Long-range two-particle correlations of strange hadrons with charged particles in pPb and PbPb collisions at LHC energies

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Long-range

two-particle

correlations

of

strange

hadrons

with

charged

particles

in

pPb

and

PbPb

collisions

at

LHC

energies

.

CMS

Collaboration

⋆

CERN,Switzerland

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received11September2014

Receivedinrevisedform8December2014 Accepted23January2015

Availableonline26January2015 Editor:M.Doser Keywords: CMS Ridge Long-range Correlations Flow High-multiplicity

Measurements oftwo-particleangularcorrelationsbetweenan identifiedstrangehadron (K0 S orΛ/Λ)

and acharged particle, emittedin pPbcollisions, are presented overawiderange inpseudorapidity and fullazimuth.Thedata,correspondingtoanintegratedluminosityofapproximately35 nb−1_,were

collected ata nucleon–nucleon center-of-mass energy (√sN N) of 5.02 TeV withthe CMS detectorat

theLHC.Theresultsarecomparedtosemi-peripheralPbPbcollisiondataat√sN N=2.76 TeV,covering similarcharged-particlemultiplicitiesintheevents.Theobservedazimuthalcorrelationsatlargerelative pseudorapidityareusedtoextractthesecond-order(v2)andthird-order(v3)anisotropyharmonicsofK0S

andΛ/Λparticles.Thesequantitiesarestudiedasafunctionofthecharged-particlemultiplicityinthe event andthetransverse momentumoftheparticles.Forhigh-multiplicitypPbevents,a clearparticle speciesdependenceofv2andv3isobserved.ForpT<2 GeV,thev2and v3valuesofK0S particlesare

largerthanthoseofΛ/ΛparticlesatthesamepT.Thissplittingeffectbetweentwoparticlespeciesis

foundtobestrongerinpPb thaninPbPb collisionsinthesamemultiplicityrange.Whendividedbythe number ofconstituent quarksandcomparedatthesametransversekineticenergyperquark,bothv2

and v3forK0S particlesare observedtobeconsistentwiththoseforΛ/Λparticlesatthe10%levelin

pPb collisions.Thisconsistencyextendsoverawiderangeofparticletransversekineticenergyandevent multiplicities.

©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

Studies of multiparticle correlations provide an important in-sight into the underlying mechanism of particle production in high-energycollisionsofprotonsandnuclei.A keyfeatureofsuch correlations in ultrarelativistic nucleus–nucleus (AA) collisions is theobservationofapronouncedstructure onthenearside (rela-tiveazimuthal angle

|#φ|

≈

0) that extendsoveralarge rangein relativepseudorapidity(

|#

η

|

upto4unitsormore).Thisfeature, knownasthe“ridge”,hasbeenfoundoverawiderangeofAA en-ergiesandsystemsizesatboththeRelativisticHeavyIonCollider (RHIC) [1–5] and the Large Hadron Collider (LHC) [6–10] and is interpreted asarising primarily fromthecollective hydrodynamic flowofastronglyinteracting,expandingmedium

[11,12]

.

Similar long-range correlations have also been discovered in proton–proton(pp)[13],proton–lead(pPb)[14–16],anddeuteron– gold(dAu)[17]collisionswithhighfinal-stateparticlemultiplicity. As the collision volume size is reduced, it is possible that the

⋆ _{E-mailaddress:cms-publication-committee-chair@cern.ch.}

system willnot be ableto equilibrate andthehydrodynamic de-scriptionwill breakdown. As such,there hasbeen noconsensus ontheoriginoftheparticlecorrelationstructure inthesesmaller systems. A variety of theoretical models have been proposed to interpretthisphenomenoninpp[18],pPb,anddAu collisions. Be-sides hydrodynamic effects in a high-density system [19,20], an alternatemodelincludinggluonsaturationintheincoming nucle-onshasalsobeenshowntodescribethesedata[21,22].

Inhydrodynamicaldescriptions,thecollectiveflowmanifests it-self as an azimuthal anisotropy in the distribution of final-state particles. An additional key consequence of thesemodels is that the measured anisotropies will depend on the mass of the par-ticle [23–25]. Morespecifically, for particles withtransverse mo-mentum below about 2 GeV, the anisotropy will be larger for lighter particles.Thepresenceofthismassorderingiswell estab-lishedinAA collisionsatRHICandLHCenergies

[26–30]

.This phe-nomenonhasrecentlyalsobeenobservedinpPb[31]anddAu[17]

collisions, consistent with expectationsfrom hydrodynamic mod-els [32,33]. The analysis presented in thispaper aims to further explore thiseffectby extractinganisotropies ofidentifiedstrange

http://dx.doi.org/10.1016/j.physletb.2015.01.034

0370-2693/©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

mesons(K_S)andbaryons(

Λ

and

Λ

)inpPb andinPbPb collisions

thatproducesimilarfinal-stateparticlemultiplicity.

The azimuthal correlations of emitted particle pairs are typ-ically characterized by their Fourier components, dN_d#φpair

∝

1

+

!

n2Vn#cos

(

n

#φ)

, where Vn# are the two-particle Fourier

co-efficients and vn

=

√

Vn# denote the single-particle anisotropy

harmonics [34]. In particular, the second and third Fourier com-ponents are known as elliptic (v2) and triangular (v3) flow,

re-spectively[12].Inhydrodynamicalmodels, v2 and v3 aredirectly

relatedtothe responseofthe medium totheinitial collision ge-ometryanditsfluctuations[35–37].Assuch,theseFourier compo-nents canprovide insightinto thefundamental transport proper-tiesofthemedium.

In AA collisions at RHIC, a scaling of v2 as a function of pT

withthenumberofconstituentquarks(nq)hasbeenobservedin

the range 2

<

pT

<

6 GeV [38]. Specifically, the values of v2

/

nq

arefound tobe very similarforall mesons (nq

=

2)andbaryons

(nq

=

3)whencomparedatthesamevalueofpT

/

nq.Thisempirical

scalingmay indicate that final-statehadrons are formed through recombinationofquarksin this pT regime [39–41],possibly

pro-viding evidence of deconfinement of quarks andgluons inthese systems.AtlowerpT(pT

<

2 GeV),a similarscalingbehavioris ob-served,although,accordingtoperfectfluid hydrodynamics,v2

/

nq

values must be compared at the same transverse kinetic energy perconstituentquark(KET

/

nq,whereKET

=

"

m2

₊

_p2

T

−

m)to

ac-countforthemassdifferenceofhadrons[42,43].

Thispaperpresentsananalysisoftwo-particlecorrelationswith identified strange hadrons, K0_S and

Λ/Λ

, in pPb collisions at a center-of-massenergypernucleonpair(

√

sN N) of5.02 TeV.With

the implementation of a dedicated high-multiplicity trigger, the 2013 pPb data sample gives access to multiplicities comparable to those insemi-peripheral PbPb collisions. Two-particle correla-tionfunctionsareconstructedbyassociatingaK0

S or

Λ/Λ

particle

with a charged particle (pairs of K0

S or

Λ/Λ

particles are not

studied due to their limited statistical precision). In the context ofhydrodynamicmodels, Fouriercoefficientsofdihadron correla-tions can be factorizedinto products of single-particle azimuthal anisotropies.Assumingthatthisrelationship holds,v2 and v3 are

extractedfromlong-rangetwo-particlecorrelationsasafunctionof strangehadron pT andeventmultiplicity.Toexaminethevalidity

ofconstituentquarknumberscaling,v2

/

nqandv3

/

nqareobtained

asa function of KET

/

nq for both K0S and

Λ/Λ

particles.A direct

comparisonofthepPb andPbPb resultsoverabroadrangeof sim-ilarmultiplicitiesispresented.

2. TheCMSexperimentanddatasample

Adescriptionof theCMSdetectorin theLHC atCERNcan be foundinRef.[44].The maindetectorcomponentusedinthis pa-per is the tracker,located ina superconducting solenoidof 6 m internaldiameter,providinga magneticfieldof3.8 T.The tracker consistsof1440siliconpixeland15 148siliconstripdetector mod-ules,coveringthepseudorapidityrange

|

η

|

<

2

.

5.Forhadronswith pT

≈

1 GeV and

|

η

|

≈

0,theimpactparameter(distanceofclosest

approachfromtheprimary collisionvertex)resolutionis approxi-mately100 µmandthepT resolutionis0.8%.

Also located inside the solenoid are the electromagnetic cal-orimeter (ECAL) and the hadron calorimeter (HCAL). The ECAL consists of 75 848 lead tungstate crystals, arranged in a quasi-projectivegeometryanddistributedinabarrelregion(

|

η

|

<

1

.

48)

andtwo endcaps that extend to

|

η

|

=

3

.

0. The HCAL barrel and

endcapsaresamplingcalorimeterscomposed ofbrass and scintil-latorplates,covering

|

η

|

<

3

.

0.Iron/quartz-fiberforward

calorime-ters(HF)areplacedoneachsideoftheinteractionregion,covering

2

.

9

<

|

η

|

<

5

.

2.The detailedMonte Carlo(MC) simulationof the

CMSdetectorresponseisbasedon geant4

[45]

.

The data sample used in this analysiswas collected withthe CMSdetectorduringtheLHCpPb runin2013.Thetotalintegrated luminosity of the data set is about35 nb−1 _{[46].The beam}

en-ergies are 4 TeV for protons and 1.58 TeV per nucleon for lead nuclei, resulting in a center-of-mass energy per nucleon pair of 5.02 TeV.The directionoftheprotonbeamwasinitiallysetup to be clockwise (20 nb−1_{), andwas later reversed (15 nb}−1_{). As a}

result ofthe energy difference betweenthe colliding beams, the nucleon–nucleoncenter-of-massinthepPb collisionsisnotatrest with respect to the laboratory frame. Massless particles emitted at

η

cm

=

0 in the nucleon–nucleon center-of-massframe will be

detected at

η

= −

0

.

465 (clockwise protonbeam)or 0

.

465

(coun-terclockwise proton beam) in the laboratory frame. A sample of peripheralPbPb dataat

√

sN N

=

2

.

76 TeV correspondingtoan

in-tegratedluminosity of about2.3 µb−1_{,collected during the 2011}

LHCheavy-ionrun,isalsoanalyzedforcomparisonwithpPb data atsimilarcharged-particlemultiplicityranges.

3. Onlinetriggeringandofflinetrackreconstructionand selection

The onlinetriggeringandtheoffline reconstruction and selec-tion follow the same procedure as described in Ref. [47]. Mini-mumbiaspPb eventsaretriggeredbyrequiringatleastonetrack with pT

>

0

.

4 GeV to be found in the pixel tracker for a pPb

bunchcrossing.Becauseofhardwarelimitsonthedataacquisition rate,onlyasmallfraction(

∼

10−3_{)ofallminimumbiastriggered}

events are recorded. In order to collect a large sample of high-multiplicity pPb collisions, a dedicatedhigh-multiplicitytrigger is alsoimplementedusingtheCMSLevel1(L1)andhigh-leveltrigger (HLT)systems.AtL1,twoeventstreamsweretriggered by requir-ingthetotaltransverseenergysummedoverECALandHCALtobe greaterthan 20or40 GeV. Chargedtracksarethenreconstructed onlineattheHLTusingthethreelayersofpixeldetectors,and re-quiringa trackoriginwithin a cylindricalregion of30 cmlength alongthebeamand0.2 cmradiusperpendiculartothebeam.For eachevent,thenumberofpixeltracks(Nonline

trk )with

|

η

|

<

2

.

4 and

pT

>

0

.

4 GeV is counted separately for each vertex. Only tracks

with a distance of closest approach of 0.4 cm or less to one of theverticesareincluded.Theonlineselection requiresNonline

trk for

the vertex withthe most tracks to exceed a specific value. Data aretakenwiththresholdsofNonline

trk

>

100

,

130 (fromeventswith

L1thresholdof20 GeV),and160,190(fromeventswithL1 thresh-old of40 GeV).While alleventswith Nonline

trk

>

190 are accepted,

only a fractionofthe events fromthe other thresholdsare kept. This fractionis dependent on the instantaneous luminosity.Data fromboththeminimumbias triggerandhigh-multiplicity trigger areretainedforofflineanalysis.

In the offline analysis, hadroniccollisions are selected by the presence ofatleastone tower withenergyabove 3 GeV ineach of the two HF calorimeters. Events are also required to contain at least one reconstructed primary vertex within 15 cm of the nominalinteractionpointalongthebeamaxisandwithin0.15 cm transversetothebeamtrajectory.Atleasttworeconstructedtracks are required to be associated withthe primary vertex, a condi-tionthatisimportantonlyforminimumbiasevents.Beamrelated backgroundissuppressed byrejecting eventsforwhich lessthan 25% of all reconstructed tracks pass the high-purity selection (as definedinRef.[48]).ThepPb instantaneousluminosityprovidedby theLHCinthe2013runresultedina3% probabilityofhavingat leastoneadditionalinteractionpresentinthesamebunchcrossing (pile-upevents).Theprocedureusedforrejectingpile-upeventsis described inRef. [47] and is based on the numberof tracks

as-sociatedwitheachreconstructedvertexandthedistancebetween differentvertices.A purityof99.8%forsinglepPb collisionevents isachievedforthehighestmultiplicitypPb interactionsstudiedin thispaper.Withtheselectioncriteriaabove,97–98%oftheevents are foundto beselected amongthosepPb interactions simulated withthe epos lhc [49] and hijing 2.1[50] eventgenerators that have at least one particle from the pPb interaction with energy

E

>

3 GeV ineachofthe

η

ranges

−

5

<

η

<

−

3 and3

<

η

<

5.

In this analysis, high-purity tracks are used to select primary tracks(tracksoriginatingfromthepPb interaction).Additional re-quirements are applied to enhance the purity of primary tracks. Thesignificanceoftheseparationalongthebeamaxis(z)between the track and the best vertex, dz/

σ

(

dz), and the significance of

theimpactparameterrelativetothebestvertextransversetothe beam, dT

/

σ

(

dT

)

, mustbe less than3, andthe relative pT

uncer-tainty,

σ

(

pT

)/

pT,must belessthan 10%. Toensurehightracking

efficiencyand toreduce therate ofmisreconstructed tracks, pri-marytrackswith

|

η

|

<

2

.

4 andpT

>

0

.

3 GeV areusedinthe

analy-sis(apTcutoffof0.4 GeVisusedinthemultiplicitydetermination

tomatchtheHLTrequirement).Basedonsimulationstudiesusing geant4topropagateparticlesfromthe hijing eventgenerator,the combinedgeometricalacceptanceandefficiencyforprimary track reconstructionexceeds60%forpT

≈

0

.

3 GeV and

|

η

|

<

2

.

4.The

ef-ficiencyisgreaterthan90%inthe

|

η

|

<

1 regionforpT

>

0

.

6 GeV.

Forthe eventmultiplicity rangestudied inthispaper, no depen-denceof the tracking efficiencyon multiplicity is found andthe rateofmisreconstructedtracksis1–2%.

TheentirepPb datasetisdividedintoclassesbasedonthe re-constructedtrack multiplicity, Noffline

trk ,whereprimary tracks with

|

η

|

<

2

.

4 and pT

>

0

.

4 GeV arecounted.Details ofthe

multiplic-ityclassificationinthisanalysis, includingthe fractionofthefull multiplicitydistributionandtheaveragenumberofprimarytracks beforeandaftercorrectingfordetectoreffectsineachmultiplicity range,areprovidedinRef.[47].

A subset of semi-peripheral PbPb data collected during the 2011 LHC heavy-ion run with a minimum bias trigger are also reanalyzed in order to directly compare pPb and PbPb systems atthe same collisionmultiplicity. The reanalyzed eventswere in the range of 50–100% centrality, where centrality is defined as the fractionofthe total inelasticcross section, with0% denoting themostcentralcollisions.Thissamplewas reprocessedusingthe sameeventselectionandtrackreconstructionalgorithmasforthe presentpPb analysis.A description ofthe2011PbPb data canbe foundinRefs.[47,51].

4. ReconstructionofK0

Sand

Λ/Λ

candidates

ThereconstructiontechniqueforK0

S and

Λ/Λ

candidates

(gen-erallyreferred toas V0_{s) atCMSwas first describedin Ref.[52].}

Toincreasetheefficiencyfortrackswithlowmomentumandlarge impact parameters, both characteristic of the K0

S and

Λ/Λ

de-cayproducts, thestandard looseselectionoftracks(as definedin Ref. [48]) is used in reconstructing the K0

S and

Λ/Λ

candidates.

Oppositelychargedtrackswithatleast4hitsandbothtransverse and longitudinal impact parameter significances greater than 1 (with respect to the primary vertex) are first selected to forma secondary vertex. The distanceofclosest approach ofthe pairof tracksisrequiredtobelessthan0.5 cm.Thefittedvertexinx, y,

z ofeachpairoftracksisrequiredtohavea

χ

2_{value normalized}

bythenumberofdegreesoffreedomlessthan7.Thepairoftracks isassumedtobe

π

+

π

− inK0_S reconstruction, whilethe assump-tionof

π

−_p

₍

_π

+_p

₎

_isusedin

_Λ

₍

_Λ

_{)reconstruction.For}

_Λ/Λ

_,the

lower-momentumtrackisassumedtobethepion.

Due to the long lifetime of K0_S and

Λ/Λ

particles, a require-ment on the significance of the V0 _{decay length, which is the}

three-dimensional distancebetweenthe primary and V0 _vertices

dividedbyitsuncertainty,tobegreaterthan5isappliedtoreduce background contributions. Toremove K0

S candidates misidentified

as

Λ/Λ

particles andvice versa, the

Λ/Λ

(K0_S) candidates must haveacorresponding

π

+

π

−

(

p

π

−

)

massmorethan20(10) MeV awayfromthePDGvalueoftheK0

S (

Λ

)mass[53].Theangle

θ

point

betweentheV0 _{momentumvectorandthevectorconnectingthe}

primary and V0 _{vertices is required to satisfy cos}

_θ

point

_>

₀

_.

_999.

This reduces theeffect ofnuclear interactions, random combina-tions oftracks, and

Λ/Λ

particles originatingfrom weak decays

of

Ξ

and

Ω

− particles. From MC simulations using Geant4 and

the hijing event generator, it is found that the contribution of

Λ/Λ

particlesfromweakdecaysislessthan3%afterthis require-ment. The K0_S (

Λ/Λ

) reconstruction efficiency is about 6% (1%) for pT

≈

1 GeV and 20%

(

10%

)

for pT

>

3 GeV within

|

η

|

<

2

.

4. Thisefficiencyincludes theeffects ofacceptanceandthe branch-ingratioforV0particledecaysintoneutralparticles.Therelatively lowreconstructionefficiencyoftheV0_{candidatesisprimarilydue}

to the decaylength cut andthelow efficiency forreconstructing daughtertrackswithpT

<

0

.

3 GeV orlargeimpactparameters.

Examples of invariant mass distributions of reconstructed K0 S

and

Λ/Λ

candidates are shownin Fig. 1 for pPb data, with V0 pT inthe rangeof 1–3 GeV and eventmultiplicity in the range

220

≤

Noffline

trk

<

260.Sincetheresultsfor

Λ

and

Λ

arefoundtobe

consistent,theyhavebeencombinedinthisanalysis.TheV0_peaks

can be clearly identified withlittlebackground.The true V0

sig-nal peakiswell described bya doubleGaussian function(witha commonmean),whilethebackgroundismodeled bya4th-order polynomialfunctionfitovertheentiremassrangeshownin

Fig. 1

. The mass window of

±

2

σ

wide around the center of the peak isdefinedasthe“peakregion”,where

σ

representstherootmean squareofthetwostandarddeviationsofthedoubleGaussian func-tions weightedbytheyields (withtypicalvalueof

σ

indicatedin

Fig. 1). Toestimate the contributionof backgroundcandidates in the peak region to the correlation measurement,a “sideband re-gion”ischosenthatincludesV0_{candidatesfromoutsidethe}

_±

₃

_σ

massrangearoundthe V0 _{masstothelimitofthemass}

distribu-tionsshownin

Fig. 1

.

5. Analysisoftwo-particlecorrelations

The construction of the two-particle correlation function fol-lowsthesameprocedureestablishedinRefs. [6,7,14,47].However, inthispaper,reconstructedV0_{candidatesfromeitherthepeakor}

sidebandregionaretakenas“trigger”particleswithinagivenptrig_T

range,instead ofchargedtracks asusedinprevious publications. The number oftrigger V0 _{candidates inthe eventis denoted by}

Ntrig.Particle pairsareformedbyassociatingeachtriggerparticle

withtheremainingchargedprimarytracksinaspecifiedpassoc

T

in-terval(whichcanbeeitherthesameasordifferentfromthe ptrig_T

range).Thetwo-dimensional(2D)correlationfunctionisdefinedin thesamewayasinpreviousanalysesas

1 N_trig d2_Npair d

#

η

d

#φ

=

B

(

0

,

0

)

×

S

(#

η

, #φ)

B

(#

η

, #φ)

,

(1)

where

#

η

and

#φ

arethedifferencesin

η

and

φ

ofthepair.The

same-event pair distribution, S

(#

η

,

#φ)

, represents the yield of

particlepairsnormalizedbyNtrigfromthesameevent,

S

(#

η

, #φ)

=

1 Ntrig

d2_Nsame

Fig. 1. InvariantmassdistributionofK0

S(left)andΛ/Λ(right)candidatesinthepTrangeof1–3 GeVfor220≤Nofflinetrk <260 inpPb collisionsat√sN N=5.02 TeV.Thesolid lineshowsthefitfunctionofadoubleGaussianplusa4th-orderpolynomial(dashedline).

Themixed-eventpairdistribution,

B

(#

η

, #φ)

=

1 Ntrig

d2_Nmix

d

#

η

d

#φ

,

(3)

isconstructedby pairingthetrigger V0 _{candidatesineach event}

withtheassociatedchargedprimary tracksfrom20different ran-domlyselectedeventsinthesame2 cmwiderangeofvertex po-sitioninthezdirectionandfromthesametrackmultiplicityclass. Here, Nmix _{denotes the number of pairs taken from the mixed}

events.Theratio B

(

0

,

0

)/

B

(#

η

,

#φ)

mainlyaccountsforthepair acceptanceeffects,with B

(

0

,

0

)

representingthemixed-event

as-sociatedyieldforbothparticlesofthepairgoinginapproximately the same direction and thus having maximum pair acceptance (withabinwidthof0.3in

#

η

and

π

/

16 in

#φ

).Thus,the

quan-tityinEq.(1)iseffectivelytheper-trigger-particleassociatedyield. A pairisremoved ifthe associatedparticlebelongstoa daughter trackof any trigger V0 _{candidate (this contribution is negligible}

sinceassociatedparticlesaremostlyprimarytracks).

The same-event and mixed-event pair distributions are first calculated foreach event, andthen averaged over all the events within the track multiplicity class. The range of 0

<

|#

η

|

<

4

.

8

and0

<

|#φ|

<

π

isusedtofillonequadrantofthe

(#

η

,

#φ)

his-tograms,withtheotherthreequadrantsfilled(forillustration pur-poses)byreflectiontocovera(

#

η

,

#φ

)rangeof

−

4

.

8

< #

η

<

4

.

8

and

−

π

/

2

< #φ <

3

π

/

2 for the2D correlation functions,aswill be shown later in Fig. 2. In performing the correlation analyses, eachreconstructedprimarytrackandV0 _{candidateisweightedby}

acorrection factor, followingthe procedure described inRefs. [6, 7,14,47]. This correction is also applied in calculating Ntrig. This

factor accounts for detector effects including the reconstruction efficiency,the detector acceptance,andthe fraction of misrecon-structedtracks.Thiscorrectionfactorisfoundtohaveanegligible effectontheazimuthalanisotropyharmonics.

5.1.Extractionofvnharmonics

Motivatedby hydrodynamic models oflong-rangecorrelations inpPb collisions, azimuthalanisotropy harmonicsofK0

S and

Λ/Λ

particles are extractedvia a Fourierdecomposition of

#φ

corre-lation functions averaged over

|#

η

|

>

2 (to remove short-range

correlationssuchasjetfragmentation), 1 Ntrig dNpair d

#φ

=

Nassoc 2

π

#

1

+

$

n 2Vn#cos

(

n

#φ)

%

,

(4)

aswas doneinRefs. [6,7,14,47].Here, Vn# arethe Fourier

coeffi-cientsand Nassoc represents thetotalnumberofpairs pertrigger

V0 _{particle for a given}

₍

_ptrig

T

,

passocT

)

bin. The first three Fourier

termsareincludedinthefitstothecorrelation functions. Includ-ing additional terms hasa negligible effect onthe results ofthe Fourierfit.

Iftheobservedtwo-particleazimuthalcorrelationsarisepurely astheresultofconvolutinganisotropicdistributionsofsingle par-ticles,thentheVn# coefficientscanbefactorizedintotheproduct

ofsingle-particleanisotropies[47],

Vn#

&

ptrig_T

,

passoc_T

'

=

vn

&

ptrig_T

'

×

vn

&

passocT

'

.

(5)

Following this assumption, the elliptic

(

v2

)

and triangular

(

v3

)

anisotropy harmonicsof V0 _{particlescan be extractedasa}

func-tionofpT fromthefittedFouriercoefficients,

vn

&

pV 0 T

'

=

Vn#

(

pV 0 T

,

prefT

)

"

Vn#

(

prefT

,

prefT

)

,

n

=

2

,

3

.

(6)

Here, a fixed pref_T rangefor the“reference” chargedprimary par-ticles is chosen to be 0

.

3

<

pT

<

3

.

0 GeV (the lowest pT region

accessiblebyCMSandthesameaswasusedinRef.[47]),to min-imizecorrelationsfromback-to-backjetsathigherpT.

The vn values are first extracted for V0 candidates from the

peakregion(which containssmallcontributionsfrombackground

V0_{s)andsidebandregion,denotedas v}obs

n andvbkgn ,respectively.

The vn signal of true V0 particles is denoted by vsign andis

ob-tainedby

vsign

=

v

obs

n

− (

1

−

fsig

)

×

vbkgn

fsig

,

n

=

2

,

3

,

(7)

assumingvsign andvbkgn areindependentfromeachother.Here, fsig

representsthesignalyieldfractioninthepeakregiondetermined by thefits tothemass distributionshownin

Fig. 1

.Thisfraction exceeds80% for

Λ/Λ

candidatesatpT

>

1 GeV andisabove95% forK0

S candidatesovertheentirepTrange.

5.2. Systematicuncertainties

Table 1summarizes differentsources of systematic uncertain-tiesinvsign (identicalforK0S and

Λ/Λ

particles)forpPb andPbPb

data.Thedominantsourcesofsystematicuncertaintiesarerelated tothereconstruction ofV0 _{candidates.The systematiceffectsare}

found tohave nodependenceon pT sothe estimatedsystematic

uncertaintiesareassumedtobeconstantpercentagesoverthe en-tire pT range.Systematicuncertainties in vsig3 areassumed tobe

Table 1

Summaryofsystematicuncertaintiesinvsig_n forpPb andPbPb data.

Source pPb (%) PbPb (%)

V0_{mass distribution range used in fit 1 1}

Size of V0_{mass region for signal 2 2}

Size and location of V0_{mass sideband region 2.2 2.2}

Misidentified V0_{mass region 2 2}

V0_{selection criteria 3 3} Tracker misalignment 2 2 MC closure test 4 4 Trigger efficiency 2 – Pile-up 1 – Total 6.9 6.6

TherangeoftheV0 _{massdistributionsusedinfittingthe}

sig-nalplus background(Fig. 1) isvaried by10%.Thischange,which could affectthevalue of fsig _{usedin Eq.(7),yields a systematic}

uncertaintyoflessthan1%forthevsig₂ results.Changingthemass rangeincludedinthepeakregioncouldimpactthevaluesofboth

fsig_andvobs

2 .Foravariationfrom

±

1

σ

to

±

3

σ

,thevsig2 valuesare

foundtobe consistentwithin 2%.Systematicuncertaintiesdueto selection ofdifferentsideband massregions, whichcould change

vbkg₂ , are estimated to be 2.2%. Possible contamination by resid-ualmisidentified V0 _{candidates (i.e.,K}0

S as

Λ/Λ

,andvice versa)

isalsoinvestigated.Variationoftheinvariantmassrangeusedto rejectmisidentified V0 _{candidatesleads tovariations oflessthan}

2% on vsig₂ .Systematiceffects relatedto selectionof the V0

can-didates are evaluated by varying the requirements on the decay lengthsignificanceandcos

θ

point,resultinginanuncertaintyof3%.

As misalignment of the tracker detector elements can affect the

V0 _{reconstruction performance, an alternative detectorgeometry}

isstudied.Comparedtothestandardconfiguration,thisalternative hasthetwohalvesofthebarrelpixeldetectorshiftedinopposite directionsalong thebeamby adistanceon theorderof100 µm. Thevaluesofvsig₂ foundusingtheshiftedconfigurationdifferedby lessthan2%fromthedefaultones.

To test the procedure of extracting the V0 _{signal v2 from}

Eq. (7), a study using epos lhc pPb MC events is performed to comparetheextractedvsig₂ resultswiththegenerator-levelK0

Sand

Λ/Λ

values.Theagreement isfound tobe betterthan 4%.Other

systematicuncertaintiesintroducedbythehigh-multiplicitytrigger efficiency(1%)andpossibleresidualpile-upeffects(1–2%)forpPb dataare estimatedinthesamewayasinRef. [47],andfound to makeonlyasmallcontribution.Thevarioussources ofsystematic uncertaintiesareaddedtogetherinquadraturetoarriveatthefinal systematic uncertainties (6.9%forpPb and 6.6% for PbPb),which areshownasshadedboxesin

Figs. 4–7

.

6. Results

The2Dtwo-particlecorrelationfunctionsmeasuredinpPb col-lisions for pairs of a K0

S (left) and

Λ/Λ

(right) trigger particles

anda chargedassociatedparticle(h±_{) areshownin Fig. 2inthe} pT range of1–3 GeV. The 2D correlation functionsare corrected

for the background V0 _{candidates, following the same approach}

of correcting vn in Eq. (7). The correction is negligible in this pT range because of the high signal yield fraction of V0

candi-dates. Forlow-multiplicityevents(Noffline

trk

<

35,

Fig. 2

(a)and (b)),

a sharp peak near

(#

η

,

#φ)

= (

0

,

0

)

due to jet fragmentation

Fig. 2. The2Dtwo-particlecorrelationfunctionsinpPb collisionsat√sN N=5.02 TeV forpairsofaK0_S(a), (c) orΛ/Λ(b), (d) triggerparticleandachargedassociated

particle(h±_),with1<ptrig

T <3 GeV and1<passocT <3 GeV,inthemultiplicityrangesNofflinetrk <35 (a), (b) and220≤Ntrkoffline<260 (c), (d).Thesharpnear-sidepeakfrom

Fig. 3. The1D#φcorrelationfunctionsfrompPb dataafterapplyingtheZYAMprocedure,inthemultiplicityrangeNoffline

trk <35 (open)and220≤Nofflinetrk <260 (filled),for

triggerparticlescomposedofinclusivechargedparticles(left),K0

Sparticles(middle),andΛ/Λparticles(right).SelectionofafixedptrigT andpassocT rangeofboth1–3 GeVis

shownforthelong-rangeregion(|#η_|>2)ontopandtheshort-range(|#η_|<1)minuslong-rangeregiononthebottom.Thecurvesonthetoppanelscorrespondtothe Fourierfitsincludingthefirstthreeterms.Statisticaluncertaintiesaresmallerthanthesizeofthemarkers.

(truncatedfor better illustrationof the full correlation structure) canbeclearlyobservedforbothK0

S–h±and

Λ/Λ

–h± correlations.

Movingtohigh-multiplicity events(220

≤

Noffline_trk

<

260,

Fig. 2

(c)

and (d)), in addition to the peak from jet fragmentation, a pro-nouncedlong-rangestructureisseenat

#φ

≈

0,extendingatleast

4.8unitsin

|#

η

|

.Thisstructurewaspreviously observedin high-multiplicity (Noffline

trk

∼

110) pp collisions at

√

s

=

7 TeV [13] and pPb collisionsat

√

sN N

=

5

.

02 TeV[14–16,47]forinclusivecharged

particles,andalsoforidentifiedchargedpions,kaons,andprotons in pPb collisions at

√

sN N

=

5

.

02 TeV [31]. A similar long-range

correlationstructure hasalsobeenextensivelystudied inAA col-lisions over a wide range of energies [1–9], where it is believed to arise primarily from collective flow of a strongly interacting medium[34].

Toinvestigatethecorrelation structure fordifferentspeciesof particles in detail, one-dimensional (1D) distributions in

#φ

are

found by averaging the signal andmixed-event 2D distributions over

|#

η

|

<

1 (defined asthe “short-rangeregion”)and

|#

η

|

>

2 (definedasthe“long-rangeregion”),asdoneinRefs.[6,7,13,14,47].

Fig. 3shows the1D

#φ

correlation functions frompPb data for

triggerparticlescomposedofinclusivechargedparticles(left)[47], K0_S particles(middle), and

Λ/Λ

particles(right), inthe multiplic-ityrange Noffline

trk

<

35 (open)and220

≤

Nofflinetrk

<

260 (filled).The

curves show the Fourier fits from Eq. (4) to the long-range re-gion,whichwillbediscussedindetaillater.Followingthestandard zero-yield-at-minimum(ZYAM)procedure[47],eachdistributionis shiftedtohavezeroassociatedyieldatitsminimumtorepresent the correlated portion of the associated yield. Selection of fixed

ptrig_T and passoc

T ranges of 1–3 GeV is shown for the long-range

region (top) and for the difference of the short- and long-range regions (bottom) in Fig. 3. As illustrated in Fig. 2, the near-side long-range signal remains nearly constant in

#

η

. Therefore, by takinga difference of1D

#φ

projectionsbetweenthe short- and

long-rangeregions,thenear-sidejetcorrelationscanbeextracted. As shown in the bottom panels of Fig. 3, dueto biasesin mul-tiplicity selection toward higher pT jets, a larger jet peak yield

isobservedforeventsselectedwithhighermultiplicities.Because chargedparticles are directly used in determining the

multiplic-ityin the event, thisselection bias ismuch stronger forcharged particlesthanK0

S and

Λ/Λ

hadrons.ForNofflinetrk

<

35,nonear-side

correlationsareobservedinthelong-rangeregionforanyparticle species.ThePbPb datashowqualitativelythesamebehaviorasthe pPb data,andthusarenotpresentedhere.

Recently,thev2anisotropyharmonicsforchargedpions,kaons,

andprotons have beenstudied usingtwo-particle correlations in pPb collisions [31], and are found to be qualitatively consistent withhydrodynamicmodels[32,33].Inthispaper,theelliptic(v2)

andtriangular (v3) flow harmonics ofK0S and

Λ/Λ

particles are

extracted from the Fourier decomposition of 1D

#φ

correlation functions for the long-range region (

|#

η

|

>

2) in a significantly

larger sample ofpPb collisions such that theparticle species de-pendence of vn can be investigated in detail. In Fig. 4, the vsig2

of K0

S and

Λ/Λ

particles are plotted asa function of pT for the

threelowestmultiplicity rangesinPbPb andpPb collisions.These data were recorded using a minimum bias trigger. The range of the fraction of the full multiplicity distribution that each multi-plicityselectioncorrespondsto,asdeterminedinRef.[47],isalso specifiedinthefigure.IncontrasttomostotherPbPb analyses,the presentworkusesmultiplicitytoclassifyevents,insteadofthe to-tal energy deposited in HF (the standard procedure of centrality determination inPbPb)[47,51]. Byexaminingthe HFenergy dis-tribution for PbPb events in each of the multiplicity ranges, the correspondingaverageHFfractionalcrosssection(anditsstandard deviation)can be determined,whichare presentedforPbPb data inthefigure.

Inthelowmultiplicity region(Fig. 4),the v2 valuesofK0S and

Λ/Λ

particles are compatible within statistical uncertainties. As

thereisnoevident long-rangenear-sidecorrelation seeninthese low-multiplicityevents,theextractedv2 mostlikelyreflects

back-to-back jet correlations on the away side. Away-side jet correla-tions typicallyappearasa peakstructure around

#φ

≈

π

,which contributestovariousordersofFourierterms.

The top row of Fig. 5 shows the measured v2 values for K0S

and

Λ/Λ

particles as a function of pT from the high

multiplic-ity pPb data, along withthe previously published results for in-clusive charged particles [47]. In the pT

!

2 GeV region for all

Fig. 4. Thev2resultsforK0S(filledsquares)andΛ/Λ(filledcircles)particlesasafunctionofpTforthreemultiplicityrangesobtainedfromminimumbiastriggeredPbPb

sampleat√sN N=2.76 TeV (toprow)andpPb sampleat√sN N=5.02 TeV (bottomrow).Theerrorbarscorrespondtostatisticaluncertainties,whiletheshadedareas

denotethesystematicuncertainties.ThevaluesinparenthesesgivethemeanandstandarddeviationoftheHFfractionalcrosssectionforPbPb andtherangeofthefraction ofthefullmultiplicitydistributionincludedforpPb.

Fig. 5. Toprow:thev2resultsforK0S(filledsquares),Λ/Λ(filledcircles),andinclusivechargedparticles(opencrosses)asafunctionofpT forfourmultiplicityranges

obtainedfromhigh-multiplicitytriggeredpPb sampleat√sN N=5.02 TeV.Middlerow:thev2/nqratiosforK0S(filledsquares)andΛ/Λ(filledcircles)particlesasafunction

ofKET/nq,alongwithafittotheK0Sresultsusingapolynomialfunction.Bottomrow:ratiosofv2/nqforK0SandΛ/Λparticlestothefittedpolynomialfunctionasafunction

ofKET/nq.Theerrorbarscorrespondtostatisticaluncertainties,whiletheshadedareasdenotethesystematicuncertainties.Thevaluesinparenthesesgivetherangeofthe

fractionofthefullmultiplicitydistributionincludedforpPb.

high-multiplicity ranges, the v2 values of K0_S particles are larger

than those for

Λ/Λ

particles at each pT value. Both of them

are consistently below the v2 values of inclusive charged

parti-cles. As most charged particles are pions, the data indicate that lighterparticlespeciesexhibitastrongerazimuthalanisotropy sig-nal. This mass ordering behavior is consistent withexpectations

in hydrodynamic models and the observation in 0–20% central-ity pPb collisions [31]. A similar trend was first observed in AA collisionsatRHIC[28,29].AthigherpT,thev2 valuesof

Λ/Λ

par-ticles are larger than those of K0

S. The inclusive charged particle

v2 values fall between the values of the two identified strange

parti-Fig. 6. Toprow:the v2resultsforK0S (filledsquares),Λ/Λ(filledcircles),andinclusivechargedparticles(opencrosses)asafunctionofpTforfourmultiplicityranges

obtainedfromminimumbiastriggeredPbPb sampleat√sN N=2.76 TeV.Middlerow:thev2/nqratiosforK0S(filledsquares)andΛ/Λ(filledcircles)particlesasafunction

ofKET/nq.Bottomrow:ratiosofv2/nqforK0SandΛ/Λparticlestoasmoothfitfunctionofv2/nqforK0SparticlesasafunctionofKET/nq.Theerrorbarscorrespondto

statisticaluncertainties,whiletheshadedareasdenotethesystematicuncertainties.ThevaluesinparenthesesgivethemeanandstandarddeviationoftheHFfractional crosssectionforPbPb.

cles.NotethattheratioofbaryontomesonyieldinpPb collisions isenhancedathigher pT,aneffectthatbecomes strongeras

mul-tiplicityincreases[54,55]. Thisshould alsobe takeninto account whencomparing vn valuesbetweeninclusiveandidentified

parti-cles.Comparingtheresultsin

Fig. 4

and

Fig. 5

,thedependenceof

v2 ontheparticlespeciesmayalreadybeemerginginthe

multi-plicityrangeof60

≤

Noffline trk

<

120.

The scaling behavior of v2 divided by the number of

con-stituent quarks as a function of transverse kinetic energy per quark, KET

/

nq,is investigated for high-multiplicity pPb eventsin

themiddle rowof

Fig. 5

.Afterscaling by the numberofquarks, the v2 distributions for K0_S and

Λ/Λ

particles are found to be inagreement. The middle rowof

Fig. 5

also showsthe resultof fittinga polynomial function to the K0

S data. The bottom rowof Fig. 5showsthenq-scaledv2 resultsforK0S and

Λ/Λ

particles

di-videdby thispolynomial function fit, indicating that the scaling isvalidto betterthan10% overmostoftheKET

/

nq range,except

forKET

/

nq

<

0

.

2 GeV wherethedeviationgrowstoabout20%.In

AA collisions,thisapproximatescaling behavior isconjectured to berelatedto quarkrecombination[39–41],whichpostulates that collectiveflowisdevelopedamongconstituentquarksbeforethey combineintofinal-statehadrons.Notethatthescalingofv2 with

the number of constituent quarks was originally observed as a functionof pT,insteadofKET,fortheintermediate pT rangeofa

fewGeV[38],andinterpretedinasimplepictureofquark coales-cence[39].However, itwaslaterdiscovered thatwhenplottedas afunctionofKETinordertoremovethemassdifferenceof

identi-fiedhadrons,thescalingappearstoholdovertheentirekinematic range[42,43].However,thisscalingbehaviorisnotexpectedtobe exactatlow pTinhydrodynamic modelsbecauseoftheimpactof

radialflow.Asthevn datatendtoapproachaconstantvalue asa

functionofpTorKETforpT

"

2 GeV,thescalingbehaviorinterms

of pT andKET cannot be differentiated inthat regime. Therefore,

thenq-scaled vn resultsinthispaperarepresentedasafunction

of KET

/

nq in order to explore the scaling behavior over a wider

kinematicrange.

The particle species dependence of v2 and its scaling

behav-iorisalsostudiedinPbPb data overthesamemultiplicityranges asfor the pPb data, as shownin Fig. 6. The mean andstandard deviation ofthe HF fractional cross section ofthe PbPb data are indicated ontheplots.Qualitatively, a similarparticle-species de-pendence of v2 is observed. However, the mass ordering effect

is found to be less evident in PbPb data than in pPb data for all multiplicity ranges. In hydrodynamic models, this may indi-cate a stronger radial flow is developed in the pPb system as its energy density is higher than that of a PbPb system due to having a smaller size systemat the same multiplicity.Moreover, the nq-scaled v2 data in PbPb at similar multiplicities suggest

a stronger violation of constituent quark number scaling, up to 25%, than is observed in pPb, especially for higher KET

/

nq

val-ues. This is also observed in peripheral AuAu collisions at RHIC, while the scaling applies more closely for central AuAu colli-sions[56].

The triangularflow harmonic, v3,of K0S and

Λ/Λ

particles is

alsoextractedinpPb and PbPb collisions,asshownin

Fig. 7

.Due to limited statisticalprecision, only the resultin the multiplicity range 185

≤

Noffline

trk

<

350 is presented.A similar species

depen-dence of v3 to that of v2 isobserved and, within the statistical

uncertainties,the v3 valuesscaledbytheconstituentquark

num-ber forK0

S and

Λ/Λ

particles matchatthelevelof20% overthe

full KET

/

nq range. To date, no calculations of the quark number

scalingof triangularflow, v3, havebeenperformedintheparton

Fig. 7. Top:the v3 resultsforK0S(filledsquares),Λ/Λ(filledcircles),andinclusivechargedparticles(opencrosses)asafunctionofpTforthemultiplicityrange185≤ Noffline

trk <350 inpPb collisionsat√sN N=5.02 TeV (left)andinPbPb collisionsat√sN N=2.76 TeV (right).Bottom:thenq-scaledv3valuesofK0S(filledsquares)andΛ/Λ

(filledcircles)particlesasafunctionofKET/nqforthesametwosystems.Ratiosofvn/nqtoasmoothfitfunctionofvn/nqforKS0 particlesasafunctionofKET/nqare alsoshown.Theerrorbarscorrespondtostatisticaluncertainties,whiletheshadedareasdenotethesystematicuncertainties.Thevaluesinparenthesesgivethemeanand standarddeviationoftheHFfractionalcrosssectionforPbPb andtherangeofthefractionofthefullmultiplicitydistributionincludedforpPb.

7. Summary

Measurementsoftwo-particlecorrelationswithanidentifiedK0 S

or

Λ/Λ

triggerparticle havebeenpresented overa broad

trans-verse momentum and pseudorapidity range in pPb collisions at

√

_s

N N

=

5

.

02 TeV and PbPb collisions at

√

sN N

=

2

.

76 TeV. With

theimplementation ofa high-multiplicity trigger during theLHC 2013pPb run,the identified particlecorrelation datain pPb col-lisionsareexplored overabroadparticlemultiplicityrange, com-parabletothatcoveredby50–100%centralityPbPb collisions.The long-range (

|#

η

|

>

2) correlations are quantified in terms of

az-imuthalanisotropyFourierharmonics(vn)motivatedby

hydrody-namicmodels.Inlow-multiplicitypPb andPbPb events,similarv2

valuesofK0

Sand

Λ/Λ

particlesareobserved,whichlikelyoriginate

fromback-to-backjetcorrelations. Forhighereventmultiplicities, a particle speciesdependence of v2

(

pT

)

and v3

(

pT

)

is observed.

For pT

!

2 GeV, thevaluesof vn forK0S particles arefound tobe

larger than those of

Λ/Λ

particles, while this order is reversed

athigher pT. This behavior is consistent with RHIC andLHC

re-sults in AA collisions and for identified charged hadrons in pPb and dAu collisions. For similar event multiplicities, the particle species dependence of v2 and v3 at low pT is observed to be

more pronounced in pPb than in PbPb collisions. In the context ofhydrodynamic models,thismayindicate that astronger radial flowboostisdevelopedinpPb collisions.Furthermore,constituent quark numberscaling of v2 and v3 between K0S and

Λ/Λ

parti-cles isfound to apply forPbPb and high-multiplicity pPb events. Theconstituentquarknumberscalingisfound toholdatthe10% (25%)levelinpPb (PbPb)collisions,forsimilareventmultiplicities. It will be interesting tosee if thisscaling lawcontinues to hold forotherparticles.Theresultspresentedinthispaperprovide im-portant input tothe furtherexploration ofthepossible collective floworiginoflong-rangecorrelations,andcanbeusedtoevaluate modelsofquarkrecombinationinadeconfinedmediumofquarks andgluons.

Acknowledgements

WecongratulateourcolleaguesintheCERNaccelerator depart-ments for the excellent performance of the LHC and thank the technical andadministrativestaffs atCERNand atother CMS in-stitutes for their contributions to the success of the CMS effort. Inaddition,wegratefullyacknowledgethecomputingcentresand personneloftheWorldwideLHCComputingGridfordeliveringso effectively thecomputinginfrastructure essentialto our analyses. Finally, we acknowledge the enduring support for the construc-tion andoperationofthe LHCandtheCMSdetectorprovided by thefollowingfundingagencies:BMWFWandFWF(Austria);FNRS and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES(Bulgaria);CERN;CAS,MOST,andNSFC(China);COLCIENCIAS (Colombia);MSESandCSF(Croatia);RPF(Cyprus);MoER,ERCIUT andERDF(Estonia); AcademyofFinland,MEC,andHIP (Finland);

CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH (Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); NRF and WCU (Republic ofKorea); LAS (Lithuania);MOE andUM (Malaysia); CINVESTAV, CONACYT,SEP,andUASLP-FAI(Mexico);MBIE(NewZealand);PAEC (Pakistan);MSHEandNSC(Poland);FCT(Portugal);JINR(Dubna); MON,RosAtom,RASandRFBR(Russia);MESTD(Serbia);SEIDIand CPAN(Spain);SwissFundingAgencies(Switzerland);MST(Taipei); ThEPCenter,IPST,STARandNSTDA(Thailand);TÜBITAK and TAEK (Turkey);NASUandSFFR(Ukraine);STFC (UnitedKingdom);DOE andNSF(USA).

Individuals have received support from the Marie-Curie pro-gramme and the European Research Council and EPLANET (Eu-ropean Union); the Leventis Foundation; the A.P. Sloan Founda-tion; the Alexander von Humboldt Foundation; the Belgian Fed-eral Science Policy Office; the Fonds pour la Formation à la Recherchedansl’Industrieetdansl’Agriculture(FRIA-Belgium);the AgentschapvoorInnovatiedoorWetenschapenTechnologie (IWT-Belgium);the Ministryof Education,Youth andSports(MEYS) of theCzechRepublic;theCouncilofScienceandIndustrialResearch, India; the HOMING PLUS programme of Foundation For Polish Science, cofinanced fromEuropean Union, Regional Development Fund;theCompagnia diSanPaolo (Torino); theConsorzioper la Fisica(Trieste); MIURproject 20108T4XTM(Italy); theThalisand Aristeia programmes cofinancedby EU-ESF and the Greek NSRF; andtheNationalPrioritiesResearchProgrambyQatarNational Re-searchFund.

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CMSCollaboration

V. Khachatryan,

A.M. Sirunyan,

A. Tumasyan

YerevanPhysicsInstitute,Yerevan,Armenia

W. Adam,

T. Bergauer,

M. Dragicevic,

J. Erö,

C. Fabjan

1

,

M. Friedl,

R. Frühwirth

1

,

V.M. Ghete,

C. Hartl,

N. Hörmann,

J. Hrubec,

M. Jeitler

1

_,

_{W. Kiesenhofer,}

_{V. Knünz,}

_{M. Krammer}

1

_,

_{I. Krätschmer,}

_{D. Liko,}

I. Mikulec,

D. Rabady

2

,

B. Rahbaran,

H. Rohringer,

R. Schöfbeck,

J. Strauss,

A. Taurok,

W. Treberer-Treberspurg,

W. Waltenberger,

C.-E. Wulz

1

InstitutfürHochenergiephysikderOeAW,Wien,Austria

V. Mossolov,

N. Shumeiko,

J. Suarez Gonzalez

NationalCentreforParticleandHighEnergyPhysics,Minsk,Belarus

S. Alderweireldt,

M. Bansal,

S. Bansal,

T. Cornelis,

E.A. De Wolf,

X. Janssen,

A. Knutsson,

S. Luyckx,

S. Ochesanu,

R. Rougny,

M. Van De Klundert,

H. Van Haevermaet,

P. Van Mechelen,

N. Van Remortel,

A. Van Spilbeeck

UniversiteitAntwerpen,Antwerpen,Belgium

F. Blekman,

S. Blyweert,

J. D’Hondt,

N. Daci,

N. Heracleous,

J. Keaveney,

S. Lowette,

M. Maes,

A. Olbrechts,

Q. Python,

D. Strom,

S. Tavernier,

W. Van Doninck,

P. Van Mulders,

G.P. Van Onsem,

I. Villella

VrijeUniversiteitBrussel,Brussel,Belgium

C. Caillol,

B. Clerbaux,

G. De Lentdecker,

D. Dobur,

L. Favart,

A.P.R. Gay,

A. Grebenyuk,

A. Léonard,

A. Mohammadi,

L. Perniè

2

,

T. Reis,

T. Seva,

L. Thomas,

C. Vander Velde,

P. Vanlaer,

J. Wang,

F. Zenoni

J. Mccartin,

A.A. Ocampo Rios,

D. Ryckbosch,

S. Salva Diblen,

M. Sigamani,

N. Strobbe,

F. Thyssen,

M. Tytgat,

E. Yazgan,

N. Zaganidis

GhentUniversity,Ghent,Belgium

S. Basegmez,

C. Beluffi

3

,

G. Bruno,

R. Castello,

A. Caudron,

L. Ceard,

G.G. Da Silveira,

C. Delaere,

T. du Pree,

D. Favart,

L. Forthomme,

A. Giammanco

4

_,

_{J. Hollar,}

_{A. Jafari,}

_{P. Jez,}

_{M. Komm,}

_{V. Lemaitre,}

C. Nuttens,

D. Pagano,

L. Perrini,

A. Pin,

K. Piotrzkowski,

A. Popov

5

,

L. Quertenmont,

M. Selvaggi,

M. Vidal Marono,

J.M. Vizan Garcia

UniversitéCatholiquedeLouvain,Louvain-la-Neuve,Belgium

N. Beliy,

T. Caebergs,

E. Daubie,

G.H. Hammad

UniversitédeMons,Mons,Belgium

W.L. Aldá Júnior,

G.A. Alves,

L. Brito,

M. Correa Martins Junior,

T. Dos Reis Martins,

C. Mora Herrera,

M.E. Pol

CentroBrasileirodePesquisasFisicas,RiodeJaneiro,Brazil

W. Carvalho,

J. Chinellato

6

_,

_{A. Custódio,}

_{E.M. Da Costa,}

_{D. De Jesus Damiao,}

_{C. De Oliveira Martins,}

S. Fonseca De Souza,

H. Malbouisson,

D. Matos Figueiredo,

L. Mundim,

H. Nogima,

W.L. Prado Da Silva,

J. Santaolalla,

A. Santoro,

A. Sznajder,

E.J. Tonelli Manganote

6

,

A. Vilela Pereira

UniversidadedoEstadodoRiodeJaneiro,RiodeJaneiro,Brazil

C.A. Bernardes

b

,

S. Dogra

a

,

T.R. Fernandez Perez Tomei

a

,

E.M. Gregores

b

,

P.G. Mercadante

b

,

S.F. Novaes

a

_,

_{Sandra S. Padula}

a

a_{UniversidadeEstadualPaulista,SãoPaulo,Brazil} b_{UniversidadeFederaldoABC,SãoPaulo,Brazil}

A. Aleksandrov,

V. Genchev

2

_,

_{P. Iaydjiev,}

_{A. Marinov,}

_{S. Piperov,}

_{M. Rodozov,}

_{S. Stoykova,}

_{G. Sultanov,}

V. Tcholakov,

M. Vutova

InstituteforNuclearResearchandNuclearEnergy,Sofia,Bulgaria

A. Dimitrov,

I. Glushkov,

R. Hadjiiska,

V. Kozhuharov,

L. Litov,

B. Pavlov,

P. Petkov

UniversityofSofia,Sofia,Bulgaria

J.G. Bian,

G.M. Chen,

H.S. Chen,

M. Chen,

R. Du,

C.H. Jiang,

R. Plestina

7

_,

_{J. Tao,}

_{Z. Wang}

InstituteofHighEnergyPhysics,Beijing,China

C. Asawatangtrakuldee,

Y. Ban,

S. Liu,

Y. Mao,

S.J. Qian,

D. Wang,

L. Zhang,

W. Zou

StateKeyLaboratoryofNuclearPhysicsandTechnology,PekingUniversity,Beijing,China

C. Avila,

L.F. Chaparro Sierra,

C. Florez,

J.P. Gomez,

B. Gomez Moreno,

J.C. Sanabria

UniversidaddeLosAndes,Bogota,Colombia

N. Godinovic,

D. Lelas,

D. Polic,

I. Puljak

UniversityofSplit,FacultyofElectricalEngineering,MechanicalEngineeringandNavalArchitecture,Split,Croatia

Z. Antunovic,

M. Kovac

UniversityofSplit,FacultyofScience,Split,Croatia

V. Brigljevic,

K. Kadija,

J. Luetic,

D. Mekterovic,

L. Sudic