DSpace at VNU: Measurements of long-range near-side angular correlations in root sNN=5TeV proton-lead collisions in the forward region

The correlations are measured in the laboratory system as a function of relative pseudorapidity, η, and relative azimuthal angle, φ, for events in different classes of event activity and

11 76

Measurements of long-range near-side angular correlations in

√

s N N = 5 TeV proton-lead collisions in the forward region

The LHCb Collaboration

Article history:

Received 10 December 2015

Received in revised form 27 September

2016

Accepted 30 September 2016

Available online xxxx

Editor: W.-D Schlatter

Two-particle angular correlations are studied in proton-lead collisions at a nucleon–nucleon centre-of-mass energy of √s

N N=5 TeV, collected with the LHCb detector at the LHC The analysis is based on data recorded in two beam configurations, in which either the direction of the proton or that of the lead ion is analysed The correlations are measured in the laboratory system as a function of relative pseudorapidity, η, and relative azimuthal angle, φ, for events in different classes of event activity and for different bins of particle transverse momentum In high-activity events a long-range correlation

on the near side, φ≈0, is observed in the pseudorapidity range 2.0 <η<4.9 This measurement of long-range correlations on the near side in proton-lead collisions extends previous observations into the forward region up to η=4.9 The correlation increases with growing event activity and is found to be more pronounced in the direction of the lead beam However, the correlation in the direction of the lead and proton beams are found to be compatible when comparing events with similar absolute activity in the direction analysed

©2016 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

Studies of two-particle angular correlations are an important

experimentalmethodtoinvestigatethedynamicsofmulti-particle

production in QCD and to probe collective effects arising in the

dense environment of a high-energy collision The highest

parti-cledensitiesandmultiplicitiesreachedinproton–proton(pp)and

proton-lead collisions (pPb) at the LHC are of a similar size to

those in non-central nucleus–nucleus (AA) collisions This

moti-vates lookingfor signatureswhich were sofar mainly studiedin

AAcollisions

Two-particle correlations are conveniently described by

two-dimensional ( η , φ)-correlation functions For pairs of prompt

chargedparticles their separations inpseudorapidity,  η, andin

theazimuthal angle, φ, aremeasured in thelaboratory system

Structuresinthecorrelationfunctionareclassifiedintoshort-range

(| η | 2) and long-range (| η | 2) effects On the near-side

(|φ| ≈0) a short-range “jet peak” at  η ≈0 is the dominant

structure, caused by the fact that in the fragmentation process

thefinal-stateparticlesarecollimatedaroundtheinitialparton.To

balancethemomentum,thepeakisaccompaniedbyalong-range

structureontheawayside(|φ| ≈ π)causedbyparticlesthatare

oppositeinazimuthalangle

Due to the different momentum fractions carried by the

col-lidingpartons and the resulting individual boosts, the away-side

structureisnotrestrictedin  η,butelongatedoveralargerange

Incomplexheavy-ioncollisions,theseshort- andlong-range

struc-turesare modified asa resultofthestronglyinteractingmedium that isformeddepending onthe centralityofthecollision Long-rangecorrelationsonthenear- andaway-sideareobserved,which are typically explained as being the result of a hydrodynamical flowofthedeconfinedmedium[1].Measurementsinveryrare pp

collisionsthathaveanextremelyhighparticlemultiplicityrevealed

asimilarunexpectedlong-rangecorrelationonthenearside[2–4] Thisstructure, oftenreferred toasthenear-side “ridge”, hasalso beenconfirmedinhigh-multiplicity pPb collisions[5–9],whereit wasfoundtobemuchmorepronouncedthanin pp collisions.

Thetheoreticalinterpretationofthemechanismresponsiblefor the ridge in pp and pPb is still underdiscussion Various mod-els have been proposed such as gluon saturation in the frame-work of a colour-glass condensate [10–13] or the hydrodynamic evolutionofahighdensitypartonicmedium [14],multiparton in-teractions[15–17],jet-mediuminteractions [18,19],andcollective effects[20–24]inducedbytheformationandexpansionofa high-densitysystempossiblyproducedinthesecollisions.Analysesthat

in the central rapidity region, probing ranges up to | η | =2.5 In

a recent analysis [9] larger pseudorapidities were also accessed

for-ward (2.5 < | η | <4.0) andthe central (| η | <1.0) region.For the

detec-tor,uniqueamongtheLHCexperiments,isusedtostudytheridge phenomenonin pPb collisions.Proton-lead collisionsareanalysed

inthe rangeof2.0 < η <4.9 andin thedirectionsofthe proton

http://dx.doi.org/10.1016/j.physletb.2016.09.064

0370-2693/©2016 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 SCOAP 3

1 66

andtheleadbeamsseparately.Confirmationofthe ridge

correla-tionatlargepseudorapiditiesandcomparisonofitsmagnitudefor

thetwobeamdirectionsprovidenewinputtothetheoretical

un-derstandingoftheunderlyingmechanisms

2 Experimental setup

The analysis isbased on data collected with the LHCb

detec-tor during the proton-lead data-taking period in 2013 The LHC

provided pPb collisions ata nucleon–nucleon centre-of-mass

en-ergyof√

s N N=5 TeV,correspondingtoa protonbeamenergyof

thisasymmetric beamconfiguration,thereis a relativeboost

be-tweentherapidityintheLHCblaboratoryframe,ylab,andy inthe

nucleon–nucleoncentre-of-massframe,correspondingtoashiftof

0.47 units

TheLHCb detector[25,26]is asingle-arm forward

spectrome-tercoveringthepseudorapidity range2 < η <5 inthelaboratory

beam, two different configurations are distinguished In the

for-ward configurationtheprotonbeampointstopositiverapidity,into

the LHCb spectrometer, and the recorded collisions are referred

to as p+Pb.The opposite backward configuration, in whichthe

leadbeampointstopositiverapidity,isreferredtoasPb+p.The

prob-ing rapidities y in the nucleon–nucleon centre-of-mass frame of

1.5 <y <4.4 in the p+Pb configuration and−5.4 <y < −2.5

cor-respondto an integrated luminosity of 0.46 nb−1 inthe p+Pb

configurationand0.30 nb−1 forthePb+p configuration.

The LHCb detector, designed for the study of particles

con-taining b or c quarks, includes a high-precision tracking system

consisting of a silicon-strip vertex detector (VELO) surrounding

the interaction region, a large-area silicon-strip detector located

4 Tm,andthreestations ofsilicon-stripdetectorsandstraw drift

av-erage over small asymmetries in the detection of charged

of charged particles with a relative uncertainty that varies from

ofchargedhadronsaredistinguished usinginformationfromtwo

ring-imagingCherenkovdetectors.Photons,electronsandhadrons

are identified by a calorimetersystem consisting of

scintillating-padandpreshowerdetectors,anelectromagneticcalorimeteranda

hadroniccalorimeter.Muonsareidentifiedbya systemcomposed

ofalternating layersofiron andmultiwireproportionalchambers

Theonlineeventselectionisperformedbyatrigger,whichconsists

of a hardware stage, based on information from the calorimeter

andmuon systems,followedbyasoftwarestage, whichappliesa

fulleventreconstruction Duringdatatakingof pPb collisions,an

activity trigger in the hardware stage acceptednon-empty beam

bunch crossings, and the softwarestage acceptedevents with at

leastonereconstructedtrackintheVELO

3 Data selection and corrections

MonteCarlosimulationsare usedto evaluatethe efficiencyof

thefollowingselectionsandtoestimatetheremaining

contamina-tionintheselectedtracksample.Proton-lead collisionsin p+Pb

gen-erator [27] inversion 1.383bs.2 As a cross-check, proton–proton

collisions at a centre-of-mass energy of 8 TeV aresimulated

us-ing Pythia [28] in a special LHCb configuration [29] and witha

highaverageinteractionrate(largepile-up)toreproducethelarger particle multiplicity in proton-lead collisions Particle decays are simulatedby EvtGen[30].Theinteractionofthe generated parti-cleswiththedetector,anditsresponse,areimplementedusingthe Geant4toolkit[31]asdescribedinRef.[32]

Themeasurementsarebasedonproton-leadcollisionsthatare

crossingshavemorethanoneinteraction.Eacheventisrequiredto haveexactlyone reconstructedprimary vertexcontainingatleast five tracks Beam-related background interactions are suppressed

byrequiringthepositionofthereconstructedprimaryvertextobe within ±3 standarddeviationsaroundthemeaninteractionpoint, separately foreach coordinate.The meanvalue andthe widthof this luminous region are determined separately from a Gaussian

fit to the distribution of reconstructed primary vertices of each datasample Dependingonthepolarityofthemagneticfieldand theresultingbeamoptics,thesizeofthestandarddeviationalong

transverse direction it is around 30 μm While pPb interactions

are mostlikely producedinthisregion,beam-relatedbackground extends furtheralongthe beamline.Beam gaseventsor interac-tions withdetectormaterial can produce a very highnumber of particles; however, insuch cases the total energydeposit in the calorimeter is much smaller than that of typical pPb collisions.

Events with too small a ratio of the number of clusters in the electromagnetic calorimeterto that inthe VELO are rejected; in-dividuallowerboundsaredefinedforcollisions inthe p+Pb and

The angular correlationsare determined for charged particles that are directly produced in the pPb interaction. The measure-mentisbasedontrackstraversingthefulltrackingsystem,which restricts charged particles in pseudorapidity to 2.0 < η <4.9 In addition, particles are required to have a transverse momentum

pT>0.15 GeV/c andatotalmomentump>2 GeV/c. Reconstruc-tion artefacts, such as fake tracks,are suppressed using a multi-variate classifier.The remaining average fractionoffake tracks is

oftheorderof7% and12% inthe p+Pb andPb+p samples, re-spectively The probability ofreconstructing fake tracks increases with thenumber of hitsin thetracking detectors.Thus, the

particleandhitmultiplicitythat ispresentinthedirectionofthe leadremnant.Toselectprimarytracksoriginatingdirectlyfromthe

the reconstructed primary vertexmust not exceed 1.2 mm, after whichthefractionofremainingtracksfromsecondaryparticlesis estimatedtobelessthan3.5%

Theinefficiencyinfindingchargedparticlesarisesfromtwo ef-fects: limited detector acceptance in the range of 2.0 < η <4.9, andlimitations ofthe trackreconstruction Forparticles fulfilling thekinematicrequirements,theacceptancedescribesthefraction

about 70% on average In contrast, the track reconstruction ef-ficiency varies from 96% for low-multiplicity events to 60% for eventswiththehighestmeasuredmultiplicity

Afterapplyingtheselectionrequirements,theremaining proba-bilitiesofselectingfaketracks, Pfake,andsecondaryparticles, Psec,

aswellastheefficienciesrelatedtothedetectoracceptance, acc, and the track reconstruction, tr, are estimated in simulation as

a function of the angularvariables η and φ, the transverse mo-mentum pT, and the hit-multiplicity in the VELO, Nhit

VELO Each reconstructedtrackisassignedaweight, ω,thataccountsforthese effects:

ω ( η , φ,pT, Nhit

VELO) = (1− Pfake− Psec)/( acc · tr ). (1)

11 76

Fig 1 Hit-multiplicitydistribution in the VELO for selected events of the minimum-bias samples in the (left)p+Pb and (right) Pb+p configurations.The activity classes are defined as fractions of the full distribution, as indicated by colours (shades) The 0–3% class is a sub-sample of the 0–10% class (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1

Relative event-activity classes defined by the VELO-hit multiplicity,Nhit

VELO , of an event The classes are defined as fractions of theNhit

VELO distribution for minimum-bias recorded events in thep+Pb or Pb+p configuration.The 0–3% class is a

sub-sample of the 0–10% class For illustration purposes the average number, N chMC , of

prompt charged particles withp >2 GeV/ c, pT>0.15 GeV/ c and2.0< η <4.9 is

listed for events simulated with the Hijing event generator Statistical uncertainties

are negligible.

Relative activity class p+Pb Pb+p

RangeNhit VELO N chMC RangeNhit

VELO N chMC

50–100% very low 0–1200 18.9 0–1350 29.2

30–50% low 1200–1700 30.0 1350–2000 47.4

10–30% medium 1700–2400 42.8 2000–3000 70.9

0–10% high 2400–max 63.6 3000–max 106.7

0–3% very high 3000–max 73.7 3800–max 126.4

4 Activity classes and data samples

number of particles produced within a collision The hit

multi-plicityin the VELO is proportional to this global eventproperty

Withitscoverageinpseudorapidityrangingfrom1.9 < η <4.9 in

theforwarddirectionand−2.5< η < −2.0 inthebackward

direc-tion,theVELOcanprobethetotalnumberofchargedparticlesper

eventmorecomprehensivelythanothersub-detectorsofLHCb

Theanalysispresentedinthispaperisbasedonasubsetofthe

totaldata setrecorded during the2013 pPb runningperiod.The

1.1×108 eventswhichare randomlyselectedfromtheabout10

timeslargerfull sample.Thehigh-multiplicitysamplescontainall

recordedeventswithatleast2200 hits intheVELO andamount

toabout1.1×108eventsinPb+p and1.3×108 eventsinPb+p

collisions

Five event-activity classes are defined as fractions of the

hit-multiplicity distributions of the minimum-bias samples, as

indi-catedinFig 1.SincecollisionsrecordedinthePb+p configuration

reach larger hit-multiplicities compared to those in the p+Pb

configuration,the relative classesare definedseparately for each

configuration.The 50–100%class contains approximatelythe 50%

ofeventswiththelowesteventactivities,followedbythe30–50%

and10–30% classesrepresentingmedium-activityevents,andthe

0–10%and0–3% classesofhigh-activity events.Theranges

defin-ingtheactivityclassesarelistedinTable 1.Foreachclass,average

numbersofchargedparticles, N chMC,are quotedforillustration,

basedonthe Hijing eventgenerator

Thelong-rangecorrelationsinthedirectionofthefragmenting

proton(p+Pb configuration)andthe directionof the

fragment-ingleadion(Pb+p configuration)arecomparedforclassesofthe

same absoluteactivity in the pseudorapidity range of 2.0 < η <

Table 2

Common absolute activity bins for the p +Pb and Pb+p samples.The activity

of p +Pb events is scaled to match the same activity ranges of Pb+p events,

as explained in the text For illustration purposes the average number, N chMC , of prompt charged particles with p >2 GeV/ c, pT>0.15 GeV/ c and2.0< η <4.9 is listed for events simulated with the Hijing event generator The uncertainties are due to the scaling factor of 0.77±0.08 Statistical uncertainties are negligible.

Common absolute activity bin

Nhit VELO -range

in Pb+p scale

p+Pb Pb+p

N chMC N chMC

4.9.Here aproperassignmentofequivalent activityclassesneeds

to take into account the fact that the VELO acceptance is larger thanthepseudorapidityintervalofinterest.Assumingalinear re-lationbetweenthetotalnumberofVELOhitsandthenumberof tracks inthe range 2.0 < η <4.9, one findsthat N VELO hitsin thePb+p configurationcorrespondtoN/(0.77±0.08)VELOhits

inthePb+p case.The uncertaintyinthescalingfactoraccounts for deviations fromperfect linearity in the data that are not re-producedinthesimulation,andispropagatedintothesystematic uncertaintiesoftheresults.Fivecommonabsoluteactivityclasses, labelledI–V,aredefinedinthehigh-activityregionandarelisted

in Table 2 with the corresponding average numbers of charged particles fromsimulation.Thequoted uncertainties inthe p+Pb samplearerelatedtothesystematicuncertaintyofthescaling fac-tor

Theanalysisisrepeatedusinganalternativeevent-activity clas-sification,basedonthemultiplicityofselectedtracksintherange

2.0< η <4.9.Inanalogytothenominalapproachusingthe VELO-hitmultiplicity,thesamefractionsofthefulldistributionareused

to define relative activity classes for both beam configurations Similarly, five common activity bins for the p+Pb and Pb+p

samples are defined in the intermediate to high-activity classes The results are found to be independent of thedefinition of the activityclasses

5 Analysis method

Two-particle correlations are measured separately for events

in each activity class The track sample containing the selected candidates of primary charged particles is divided into three pT

intervals: 0.15–1.0 GeV/c,1.0–2.0 GeV/c and 2.0–3.0 GeV/c.For each event, all candidates within a given pT interval are identi-fiedastrigger particles.Byselectingatriggerparticleallremaining candidateswithin thesameintervalcomposethegroup of

associ-1 66

Fig 2 Two-particlecorrelation functions for events recorded in the p+Pb configuration, showing the (left) low and (right) high event-activity classes The analysed pairs

of prompt charged particles are selected in apT range of 1–2 GeV/ c.The near-side peak around η = φ =0 is truncated in the histograms (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

ated particles.Particle pairs are formed by combiningevery

trig-ger particle with each associated particle Due to the symmetry

aroundtheorigin,differencesinazimuthalangle φ aretakenin

the range [0, π ] and asabsolute values in  η For visualisation

purposesplotsaresymmetrized.Thetwo-particlecorrelation

func-tion is composed ofa signal part S( η , φ), a background part

B( η , φ),anda normalizationfactor B(0, 0).The totalfunction

isdefinedastheassociatedyieldpertriggerparticle,givenby

1

Ntrig

d2Npair

d ηdφ = S( η , φ)

B( η , φ) ×B(0,0), (2)

where Npair is thenumber ofparticle pairsfound in a ( η , φ)

bin.ThenumberoftriggerparticleswithinagivenpTintervaland

activityclassisdenotedbyNtrig.ThesignaldistributionS( η , φ)

describestheassociatedyieldpertriggerparticleforparticlepairs,

Nsame,formedfromthesameevent,andisdefinedas

S( η , φ) = 1

Ntrig

d2Nsame

FollowingtheapproachinRef.[6],thesumovertheeventsis

per-formed separately forNtrig andford2Nsame/d ηdφ beforethe

ratioiscalculated.The backgrounddistribution B( η , φ) is

de-finedforparticlepairsofmixedevents,

B( η , φ) = d2Nmix

anddescribes the yield of uncorrelatedparticles The Nmix pairs

areconstructedbycombiningalltriggerparticlesofaneventwith

theassociatedparticlesoffivedifferentrandomeventsinthesame

activity class, whose vertex positions in the beam direction are

within 2 cm ofthe original event.As a result, effects duetothe

detectoroccupancy, acceptanceandmaterialare accountedforby

dividingthesignalby thebackgrounddistribution,wherethe

lat-ter is normalised to unity around the origin The factor B(0, 0)

describesthe associated yield forparticles of a pairtravelling in

pairacceptance

All trigger and associated particles in the signal and

back-ground distributions are weighted with the correction factors ω

described inSection3.Furthermore,alternative correctionfactors

determinedfromthelargepile-uppp simulationusingPythiaare

appliedto evaluatesystematicuncertainties Theresulting

associ-atedcorrelation yields agree within 3% withthenominal results

To estimate the influence of the track selection, the correction

relaxedtotwicethenominalvalue,andthevalueofthe multivari-ate classifier used to suppress fake tracks isvaried by ±5% The resulting different correction factors are applied to the measure-ments whichare thencomparedtothenominalcorrectedresults The differencedueto thedifferentpromptselection isnegligible, whilethealternativefaketracksuppressionresultsinamaximum variationof3%.Typical variationsaremuchsmaller.Theeffecton thefinalresults,obtainedaftersubtractingaglobaloffset,is negli-gible

6 Results

Two-particle correlation functions for events recorded in the

p+Pb configurationarepresentedinFig 2.Thecorrelationfor par-ticleswith1 <pT<2 GeV/c isshownforeventsofthe50–100% and0–3%class,representinglowandvery-higheventactivities, re-spectively.Both histogramsaredominatedbythejetpeak around

 η ≈ φ ≈0 which is due to correlations of particles originat-ing fromthesamejet-like objectsandthusbeingboostedclosely together.Forbettervisualisation ofadditionalstructures,inall 2D-histogramsthejetpeakistruncated.Thesecondprominentfeature

isvisibleontheaway-side(φ ≈ π)overalongrangein  ηand combinesjetand(potential)ridgecontributions.Theeventsample with very high eventactivity (Fig 2, right) shows an additional, lesspronounced,long-rangestructurecentredat φ =0,whichis not presentin the corresponding low-activity sample The struc-ture,oftenreferredtoasthenear-sideridge,iselongatedoverthe fullmeasured  ηrangeof2.9 units.Thisobservationoftheridge forparticles producedin proton-leadcollisions atforward rapidi-ties,2.0 < η <4.9,extendspreviousmeasurementsattheLHC

Two-particlecorrelationsforeventsrecordedinthePb+p

con-figuration are shown in Fig 3, for particle pairs with 1 <pT<

2 GeV/c. The 50–100% and 0–3% activity classes in the Pb+p

sample exhibitthesamecorrelationstructuresasthe correspond-ing classesinthe p+Pb sample.Whiletheshapeandmagnitude

of the jet peak and the away-side ridge appear to be of similar sizesinbothbeamconfigurations,thenear-sideridgeismore pro-nouncedforparticlesinthedirectionoftheleadbeam.Forthe3%

ofeventswiththehighesteventactivity,thenear-sideridgeinthe

sample

Similar behaviour is found whenanalysing particlepairs with larger transverse momenta in the interval 2 <pT<3 GeV/c. In

Fig 4the correlationfunctionsinthis pT rangearepresentedfor the 3%highest-activity eventsrecordedinthe p+Pb andPb+p

configurations The near-side ridge is present in both samples; howeverin the p+Pb sampleit isonly marginally visiblewhile

11 76

Fig 3 Two-particlecorrelation functions for events recorded in the Pb+p configuration,showing the (left) low and (right) high event-activity classes The analysed pairs

of prompt charged particles are selected in apT range of 1–2 GeV/ c.The near-side peak around( η = φ =0)is truncated in the histograms (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig 4 Two-particlecorrelation functions for events recorded in thep+Pb (left) and Pb+p (right)configurations, showing the 0–3% event-activity class The analysed pairs

of prompt charged particles are selected in apT range of 2–3 GeV/ c.The near-side peak around( η = φ =0)is truncated in each histogram (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

short-rangejet peakinthishigher pT interval ismorecollimated

comparedtothe1–2 GeV/c interval,becauseofthehigheraverage

totalmomentumoftheparticles.Asaresult,thenear-sideridgeis

visibletowards | η |valuesslightlybelow2.0 withoutbeing

cov-eredbythejetpeak

Inorder to studythe evolution ofthe long-rangecorrelations

onthenearandaway sidesin moredetail,one-dimensional

pro-jectionsofthecorrelationfunctionon φ arecalculated,

Y(φ) ≡ 1

Ntrig

dNpair

 ηb−  ηa

 η b



 η a

1

Ntrig

d2Npair

d ηdφd η

(5)

Theshort-range correlations, e.g ofthejet-peak, are excluded by

averagingthetwo-dimensionalyieldovertheintervalfrom  ηa=

2.0 to  ηb=2.9.Since random particle combinationsproduce a

flatpedestalintheyield,thecorrelation structuresofinterestare

extractedbyusingthezero-yield-at-minimum(ZYAM)method[33,

34].By fittinga second-order polynomial to Y(φ) in therange

0.1 < φ <2.0, the offset is estimated as the minimum of the

polynomial This value, further denoted as CZYAM, is subtracted

fromY(φ)toshiftits minimumtobe atzeroyield.The

uncer-taintiesonCZYAM duetothelimitedsamplesizeandthefitrange

arebelow0.002 forallindividualmeasurements

Thesubtractedone-dimensional yieldsforthe p+Pb (full

cir-cles)and Pb+p (opencircles) data samplesare shownin Fig 5

forall activity classesand p intervals The correlation increases

witheventactivity,butdecreasestowardshigher pT wherefewer particles arefound Since moreparticles areemitted intothe

configuration,alargeroffsetisobserved,asindicatedbytheZYAM constants.AlldistributionsinFig 5showamaximumat φ = π,

momentumofthenear-side(the jetpeakisexcluded inthis rep-resentation) Thelower activityclasses,50–100% and30–50%, do

class of thePb+p sampleshows a first changein shape ofthe distributionat φ =0.Thepicturechangeswhenprobingthe in-termediate activityclass10–30%.Inall pT intervalsofthe Pb+p

sampletheemergenceofthenear-sideridgewithasecond maxi-mumat φ =0 isclearlyvisible.Inthe p+Pb sampletheevent activityisstillnothighenoughtoformaclearnear-sidestructure

Inthehigh-activityclasses,0–10%and0–3%,thenear-sideridgeis strongly pronounced in the Pb+p sample inall pT intervals In the p+Pb samplethenear-sidestructureislessdistinct;however the1<pT<2 GeV/c intervalshowsaclearnear-sideridge

Aqualitativelysimilarbehaviourisseenintheforward-central correlationsstudied by theALICEexperiment[9],witha forward muon trigger and central associated particles Here also a clear ridge effect is observed, which grows with increasing event ac-tivity,andindicationsare seenthat itis morepronounced inthe hemisphereofthePbnucleus

ridge.Theridgeontheaway-sideisonlyweaklydependenton pT,

1 66

Fig 5 One-dimensionalcorrelation yield as a function ofφobtained from the

ZYAM-method by averaging over 2.0<  η <2.9 The subtracted yields are

pre-sented for √

s N N=5 TeV proton-lead collisions recorded in p +Pb (full green

circles) and Pb+p (openblue circles) configurations The ZYAM constant is given

in each panel Event classes are compared for low to very-high activities from top

to bottom, and different intervals of increasingpT from left to right Only statistical

uncertainties are shown Error bars are often smaller than the markers (For

inter-pretation of the references to colour in this figure legend, the reader is referred to

the web version of this article.)

1 <pT<2 GeV/c. Comparing p+Pb and Pb+p, one finds that

especiallyforhigheventactivitiesthenear-sideridgeismore

pro-nouncedinthePbhemisphere

Study of the one-dimensional yields within a pT interval for

different activity classes shows that the away side remains

ap-proximately unchanged, while the near side starts to form the

additionalridgewhenacertaineventactivityisreached.This

turn-on,however,appearstobeatdifferentactivitiesinthep+Pb and

Thesame qualitativeobservationsin thevariousanalysisbins,

including the emergence of the near-side ridge, are found when

using the track-based approach for the definition of the activity

classesasasystematiccheck.Thetotalcorrelationyieldvariesby

10% inthelow-pT range.Theemergenceofthenear-sideridge in

theZYAM-subtractedyieldisunaffectedbythechangeoftheevent

activitydefinition

Further systematic effects related to the event selection are

evaluatedbyincludingeventswithmultiplereconstructedprimary

vertices.Thechangeofthefinal correlation yieldisnegligible.As

po-laritiesareanalysedseparately.Theresultsareingoodagreement witheachother

Toinvestigatetheactivitydependenceofthelong-range corre-lationsinthep+Pb andPb+p samplesinmoredetail,common bins in absolute activity for both samples are studied For this

number of charged particles are emitted into the forward direc-tion.Eventsofbothsamplesaregroupedintofivenarrowactivity bins, asdefinedinTable 2.Fig 6 comparesthe ZYAM-subtracted two-particle correlation yields in therange 1 <pT<2 GeV/c,in

bands representthe systematic uncertaintyon the scaling factor, which translates the activity of the p+Pb configurationto that

sameactivityin theforwardregion,theobservedlong-range cor-relations become compatible within the uncertainties, except for binIinwhichtheaway-side yieldin p+Pb isstillslightlymore pronounced Thenear-sidecorrelation inthebeam(p) andtarget (Pb) fragmentationhemispheresshowsaconsistent increasewith increasingeventactivity

7 Summary and conclusions

Two-particleangularcorrelationsbetweenpromptcharged par-ticlesproducedinpPb collisionsat√

s N N=5 TeV havebeen

detector The angular correlations are studied in the laboratory frameinthepseudorapidityrange2.0< η <4.9 overthefullrange

of azimuthal angles, probing particle pairs in different common

pT intervals With the asymmetric detector layout, the analysis

config-urations, which probe rapidities in the nucleon–nucleon centre-of-mass frame of 1.5 <y <4.4 and −5.4 < y < −2.5, respec-tively The strength ofthe near-side ridge observed in the back-ward(Pb+p configuration)regionappearstobeofsimilarsizeto that found in theforward(p+Pb configuration)region The rel-ative shift of aboutone unit in nucleon–nucleon centre-of-mass rapidity betweenthe twoconfigurations produces nosizeable ef-fect on the near-side ridge within the accuracy of the measure-ment.Foreventswithhigheventactivityalong-rangecorrelation

on the near side (the ridge) is observed in both configurations While thecorrelationstructureontheaway sideshrinkswith in-creasing pT,thenear-side ridgeis mostpronouncedin therange

1 <pT<2 GeV/c. The observation of the ridge in the forward

qualitative features Furthermore, the correlation dependence on the eventactivity is investigated for relative and absolute activ-ityranges.Thecorrelation structuresonthenearsideandonthe away side both grow stronger withincreasing eventactivity For identicalabsoluteactivityrangesinthep+Pb andPb+p

config-urations theobservedlong-rangecorrelationsarecompatiblewith eachother

Acknowledgements

We express our gratitude to our colleagues in the CERN ac-celerator departments for the excellent performance of the LHC

We thank the technical andadministrative staff at the LHCb

(Spain);SNSFandSER(Switzerland);NASU(Ukraine);STFC(United

11 76

Fig 6 One-dimensionalcorrelation yield as a function ofφobtained from the ZYAM-method by averaging the two-dimensional distribution over 2.0<  η <2.9 The results for thep+Pb and Pb+p samplesare compared in five event classes which probe identical activities in the range 2.0< η <4.9 The measured hit-multiplicities of thep+Pb sample are scaled to agree with the hit-multiplicities of the Pb+p sample.The uncertainty band represents the systematic limitation of the scaling procedure The error bars represent the statistical uncertainty (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

that are provided by CERN, IN2P3 (France), KIT and DESY

(Ger-many), INFN (Italy), SURF (The Netherlands), PIC (Spain), GridPP

(United Kingdom), RRCKI (Russia), CSCS (Switzerland), IFIN-HH

areindebtedtothecommunitiesbehind themultipleopensource

softwarepackagesonwhich wedepend.Weare alsothankfulfor

the computing resources and the access to software R&D tools

provided by Yandex LLC (Russia) Individual groups or members

havereceived supportfromAvHFoundation(Germany),EPLANET,

MarieSkłodowska-Curie Actions andERC (EuropeanUnion),

Ré-gionAuvergne(France),RFBR(Russia),GVA,XuntaGalandGENCAT

(Spain),TheRoyalSocietyandRoyalCommissionfortheExhibition

of1851(UnitedKingdom)

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1 66

LHCb Collaboration

J.J Back49, A Badalov37, C Baesso61, W Baldini17,39, R.J Barlow55, C Barschel39, S Barsuk7,

I Bediaga1, L.J Bel42, V Bellee40, N Belloli21,j, I Belyaev32, E Ben-Haim8, G Bencivenni19,

M van Beuzekom42, A Bien12, S Bifani46, P Billoir8, T Bird55, A Birnkraut10, A Bizzeti18,h,

L Capriotti55, A Carbone15,d, G Carboni25,k, R Cardinale20,i, A Cardini16, P Carniti21,j, L Carson51,

H.V Cliff48, J Closier39, V Coco39, J Cogan6, E Cogneras5, V Cogoni16,e, L Cojocariu30,

R Ekelhof10, L Eklund52, I El Rifai5, Ch Elsasser41, S Ely60, S Esen12, H.M Evans48, T Evans56,

F Ferrari15, F Ferreira Rodrigues1, M Ferro-Luzzi39, S Filippov34, M Fiore17,39, , M Fiorini17, ,

M Firlej28, C Fitzpatrick40, T Fiutowski28, K Fohl39, P Fol54, M Fontana16, F Fontanelli20,i,

D.C Forshaw60, R Forty39, M Frank39, C Frei39, M Frosini18, J Fu22, E Furfaro25,k,

M Kecke12, M Kelsey60, I.R Kenyon46, M Kenzie39, T Ketel43, E Khairullin66, B Khanji21,39,j,

11 76

W Parker59, C Parkes55, G Passaleva18, G.D Patel53, M Patel54, C Patrignani20,i, A Pearce55,50,

A Pellegrino42, G Penso26,l, M Pepe Altarelli39, S Perazzini15,d, P Perret5, L Pescatore46,

K Petridis47, A Petrolini20,i, M Petruzzo22, E Picatoste Olloqui37, B Pietrzyk4, T Pilaˇr49, D Pinci26,

A Pistone20, A Piucci12, S Playfer51, M Plo Casasus38, T Poikela39, F Polci8, A Poluektov49,35,

M Santimaria19, E Santovetti25,k, A Sarti19,l, C Satriano26,m, A Satta25, D.M Saunders47,

N Tuning42,39, A Ukleja29, A Ustyuzhanin66,65, U Uwer12, C Vacca16,39,e, V Vagnoni15, G Valenti15,

1 66

1Centro Brasileiro de Pesquisas Físicas (CBPF), Rio de Janeiro, Brazil

2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil

3Center for High Energy Physics, Tsinghua University, Beijing, China

4LAPP, Université Savoie Mont-Blanc, CNRS/IN2P3, Annecy-Le-Vieux, France

5Clermont Université, Université Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France

6CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France

7LAL, Université Paris-Sud, CNRS/IN2P3, Orsay, France

8LPNHE, Université Pierre et Marie Curie, Université Paris Diderot, CNRS/IN2P3, Paris, France

9I Physikalisches Institut, RWTH Aachen University, Aachen, Germany

10Fakultät Physik, Technische Universität Dortmund, Dortmund, Germany

11Max-Planck-Institut für Kernphysik (MPIK), Heidelberg, Germany

12Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany

13School of Physics, University College Dublin, Dublin, Ireland

14Sezione INFN di Bari, Bari, Italy

15Sezione INFN di Bologna, Bologna, Italy

16Sezione INFN di Cagliari, Cagliari, Italy

17Sezione INFN di Ferrara, Ferrara, Italy

18Sezione INFN di Firenze, Firenze, Italy

19Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy

20Sezione INFN di Genova, Genova, Italy

21Sezione INFN di Milano Bicocca, Milano, Italy

22Sezione INFN di Milano, Milano, Italy

23Sezione INFN di Padova, Padova, Italy

24Sezione INFN di Pisa, Pisa, Italy

25Sezione INFN di Roma Tor Vergata, Roma, Italy

26Sezione INFN di Roma La Sapienza, Roma, Italy

27Henryk Niewodniczanski Institute of Nuclear, Physics Polish Academy of Sciences, Kraków, Poland

28AGH – University of Science and Technology, Faculty of Physics and Applied Computer Science, Kraków, Poland

29National Center for Nuclear Research (NCBJ), Warsaw, Poland

30Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania

31Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia

32Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia

33Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia

34Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia

35Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia

36Institute for High Energy Physics (IHEP), Protvino, Russia

37Universitat de Barcelona, Barcelona, Spain

38Universidad de Santiago de Compostela, Santiago de Compostela, Spain

39European Organization for Nuclear Research (CERN), Geneva, Switzerland

40Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland

41Physik-Institut, Universität Zürich, Zürich, Switzerland

42Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands

43Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, The Netherlands

44NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine

45Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine

46University of Birmingham, Birmingham, United Kingdom

47H.H Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom

48Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom

49Department of Physics, University of Warwick, Coventry, United Kingdom

50STFC Rutherford Appleton Laboratory, Didcot, United Kingdom

51School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom

52School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom

53Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom

54Imperial College London, London, United Kingdom

55School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom

56Department of Physics, University of Oxford, Oxford, United Kingdom

57Massachusetts Institute of Technology, Cambridge, MA, United States

58University of Cincinnati, Cincinnati, OH, United States

59University of Maryland, College Park, MD, United States

60Syracuse University, Syracuse, NY, United States

61Pontifícia Universidade Católica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil u

62Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China v

63Departamento de Fisica, Universidad Nacional de Colombia, Bogota, Colombia w

64Institut für Physik, Universität Rostock, Rostock, Germany x

65National Research Centre Kurchatov Institute, Moscow, Russia y

66Yandex School of Data Analysis, Moscow, Russia y