthe absenceofinitial-state andnucleareffects, the ratio R_pPb is expected to be unity at high pT [28]. Another measureofnuclearmodiﬁcationisthequantity RCP,whichisde- ﬁnedtobe:

R_CP(p_T,η)=^TPb,P

^TPb,C

(1/Nevt,C)d²N_pPb_,_C/dηdpT

(1/N_evt_,_P)d²N_pPb_,_P/dηdp_T, (2) andcanbeconstructedwithouttheneedfora pp referencespec- trum. The indices“P” and“C” labelperipheral (large impact pa- rameter)andcentral(smallimpactparameter)centralityintervals, respectively. The RCP is presented as a function of pseudorapidity(η) ratherthan y^∗ sincebothnumeratoranddenominatorare fromthesame collidingsystems.Measurementsof R_pPb and RCP provideusefulinputforconstrainingmodelsofshadowing,energy lossandradial ﬂoweffects.Theyshould alsoprovideusefulinput forthe determinationof nuclear partondistribution functions,in particularasafunctionofprotonimpactparameter[6].Theabso- lutevaluesofthenuclearmodiﬁcationdependonthe^TPb^values andshouldbeinterpretedwithrespecttotheassumptionsunder- lyingtheparticularmodelusedtocalculatethenormalization.

A recent ATLAS publication [29] has reported measurements of the mean charged-particle multiplicity asa function of pseudorapidity and collision centrality and explored the relationship betweenthecentralitydependenceoftheparticleproductionand modelsoftheinitialnucleargeometry.Theresultspresentedhere utilize the same centrality deﬁnition and geometric models, but build uponthat workby exploring the pT, η and y^∗ dependence ofper-eventcharged-particleyieldsinp+Pb collisionsatacentre- of-massenergy√

sNN=⁵.02TeV andcomparing that dependence totheexpectationsfrom pp collisionsthroughthequantitiesRpPb andR_CP.

Thesemeasurementsare anextension ofasimilar programme carried out at the Relativistic Heavy Ion Collider, where all ex- perimentsreportedtheabsenceofcharged-particlesuppressionat 2<pT<10GeV in d+Au collisions [30–35], in contrast to the strong suppressionfound in Au+Au collisions [31,33].Measure- ments ofnuclear modiﬁcation factors asa function oftransverse momentum in a narrow pseudorapidity window relative to the centre-of-mass frame |ηCM|<0.3 have been reported by ALICE integratedovercentrality[36,37]anddifferentiallyforseveralcen- trality classes [38,39]. Similarly, CMS results have been reported integratedovercentralityandinabroaderpseudorapiditywindow,

|ηCM|<1[40].

2. TheATLASdetector

TheATLASdetector[41]attheLargeHadronCollider(LHC)covers almost the entire solid angle¹ around the collision point. It

1 ATLASusesaright-handed coordinatesystemwith itsoriginat thenominal interactionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeam pipe.Thex-axispointsfromtheIPtothecentreoftheLHCring,andthe y-axis pointsupwards.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φ beingtheazimuthalanglearoundthez-axis.Thepseudorapidityisdeﬁnedinterms ofthepolarangleθasη= −^{ln tan}(θ/2).Angulardistanceismeasuredinunitsof

R≡

(η)²+ (φ)²^.

consistsofaninnertrackingdetectorsurroundedbyathinsuper- conducting solenoid, electromagnetic and hadronic calorimeters, andamuonspectrometerincorporatingthreelargesuperconduct- ingtoroidalmagnets.

The inner detector (ID) system is immersed in a 2 T axial magneticﬁeldandprovidescharged-particletrackinginthepseu- dorapidity range |η|<2.5. The ID tracker is composed of three detector subsystems. Closest to the interaction point is a high- granularitysiliconpixeldetectorcovering|η_|<2.7,whichtypically providesthreemeasurementspertrack.Nextisasiliconmicrostrip tracker (SCT), which typically yields four pairs of hits per track, eachprovidingatwo-dimensionalmeasurementpoint.Thesilicon detectorsarecomplementedbythestraw-tubetransitionradiation tracker,whichenablesradiallyextendedtrackreconstructionupto

|η_|₌2.0.

The calorimetersystem covers thepseudorapidity range |η|<

4.9. Within the region |η|<3.2, electromagnetic calorimetry is provided by high-granularity lead/liquid-argon (LAr) electromagnetic calorimeters, with an additional thin LAr presampler covering |η_|<1.8, to measure the contribution of showers ini- tiated in the material upstream of the calorimeters. Hadronic calorimetryisprovidedby asteel/scintillator-tilecalorimeter,seg- mentedintothreebarrelstructureswithin|η|<1.7,andtwocop- per/LArhadronicendcapcalorimeterscovering1.5<|η_|<3.2.The calorimeter coverage is completed with forward copper/LAr and tungsten/LAr calorimeter modules optimized for electromagnetic andhadronicmeasurements,respectively,covering3.1<|η|<4.9.

Theminimum-biastriggerscintillators(MBTS)detectchargedpar- ticlesover2.1<|η|<3.9 usingtwohodoscopes, eachofwhichis subdividedinto16counterspositionedatz= ±³.6 m.

A three-leveltrigger systemis usedto selectevents [42]. The Level-1 trigger isimplemented inhardware and usesa subset of detectorinformationto reduce theeventrateto100 kHz. Thisis followed by two software-basedtriggerlevels which together reduce theeventratetoabout1000 Hz,whichisrecordedfordata analysis.

3. Datasetsandeventselection 3.1. Eventselectioninp+Pb collisions

The p+Pb collisionswere recorded bythe ATLAS detectorin September 2012usingatriggerthatselected eventswithatleast one hit ineach side oftheMBTS, withthe resultingdatasetcor- respondingtoanintegratedluminosityof1 μb⁻¹.Duringthatrun theLHCwas conﬁguredwithaclockwise4 TeV protonbeamand an anti-clockwise1.57 TeV per-nucleon ²⁰⁸Pbbeamthattogether producedcollisionswithanucleon–nucleoncentre-of-massenergy of√

s=⁵.02 TeV andalongitudinalrapidityboostofy_lab=⁰.465 unitswithrespecttotheATLASlaboratoryframe.Followingacom- monconventionusedforp+A measurements,therapidityistaken tobepositiveinthedirectionoftheprotonbeam,i.e.oppositeto theusualATLASconventionforpp collisions.Withthisconvention, theATLAS laboratoryframerapidity, y,andthe p+Pb centre-of- masssystemrapidity,y^∗,arerelatedby y^∗=^y−⁰.465.

Charged-particletracks andcollisionverticesare reconstructed from clusters in the pixel detector and the SCT using an algo- rithm optimized for minimum-bias pp measurements [43]. The p+^{Pb events} ^are ^required ^to ^have ^a ^collision ^vertex ^satisfying

|^zvtx|<150 mm, at least one hit in each side of the MBTS, and a difference between the time measurements in the two MBTS hodoscopes of lessthan10 ns. Events containing multiple p+^Pb collisions(pile-up)aresuppressedbyrejectingeventsthatcontain asecondreconstructedvertexwithascalartransversemomentum

Transverse momentum, rapidity, and centrality dependence of inclusive charged-particle production in $\sqrt{^{S}NN}=5.02$ TeV p + Pb collisions measured by the ATLAS experiment

Physics Letters B

Transverse momentum, rapidity, and centrality dependence of inclusive charged-particle production in √

s

= 5 . 02 TeV p + Pb collisions measured by the ATLAS experiment

= ⁵ . 02 TeV p + ^Pb collisions measured by the ATLAS experiment