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

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DOI: 10.1016/j.physletb.2015.01.034

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by Elsevier. All rights reserved.

DIRETORIA DE TRATAMENTO DA INFORMAÇÃO

Cidade Universitária Zeferino Vaz Barão Geraldo

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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)anisotropyharmonicsofK0_S

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 v3forK0_S 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_.

S

thatproducesimilarfinal-stateparticlemultiplicity.

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

∝

1

+



n2Vncos

(

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 K0_S 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

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 aretakenwiththresholdsofN_trkonline

>

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 V0s) atCMSwas first describedin Ref.[52]. Toincreasetheefficiencyfortrackswithlowmomentumandlarge impact parameters, both characteristic of the K0_S and

Λ/Λ

de-cayproducts, thestandard loose selectionoftracks(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

−

(

pπ−

)

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

≤

N_trkoffline

<

260.Sincetheresultsfor

Λ

and

Λ

arefoundtobe consistent,theyhavebeencombinedinthisanalysis.TheV0peaks can be clearly identified withlittlebackground.The true V0 sig-nal peakiswell described bya doubleGaussian function(witha commonmean),whilethebackgroundismodeled bya4th-order polynomialfunctionfitovertheentiremassrangeshowninFig. 1. The mass window of

±

2σ 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}

_±

_3σ massrangearoundthe V0 _{masstothelimitofthemass} distribu-tionsshowninFig. 1.

5. Analysisoftwo-particlecorrelations

The construction of the two-particle correlation function fol-lowsthesameprocedureestablishedinRefs. [6,7,14,47].However, inthispaper,reconstructedV0candidatesfromeitherthepeakor 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 Ntrig d2Npair 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

d2Nmix

d



η

d

φ

,

(3)

isconstructedby pairingthetrigger V0 candidatesineach event withtheassociatedchargedprimary tracksfrom20different ran-domlyselectedeventsinthesame2 cmwiderangeofvertex po-sitioninthez directionandfromthesametrackmultiplicityclass. 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 <

3π

/

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 2Vncos

(

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

,

passoc_T

)

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



passoc_T



.

(5)

Following this assumption, the elliptic

(

v2

)

and triangular

(

v3

)

anisotropy harmonicsof V0 _{particlescan be extractedasa} func-tionofpT fromthefittedFouriercoefficients,

vn



pV_T0



=

Vn

(

p V0 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

V0s)andsidebandregion,denotedas vobs_n andvbkgn ,respectively.

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

ob-tainedby vsign

=

vobs_n

− (

1

−

fsig

)

×

vbkgn

fsig

,

n

=

2

,

3

,

(7)

assumingvsign andv

bkg

n areindependentfromeachother.Here, fsig

representsthesignalyieldfractioninthepeakregiondetermined by thefits tothemass distributionshowninFig. 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 thesameasthosein vsig₂ ,aswasdoneinRef.[47].

Table 1

Summaryofsystematicuncertaintiesinvsign 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

fsigandvobs₂ .Foravariationfrom

±

1σto

±

3σ,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 areshownasshadedboxesinFigs. 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<passoc

T <3 GeV,inthemultiplicityrangesN offline

trk <35 (a), (b) and220≤N offline

trk <260 (c), (d).Thesharpnear-sidepeakfrom jetcorrelationsistruncatedtoemphasizethestructureoutsidethatregion.

Fig. 3. The1DφcorrelationfunctionsfrompPb dataafterapplyingtheZYAMprocedure,inthemultiplicityrangeNoffline

trk <35 (open)and220≤N offline

trk <260 (filled),for triggerparticlescomposedofinclusivechargedparticles(left),K0

Sparticles(middle),andΛ/Λparticles(right).Selectionofafixedp trig

T 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 (N_trkoffline

∼

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

≤

Noffline_trk

<

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.ForNoffline_trk

<

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 ofK0_S 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 valuesofK0_S 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 K0_S 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 hadron speciesbutaremuchclosertothe v2 valuesforK0S

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.ComparingtheresultsinFig. 4andFig. 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 rowofFig. 5.Afterscaling by the numberofquarks, the v2 distributions for K0_S and

Λ/Λ

particles are found to be inagreement. The middle rowofFig. 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 K0_S and

Λ/Λ

particles is alsoextractedinpPb and PbPb collisions,asshowninFig. 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 recombinationmodel.

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

√

sN 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 andhigh-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);

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