Measurement of prompt D-0 and D-0 meson azimuthal anisotropy and search for strong electric fields in PbPb collisions at root S-NN=5.02 TeV

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Measurement

of

prompt

D

and

D

meson

azimuthal

anisotropy

and

search

for

strong

electric

fields

in

PbPb

collisions

at

√

s

=

5

.

02 TeV

.The CMS Collaboration CERN,Switzerland

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

Articlehistory:

Received29September2020

Receivedinrevisedform21February2021 Accepted25March2021

Availableonline29March2021 Editor:M.Doser Keywords: CMS Heavy-flavor Charm Electromagneticfields

ThestrongCoulombfieldcreatedinultrarelativisticheavyioncollisionsisexpectedtoproducea

rapidity-dependentdifference(v2)inthesecondFouriercoefficientoftheazimuthaldistribution(ellipticflow,

v2)betweenD0_(uc)andD0 _{(uc)mesons. Motivatedbythesearchforevidenceofthisfield,theCMS}

detectoratthe LHCisused toperformthe firstmeasurementof v2.Therapidity-averagedvalue is

foundtobe_v2=0.001±0.001 (stat)±0.003 (syst) inPbPbcollisionsat√sNN=5.02 TeV.Inaddition,

theinfluenceofthecollisiongeometryisexploredbymeasuringtheD0andD0mesonsv2andtriangular

flowcoefficient(v3)asfunctionsofrapidity,transversemomentum(pT),andeventcentrality(ameasure

oftheoverlapofthetwo Pbnuclei).Aclear centralitydependenceofprompt D0 _{meson v}

2 valuesis

observed,whilethev3islargelyindependentofcentrality.Thesetrendsareconsistentwithexpectations

offlowdrivenbytheinitial-stategeometry.

©2021TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense

(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

The observation of a strongly-coupled quark-gluon plasma (QGP), astate ofmattercomposed ofdeconfinedquarks and glu-ons, was established by experiments investigatingultrarelativistic heavy ioncollisions at the BNL RHIC [1–4] and CERN LHC [5,6]. The azimuthal particle correlationsconstitute an effectivetool to probethe propertiesoftheQGP [1–9].Thesecorrelationsare pa-rameterized by aFourier expansion [10–12],with themagnitude of the Fourier coefficients, vn, providing information about the initial collision geometry and its fluctuations [12]. The second-(v2) and third- (v3) order Fourier coefficients are referred to as “elliptic”and“triangular”flow harmonics,respectively.Measuring thesecoefficientsforparticlespecieswithdifferentquark compo-sition provides additional information about this hot and dense medium [13]. Because of their large mass, charm and bottom quarksareproducedearlierinthecollisions thanthelightquarks (upanddown) [14,15].Inaddition,thecharmandbottomquarks have massesmany timeslarger than the typical temperatures in theQGP [16].Theseheavyquarksexperiencethefullevolutionof themedium untilthehadronizationphase.Asaconsequence,the

vn of charmedD0 (uc)and D 0

(uc) mesons (henceforthreferred toasD0_{mesons,exceptwhereexplicitlystatedotherwise)are}

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

pected to receive important contributions from medium energy lossandcoalescenceeffects [17,18].

In ultrarelativistic heavy ion collisions, very strong and tran-sient (∼10−1 _{fm/c) magnetic andelectric fields are expected to} beinduced bythecollision spectatorsandparticipants [19].Such electromagnetic(EM)fieldsare predictedto producea difference in the vn harmonics for positively and negatively charged parti-cles [19].Insuchapicture,themagneticfieldismainlyresponsible for splitting the rapidity ( y)-odd directed flow (v1) [19,20]. The electricfieldispredictedtoinduceacharge-dependentsplittingin the v2 coefficientandintheaveragetransversemomentum(pT) valuesoftheemittedparticles [19].Ascharmquarksareexpected tobecreatedveryearlyinthecollision,theyhaveahigher proba-bilityofinteractingwiththisstrongEMfield thanthelightflavor quarks [20,21].

In this letter, measurements of the v2 and v3 coefficients as functionsofD0 mesonrapidity, pT,andlead-lead (PbPb)collision centralityare presented.Thecollision centralitybins aregivenin percentagerangesofthetotalinelastichadroniccrosssection,with the 0–10% centrality bin corresponding to the 10% of collisions havingthe largestoverlapof thetwo nuclei. Theflow harmonics aremeasuredusingthescalarproductmethod [22,23].Inthis anal-ysis, the selection of D0 _{mesons uses multivariate methods [24]} forselectingD0 _{candidatesandtheirantiparticles.The} contamina-tionfromnonpromptD0candidates,arisingfromBmesondecay,is consideredasasystematicuncertainty.Usingthedatarecordedin PbPbcollisions duringthe2018LHCrunperiod,correspondingto

https://doi.org/10.1016/j.physletb.2021.136253

0370-2693/©2021TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

0.58 nb−1 ofintegratedluminosity,theflowcoefficientsare mea-suredwithintherapidity range|y| <2,whichistwiceaslargeas achievedinpreviousCMSmeasurements [25].Theextensionofthe measurements tothislargerrapidity range,together withsmaller statisticaluncertainties provided bya larger dataset,furnish im-portantinputsforabetterunderstandingofthethree-dimensional evolutionoftheQGPformedinheavyioncollisions.Measurements ofthe v2 difference betweenD0 andD

0

mesons,v2,asa func-tionofrapidityarepresentedasamethodtoprobepossibleeffects originatingfromtheCoulombfields.

2. Experimentalapparatusanddatasample

The central feature of the CMS apparatus is a superconduct-ing solenoidof6 m internal diameter,providinga magneticfield of 3.8 T. Within the solenoid volume, there are four primary subdetectors including a silicon pixel and strip tracker detector, a lead tungstate crystalelectromagnetic calorimeter, and a brass and scintillator hadron calorimeter, each composed of a barrel andtwoendcapsections.Ironandquartz-fiberCherenkovhadron forward (HF) calorimeters cover the pseudorapidity range 2.9<

|η| <5.2.TheHFcalorimetersaresegmentedtoform0.175×0.175 (η×φ) towers. Muons are measured in gas-ionization detec-tors embedded inthesteelflux-returnyokeoutsidethesolenoid. The silicon tracker measures charged particles within the range

|η| <2.5.AdetaileddescriptionoftheCMSdetector,togetherwith a definitionofthecoordinatesystemusedandtherelevant kine-maticvariables,canbefoundinRef. [26].

Theanalysispresentedinthisletterusesapproximately4.27× 109minimumbias(MB)PbPbcollisioneventscollectedbytheCMS experimentduringthe2018LHCrun.TheMBeventsaretriggered byrequiringsignalsinbothforwardandbackwardsidesoftheHF calorimeters [27]. Further selections are applied offline to reject eventsfrombackgroundprocesses(beam-gasinteractionsand non-hadroniccollisions),seeRef. [28] fordetails.Eventsarerequiredto have atleast one interaction vertex, reconstructed basedon two tracks ormore,andwithadistance oflessthan15 cm fromthe centerof thenominalinteraction pointalong thebeamaxis.The primary interaction vertex is defined as the one with the high-est track multiplicity in the event. The shapesof the clusters in thepixeldetectorhavetobecompatiblewiththoseexpectedfrom particles producedattheprimary vertexlocation. ThePbPb colli-sioneventsarealsorequiredtohaveatleasttwocalorimeter tow-ersineachHFdetectorwithenergydepositsofmorethan4 GeV pertower.Thesecriteriaselect(99±2)%ofinelastichadronicPbPb collisions.The possibilitytohavevalueshigherthan100% reflects the possible presence of ultra-peripheral (nonhadronic)collisions intheselectedeventsample.

Events fromMonte Carlo (MC) simulations are used to study both promptandnonpromptD0 _{mesonprocesses.The eventsare} generated using an embedding procedure, in which D0 mesons generatedby pythia 8.212 [29] (tuneCP5 [30])areembeddedinto MB eventsfrom hydjet 1.9 [31]. Afull simulationoftheCMS de-tectorisperformedusing Geant4 [32].ThepromptD0 _mesonMC simulationisemployedtodefinesignalselectionsandmeasure ef-ficiencycorrections,whilethenonpromptD0 mesonMCsampleis usedtoestimatesystematicuncertaintiescomingfromnonprompt D0_{contamination.}

3. ReconstructionandselectionofD0mesons

Prompt D0 mesons are reconstructed from the decay D0→

π++K− andD0→π−+K+withabranchingfractionof(3.94± 0.04)%,usingselected trackswith pT>1.0 GeV/c andwithin the acceptanceof|η| <2.4.Candidatesareformedbycombiningpairs

of tracks from oppositely charged particles andrequiring an in-variantmass(minv) withina ±200 MeV/c2 windowofthe world-averageD0_{mesonmassof(1864.83±0.05)MeV/c}2_[33].Foreach pair of selected tracks, two possible candidates for D0 _{and D}0 mesonsareconsideredbyassumingoneofthetrackshasthepion mass, while the other track has the kaon mass, and vice versa. Kinematicvertexfits are performedto reconstructthe secondary verticesofD0 candidatedecays.

After the D0 candidate reconstruction, a selection using a boosted decision tree (BDT) algorithm from the tmva pack-age [24] isemployed.FortheBDTtraining,misidentifiedD0 candi-datesindataevents,wherepionandkaonhavethesamecharge, areusedtomimicthecombinatorialbackground.Thesignal candi-datesaretakenfromMCsimulationsofpromptD0 mesonsandare requiredtomatchD0 particlesatthegeneratorlevel.Thevariables related to D0 _{mesons used to discriminate the signal from the} backgroundare: χ2 _{probability fortheD}0 _{vertexfit, 3D distance} betweenthe secondaryand primary verticesand its significance, the decay length significance projected in the xy-plane, and the angleintwoandthreedimensionsbetweenthemomentumofthe D0 mesoncandidateandthelineconnectingtheprimary andthe secondaryvertices(pointingangle).Relatedtothedecayproducts oftheD0mesoncandidate,thevariablesusedare:theuncertainty in pT returned by thetrack fittingprocedure, the significanceof the z and the xy distances ofclosestapproachtotheprimary ver-tex,andthenumberofhitsinthetrackerdetector.Thesevariables are chosen by analyzing their BDT ranking (variables more fre-quentlyused in the decisiontree) and correlation matrixamong all variables. Different BDT boost algorithms are tested, choosing theadaptive boost algorithm [24] asdefault. Overtrainingchecks aredone forall analysisbins bycomparing theBDT distributions fromtrainingandtestingD0mesonsamples.Inaddition,aBDTcut optimization is performedin bins of centrality, pT, and rapidity, doingascanindifferentBDTscoresandfindingtheoneresulting in maximal D0 mesons signal significance for each analysis bin. Comparedto acutoff-based procedure, thisBDT selection almost doublesthe signalsignificancefor D0 _{mesonsin1<|y| <2,and} increasesthesignal significanceby30%forD0 mesonsin|y| <1, for events with collision centrality in the range 0–30%. For the remaininganalysisbins asimilar performance ofBDTand cutoff-basedmethodsisobserved.

4. Analysistechnique

The elliptic andtriangular flow coefficientsof D0 _{mesons are} extractedusingthescalarproduct(SP) method,similarly towhat wasdone inapreviousCMSpublication [25].Inthismethod,the

vn coefficientsofD0 candidates(includingbackgrounds)are mea-suredusing vn{SP} ≡  Q_nD0Q_nA∗   QnAQnB∗QnAQ∗nC QnBQnC∗ , (1)

with the Q -vectors expressed as Qn≡Mj=1wjeinφj, where the sumis over the total number(M) of HF towers above a certain energy threshold (with the weights wj taken as the energy de-positedintheHFtoweratazimuthalangle φj),oftrackswith pT aboveacertainthreshold(with wj takenastrackpT inφjangle), orofselectedD0 _{mesoncandidates(with w}

jtakenequalto1). The Q -vectors relatedtoHFandthetrackeraremeasuredand correctedfordetectorirregularitiesbyapplyingaflattening anda recenteringprocedure [12,34].The QnAandQnBaredefinedusing the event-planemeasurements fromthe negative (−5<η<−3, HF−) and thepositive (3<η<5, HF+) sidesofHF, and QnC is measuredusingthetrackerinformationintheregionof|η| <0.75,

allowing to minimize the correlations among the three regions, with a gap of more than two units of rapidity. The Q_nD0 vector is definedforeach D0 mesoncandidate.The averages QnAQnB∗ ,

QnAQnC∗ ,andQnBQnC∗ aremadeconsideringallselectedevents, while the average QD0

n QnA∗  is made considering all D0 meson candidates in all selected events. To avoid autocorrelations, the termsQ_nD0Q_nA∗ andQnAQnB∗ useA=HF−(HF+)whentheD0 mesoncandidateisatpositive(negative)pseudorapidity.

One goal of this analysis is to measure the difference (vn) between D0 and D0meson flow coefficients, vn, as a function of rapidity, to probe effects from EM fields. The difference vn is measuredas: vn{SP} ≡ Q_nD0Q_nA∗ − Q_nD0Q_nA∗  QnAQnB∗QnAQnC∗ QnBQnC∗ . (2)

The vn and vn of D0 meson candidates are first measured as a function oftheir minv. The extraction of the D0 mesons signal

vn (vn), vsign (vsign ), is performed via a simultaneous binned

χ2 _{fit of the m}

inv distribution and of vn (vn). The minv distri-bution is fitwith threecomponents: a third-order polynomial to modelthecombinatorialbackground, B(minv);twoGaussianswith the same meanbut differentwidths to describe the minv in dif-ferent kinematic regions for the D0 mesons signal, S(minv); and oneadditionalGaussiandistributionfortheswapcomponent cor-respondingtotheincorrectmassassignmentfortheassumedpion andkaonparticles, S W(minv).Thewidthof S W(minv)andthe ra-tiobetweenthe yieldsof S W(minv)and S(minv)are fixedby the values extractedfromMC simulations. Inthiscase, the following expressioncanbeusedforextractingvsign :

vsign +bkg(minv)=α(minv)vnsig+ [1−α(minv)]vbkgn (minv). (3) The α(minv)parameter, whichcharacterizes thesignal fractionas afunctionofmass,isdefinedasfollows:

α(minv)

= [S(minv)+S W(minv)]/[S(minv)+S W(minv)+B(minv)]

=αsignal(minv)+αswap(minv). (4)

Forextractingthedifferencevsign ,thefollowingexpressionis em-ployed:

vsign +bkg(minv)= vsign (αsignal(minv)−αswap(minv))+const.

(5) Theterm vbkgn (minv)fromEq. (3) ismodeledwithalinearfunction, whiletheconstantparameter const in Eq. (5) isaddedtoaccount forpossiblefluctuationsinthebackground vncomponent.The rel-evance ofthis const parameter was investigated by redoing vn measurements in MC simulation (without azimuthal correlations oreffectsfromEMfields),indicatingthatthisparameterimproves the fit quality anddoes not introduce artificial signals. A cross-check is performedby redoing the measurements using a linear function insteadofa constant.No significantchanges inthe cen-tralvaluesof v2 andon theiruncertainties are observed.Fig.1 showsanexampleofasimultaneousfitfor v2 andv2.

After performing the fits forextractingthe signal vn,there is stillasizablefractionofnonpromptD0 mesonsembeddedin vsign . Theextractedvncanbewrittenas

vsig_n =fpromptvpromptn + (1−fprompt)vnonpromptn . (6)

ThenonpromptD0_{mesoncontaminationistakenintoaccountasa} systematicuncertainty,bycheckingthatthenonpromptD0 meson fraction is always smaller than 12% (i.e., comparable to the un-certaintiesinthereconstructedD0 _{mesonyield).Thisimpliesthat} thecentral valuesof vn willnot be considerablyaffected bythis component,beingcompatiblewithin statisticaluncertainties.Such alowfractionarisesfromtheuseofpromptD0 _{mesonsignalsin} theBDTtraining,togetherwithvariablesthatarehighlycorrelated withthe distance ofclosest approach (DCA) to theprimary ver-tex,whichisdefinedastheflightdistanceoftheD0_{particletimes} thesineofthepointingangleinthreedimensions.AdditionalDCA selectionanddedicatedtraining,involvingpromptandnonprompt D0 _{mesonsignals,donotbringconsiderableimprovementsin} per-formance.ThepromptandnonpromptD0 _{mesonfractionsare} ob-tainedusingtheDCA variable.ForpromptD0mesons,thenonzero DCA correspondstothedetectorresolution,andisexpectedtobe concentrated around zero.Fornonprompt D0 _{mesons, larger} val-uesofDCA resultfromtheBmesondecay.Toextracttheprompt andnonpromptD0 mesonfractions,afittotheDCA distributions isperformedindataconsideringDCA shapesfromMCsimulations forpromptandnonpromptD0mesoncomponents.Thenonprompt D0 meson vnisestimatedbyconsideringtworegionsintheDCA: one withvery low fraction (2.7–8.0%) ofnonprompt D0 _particles (DCA<0.012 cm), and one witha highfraction (62.0–88.0%) of nonpromptD0 particles(DCA>0.012 cm).Usingthisinformation togetherwithEq. (6),itispossibletoestimate vnonpromptn by solv-ing asystemof two equationsfromthe two DCA regions.Inthe currentanalysisthisprocedurecanonlybe doneinwide pT, cen-trality, andrapidity bins, because ofthe limitedamount of data availableintheregionwithDCA>0.012 cm.

5. Systematicuncertainties

Thesources of systematicuncertainties includethe D0 identi-fication requirements(BDTselection); theprobability distribution function(PDF)formodelingthebackgroundintheinvariantmass fit;theimpactofacceptanceandefficiencyoftheD0 _mesonyield; thevariationofthePDFformodelingthebackground vn;andthe remainingnonpromptD0_{contamination.Withtheexceptionofthe} last component, the uncertainties are quoted as absolute values ofvnandvnaftercomparing thedefaultanalysisconfiguration withthevariations.Todiminishtheinfluenceofstatistical fluctua-tions,afterobservingnospecialtrendsinthedeviationsfromthe defaultmeasurements, the systematicuncertainties are evaluated byaveragingthedeviationswithaconstantfitasafunctionofthe analysisbins.

Inordertotakeintoaccountthesystematicuncertainty associ-atedwiththeBDTselection,theBDTcutisvariedupanddownby themaximaldeviationbetweentheBDToptimizedselectionbased onMCsimulationsanddata.TheBDTcuts(andvariationsfor sys-tematicuncertainties)aredefinedinbinsofcollisioncentrality, pT, and rapidity, ranging from 0.28 to 0.47 (±0.02–0.03). Regarding theeffectofthebackgroundmassmodeling,eitheranexponential functiontogetherwithasecondorderpolynomial,orjustasecond orderpolynomial,areconsideredinsteadofthedefaultfitfunction usingathird-orderpolynomial.Tofit vnasafunctionofmass,the defaultconfigurationusingalinearfunction isreplaced byeither aconstantora secondorderpolynomial.Althoughthe D0 meson selectionefficiencyessentiallycancelsin vnmeasurements,a sys-tematicuncertaintyisassignedbycomparingtheresultswithand withoutapplyingcorrections basedon MC simulationsin binsof

pT and rapidity.The D0 meson selection efficiency times accep-tancevaries from0.5 to 12.5% inthe pT rangeof 1.0–8.0 GeV/c, reachingaplateauofapproximately17.0%for pT>15.0 GeV/c.

The systematic uncertainties regarding contamination from nonpromptD0 _{mesonsareestimatedbymeasuringnonpromptD}0

Fig. 1. Simultaneous fit of theπK invariant mass (left) and v2(v2) as function of invariant mass (right) for 3.0<pT<3.5 GeV/c, centrality 20–70%, and−0.6<y<0.0.

Table 1

Summaryofsystematicuncertaintiesinabsolutevaluesforv2,v3,andv2.Rangesofthevariationofuncertaintiesforallthebinsarepresented.Thecellsfilledwith“—”

refertothecaseswheretheuncertaintycancelsout.

Systematic sources pTbins y bins Centrality bins

v2

BDT selection 0.002–0.014 0.0065 0.005

Bkg. mass PDF 0.0002–0.0017 0.0007–0.0015 0.0007–0.0011

Bkg. vnPDF 0.01–0.05 0.004–0.007 0.003–0.005

D0_{efficiency correction — 0.004–0.007 0.0040–0.0045}

Nonprompt D0_{meson contamination 0.0002–0.0077 0.004 0.002–0.005}

v3

BDT selection 0.002–0.023 0.001–0.009 0.002–0.006

Bkg. mass PDF 0.0001–0.0040 0.0005–0.0008 0.0012–0.0040

Bkg. vnPDF 0.01–0.05 0.003–0.004 0.0011

D0_{efficiency correction — 0.002–0.004 0.003–0.005}

Nonprompt D0_{meson contamination 0.0001–0.0090 0.0010–0.0015 0.0001–0.0008}

v2

BDT selection 0.001–0.009

Bkg. mass PDF 0.00015–0.00030

D0_{efficiency correction 0.001–0.004}

Nonprompt D0_{meson contamination 0.00002–0.00010}

meson vn in wide bins of pT,rapidity, and centrality. A relative systematic uncertainty is obtainedby comparing vn frommixed prompt and nonprompt D0 mesons to the vn derived from non-promptD0mesons.

Table1summarizestheestimatesofsystematicuncertaintiesin absolutevaluesfor v2, v3,andv2.Therangesofvariationofthe uncertaintiesarepresentedforeachbinning.

6. Results

Results for prompt D0 _{meson v}

2 and v3 anisotropic flow co-efficients, obtained with2018 PbPb data, asfunctionsof pT and for|y| <1,areshowninFig.2forthreecentralityranges:0–10%, 10–30%,and30–50%.Theresultsextendpreviouslypublisheddata fromCMS [25],byextendingthehigh-pT coverageto∼60.0 GeV/c andbyprovidingfiner pTbins.Thesehigh-precisiondataare com-patible with previous measurements from Ref. [25], and a clear trend of rise andfall from low to high pT is observed for both

v2 and v3 across thefull centrality range.This behavior is simi-larto that observed forinclusivechargedparticles [35] for |η| <

1.0, also shown in Fig. 2. For noncentral collisions (i.e., central-ity 10–50%), values of prompt D0 _{meson v}

2 are positive up to

pT∼30.0–40.0 GeV/c, whereas the v3 values become consistent withzeroatpT∼10.0 GeV/c.

Calculations from theoretical models at midrapidity (|y| <1) arealsopresented.Thesemodelsusedifferentassumptionsofthe QGPproperties,forexampleinthethermalevolutionofthe colli-sionsystemandintheinitial-stateconditionsbeforetheformation ofthe QGP. In addition, differentmechanisms are considered re-garding the interaction of heavy quarks with the medium and forthe hadronization process. Results fromthe models LBT [36], CUJET 3.0 [37], and SUBATECH [38] include collisional and ra-diative energy losses, while those from the models TAMU [39], PHSD [15], and TAMU SMCs [40] include only collisional energy loss. Initial-state fluctuations are included in the calculations by LBT,SUBATECH,andPHSD, andcalculationsforthe v3 coefficient are only available from these three models. Coalescence mecha-nismsarealsoincludedinLBT,SUBATECH,TAMU,PHSD,andTAMU SMCs. While most models seem to capture the qualitative trend of the data (except for the v2 description provided by TAMUin the10–50%centralityrange),mostofthemodelsdonotprovidea quantitativedescriptionoverthefullrange,exceptforTAMUSMCs. TheTAMUSMCsversionimprovestheTAMUmodelby implement-ingevent-by-eventspace-momentumcorrelations(SMCs)between charmquarksandthehigh-flowpartonsintheQGPmedium [40]. Since it does not include initial-state fluctuations, TAMU SMCs does not provide v2 calculations for centrality values between 0–10%. Thisputs more stringent constraints on the development ofthecollectiveflowforcharmquarksintheQGPmedium,giving

Fig. 2. Prompt D0 _{mesonandchargedparticleflow coefficientsv}

2 (upper)and v3 (lower)at midrapidity(|y| <1.0 forpromptD0 mesonsand|η| <1.0 forcharged

particles)forthecentralityclasses0–10%(left),10–30%(middle),and30–50%(right).Theverticalbarsandopenboxesrepresentthestatisticalandsystematicuncertainties, respectively.ThehorizontalbarsrepresentthewidthofeachpT bin.Theoreticalcalculationsfor vn coefficientsofpromptD0 mesonsarealsoplottedforcomparison:

LBT [36],CUJET3.0 [37],SUBATECH [38],TAMU [39],PHSD [15].TheTAMUSMCsmodel [40] isavailableonlyinthe10–50%centralitybins.

Fig. 3. PromptD0 _{mesonflowcoefficientsv}

2(upper)andv3(lower)atmidrapidity(|y| <1,redopencircles)andforwardrapidity(1<|y| <2,blueopendiamonds)for

thecentralityclasses0–10%(left),10–30%(middle),and30–50%(right).Theverticalbarsandopenboxesrepresentthestatisticalandsystematicuncertainties,respectively. ThehorizontalbarsrepresentthewidthofeachpTbin.

furtherinputsforunderstandingheavy-quarkinteractionswiththe medium(forexample,energylossandcoalescencemechanisms).

Results for the rapidity dependence of heavy-flavor collective flowarepresentedforthefirsttimeforpromptD0 _{meson v}

2 and

v3 as functions of pT, both at midrapidity (|y| <1) and in the forward(1<|y| <2) region,asshowninFig.3.No clearrapidity dependenceisobservedforboth v2 and v3asfunctionsof pT.This

observation is similar to that for inclusivecharged-hadron mea-surements [41].

In Fig. 4 (left), resultsfor prompt D0 mesons v2 and v3, av-eragedover2.0<pT<8.0 GeV/c,for|y| <1 and1<|y| <2,are presentedasafunctionofcollisioncentrality.ThispTrangeis cho-seninordertocoverthewidestpossible pTrange,while maximiz-ingtheD0 _{mesonsignalyieldsignificance.Thesep}

rapidity-Fig. 4. PromptD0_mesonv

2andv3asfunctionsofcentrality,for2.0<pT<8.0 GeV/c andforrapidityranges|y| <1 and1<|y| <2.Theresultsarecomparedwithcharged

particlev2andv3inthesamepTrangeandwith|η| <1 (left).PromptD0 v2andv3asfunctionsofrapidity,for2.0<pT<8.0 GeV/c andforcentrality20–70%(right).

Theverticalbarsrepresentstatisticaluncertaintiesandopenboxesrepresentsystematicuncertainties.Thehorizontalbarsrepresentthewidthofeachbin. integrated results include an additional centrality bin (50–70%),

whichhasaninsufficientnumberofeventsforthefulldifferential analysis.Forbothmid- andforward-rapidityregions,the v2results showaclearincreasefromthemostcentraltomid-centralevents, andthenadecliningtrendtowardthemostperipheralevents.This trend is similar to that observed for inclusive charged particles (also shownin Fig. 4), and can be understood in terms of colli-siongeometryandviscosityeffects.Inparticular,a fasterincrease of v2 isobservedfromcentraltoperipheralcollisions forcharged particlescomparedtopromptD0_{mesons.Thisfeaturewasalso} ob-servedwhencomparing v2oflow-pTJ/ψwithchargedpions [42], whereitisclaimedthatthiscouldbeunderstoodintermsoftwo phenomena: one,associated withtransport models predicting an increasingfractionofregeneratedJ/ψ atlow-pT,whengoingfrom peripheral tocentral collisions;the other,not relatedto regener-ation, isassociatedwitha possiblepartialorlater thermalization ofcharmquarkscompared tolight quarks [42].The v3 showsno centralitydependence,which isalsoconsistent withexpectations fromcollisiongeometryfluctuations [43].

Fig. 4 (right) presents results for the rapidity dependence of promptD0meson v2and v3,forcentrality20–70%,averagedover 2.0<pT<8.0 GeV/c.Aweakrapiditydependenceof v2and v3 is observedinthedata.

Finally,tosearch foreffectsofstrongEMfields,thedifference

v2 between the v2 values of D0 and D 0

mesons is measured. TheseresultsarepresentedinFig.5,asafunctionofrapidity, av-eragedover2.0<pT<8.0 GeV/c andforcentrality20–70%.Forall rapidity bins,the v2 valuesarecompatiblewithzero.The aver-ageoverthefullrapidityregionisv2 =0.001±0.001 (stat)± 0.003 (syst). In Ref. [19], the predicted v2 splittingfor inclusive chargedparticlesduetoelectricfieldsis∼0.001 at theLHC ener-gies. Whilequantitative predictionsfor v2 splittingofD0 mesons are not yet available, they are expected to be much larger than thoseforinclusivechargedparticles.Inthecaseofv1,theALICE collaboration reported results about three orders of magnitude larger thanmeasurementsforchargedhadrons [44],although the uncertainties prevent a clear conclusion.The main reasonisthat heavy-flavorquarksareusuallyproduced muchearlierthan light-flavorquarks,theformerbeingpredominantlyproducedsoonafter thecollisiontakesplace,whentheEMfieldstrengthisseveral

or-Fig. 5. PromptD0_meson

v2 asafunctionofrapidity,for2.0<pT<8.0 GeV/c

andcentrality20–70%.Theverticalbarsrepresentstatisticaluncertaintiesandopen boxesrepresentsystematicuncertainties.Thehorizontalbarsrepresentthewidth ofeachbin.

dersofmagnitudestronger [20].The resultspresented herepose constraintsonpossibleEMeffectsoncharmquarks.

7. Summary

Measurementsoftheelliptic(v2)andtriangular(v3)flow coef-ficientsofpromptD0 mesonsarepresentedasfunctionsof trans-verse momentum (pT), rapidity, and collision centrality, in PbPb collisionsat√sNN=5.02 TeV.Theresultsimprovepreviously pub-lishedCMSdatabyextendingthe pT andrapiditycoverageandby providing more differential information in pT, rapidity,and cen-trality. Aclear centrality dependence ofprompt D0 meson v2 is observed,whilev3 islargely centralityindependent.Thesetrends areconsistent withtheexpectationthat v2 andv3 aredriven by initial-stategeometry.A weakrapidity dependence ofprompt D0

meson v2 and v3 isobserved. When comparing various theoret-ical calculationsto the data at midrapidity, no model is able to describethedataoverthefullcentralityandpT ranges.

Motivatedbythesearchforevidenceofthestrongelectricfield expected in PbPb collisions, a first measurement of the v2 flow coefficientdifference(v2)betweenD0 andD

0

mesonsasa func-tionofrapidityispresented.Therapidity-averaged v2differenceis measuredtobev2=0.001±0.001 (stat)±0.003(syst).This in-dicatesthatthereisnoevidencethatcharmhadroncollectiveflow isaffected bythe strongCoulomb field createdinultrarelativistic heavyioncollisions.Futurecomparisonsoftheoreticalmodelswith theseresultsmayprovideconstraints ontheelectricconductivity ofthequark-gluonplasma.

Declarationofcompetinginterest

Theauthorsdeclarethattheyhavenoknowncompeting finan-cialinterestsorpersonalrelationshipsthatcouldhaveappearedto influencetheworkreportedinthispaper.

Acknowledgements

WecongratulateourcolleaguesintheCERNaccelerator depart-ments for the excellent performance of the LHC and thank the technical andadministrativestaffsat CERNandat other CMS in-stitutes for their contributions to the success of the CMS effort. Inaddition,wegratefullyacknowledgethecomputingcentersand personneloftheWorldwideLHCComputingGridfordeliveringso effectivelythe computinginfrastructure essentialto ouranalyses. Finally, we acknowledge the enduring support for the construc-tion andoperation oftheLHC andtheCMSdetectorprovided by the followingfundingagencies:BMBWF andFWF(Austria); FNRS and FWO (Belgium); CNPq, CAPES,FAPERJ, FAPERGS, and FAPESP (Brazil); MES (Bulgaria); CERN; CAS, MOST, and NSFC (China); COLCIENCIAS (Colombia); MSES and CSF (Croatia); RIF (Cyprus); SENESCYT (Ecuador); MoER, ERC IUT, PUT and ERDF (Estonia); AcademyofFinland,MEC,andHIP(Finland);CEAandCNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); NK-FIA (Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN(Italy);MSIPandNRF(RepublicofKorea);MES(Latvia);LAS (Lithuania);MOE andUM(Malaysia);BUAP,CINVESTAV,CONACYT, LNS,SEP,andUASLP-FAI(Mexico);MOS(Montenegro);MBIE(New Zealand); PAEC (Pakistan); MSHE and NSC (Poland); FCT (Portu-gal);JINR(Dubna);MON,ROSATOM,RAS, RFBR,andNRCKI (Rus-sia);MESTD(Serbia);SEIDI,CPAN,PCTI,andFEDER(Spain);MoSTR (Sri Lanka); Swiss Funding Agencies (Switzerland); MST (Taipei); ThEPCenter,IPST,STAR, andNSTDA(Thailand);TÜBITAK andTAEK (Turkey); NASU (Ukraine); STFC (United Kingdom); DOEand NSF (USA).

Individuals have received support from the Marie-Curie pro-gramandtheEuropeanResearchCouncilandHorizon2020Grant, contract Nos. 675440, 752730, and 765710 (European Union); the Leventis Foundation; the A.P. Sloan Foundation; the Alexan-der von Humboldt Foundation; the Belgian Federal Science Pol-icy Office; the Fonds pour la Formation à la Recherche dans l’Industrie et dans l’Agriculture (FRIA-Belgium); the Agentschap voor Innovatie door Wetenschap en Technologie (IWT-Belgium); the F.R.S.-FNRS andFWO(Belgium) under the“Excellence of Sci-ence – EOS” – be.h project n. 30820817; the Beijing Municipal Science & Technology Commission, No. Z191100007219010; The Ministry of Education, Youth and Sports (MEYS) of the Czech Republic;theDeutscheForschungsgemeinschaft(DFG)under Ger-many’s Excellence Strategy – EXC 2121 “Quantum Universe” – 390833306; the Lendület (“Momentum”) Program and the János Bolyai Research Scholarship of the Hungarian Academy of Sci-ences, the New National Excellence Program ÚNKP, the

NK-FIA research grants 123842, 123959, 124845, 124850, 125105, 128713, 128786, and 129058 (Hungary); the Council of Science andIndustrialResearch,India; theHOMING PLUSprogramofthe Foundation for Polish Science, cofinanced from European Union, Regional Development Fund, the Mobility Plus program of the Ministry of Science and Higher Education, the National Science Center (Poland), contracts Harmonia 2014/14/M/ST2/00428, Opus 2014/13/B/ST2/02543, 2014/15/B/ST2/03998, and 2015/19/B/ST2/ 02861,Sonata-bis2012/07/E/ST2/01406;theNationalPriorities Re-searchProgram byQatar NationalResearchFund;theMinistry of ScienceandHigherEducation,projectno.02.a03.21.0005(Russia); the Tomsk Polytechnic University Competitiveness Enhancement Program; the Programa Estatal de Fomento de la Investigación CientíficayTécnica de ExcelenciaMaría de Maeztu, grant MDM-2015-0509 and the Programa Severo Ochoa del Principado de Asturias; the Thalis andAristeia programs cofinanced by EU-ESF andthe GreekNSRF;theRachadapisek SompotFund for Postdoc-toralFellowship,ChulalongkornUniversity andtheChulalongkorn AcademicintoIts2ndCenturyProjectAdvancementProject (Thai-land);theKavliFoundation;theNvidiaCorporation;the SuperMi-croCorporation; theWelchFoundation, contract C-1845;andthe WestonHavensFoundation(USA).

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TheCMSCollaboration

A.M. Sirunyan†,A. Tumasyan

YerevanPhysicsInstitute,Yerevan,Armenia

W. Adam, F. Ambrogi, T. Bergauer, M. Dragicevic,J. Erö, A. Escalante Del Valle,R. Frühwirth1,M. Jeitler1,

N. Krammer, L. Lechner,D. Liko, T. Madlener, I. Mikulec,N. Rad, J. Schieck1,R. Schöfbeck, M. Spanring,

S. Templ,W. Waltenberger, C.-E. Wulz1,M. Zarucki

InstitutfürHochenergiephysik,Wien,Austria

V. Chekhovsky, A. Litomin, V. Makarenko, J. Suarez Gonzalez

InstituteforNuclearProblems,Minsk,Belarus

M.R. Darwish, E.A. De Wolf,D. Di Croce, X. Janssen,T. Kello2,A. Lelek, M. Pieters, H. Rejeb Sfar,

H. Van Haevermaet, P. Van Mechelen,S. Van Putte, N. Van Remortel

UniversiteitAntwerpen,Antwerpen,Belgium

F. Blekman, E.S. Bols,S.S. Chhibra, J. D’Hondt, J. De Clercq, D. Lontkovskyi,S. Lowette, I. Marchesini,

S. Moortgat, Q. Python, S. Tavernier, W. Van Doninck, P. Van Mulders

D. Beghin, B. Bilin, B. Clerbaux, G. De Lentdecker, H. Delannoy, B. Dorney, L. Favart, A. Grebenyuk,

A.K. Kalsi, I. Makarenko, L. Moureaux, L. Pétré, A. Popov, N. Postiau,E. Starling, L. Thomas,

C. Vander Velde, P. Vanlaer, D. Vannerom,L. Wezenbeek

UniversitéLibredeBruxelles,Bruxelles,Belgium

T. Cornelis, D. Dobur,I. Khvastunov3,M. Niedziela, C. Roskas, K. Skovpen, M. Tytgat, W. Verbeke,

B. Vermassen, M. Vit

GhentUniversity,Ghent,Belgium

G. Bruno,F.J.J. Bury, C. Caputo, P. David,C. Delaere, M. Delcourt,I.S. Donertas, A. Giammanco,

V. Lemaitre, J. Prisciandaro, A. Saggio, A. Taliercio, M. Teklishyn,P. Vischia, S. Wuyckens, J. Zobec UniversitéCatholiquedeLouvain,Louvain-la-Neuve,Belgium

G.A. Alves, G. Correia Silva,C. Hensel,A. Moraes

CentroBrasileirodePesquisasFisicas,RiodeJaneiro,Brazil

W.L. Aldá Júnior, E. Belchior Batista Das Chagas, W. Carvalho,J. Chinellato4, E. Coelho, E.M. Da Costa,

G.G. Da Silveira5, D. De Jesus Damiao,S. Fonseca De Souza, H. Malbouisson, J. Martins6,

D. Matos Figueiredo, M. Medina Jaime7,M. Melo De Almeida, C. Mora Herrera,L. Mundim, H. Nogima,

P. Rebello Teles, L.J. Sanchez Rosas,A. Santoro, S.M. Silva Do Amaral, A. Sznajder,M. Thiel,

E.J. Tonelli Manganote4, F. Torres Da Silva De Araujo,A. Vilela Pereira UniversidadedoEstadodoRiodeJaneiro,RiodeJaneiro,Brazil

C.A. Bernardesa,L. Calligarisa,T.R. Fernandez Perez Tomeia, E.M. Gregoresb, D.S. Lemosa, P.G. Mercadanteb, S.F. Novaesa, Sandra S. Padulaa

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

A. Aleksandrov, G. Antchev, I. Atanasov, R. Hadjiiska, P. Iaydjiev, M. Misheva,M. Rodozov, M. Shopova,

G. Sultanov

InstituteforNuclearResearchandNuclearEnergy,BulgarianAcademyofSciences,Sofia,Bulgaria

M. Bonchev, A. Dimitrov, T. Ivanov, L. Litov,B. Pavlov, P. Petkov,A. Petrov UniversityofSofia,Sofia,Bulgaria

W. Fang2,X. Gao2, Q. Guo, H. Wang, L. Yuan BeihangUniversity,Beijing,China

M. Ahmad, Z. Hu, Y. Wang

DepartmentofPhysics,TsinghuaUniversity,Beijing,China

E. Chapon, G.M. Chen8, H.S. Chen8,M. Chen, C.H. Jiang, D. Leggat, H. Liao, Z. Liu, A. Spiezia,J. Tao, J. Wang, E. Yazgan, H. Zhang, S. Zhang8, J. Zhao

InstituteofHighEnergyPhysics,Beijing,China

A. Agapitos, Y. Ban, C. Chen, G. Chen, A. Levin,J. Li, L. Li,Q. Li, X. Lyu, Y. Mao, S.J. Qian, D. Wang,

Q. Wang, J. Xiao

StateKeyLaboratoryofNuclearPhysicsandTechnology,PekingUniversity,Beijing,China Z. You

M. Xiao

ZhejiangUniversity,Hangzhou,China

C. Avila, A. Cabrera,C. Florez, C.F. González Hernández, A. Sarkar, M.A. Segura Delgado

UniversidaddeLosAndes,Bogota,Colombia

J. Mejia Guisao,J.D. Ruiz Alvarez, C.A. Salazar González, N. Vanegas Arbelaez

UniversidaddeAntioquia,Medellin,Colombia

D. Giljanovic,N. Godinovic, D. Lelas,I. Puljak, T. Sculac

UniversityofSplit,FacultyofElectricalEngineering,MechanicalEngineeringandNavalArchitecture,Split,Croatia

Z. Antunovic, M. Kovac

UniversityofSplit,FacultyofScience,Split,Croatia

V. Brigljevic, D. Ferencek,D. Majumder, B. Mesic, M. Roguljic, A. Starodumov9,T. Susa

InstituteRudjerBoskovic,Zagreb,Croatia

M.W. Ather, A. Attikis, E. Erodotou,A. Ioannou, G. Kole, M. Kolosova,S. Konstantinou,

G. Mavromanolakis, J. Mousa,C. Nicolaou, F. Ptochos, P.A. Razis, H. Rykaczewski,H. Saka, D. Tsiakkouri

UniversityofCyprus,Nicosia,Cyprus

M. Finger10,M. Finger Jr.10, A. Kveton, J. Tomsa CharlesUniversity,Prague,CzechRepublic

E. Ayala

EscuelaPolitecnicaNacional,Quito,Ecuador E. Carrera Jarrin

UniversidadSanFranciscodeQuito,Quito,Ecuador

A.A. Abdelalim11,12, S. Abu Zeid13,S. Khalil12

AcademyofScientificResearchandTechnologyoftheArabRepublicofEgypt,EgyptianNetworkofHighEnergyPhysics,Cairo,Egypt

S. Bhowmik, A. Carvalho Antunes De Oliveira, R.K. Dewanjee, K. Ehataht,M. Kadastik, M. Raidal,

C. Veelken

NationalInstituteofChemicalPhysicsandBiophysics,Tallinn,Estonia

P. Eerola, L. Forthomme, H. Kirschenmann,K. Osterberg, M. Voutilainen

DepartmentofPhysics,UniversityofHelsinki,Helsinki,Finland

E. Brücken, F. Garcia, J. Havukainen, V. Karimäki, M.S. Kim,R. Kinnunen, T. Lampén, K. Lassila-Perini,

S. Laurila, S. Lehti, T. Lindén,H. Siikonen, E. Tuominen, J. Tuominiemi

HelsinkiInstituteofPhysics,Helsinki,Finland

P. Luukka, T. Tuuva

LappeenrantaUniversityofTechnology,Lappeenranta,Finland

M. Besancon, F. Couderc, M. Dejardin,D. Denegri, J.L. Faure, F. Ferri, S. Ganjour, A. Givernaud, P. Gras,

G. Hamel de Monchenault, P. Jarry,C. Leloup, B. Lenzi, E. Locci,J. Malcles, J. Rander, A. Rosowsky,

M.Ö. Sahin, A. Savoy-Navarro14, M. Titov, G.B. Yu

S. Ahuja, C. Amendola, F. Beaudette, M. Bonanomi, P. Busson, C. Charlot, O. Davignon,B. Diab,

G. Falmagne, R. Granier de Cassagnac, I. Kucher, A. Lobanov, C. Martin Perez, M. Nguyen,C. Ochando,

P. Paganini, J. Rembser,R. Salerno, J.B. Sauvan, Y. Sirois, A. Zabi,A. Zghiche LaboratoireLeprince-Ringuet,CNRS/IN2P3,EcolePolytechnique,InstitutPolytechniquedeParis,Palaiseau,France

J.-L. Agram15,J. Andrea, D. Bloch, G. Bourgatte, J.-M. Brom,E.C. Chabert, C. Collard,J.-C. Fontaine15,

D. Gelé, U. Goerlach,C. Grimault, A.-C. Le Bihan, P. Van Hove

UniversitédeStrasbourg,CNRS,IPHCUMR7178,Strasbourg,France

E. Asilar, S. Beauceron,C. Bernet, G. Boudoul, C. Camen, A. Carle,N. Chanon, R. Chierici, D. Contardo,

P. Depasse,H. El Mamouni, J. Fay, S. Gascon, M. Gouzevitch, B. Ille, Sa. Jain, I.B. Laktineh,H. Lattaud,

A. Lesauvage, M. Lethuillier, L. Mirabito,L. Torterotot, G. Touquet, M. Vander Donckt, S. Viret

UniversitédeLyon,UniversitéClaudeBernardLyon1,CNRS-IN2P3,InstitutdePhysiqueNucléairedeLyon,Villeurbanne,France

A. Khvedelidze10

GeorgianTechnicalUniversity,Tbilisi,Georgia

Z. Tsamalaidze10

TbilisiStateUniversity,Tbilisi,Georgia

L. Feld, K. Klein, M. Lipinski, D. Meuser, A. Pauls, M. Preuten,M.P. Rauch, J. Schulz,M. Teroerde

RWTHAachenUniversity,I.PhysikalischesInstitut,Aachen,Germany

D. Eliseev, M. Erdmann, P. Fackeldey,B. Fischer, S. Ghosh, T. Hebbeker,K. Hoepfner, H. Keller,

L. Mastrolorenzo, M. Merschmeyer,A. Meyer, P. Millet, G. Mocellin,S. Mondal, S. Mukherjee, D. Noll,

A. Novak, T. Pook, A. Pozdnyakov,T. Quast, M. Radziej, Y. Rath, H. Reithler,J. Roemer, A. Schmidt,

S.C. Schuler, A. Sharma, S. Wiedenbeck, S. Zaleski

RWTHAachenUniversity,III.PhysikalischesInstitutA,Aachen,Germany

C. Dziwok, G. Flügge, W. Haj Ahmad16,O. Hlushchenko, T. Kress, A. Nowack,C. Pistone, O. Pooth,D. Roy,

H. Sert, A. Stahl17, T. Ziemons

RWTHAachenUniversity,III.PhysikalischesInstitutB,Aachen,Germany

H. Aarup Petersen, M. Aldaya Martin,P. Asmuss,I. Babounikau, S. Baxter, K. Beernaert,O. Behnke,

A. Bermúdez Martínez, A.A. Bin Anuar, K. Borras18, V. Botta,D. Brunner, A. Campbell, A. Cardini,

P. Connor, S. Consuegra Rodríguez,C. Contreras-Campana, V. Danilov, A. De Wit, M.M. Defranchis,

L. Didukh, C. Diez Pardos,D. Domínguez Damiani,G. Eckerlin, D. Eckstein, T. Eichhorn, A. Elwood,

E. Eren, L.I. Estevez Banos, E. Gallo19, A. Geiser,A. Giraldi, A. Grohsjean, M. Guthoff, M. Haranko, A. Harb, A. Jafari20, N.Z. Jomhari, H. Jung,A. Kasem18, M. Kasemann, H. Kaveh, J. Keaveney,

C. Kleinwort, J. Knolle,D. Krücker, W. Lange, T. Lenz,J. Lidrych, K. Lipka,W. Lohmann21,R. Mankel,

I.-A. Melzer-Pellmann, J. Metwally, A.B. Meyer, M. Meyer, M. Missiroli, J. Mnich, A. Mussgiller,

V. Myronenko, Y. Otarid,D. Pérez Adán, S.K. Pflitsch,D. Pitzl, A. Raspereza,A. Saibel, M. Savitskyi,

V. Scheurer, P. Schütze, C. Schwanenberger, R. Shevchenko,A. Singh, R.E. Sosa Ricardo, H. Tholen,

N. Tonon, O. Turkot, A. Vagnerini,M. Van De Klundert, R. Walsh, D. Walter,Y. Wen, K. Wichmann,

C. Wissing, S. Wuchterl, O. Zenaiev, R. Zlebcik

DeutschesElektronen-Synchrotron,Hamburg,Germany

R. Aggleton, S. Bein,L. Benato, A. Benecke,K. De Leo, T. Dreyer, A. Ebrahimi, F. Feindt, A. Fröhlich,

C. Garbers, E. Garutti, D. Gonzalez, P. Gunnellini, J. Haller, A. Hinzmann, A. Karavdina, G. Kasieczka,

R. Klanner, R. Kogler, S. Kurz, V. Kutzner, J. Lange,T. Lange, A. Malara, J. Multhaup,C.E.N. Niemeyer,

A. Nigamova, K.J. Pena Rodriguez, A. Reimers, O. Rieger,P. Schleper, S. Schumann,J. Schwandt,

D. Schwarz, J. Sonneveld,H. Stadie, G. Steinbrück, B. Vormwald, I. Zoi

M. Akbiyik, M. Baselga, S. Baur, J. Bechtel,T. Berger,E. Butz, R. Caspart, T. Chwalek, W. De Boer,

A. Dierlamm, K. El Morabit,N. Faltermann, K. Flöh,M. Giffels, A. Gottmann,F. Hartmann17,C. Heidecker,

U. Husemann, M.A. Iqbal,I. Katkov22,S. Kudella, S. Maier, M. Metzler, S. Mitra, M.U. Mozer, D. Müller,

Th. Müller, M. Musich, G. Quast, K. Rabbertz, J. Rauser, D. Savoiu, D. Schäfer, M. Schnepf, M. Schröder,

I. Shvetsov, H.J. Simonis,R. Ulrich, M. Wassmer, M. Weber, C. Wöhrmann, R. Wolf,S. Wozniewski

KarlsruherInstitutfuerTechnologie,Karlsruhe,Germany

G. Anagnostou, P. Asenov, G. Daskalakis,T. Geralis, A. Kyriakis,D. Loukas, G. Paspalaki,A. Stakia InstituteofNuclearandParticlePhysics(INPP),NCSRDemokritos,AghiaParaskevi,Greece

M. Diamantopoulou, D. Karasavvas, G. Karathanasis,P. Kontaxakis, C.K. Koraka,A. Manousakis-katsikakis,

A. Panagiotou, I. Papavergou, N. Saoulidou, K. Theofilatos, K. Vellidis,E. Vourliotis NationalandKapodistrianUniversityofAthens,Athens,Greece

G. Bakas, K. Kousouris, I. Papakrivopoulos,G. Tsipolitis, A. Zacharopoulou

NationalTechnicalUniversityofAthens,Athens,Greece

I. Evangelou, C. Foudas, P. Gianneios,P. Katsoulis, P. Kokkas, S. Mallios, K. Manitara, N. Manthos,

I. Papadopoulos, J. Strologas,F.A. Triantis, D. Tsitsonis UniversityofIoánnina,Ioánnina,Greece

M. Bartók23, R. Chudasama, M. Csanad, M.M.A. Gadallah24,P. Major, K. Mandal, A. Mehta,G. Pasztor,

O. Surányi, G.I. Veres

MTA-ELTELendületCMSParticleandNuclearPhysicsGroup,EötvösLorándUniversity,Budapest,Hungary

G. Bencze, C. Hajdu,D. Horvath25,F. Sikler, V. Veszpremi, G. Vesztergombi†

WignerResearchCentreforPhysics,Budapest,Hungary

N. Beni, S. Czellar,J. Karancsi23, J. Molnar,Z. Szillasi, D. Teyssier InstituteofNuclearResearchATOMKI,Debrecen,Hungary

P. Raics, Z.L. Trocsanyi, B. Ujvari InstituteofPhysics,UniversityofDebrecen,Debrecen,Hungary T. Csorgo, S. Lökös26, F. Nemes, T. Novak EszterhazyKarolyUniversity,KarolyRobertCampus,Gyongyos,Hungary

S. Choudhury, J.R. Komaragiri, D. Kumar, L. Panwar, P.C. Tiwari

IndianInstituteofScience(IISc),Bangalore,India

S. Bahinipati27, C. Kar,P. Mal, T. Mishra, V.K. Muraleedharan Nair Bindhu, A. Nayak28, D.K. Sahoo27,

N. Sur, S.K. Swain

NationalInstituteofScienceEducationandResearch,HBNI,Bhubaneswar,India

S. Bansal, S.B. Beri,V. Bhatnagar, S. Chauhan,N. Dhingra29,R. Gupta,A. Kaur, A. Kaur, S. Kaur,P. Kumari,

M. Lohan, M. Meena, K. Sandeep, S. Sharma, J.B. Singh, A.K. Virdi

PanjabUniversity,Chandigarh,India

A. Ahmed, A. Bhardwaj,B.C. Choudhary, R.B. Garg,M. Gola, S. Keshri, A. Kumar,M. Naimuddin,

P. Priyanka, K. Ranjan, A. Shah,R. Sharma

M. Bharti30,R. Bhattacharya, S. Bhattacharya, D. Bhowmik,S. Dutta, S. Ghosh,B. Gomber31,M. Maity32, K. Mondal, S. Nandan, P. Palit,A. Purohit, P.K. Rout, G. Saha,S. Sarkar, M. Sharan, B. Singh30,S. Thakur30 SahaInstituteofNuclearPhysics,HBNI,Kolkata,India

P.K. Behera, S.C. Behera, P. Kalbhor, A. Muhammad,R. Pradhan, P.R. Pujahari, A. Sharma, A.K. Sikdar

IndianInstituteofTechnologyMadras,Madras,India

D. Dutta, V. Jha, D.K. Mishra, K. Naskar33,P.K. Netrakanti, L.M. Pant, P. Shukla BhabhaAtomicResearchCentre,Mumbai,India

T. Aziz, M.A. Bhat, S. Dugad,R. Kumar Verma,U. Sarkar

TataInstituteofFundamentalResearch-A,Mumbai,India

S. Banerjee, S. Bhattacharya, S. Chatterjee, P. Das, M. Guchait,S. Karmakar, S. Kumar, G. Majumder,

K. Mazumdar, S. Mukherjee,D. Roy, N. Sahoo

TataInstituteofFundamentalResearch-B,Mumbai,India

S. Dube, B. Kansal, A. Kapoor, K. Kothekar, S. Pandey, A. Rane, A. Rastogi,S. Sharma

IndianInstituteofScienceEducationandResearch(IISER),Pune,India

H. Bakhshiansohi34

DepartmentofPhysics,IsfahanUniversityofTechnology,Isfahan,Iran

S. Chenarani35,S.M. Etesami, M. Khakzad, M. Mohammadi Najafabadi, M. Naseri

InstituteforResearchinFundamentalSciences(IPM),Tehran,Iran

M. Felcini, M. Grunewald

UniversityCollegeDublin,Dublin,Ireland

M. Abbresciaa,b,R. Alya,b,36,C. Calabriaa,b,A. Colaleoa, D. Creanzaa,c,N. De Filippisa,c,M. De Palmaa,b, A. Di Florioa,b,A. Di Pilatoa,b,W. Elmetenaweea,b,L. Fiorea,A. Gelmia,b, G. Iasellia,c, M. Incea,b,

S. Lezkia,b,G. Maggia,c, M. Maggia,I. Margjekaa,b, J.A. Merlina,G. Minielloa,b,S. Mya,b, S. Nuzzoa,b, A. Pompilia,b,G. Pugliesea,c, A. Ranieria,G. Selvaggia,b,L. Silvestrisa, F.M. Simonea,b,R. Vendittia, P. Verwilligena

a_{INFNSezionediBari,Bari,Italy} b_{UniversitàdiBari,Bari,Italy} c_{PolitecnicodiBari,Bari,Italy}

G. Abbiendia, C. Battilanaa,b, D. Bonacorsia,b, L. Borgonovia,b, R. Campaninia,b,P. Capiluppia,b, A. Castroa,b,F.R. Cavalloa, C. Cioccaa, M. Cuffiania,b,G.M. Dallavallea,T. Diotalevia,b,F. Fabbria, A. Fanfania,b,E. Fontanesia,b, P. Giacomellia, L. Giommia,b,C. Grandia, L. Guiduccia,b,F. Iemmia,b, S. Lo Meoa,37, S. Marcellinia,G. Masettia,F.L. Navarriaa,b, A. Perrottaa,F. Primaveraa,b,T. Rovellia,b, G.P. Sirolia,b, N. Tosia

a_{INFNSezionediBologna,Bologna,Italy} b_{UniversitàdiBologna,Bologna,Italy}

S. Albergoa,b,38, S. Costaa,b, A. Di Mattiaa,R. Potenzaa,b,A. Tricomia,b,38, C. Tuvea,b

a_{INFNSezionediCatania,Catania,Italy} b_{UniversitàdiCatania,Catania,Italy}

G. Barbaglia,A. Cassesea,R. Ceccarellia,b,V. Ciullia,b,C. Civininia, R. D’Alessandroa,b, F. Fioria, E. Focardia,b_{, G. Latino}a,b_{,P. Lenzi}a,b_{, M. Lizzo}a,b_{,M. Meschini}a_{,S. Paoletti}a_{, R. Seidita}a,b_,

G. Sguazzonia,L. Viliania

b_{UniversitàdiFirenze,Firenze,Italy}

L. Benussi, S. Bianco, D. Piccolo INFNLaboratoriNazionalidiFrascati,Frascati,Italy

M. Bozzoa,b, F. Ferroa, R. Mulargiaa,b, E. Robuttia,S. Tosia,b

a_{INFNSezionediGenova,Genova,Italy} b_{UniversitàdiGenova,Genova,Italy}

A. Benagliaa, A. Beschia,b, F. Brivioa,b, F. Cetorellia,b,V. Cirioloa,b,17, F. De Guioa,b, M.E. Dinardoa,b, P. Dinia,S. Gennaia, A. Ghezzia,b, P. Govonia,b, L. Guzzia,b,M. Malbertia,S. Malvezzia,D. Menascea, F. Montia,b, L. Moronia, M. Paganonia,b,D. Pedrinia, S. Ragazzia,b, T. Tabarelli de Fatisa,b,

D. Valsecchia,b,17,D. Zuoloa,b

a_{INFNSezionediMilano-Bicocca,Milano,Italy} b_{UniversitàdiMilano-Bicocca,Milano,Italy}

S. Buontempoa,N. Cavalloa,c,A. De Iorioa,b, F. Fabozzia,c,F. Fiengaa,G. Galatia,A.O.M. Iorioa,b, L. Layera,b,L. Listaa,b,S. Meolaa,d,17,P. Paoluccia,17,B. Rossia, C. Sciaccaa,b, E. Voevodinaa,b

a_{INFNSezionediNapoli,Napoli,Italy} b_{UniversitàdiNapoli‘FedericoII’,Napoli,Italy} c_{UniversitàdellaBasilicata,Potenza,Italy} d_{UniversitàG.Marconi,Roma,Italy}

P. Azzia, N. Bacchettaa, D. Biselloa,b,A. Bolettia,b,A. Bragagnoloa,b,R. Carlina,b,P. Checchiaa, P. De Castro Manzanoa, T. Dorigoa,U. Dossellia,F. Gasparinia,b,U. Gasparinia,b, S.Y. Hoha,b,

M. Margonia,b,A.T. Meneguzzoa,b, M. Presillab,P. Ronchesea,b, R. Rossina,b, F. Simonettoa,b,G. Strong, A. Tikoa,M. Tosia,b, M. Zanettia,b,P. Zottoa,b,A. Zucchettaa,b,G. Zumerlea,b

a_{INFNSezionediPadova,Padova,Italy} b_{UniversitàdiPadova,Padova,Italy} c_{UniversitàdiTrento,Trento,Italy}

A. Braghieria,S. Calzaferria,b, D. Fiorinaa,b,P. Montagnaa,b, S.P. Rattia,b,V. Rea,M. Ressegottia,b, C. Riccardia,b,P. Salvinia,I. Vaia, P. Vituloa,b

a_{INFNSezionediPavia,Pavia,Italy} b_{UniversitàdiPavia,Pavia,Italy}

M. Biasinia,b,G.M. Bileia, D. Ciangottinia,b,L. Fanòa,b, P. Laricciaa,b,G. Mantovania,b,V. Mariania,b, M. Menichellia, A. Rossia,b,A. Santocchiaa,b, D. Spigaa,T. Tedeschia,b

a_{INFNSezionediPerugia,Perugia,Italy} b_{UniversitàdiPerugia,Perugia,Italy}

K. Androsova,P. Azzurria,G. Bagliesia,V. Bertacchia,c, L. Bianchinia, T. Boccalia,R. Castaldia,

M.A. Cioccia,b, R. Dell’Orsoa,M.R. Di Domenicoa,b,S. Donatoa,L. Gianninia,c,A. Giassia,M.T. Grippoa, F. Ligabuea,c, E. Mancaa,c,G. Mandorlia,c,A. Messineoa,b,F. Pallaa, A. Rizzia,b, G. Rolandia,c,

S. Roy Chowdhurya,c,A. Scribanoa,N. Shafieia,b,P. Spagnoloa,R. Tenchinia,G. Tonellia,b,N. Turinia, A. Venturia,P.G. Verdinia

a_{INFNSezionediPisa,Pisa,Italy} b_{UniversitàdiPisa,Pisa,Italy}

c_{ScuolaNormaleSuperiorediPisa,Pisa,Italy}

F. Cavallaria, M. Cipriania,b,D. Del Rea,b,E. Di Marcoa, M. Diemoza,E. Longoa,b,P. Meridiania, G. Organtinia,b, F. Pandolfia,R. Paramattia,b,C. Quarantaa,b,S. Rahatloua,b, C. Rovellia,

F. Santanastasioa,b,L. Soffia,b, R. Tramontanoa,b

a_{INFNSezionediRoma,Rome,Italy} b_{SapienzaUniversitàdiRoma,Rome,Italy}

N. Amapanea,b,R. Arcidiaconoa,c, S. Argiroa,b, M. Arneodoa,c, N. Bartosika, R. Bellana,b,A. Belloraa,b, C. Biinoa,A. Cappatia,b, N. Cartigliaa, S. Comettia, M. Costaa,b,R. Covarellia,b, N. Demariaa,B. Kiania,b, F. Leggera,C. Mariottia,S. Masellia, E. Migliorea,b,V. Monacoa,b,E. Monteila,b,M. Montenoa,

M.M. Obertinoa,b, G. Ortonaa, L. Pachera,b, N. Pastronea,M. Pelliccionia, G.L. Pinna Angionia,b, M. Ruspaa,c, R. Salvaticoa,b, F. Sivieroa,b, V. Solaa,A. Solanoa,b,D. Soldia,b,A. Staianoa,D. Trocinoa,b

a_{INFNSezionediTorino,Torino,Italy} b_{UniversitàdiTorino,Torino,Italy}

c_{UniversitàdelPiemonteOrientale,Novara,Italy}

S. Belfortea, V. Candelisea,b, M. Casarsaa,F. Cossuttia, A. Da Rolda,b, G. Della Riccaa,b,F. Vazzolera,b

a_{INFNSezionediTrieste,Trieste,Italy} b_{UniversitàdiTrieste,Trieste,Italy}

S. Dogra, C. Huh,B. Kim, D.H. Kim, G.N. Kim, J. Lee, S.W. Lee, C.S. Moon, Y.D. Oh,S.I. Pak, S. Sekmen,

D.C. Son, Y.C. Yang

KyungpookNationalUniversity,Daegu,RepublicofKorea

H. Kim, D.H. Moon

ChonnamNationalUniversity,InstituteforUniverseandElementaryParticles,Kwangju,RepublicofKorea B. Francois, T.J. Kim, J. Park

HanyangUniversity,Seoul,RepublicofKorea

S. Cho, S. Choi, Y. Go,S. Ha, B. Hong,K. Lee, K.S. Lee, J. Lim, J. Park, S.K. Park,Y. Roh, J. Yoo KoreaUniversity,Seoul,RepublicofKorea

J. Goh, A. Gurtu

KyungHeeUniversity,DepartmentofPhysics,Seoul,RepublicofKorea

H.S. Kim, Y. Kim

SejongUniversity,Seoul,RepublicofKorea

J. Almond, J.H. Bhyun, J. Choi, S. Jeon, J. Kim,J.S. Kim, S. Ko, H. Kwon, H. Lee,K. Lee, S. Lee, K. Nam,

B.H. Oh, M. Oh, S.B. Oh,B.C. Radburn-Smith, H. Seo, U.K. Yang,I. Yoon

SeoulNationalUniversity,Seoul,RepublicofKorea

D. Jeon, J.H. Kim, B. Ko, J.S.H. Lee, I.C. Park, I.J. Watson UniversityofSeoul,Seoul,RepublicofKorea

H.D. Yoo

YonseiUniversity,DepartmentofPhysics,Seoul,RepublicofKorea

Y. Choi, C. Hwang, Y. Jeong, H. Lee,J. Lee,Y. Lee, I. Yu SungkyunkwanUniversity,Suwon,RepublicofKorea

Y. Maghrbi

CollegeofEngineeringandTechnology,AmericanUniversityoftheMiddleEast(AUM),Kuwait

V. Veckalns39

RigaTechnicalUniversity,Riga,Latvia

A. Juodagalvis, A. Rinkevicius, G. Tamulaitis VilniusUniversity,Vilnius,Lithuania

W.A.T. Wan Abdullah, M.N. Yusli,Z. Zolkapli NationalCentreforParticlePhysics,UniversitiMalaya,KualaLumpur,Malaysia

J.F. Benitez, A. Castaneda Hernandez, J.A. Murillo Quijada, L. Valencia Palomo

UniversidaddeSonora(UNISON),Hermosillo,Mexico

H. Castilla-Valdez, E. De La Cruz-Burelo, I. Heredia-De La Cruz40, R. Lopez-Fernandez,

A. Sanchez-Hernandez

CentrodeInvestigacionydeEstudiosAvanzadosdelIPN,MexicoCity,Mexico

S. Carrillo Moreno, C. Oropeza Barrera, M. Ramirez-Garcia, F. Vazquez Valencia

UniversidadIberoamericana,MexicoCity,Mexico

J. Eysermans, I. Pedraza, H.A. Salazar Ibarguen, C. Uribe Estrada

BenemeritaUniversidadAutonomadePuebla,Puebla,Mexico A. Morelos Pineda

UniversidadAutónomadeSanLuisPotosí,SanLuisPotosí,Mexico J. Mijuskovic3,N. Raicevic

UniversityofMontenegro,Podgorica,Montenegro D. Krofcheck

UniversityofAuckland,Auckland,NewZealand S. Bheesette, P.H. Butler UniversityofCanterbury,Christchurch,NewZealand

A. Ahmad, M. Ahmad, M.I. Asghar, M.I.M. Awan, Q. Hassan,H.R. Hoorani, W.A. Khan, M.A. Shah,

M. Shoaib, M. Waqas

NationalCentreforPhysics,Quaid-I-AzamUniversity,Islamabad,Pakistan

V. Avati, L. Grzanka,M. Malawski

AGHUniversityofScienceandTechnologyFacultyofComputerScience,ElectronicsandTelecommunications,Krakow,Poland

H. Bialkowska, M. Bluj,B. Boimska, T. Frueboes,M. Górski, M. Kazana, M. Szleper,P. Traczyk,P. Zalewski

NationalCentreforNuclearResearch,Swierk,Poland

K. Bunkowski, A. Byszuk41,K. Doroba, A. Kalinowski, M. Konecki, J. Krolikowski, M. Olszewski,

M. Walczak

InstituteofExperimentalPhysics,FacultyofPhysics,UniversityofWarsaw,Warsaw,Poland

M. Araujo, P. Bargassa, D. Bastos,A. Di Francesco, P. Faccioli, B. Galinhas, M. Gallinaro,J. Hollar, N. Leonardo, T. Niknejad,J. Seixas,K. Shchelina, O. Toldaiev, J. Varela

LaboratóriodeInstrumentaçãoeFísicaExperimentaldePartículas,Lisboa,Portugal

A. Baginyan, A. Golunov, I. Golutvin, I. Gorbunov, V. Karjavine, I. Kashunin,A. Lanev, A. Malakhov,

V. Matveev42,43,V.V. Mitsyn, P. Moisenz,V. Palichik, V. Perelygin, S. Shmatov, O. Teryaev,V. Trofimov, N. Voytishin, B.S. Yuldashev44,A. Zarubin, V. Zhiltsov