Measurement of the cross section ratio sigma(t(t)over-barb(b)over-bar)/sigma(t(t)over-barjj) in pp collisions at root s=8 TeV

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Physics

Letters

B

www.elsevier.com/locate/physletb

Measurement

of

the

cross

section

ratio

σ

/

σ

in

pp collisions

at

√

s

₌

8 TeV

Receivedinrevisedform31March2015 Accepted28April2015

Availableonline30April2015 Editor:M.Doser

Keywords:

CMS Physics Topphysics

Thefirstmeasurementofthecrosssectionratioσ_ttbb/σttjjispresentedusingadatasamplecorresponding toanintegratedluminosityof19.6 fb−1_{collectedinppcollisionsat}√_{s=8 TeV withtheCMSdetectorat} theLHC.Eventswithtwoleptons(e orµ)andfourreconstructedjets,includingtwoidentifiedasbquark jets,inthefinalstateareselected.Theratioisdeterminedforaminimumjettransversemomentum pT ofboth20 and40 GeV/c.Themeasuredratiois0.022±0.003(stat)±0.005(syst) for pT>20 GeV/c. Theabsolutecrosssectionsσ_ttbband σttjjare alsomeasured.Themeasuredratiofor pT>40 GeV/cis compatiblewithatheoreticalquantumchromodynamicscalculationatnext-to-leadingorder.

2015CERNforthebenefitoftheCMSCollaboration.PublishedbyElsevierB.V.Thisisanopenaccess articleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

With the observation of a new boson at a mass around 125 GeV/c2 _{[1–3] whoseproperties are consistent with those of} thestandardmodel(SM)HiggsbosonH[4–9],theSMappearsto becomplete. Oneofthemostsensitivechannelsin thediscovery oftheHiggsboson,H→γ γ,isexpectedtohavetopquark loops both inthe productionanddecay ofthe Higgsboson in theSM. Hence,itisimportantto determinethecouplingsofthenew bo-son to fermions, especially to the top quark. In the SM, one of themostpromisingchannelsforadirectmeasurementofthetop quarkYukawacouplingistheproductionoftheHiggsbosonin as-sociationwithatt pair(ttH),wheretheHiggsbosondecaystobb, thusleadingtoattbb finalstate.

Theexpectedquantumchromodynamics(QCD)crosssectionfor ttH productioninppcollisions at√s₌8 TeV, calculatedto next-to-leadingorder(NLO),is0.128+0.005

−0.012(scale)±0.010 pb(PDF+αS) [10], where the uncertainty labelled “scale” refers to the un-certainty from the factorization and renormalization scales (µF

andµR),andtheuncertaintylabelled“PDF+αS”comesfromthe uncertainties in the parton distribution functions(PDFs) andthe strong coupling constant αS. This final state, which has not yet been observed, has an irreducible nonresonant background from theproduction ofa top quark pairin associationwitha b quark pair. Calculations of the inclusive production cross section for tt events with additional jets have been performed to NLO

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

sion[11–16].Foraproton–protoncentre-of-massenergyof8 TeV, the predictions forthe production of a top quark pair with two additional jetsttjj and withtwo additional bquark jetsttbb are

σ_ttjj=21.0±2.9(scale) pb and σ_ttbb=0.23±0.05(scale) pb, re-spectively[16].Inthiscalculation,theadditionaljetsarerequired to have transverse momenta pT>40 GeV/c and absolute pseu-dorapidity |η|<2.5, while for the ttH production value quoted above, no such requirements are applied to the decay products of theHiggs boson.The dominant uncertainties inthese calcula-tionsarefromthefactorizationandrenormalizationscales[17,18] causedbythepresenceoftwoverydifferentscalesinthisprocess, the topquark massandthejet pT. Therefore,experimental mea-surementsof σ_ttjj and σ_ttbb productioncanprovideagoodtestof NLO QCDtheoryandimportantinput aboutthemainbackground inthesearchforthettH process.

InthisLetter,thefirstmeasurementsofthecrosssections σ_ttbb

and σttjj andtheir ratioare presented.The analyzeddatasample ofppcollisionsatacentre-of-massenergyof8 TeVwascollected withtheCMSexperimentattheCERNLHCandcorrespondstoan integratedluminosityof19.6±0.5 fb−1[19].Theprimary motiva-tion formeasuringthecross sectionratioisthat manykinematic distributionsareexpectedtobesimilarforttbb andttjj,leadingto reducedsystematicuncertaintiesintheratio.

2. CMSdetectorandeventreconstruction

The central feature of the CMS apparatus is a superconduct-ing solenoidof6 m internal diameter,providing amagneticfield of 3.8 T. Withinthe superconducting solenoid volume are a

sili-http://dx.doi.org/10.1016/j.physletb.2015.04.060

0370-2693/2015CERNforthebenefitoftheCMSCollaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

conpixelandstriptracker,aleadtungstatecrystalelectromagnetic calorimeter(ECAL), anda brass andscintillator hadron calorime-ter(HCAL),each composed of a barreland two endcapsections. Muons are measured in gas-ionization detectors embedded in thesteelflux-returnyokeoutsidethesolenoid. Extensiveforward calorimetrycomplementsthecoverageprovidedbythebarreland endcapdetectors.

The particle-flow event algorithm reconstructs and identifies eachsingleparticlewithanoptimizedcombinationofall subdetec-torinformation[20,21].Theenergyofphotonsisdirectlyobtained from the ECAL measurement, corrected for zero-suppression ef-fects. The energy ofelectrons is determined froma combination of the electron momentum at the primary interaction vertex as determinedby thetracker, theenergyofthe correspondingECAL cluster,andtheenergysumofallbremsstrahlungphotonsspatially compatiblewithoriginatingfromtheelectrontrack.Theenergyof muonsisobtainedfromthecurvatureofthecorrespondingtrack. The energy of charged hadrons is determined from a combina-tionoftheirmomentummeasuredinthetrackerandthematching ECALandHCALenergydeposits,correctedforzero-suppression ef-fectsandfortheresponsefunctionofthecalorimeterstohadronic showers. Finally, the energy ofneutral hadrons is obtainedfrom thecorrespondingcorrectedECALandHCALenergy.

Jetmomentumisdeterminedasthevectorialsumofall parti-clemomentainthejet,andisfoundfromsimulationtobewithin 5to10%ofthetruemomentumoverthewhole pT spectrumand detectoracceptance.Anoffsetcorrectionisappliedtotakeinto ac-counttheextraenergyclusteredinjetsduetoadditionalproton– proton interactions within the same bunch crossing (pileup). Jet energycorrectionsarederivedfromsimulation,andareconfirmed within situmeasurements with the energybalance of dijetand photon+jet events.Additionalselectioncriteriaareappliedtoeach eventtoremovespuriousjet-likefeaturesoriginatingfromisolated noisepatternsincertainHCALregions.

AmoredetaileddescriptionoftheCMSdetector,togetherwith adefinitionofthecoordinatesystemusedandtherelevant kine-maticvariables,canbefoundinRef.[22].

3. Simulationanddefinitionofsignalevents

MonteCarlo(MC)simulateddatasamplesforthett signalare generatedbythe MadGraph (v.5.1.3.30)eventgenerator[23]with matrixelements (ME)atleadingorder,allowinguptothree addi-tionalpartonsincludingbquarks.Thegeneratedeventsare inter-facedwith pythia (v.6.426)[24] toprovidetheshoweringofthe partons,andtoperformthematchingofthesoftradiationwiththe contributionsfromtheME.The τ leptondecaysarehandledwith tauola (v. 2.75) [25]. The powheg (v. 1.0) generator [26–28] at NLO,interfacedwith pythia,isusedforcross-checksand system-aticstudies.AZ/γ∗+jets backgroundsampleissimulatedin Mad-Graph.ThettH processismodelledusing pythia.Theelectroweak productionofsingle top quarks(pp→tW and pp→tW) is sim-ulated in powheg with an approximate next-to-next-to-leading-order(NNLO) crosssection calculation[29].The CTEQ6L1[30]set ofPDFsisusedforthe MadGraph and pythia samples,whilethe CTEQ6M [31] set is used for the powheg samples. The CMS de-tectorresponseissimulatedusing Geant4 (v. 9.4)[32].Thepileup distributionusedinthe simulationisweighted tomatchthe one observedindata.

Measurements are reported for two different regions of the phase space: a visible phase space and the full phase space. In the visible phase space, all ttbb final state particles (ttbb→ bW+_bW−_bb→b'+_νb'−_{νbb)excepttheneutrinos,i.e.thecharged} leptonsandjetsoriginatingfromthedecaysofthetop quarks,as well as the two additional b quark jets (“b jets”), are required

to be within the same experimentally accessible kinematic re-gion.Simulatedttbb eventsaredefinedto beinthevisiblephase spaceandarecategorizedascomingfromthettjj processifthey contain, at the generator level, at least four particle-level jets, includingatleasttwojetsoriginatingfrombquarks,andtwo lep-tons (ttjj→bW+_bW−_jj→b'+_νb'−_{νjj). Each lepton must have} pT>20 GeV/c,|η|<2.4,andcomefromthedecayofaW boson fromone ofthe top quarks.Electronsor muonsoriginatingfrom theleptonic decaysof τ leptons producedinW→τ ν decaysare included. Jets which are within )R=!)φ2+ )η2<0.5 of an identified electronor muonare removed, where)φ and)ηare thedifferencesinazimuthalangleandpseudorapiditybetweenthe directionsofthejetsandthelepton.Theparticle-leveljetsare ob-tainedbycombiningallfinal-stateparticles,excludingneutrinos,at thegeneratorlevelwithananti-kT clusteringalgorithm [33]with a distance parameter of0.5 andare required to satisfy |η|<2.5 and pT>20 GeV/c, whichis lowerthan the reconstructed mini-mumjet pT,asdescribedbelow.Thebandcquarkjets(“cjets”) are identified by thepresence ofcorresponding hadrons contain-inga borcquarkamongtheancestorsofthejetconstituents.In thecasewheretwojetscontainthedecayproductsofthesameb hadron,thejetwiththehigherpT isselectedasthebjet.Whena bhadronissuccessfullymatched,thecquarksarenotconsidered. Thettjj sampleiscomposedoffourcomponents,distinguished bytheflavourofthetwojetsinadditiontothetwobjetsrequired fromthetopquarkdecays.Thefourcomponentsarethettbb final state withtwo bjets, the ttbj finalstate withone bjet andone lighter-flavourjet,thettcc finalstatewithtwocjets, andthettLF final state withtwo light-flavour jets(froma gluon or u,d, ors quark)oronelight-flavourjetandonecjet.Thettbj finalstate is mainlyfromthemergingoftwo bjetsorthelossofoneoftheb jetscausedbytheacceptancerequirements.Efficiencycorrections to the measurementfor thevisible phase spaceare mainly from detectoreffects. The resultsfor thevisible phase spaceare com-paredwiththosefromMCsimulations.

The goal of the full phase space result is to provide a com-parisonto theoreticalcalculations, whicharegenerallyperformed atthe partonlevel. Toobtain a full phase spaceMC sample, the jetreconstruction isperformedonthepartons(gluons,aswellas quarks lighter than top) before hadronization, as well as τ lep-tonsthat decayhadronically.As thefull hadronization anddecay chainisknown,only τ leptonsthatdecayhadronicallyandpartons thatleadtohadronsareincluded.Thejetreconstructionalgorithm is the same asfor the visible phase space. Following the jet re-construction, b jets are identified with a )R<0.5 requirement betweenthebquarksandparton-leveljets,where)φ and)ηare the azimuthal angle and pseudorapidity differences, respectively, betweenthedirectionsofthebquarkandtheparton-leveljet.For comparisonwiththeoreticalpredictions[16],resultsarequotedfor two different jet pT thresholds of pT>20 and >40 GeV/c on the jets not arising from top quark decays. To clarify the phase space definition,the objects on which the selectionsare applied arelistedinTable 1.

4. Eventselectionandbackgroundestimation

Theeventsarerecordedusingdileptontriggerswith asymmet-ric thresholds of 8 and 17 GeV/c on the transverse momentum oftheleptons.Jetsare reconstructedusingthesamealgorithmas in thesimulations. The leptons andall charged hadronsthat are associatedwithjetsarerequiredtooriginatefromtheprimary ver-tex,definedasthevertexwiththehighest"p2

T ofits associated tracks.Muoncandidatesare reconstructedbycombining informa-tion from the silicon tracker and the muon system [34]. Muon candidatesarefurtherrequiredtohaveaminimumnumberofhits

Table 1

Theobjectsusedtodefinethevisibleandfullphasespacearelisted.Detailsoftheparton- andparticle-level definitionsaredescribedinthetext.Thesymbolt denotesatopquark.

Phase Space (PS) Parton level Particle level

Visible PS – 4 (b) jets and 2 leptons (e,µ)

Full PS t, t and 2 (b) jets (not from t or t) – inthesilicontrackerandtohaveahigh-qualityglobalfitincluding

aminimumnumberofhitsinthe muondetector.Electron candi-datesarereconstructedbycombiningatrackwithenergydeposits intheECAL,takingintoaccountbremsstrahlungphotons. Require-mentson electronidentificationvariablesbasedon showershape andtrack-clustermatchingareappliedtothereconstructed candi-dates[35,36].MuonsandelectronsmusthavepT>20 GeV/c and |η|<2.4.

Toreducethebackgroundcontributions ofmuonsorelectrons fromsemileptonic heavy-flavour decays,relative isolation criteria areapplied.Therelativeisolation parameter,Irel,isdefinedasthe ratioofthesumofthetransversemomentaofallobjectsinacone of)R<0.3 aroundthelepton pT directiontothelepton pT.The objects considered are the charged hadrons associated with the primaryvertexaswellastheneutralhadronsandphotons,whose energiesarecorrectedfortheenergyfrompileup.Thus,

Irel= "_pcharged hadron T + "_{pneutral hadron} T + "_pphoton T plepton_T . (1)

Leptons are required to have Irel<0.15. The efficiencies forthe aboveleptonidentificationrequirementsaremeasuredusingZ bo-son candidates in data and are found to be consistent with the valuesfromthesimulation.Theresidualdifferencesareappliedas acorrectiontothesimulation.

The event selection requires the presence of two isolated opposite-signleptons of invariant mass M''>12 GeV/c2.Lepton pairs ofthe sameflavour (e+e−, µ+µ−) are rejectedif their in-variantmassiswithin15 GeV/c2oftheZ bosonmass.Themissing transverseenergy(Emiss

T )isdefinedasthemagnitudeofthe vecto-rialsumof thetransversemomenta ofall reconstructedparticles in theevent [37]. In the same-flavour channels, remaining back-groundsfrom Z/γ∗+jets processesaresuppressedbydemanding Emiss

T >30 GeV. Forthe e±µ∓ channel, no EmissT requirement is applied.

Fourormorereconstructedjetsarerequiredwith|η|<2.5 and pT>30 GeV/c, ofwhich at least two jetsmust be identified as bjets,usingacombinedsecondaryvertex(CSV) algorithm,which combinessecondary vertexinformationwithlifetimeinformation ofsingletracks toproduceab-taggingdiscriminator [38].Atight b-taggingrequirementonthisdiscriminatorisapplied,whichhas anefficiencyofabout45%forbjetsandamisidentification proba-bilityof0.1%forlight-flavourjets.

Differences in the b-tagging efficiencies between data and simulation [38] are accounted for by reweighting the shape of the CSV b-tagging discriminator distribution in the simulation to match that in the data. Data/MC scale factors for this pT -and η-dependent correction are derived separately forlight- and heavy-flavourjets.Thescalefactorforcjetsisnotmeasured, ow-ingtothelimitedamountofdata,andissettounity.Light-flavour scale factors are determined from a control sample enriched in events witha Z boson and exactly two jets. Heavy-flavour scale factors are derived from a tt enriched sample with exactly two jets,excludingZ→ ''events.

The backgroundcontributions arising from Z/γ∗+jets events is estimatedin data usingthe number ofevents having a dilep-toninvariant massof76<M''<106 GeV/c2,scaledby theratio

Fig. 1. Normalizeddistributionsofthebjetdiscriminatorforthethird(top)and

fourth(bottom)jetsinanevent,sortedindecreasingorderofb-tagging discrim-inatorvalue,afterthefulleventselection.ThehistogramsareobtainedfromMC simulationandareseparatedaccordingtojetflavour.

of eventsthat fail andpassthis selectionin theDrell–Yan simu-lation[39,40].Themultijetanddibosonbackgroundcontributions arenegligibleafterthefulleventselection.

5. Measurement

Afterthefull eventselection,the threedileptoncategoriesee,

µµ,and eµ are combined, andthe ratio of the numberof ttbb events tottjj eventsis obtainedfromthedataby fittingthe CSV b-taggingdiscriminator distributions.Thedistributions ofthe dis-criminatorfromsimulationforthethirdandfourthjetsin decreas-ingorderoftheb-taggingdiscriminator,i.e.forthetwoadditional jetsnotidentifiedascomingfromthetopquarkdecays,areshown in Fig. 1. The third and fourth jets from ttjj events tend to be light-flavourjets,whiletheseareheavy-flavourjetsforttbb events. Thesetwodistributions areusedtoseparatettbb fromother pro-cesses.

Fig. 2 shows the b-tagging discriminator distributions of the thirdandfourthjetsintheeventsfromdataandsimulation,where the simulation histogramshavebeen scaled tothe fit result. The

Fig. 2. Distributionsofbjetdiscriminatorforthethird(top)andfourth(bottom) jetsineventsindecreasingorderofb-taggingdiscriminatorvalue,afterthe full eventselection.PointsarefromdataandstackedhistogramsfromMCsimulation usingresultsfromthefittodata.Theratioofthenumberofdataeventstothe totalnumberofMCeventsafterthefitisshowninthelowerpanels.

fitisperformedonbothdistributionssimultaneously,andcontains twofreeparameters,anoverallnormalizationandtheratioofthe numberof ttbb events to ttjj events.The ttcc and ttLF contribu-tionsarecombined,andtheratioofthettbb tottbj contributions isconstrainedusingthepredictionsfromtheMCsimulation. Addi-tionally,thebackgroundcontributions fromsingle topproduction andfromtt eventsthat fail thevisiblephase spacerequirements (labelled “tt other”) are scaled by the normalization parameter. ThecontributionfromZ/γ∗+jets isfixedfromdata,asdescribed above.Nuisanceparametersareusedtoaccountforthe uncertain-tiesinthebackgroundcontributions.

The b-tagged jet multiplicity distribution in Fig. 3 shows the comparisonbetween data and the MC simulation, scaled by the fitresultstothedata.Theresults,whichincludethe requirement offour jets butnot the b-taggingrequirement, indicate that the fitisagoodmatchtothe data,asmadeclearinthelower panel showingthedata/MCratio.

Table 2givesthepredictednumberofeventsforeach physics processandforeachdileptoncategoryafterfittingtothedata,as wellasacomparisonofthetotalnumberofeventsexpectedfrom the simulation and observed in data. Since the full event selec-tionrequiresatleasttwob-taggedjets, whichisusually satisfied bytt events,only3%oftheeventsarefromnon-tt processes.The expectedcontributionfromthettH processis12events.This con-tributionisnotsubtractedfromthedata.

The ratio of the number of ttbb to ttjj events at the recon-structionlevel obtainedfrom the fit iscorrected for the ratioof efficiencies. The eventselection efficiencies, definedas the

num-Fig. 3. Distributionofbjetmultiplicityafterthefour-jetrequirementbutwithout

theb-taggingrequirement.PointsarefromdataandstackedhistogramsfromMC simulationusingresultsfromthefittodata.Theratioofthenumberofdataevents tothetotalnumberofMCeventsafterthefitisshowninthelowerpanel. Table 2

Thenumberofeventsforeachphysicsprocessandforeachdileptoncategoryafter fittingtothedata,theirtotal,andtheobservedtotalnumberofevents.Theresults areafterthefinaleventselection.TheZ/γ∗_{→ ''}uncertaintyisfromdata,while allotheruncertaintiesincludeonlythestatisticaluncertaintiesintheMCsamples.

Final state e+_e− _µ+_µ− _e±_µ∓ _All

ttbb 18 26 61 105±2 ttbj 35 48 109 191±3 ttcc 13 19 45 78±2 ttLF 249 347 840 1438±9 tt others 21 25 64 109±3 Single top 7.4 11 24 43±5 Z/γ_{∗ → ''} 5.7 5.4 3.1 14±7 Total 350 483 1149 1983±13 Data 367 506 1145 2018

berofttbb andttjj eventsafterthefulleventselectiondividedby thenumberofeventsinthecorrespondingvisiblephasespaceare 18.7%and7.2%,respectively.Thettbb andttjj crosssectionsinthe visiblephasespacearemeasuredusing σvisible=N/(+L),whereL istheintegratedluminosity,N isthenumberofobservedevents, and + is the efficiency foreach process. However, the NLO the-oretical calculation is based on parton-level jets being clustered withpartonsbeforehadronizationinthefullphasespace.Forthe purpose of comparing with the theoretical prediction, the cross sections in the full phase space are extrapolated fromthe cross sectionsin thevisible phase spaceusing σfull=σvisible/A, where A is the acceptance.The acceptancesfor extending ttbb and ttjj to the full phase space based on the MadGraph simulation are 2.6%and2.4%,respectively,includingthett todileptonbranching fraction,calculatedusingtheleptonicbranchingfractionoftheW boson[41].Theacceptanceisdefinedasthenumberofeventsin the corresponding visible phase space divided by the number of eventsinthefullphasespace.

6. Estimationofsystematicuncertainties

Thesystematicuncertainties aredeterminedseparately forthe ttbb andttjj crosssectionsandtheir ratio.Intheratio,many sys-tematiceffectscancel,specificallynormalizationuncertaintiessuch astheonesrelatedtothemeasurementoftheintegrated luminos-ityandtheleptonidentificationincludingtriggerefficiencies,since theyarecommontobothprocesses.Thevarioussystematic uncer-taintiesinthemeasuredvaluesareshowninTable 3forthevisible

Table 3

Summaryofthesystematicuncertaintiesfromvarioussourcescontributingtoσ_ttbb, σ_ttjj,andtheratioσ_ttbb/σ_ttjjforajet pTthresholdofpT>20 GeV/cinthevisible phasespace.

Source σ_ttbb(%) σttjj(%) σσttbbttjj (%)

Pileup 1.0 1.0 1.0

JES & JER 11 8.0 5.0

b tag (b quark flavour) 15 <0.1 15 b tag (c quark flavour) 4.0 <0.1 4.0 b tag (light flavour) 7.0 <0.1 7.0 Ratio of ttbb and ttbj 9.0 <0.1 9.0

Bkgnd modelling 1.0 1.0 1.0

ttcc fraction in the fit 4.2 0.2 4.0

Lepton identification 4.0 4.0 – MC generator 3.0 3.0 3.0 Scale (µFandµR) 8.0 3.0 6.0 PS matching 12 5.0 3.0 PDF 4.0 4.0 <0.1 Eff. (ttcc fraction) – 1.6 1.6 Luminosity 2.6 2.6 – Total uncertainty 28 12 22

phasespaceandajetpTthresholdof20 GeV/c,includingthe lumi-nosityuncertainty [19]andlepton identification[42],whichonly affecttheabsolutecrosssectionmeasurements.Thesystematic un-certainty in the lepton identification is assessed using the scale factorobtainedfromZ boson candidates andalsotakinginto ac-countthedifferentphasespacebetweenZ bosonandtt events.

Thesystematicuncertaintiesassociatedwiththeb-tagging dis-criminatorscalefactorsforbjetsandlight-flavourjetsarestudied separately,varying their values within their uncertainties. The b-flavour scale factors are obtained using tt enriched events, and their dominant uncertaintycomes fromthe contamination when one ofthebjetsisnot reconstructed [43] (indicatedas“b quark flavour”inTable 3). Thecjet scalefactorisassumedto beunity with an uncertainty twice as large as the b-tagging scale fac-tor[38](indicatedas“cquarkflavour”inTable 3).Thelight-flavour jet scale factors are determined from Z boson enriched events. Theiruncertaintyarisesbecausethecontributionfromthe Z+bb process inthiscontrol sample isnot well modelled(indicated as “light flavour”inTable 3). Theb-taggingdiscriminator can be af-fectedbythejet energyscale (JES)variations. Thesystematic un-certainty in the jet energyscale [44] is obtained by varying the jet energy scale factor by one standard deviation foreach quark flavour. The uncertainty in the jet energy resolution (JER) is as-sessedbysmearingthesimulatedjetenergyresolutionby10%on average,takingintoaccountthe η dependence[44].

The uncertaintyarising fromconstraining theratio ofthe ttbj tottbb contributionsinthefittomatchtheMCpredictionis eval-uated by comparing the result withand without the constraint. The numberof pileup interactions in data isestimated fromthe measured bunch-to-bunchinstantaneous luminosity andthetotal inelasticcross section. The systematicuncertaintyin the number ofpileupevents isestimatedby conservatively varyingthiscross section by 5% to cover all the uncertainties in the modellingof the pileup physics. The contributions from Drell–Yan and single topquarkprocessesaresmall, andtheshapesofthedistributions fromthesebackgroundsaresimilartothoseofthettLF component. Therefore,thesebackgrounds donot affectthemeasurement sig-nificantly.Fortheefficiencyofttjj events,theuncertaintyowingto theheavy-flavourfractionisestimatedbyvaryingthecontribution by50%.Anuncertaintytoaccountforthevariationofthettcc frac-tioninthefitisalsoassignedbyvaryingthecontributionby 50%. Thisvariationischosen becausethetheoreticaluncertaintyinthe ttjj crosssection is lessthan50%, andthe fittedttcc fraction

re-mains within 50% of the input value when fitting with the ttcc contributionasafreeparameter.

The dependence of the correction factor forthe particle level ontheassumptions madeintheMC simulationisanothersource ofsystematicuncertainty:thegenerators MadGraph and powheg arecomparedandthedifferenceintheefficiencyratioistakenas the systematic uncertainty. The uncertainties from the factoriza-tion/renormalization scalesandthe matchingscalethat separates jetsfromMEandfrompartonshowersin MadGraph areestimated byvaryingthescalesafactoroftwoupanddownwithrespectto theirreferencevalues.TheuncertaintiesinthePDFsareaccounted forbyfollowingthePDF4LHCprescription[45].

The total systematic uncertainty in the cross section ratio is 22%, with the dominant contributions from the b-tagging ef-ficiency and the misidentification of light-flavoured partons, fol-lowedbytherenormalization/factorizationandmatchingscale sys-tematicuncertainties.

Theuncertaintyin σttjj issignificantlysmallerthanthatin σttbb since themeasurement ofthelatterrequires theidentificationof multiplebjets.Theuncertaintyin σ_ttbb islargerthanthat forthe cross section ratio since uncertainties that are commonbetween ttbb andttjj,such asthejetenergyscaleuncertainty,partiallyor completelycancelintheratio.

The systematic uncertainties in the measurements with a pT thresholdof40 GeV/c arefound tobe verysimilar tothosewith a 20 GeV/c threshold. Theuncertainty fromthefactorizationand renormalization scales for the higher-pT threshold of 40 GeV/c cannot be accurately determined owing to the statistical uncer-tainties in the MC sample. Thus, the pT>40 GeV/c threshold measurements usethesamescale(µF and µR)systematic

uncer-taintiesasthosefoundforthe pT>20 GeV/cthresholdresults. Inextrapolatingthemeasurementsfromthevisiblephasespace to the full phase space, the systematic uncertaintyin the accep-tanceisincluded.TheeffectoftheMCmodellingoftheacceptance is estimated by comparing the results between MadGraph and powheg. Thisuncertaintyequals 5% foreach ofthe crosssection measurementsand2%forthecrosssectionratio.

7. Results

Aftercorrectingfortheefficiencyratioandtakingintoaccount the systematic uncertainties, the cross section ratio σ_ttbb/σ_ttjj is measuredinthevisiblephasespacefromafittothemeasuredCSV b-tagging discriminator distributions shown in Fig. 2. The mea-suredcrosssectionratiointhevisiblephasespaceforeventswith particle-leveljetsandaminimumjetpTof20 GeV/c is

σ_ttbb/σ_ttjj=0.022±0.003(stat)±0.005(syst). (2)

This result isfor the visiblephase space, definedas events hav-ing twoleptons withpT>20 GeV/c and|η|<2.4,plus fourjets, including twob jetswith pT>20 GeV/c and|η|<2.5.The pre-dicted value from both MadGraph and powheg is found to be 0.016±0.002, where the MC uncertainty is the sum in quadra-tureofthestatisticaluncertaintyandthesystematicuncertainties from the factorization/renormalization and the matching scales. The measured cross sectionsare presented inTable 4.When the ttH contribution issubtracted fromthedata,the ratioisreduced by only 4%, much less than the overall uncertainty. Therefore, compared to the uncertainties, the contribution from ttH can be considered negligible.Themeasuredfull phasespaceratiowitha minimum pT of20 GeV/c forparton-leveljetsisconsistentwithin theuncertaintieswiththeresultinthevisiblephasespace.

A NLOtheoretical QCD calculationis availableforparton-level jets witha pT>40 GeV/c threshold[16].The NLO crosssection

Table 4

Themeasuredcrosssectionsσ_ttbbandσttjjandtheirratioaregivenforthevisiblephasespace(PS)definedastwoleptons withpT>20 GeV/cand|η_|<2.4 plusfourjets,includingtwobjetswithpT>20 GeV/cand|η_|<2.5,andthefullphase space,correctedforacceptanceandbranchingfractions.ThefullphasespaceresultsaregivenforjetthresholdsofpT>20 and40 GeV/c.Theuncertaintiesshownarestatisticalandsystematic,respectively.ThepredictionsofaNLOtheoretical calculationforthefullphasespaceandpT>40 GeV/carealsogiven[16].

Phase Space (PS) σ_ttbb[pb] σ_ttjj[pb] σ_ttbb/σ_ttjj Visible PS (particle) Jet pT>20 GeV/c 0.029±0.003±0.008 1.28±0.03±0.15 0.022±0.003±0.005 Full PS (parton) Jet pT>20 GeV/c 1.11±0.11±0.31 52.1±1.0±6.8 0.021±0.003±0.005 Jet pT>40 GeV/c 0.36±0.08±0.10 16.1±0.7±2.1 0.022±0.004±0.005 NLO calculation Jet pT>40 GeV/c 0.23±0.05 21.0±2.9 0.011±0.003 valuesfor σ_ttbb, σttjj,andtheratio σttbb/σttjj aregiveninTable 4.

To compare with this theoretical prediction, the analysis is re-peatedforajetthresholdofpT>40 GeV/c.Correspondinglywith a higher jet pT threshold in the event selection, 24 ttbb events and478ttjj eventsremain afterthefull eventselection,withthe acceptance(includingtheeventselection efficiency)of0.34% and 0.15%, respectively. The measured cross section ratio in the full phasespacewiththe pT>40 GeV/c thresholdis

σ_ttbb/σ_ttjj=0.022±0.004(stat)±0.005(syst). (3)

ThecrosssectionsinthefullphasespaceforthispT thresholdare summarizedinTable 4.Themeasuredcrosssectionratioishigher, butcompatiblewithin 1.6standarddeviationswiththeprediction fromtheNLOcalculationof0.011±0.003.

8. Summary

Ameasurement ofthe cross section ratio σ_ttbb/σttjj hasbeen presentedbytheCMSexperiment,usingadatasampleofpp colli-sionsatacentre-of-massenergyof8 TeV,correspondingtoan in-tegratedluminosityof19.6 fb−1_{.Theindividualcrosssections σ}

ttjj and σ_ttbb havealsobeen determined.Thecross sectionratio was measuredinavisiblephasespaceregionusingthedileptondecay modeoftt eventsandcorrectedtotheparticlelevel,corresponding tothedetectoracceptance.Themeasuredcrosssectionratiointhe visiblephasespaceis σ_ttbb/σttjj=0.022±0.003(stat)±0.005(syst) witha minimum pT forthe particle-level jets of 20 GeV/c. The cross section ratio has also been measured in the full phase space with minimum parton-jet pT thresholds of pT >20 and >40 GeV/c inorder to compare witha NLO QCD calculation of thecrosssectionratio.Themeasurementiscompatiblewithin1.6 standarddeviationswiththetheoretical prediction.These arethe firstmeasurements ofthecross sections σ_ttbb and σttjj,andtheir ratio.Theresultwillprovideimportantinformationaboutthemain backgroundinthesearchforttH andasafigureofmerit for test-ingthevalidityofNLOQCDcalculations.

Acknowledgements

WecongratulateourcolleaguesintheCERNaccelerator depart-ments for the excellent performance of the LHC and thank the technicalandadministrativestaffs atCERN andatother CMS in-stitutes for their contributions to the success of the CMS effort. Inaddition,wegratefullyacknowledgethecomputingcentresand personneloftheWorldwideLHCComputingGridfordeliveringso effectivelythecomputinginfrastructure essential toour analyses. Finally, we acknowledge the enduring support for the construc-tionandoperationofthe 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);Academy ofFinland,MEC,andHIP (Finland); CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH (Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); MSIP and NRF (Republic of Korea); LAS (Lithuania); MOE and UM (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);TUBITAKandTAEK (Turkey);NASUandSFFR(Ukraine); STFC(United Kingdom);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); theMinistry ofEducation, YouthandSports (MEYS) of theCzechRepublic;theCouncilofScienceandIndustrialResearch, India; the HOMING PLUS programme of Foundation For Polish Science, cofinanced from European Union, Regional Development Fund;the CompagniadiSan Paolo(Torino); the Consorzioper la Fisica (Trieste); MIURproject20108T4XTM (Italy);the Thalisand Aristeia programmes cofinanced by EU-ESF andthe 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. Fabjan1_{,M. Friedl, R. Frühwirth}1_{,V.M. Ghete, C. Hartl,}

N. Hörmann, J. Hrubec, M. Jeitler1, W. Kiesenhofer,V. Knünz, M. Krammer1, I. Krätschmer,D. Liko, I. Mikulec,D. Rabady2, B. Rahbaran, H. Rohringer,R. Schöfbeck, J. Strauss,A. Taurok,

W. Treberer-Treberspurg,W. Waltenberger, C.-E. Wulz1

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,B. Roland, 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. Perniè2_{,T. Reis, T. Seva, L. Thomas, C. Vander Velde, P. Vanlaer,J. Wang}

UniversitéLibredeBruxelles,Bruxelles,Belgium

V. Adler,K. Beernaert, L. Benucci, A. Cimmino,S. Costantini, S. Crucy, S. Dildick,A. Fagot, G. Garcia, 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. Beluffi3_{, G. Bruno,R. Castello, A. Caudron, L. Ceard, G.G. Da Silveira,C. Delaere,}

T. du Pree,D. Favart,L. Forthomme, A. Giammanco4,J. Hollar, P. Jez, M. Komm,V. Lemaitre, C. Nuttens, D. Pagano,L. Perrini, A. Pin, K. Piotrzkowski, A. Popov5,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. Chinellato6, 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 Manganote6, A. Vilela Pereira

UniversidadedoEstadodoRiodeJaneiro,RiodeJaneiro,Brazil

C.A. Bernardesb_{, S. Dogra}a_{,T.R. Fernandez Perez Tomei}a_{, E.M. Gregores}b_{, P.G. Mercadante}b_,

S.F. Novaesa, Sandra S. Padulaa a_{UniversidadeEstadualPaulista,SãoPaulo,Brazil} b_{UniversidadeFederaldoABC,SãoPaulo,Brazil}

A. Aleksandrov, V. Genchev2, 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

InstituteofHighEnergyPhysics,Beijing,China

C. Asawatangtrakuldee, Y. Ban, Y. Guo, Q. Li, W. Li, 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

InstituteRudjerBoskovic,Zagreb,Croatia

A. Attikis, G. Mavromanolakis, J. Mousa,C. Nicolaou, F. Ptochos, P.A. Razis

UniversityofCyprus,Nicosia,Cyprus

M. Bodlak,M. Finger, M. Finger Jr.8

CharlesUniversity,Prague,CzechRepublic

Y. Assran9, A. Ellithi Kamel10,M.A. Mahmoud11, A. Radi12,13

AcademyofScientificResearchandTechnologyoftheArabRepublicofEgypt,EgyptianNetworkofHighEnergyPhysics,Cairo,Egypt

M. Kadastik, M. Murumaa, M. Raidal, A. Tiko

NationalInstituteofChemicalPhysicsandBiophysics,Tallinn,Estonia

P. Eerola, G. Fedi,M. Voutilainen

DepartmentofPhysics,UniversityofHelsinki,Helsinki,Finland

J. Härkönen,V. Karimäki, R. Kinnunen, M.J. Kortelainen, T. Lampén, K. Lassila-Perini,S. Lehti, T. Lindén, P. Luukka, T. Mäenpää,T. Peltola, E. Tuominen, J. Tuominiemi,E. Tuovinen, L. Wendland

HelsinkiInstituteofPhysics,Helsinki,Finland

J. Talvitie, T. Tuuva

LappeenrantaUniversityofTechnology,Lappeenranta,Finland

M. Besancon, F. Couderc,M. Dejardin, D. Denegri, B. Fabbro,J.L. Faure, C. Favaro,F. Ferri, S. Ganjour, A. Givernaud, P. Gras, G. Hamel de Monchenault, P. Jarry,E. Locci, J. Malcles,J. Rander, A. Rosowsky, M. Titov

DSM/IRFU,CEA/Saclay,Gif-sur-Yvette,France

S. Baffioni,F. Beaudette, P. Busson, C. Charlot, T. Dahms, M. Dalchenko,L. Dobrzynski, N. Filipovic, A. Florent,R. Granier de Cassagnac, L. Mastrolorenzo, P. Miné, C. Mironov, I.N. Naranjo, M. Nguyen, C. Ochando, P. Paganini, S. Regnard, R. Salerno,J.B. Sauvan, Y. Sirois, C. Veelken, Y. Yilmaz, A. Zabi

LaboratoireLeprince-Ringuet,EcolePolytechnique,IN2P3-CNRS,Palaiseau,France

J.-L. Agram14_{, J. Andrea, A. Aubin, D. Bloch,J.-M. Brom, E.C. Chabert,C. Collard, E. Conte}14_,

InstitutPluridisciplinaireHubertCurien,UniversitédeStrasbourg,UniversitédeHauteAlsaceMulhouse,CNRS/IN2P3,Strasbourg,France

S. Gadrat

CentredeCalculdel’InstitutNationaldePhysiqueNucleaireetdePhysiquedesParticules,CNRS/IN2P3,Villeurbanne,France

S. Beauceron,N. Beaupere, G. Boudoul2_{,E. Bouvier, S. Brochet,C.A. Carrillo Montoya, J. Chasserat,}

R. Chierici,D. Contardo2, P. Depasse,H. El Mamouni, J. Fan, J. Fay, S. Gascon, M. Gouzevitch, B. Ille, T. Kurca, M. Lethuillier,L. Mirabito, S. Perries,J.D. Ruiz Alvarez, D. Sabes,L. Sgandurra, V. Sordini, M. Vander Donckt, P. Verdier,S. Viret, H. Xiao

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

Z. Tsamalaidze8

InstituteofHighEnergyPhysicsandInformatization,TbilisiStateUniversity,Tbilisi,Georgia

C. Autermann,S. Beranek, M. Bontenackels, M. Edelhoff,L. Feld, O. Hindrichs, K. Klein, A. Ostapchuk, A. Perieanu,F. Raupach, J. Sammet,S. Schael, H. Weber, B. Wittmer, V. Zhukov5

RWTHAachenUniversity,I.PhysikalischesInstitut,Aachen,Germany

M. Ata, M. Brodski,E. Dietz-Laursonn, D. Duchardt, M. Erdmann,R. Fischer, A. Güth, T. Hebbeker, C. Heidemann,K. Hoepfner,D. Klingebiel, S. Knutzen,P. Kreuzer, M. Merschmeyer,A. Meyer, P. Millet, M. Olschewski, K. Padeken,P. Papacz, H. Reithler,S.A. Schmitz,L. Sonnenschein, D. Teyssier,S. Thüer, M. Weber

RWTHAachenUniversity,III.PhysikalischesInstitutA,Aachen,Germany

V. Cherepanov, Y. Erdogan,G. Flügge, H. Geenen, M. Geisler, W. Haj Ahmad, A. Heister, F. Hoehle, B. Kargoll, T. Kress, Y. Kuessel,J. Lingemann2, A. Nowack,I.M. Nugent,L. Perchalla, O. Pooth,A. Stahl

RWTHAachenUniversity,III.PhysikalischesInstitutB,Aachen,Germany

I. Asin,N. Bartosik, J. Behr,W. Behrenhoff, U. Behrens,A.J. Bell, M. Bergholz15_{,A. Bethani, K. Borras,}

A. Burgmeier,A. Cakir,L. Calligaris, A. Campbell, S. Choudhury, F. Costanza, C. Diez Pardos,S. Dooling, T. Dorland,G. Eckerlin, D. Eckstein, T. Eichhorn, G. Flucke, J. Garay Garcia, A. Geiser, P. Gunnellini, J. Hauk, M. Hempel, D. Horton, H. Jung,A. Kalogeropoulos, M. Kasemann,P. Katsas, J. Kieseler,

C. Kleinwort,D. Krücker, W. Lange,J. Leonard, K. Lipka,A. Lobanov, W. Lohmann15_{, B. Lutz, R. Mankel,}

I. Marfin,I.-A. Melzer-Pellmann, A.B. Meyer, G. Mittag, J. Mnich, A. Mussgiller, S. Naumann-Emme, A. Nayak,O. Novgorodova, F. Nowak,E. Ntomari, H. Perrey,D. Pitzl, R. Placakyte, A. Raspereza, P.M. Ribeiro Cipriano,E. Ron, M.Ö. Sahin,J. Salfeld-Nebgen, P. Saxena, R. Schmidt15_,

T. Schoerner-Sadenius,M. Schröder, C. Seitz,S. Spannagel, A.D.R. Vargas Trevino, R. Walsh, C. Wissing

DeutschesElektronen-Synchrotron,Hamburg,Germany

M. Aldaya Martin,V. Blobel, M. Centis Vignali, A.R. Draeger, J. Erfle,E. Garutti, K. Goebel, M. Görner, J. Haller, M. Hoffmann,R.S. Höing, H. Kirschenmann,R. Klanner, R. Kogler, J. Lange,T. Lapsien, T. Lenz, I. Marchesini,J. Ott, T. Peiffer, N. Pietsch, J. Poehlsen,T. Poehlsen, D. Rathjens, C. Sander, H. Schettler, P. Schleper, E. Schlieckau, A. Schmidt,M. Seidel, V. Sola, H. Stadie,G. Steinbrück, D. Troendle,E. Usai, L. Vanelderen

UniversityofHamburg,Hamburg,Germany

C. Barth,C. Baus, J. Berger,C. Böser, E. Butz, T. Chwalek, W. De Boer,A. Descroix, A. Dierlamm, M. Feindt,F. Frensch, M. Giffels, F. Hartmann2, T. Hauth2,U. Husemann, I. Katkov5,A. Kornmayer2, E. Kuznetsova, P. Lobelle Pardo, M.U. Mozer,Th. Müller, A. Nürnberg, G. Quast, K. Rabbertz, F. Ratnikov, S. Röcker,H.J. Simonis, F.M. Stober,R. Ulrich, J. Wagner-Kuhr, S. Wayand, T. Weiler, R. Wolf

G. Anagnostou, G. Daskalakis,T. Geralis,V.A. Giakoumopoulou, A. Kyriakis, D. Loukas, A. Markou, C. Markou, A. Psallidas, I. Topsis-Giotis

InstituteofNuclearandParticlePhysics(INPP),NCSRDemokritos,AghiaParaskevi,Greece

A. Agapitos, S. Kesisoglou, A. Panagiotou, N. Saoulidou, E. Stiliaris

UniversityofAthens,Athens,Greece

X. Aslanoglou,I. Evangelou, G. Flouris, C. Foudas,P. Kokkas, N. Manthos, I. Papadopoulos, E. Paradas

UniversityofIoánnina,Ioánnina,Greece

G. Bencze,C. Hajdu, P. Hidas, D. Horvath16, F. Sikler,V. Veszpremi, G. Vesztergombi17,A.J. Zsigmond

WignerResearchCentreforPhysics,Budapest,Hungary

N. Beni, S. Czellar, J. Karancsi18,J. Molnar, J. Palinkas, Z. Szillasi

InstituteofNuclearResearchATOMKI,Debrecen,Hungary

P. Raics, Z.L. Trocsanyi,B. Ujvari

UniversityofDebrecen,Debrecen,Hungary

S.K. Swain

NationalInstituteofScienceEducationandResearch,Bhubaneswar,India

S.B. Beri, V. Bhatnagar,R. Gupta, U. Bhawandeep, A.K. Kalsi, M. Kaur, M. Mittal, N. Nishu, J.B. Singh

PanjabUniversity,Chandigarh,India

Ashok Kumar, Arun Kumar,S. Ahuja, A. Bhardwaj, B.C. Choudhary,A. Kumar, S. Malhotra,M. Naimuddin, K. Ranjan,V. Sharma

UniversityofDelhi,Delhi,India

S. Banerjee, S. Bhattacharya, K. Chatterjee,S. Dutta, B. Gomber, Sa. Jain, Sh. Jain,R. Khurana, A. Modak, S. Mukherjee,D. Roy, S. Sarkar, M. Sharan

SahaInstituteofNuclearPhysics,Kolkata,India

A. Abdulsalam, D. Dutta, S. Kailas,V. Kumar, A.K. Mohanty2_{, L.M. Pant, P. Shukla,A. Topkar} BhabhaAtomicResearchCentre,Mumbai,India

T. Aziz, S. Banerjee, S. Bhowmik19_{,R.M. Chatterjee, R.K. Dewanjee, S. Dugad, S. Ganguly,S. Ghosh,}

M. Guchait,A. Gurtu20,G. Kole, S. Kumar, M. Maity19, G. Majumder, K. Mazumdar,G.B. Mohanty, B. Parida,K. Sudhakar, N. Wickramage21

TataInstituteofFundamentalResearch,Mumbai,India

H. Bakhshiansohi,H. Behnamian, S.M. Etesami22_{, A. Fahim}23_{, R. Goldouzian, A. Jafari,M. Khakzad,}

M. Mohammadi Najafabadi, M. Naseri, S. Paktinat Mehdiabadi, F. Rezaei Hosseinabadi, B. Safarzadeh24, M. Zeinali

InstituteforResearchinFundamentalSciences(IPM),Tehran,Iran

M. Felcini,M. Grunewald

UniversityCollegeDublin,Dublin,Ireland

M. Abbresciaa,b_{, L. Barbone}a,b_{,C. Calabria}a,b_{,S.S. Chhibra}a,b_{,A. Colaleo}a_{,D. Creanza}a,c_,

A. Pompilia,b_{, G. Pugliese}a,c_{,R. Radogna}a,b,2_{,G. Selvaggi}a,b_{,L. Silvestris}a,2_{,G. Singh}a,b_{,R. Venditti}a,b_,

P. Verwilligena,G. Zitoa a_{INFNSezionediBari,Bari,Italy} b_{UniversitàdiBari,Bari,Italy} c_{PolitecnicodiBari,Bari,Italy}

G. Abbiendia_{,A.C. Benvenuti}a_{, D. Bonacorsi}a,b_{, S. Braibant-Giacomelli}a,b_{,L. Brigliadori}a,b_, R. Campaninia,b_{,P. Capiluppi}a,b_{,A. Castro}a,b_{, F.R. Cavallo}a_{,G. Codispoti}a,b_{, M. Cuffiani}a,b_,

G.M. Dallavallea_{,F. Fabbri}a_{,A. Fanfani}a,b_{,D. Fasanella}a,b_{, P. Giacomelli}a_{, C. Grandi}a_{, L. Guiducci}a,b_,

S. Marcellinia, G. Masettia,2_{, A. Montanari}a_{, F.L. Navarria}a,b_{,A. Perrotta}a_{, F. Primavera}a,b_{, A.M. Rossi}a,b_, T. Rovellia,b_{, G.P. Siroli}a,b_{,N. Tosi}a,b_{, R. Travaglini}a,b

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

S. Albergoa,b_{, G. Cappello}a_{,M. Chiorboli}a,b_{,S. Costa}a,b_{,F. Giordano}a,2_{,R. Potenza}a,b_{,A. Tricomi}a,b_, C. Tuvea,b

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

G. Barbaglia,V. Ciullia,b_{,C. Civinini}a_{, R. D’Alessandro}a,b_{, E. Focardi}a,b_{,E. Gallo}a_{, S. Gonzi}a,b_, V. Goria,b,2_{,P. Lenzi}a,b_{,M. Meschini}a_{,S. Paoletti}a_{, G. Sguazzoni}a_{, A. Tropiano}a,b

a_{INFNSezionediFirenze,Firenze,Italy} b_{UniversitàdiFirenze,Firenze,Italy}

L. Benussi,S. Bianco, F. Fabbri, D. Piccolo

INFNLaboratoriNazionalidiFrascati,Frascati,Italy

F. Ferroa, M. Lo Veterea,b_{, E. Robutti}a_{,S. Tosi}a,b a_{INFNSezionediGenova,Genova,Italy}

b_{UniversitàdiGenova,Genova,Italy}

M.E. Dinardoa,b_{, S. Fiorendi}a,b,2_{, S. Gennai}a,2_{,R. Gerosa}a,b,2_{, A. Ghezzi}a,b_{,P. Govoni}a,b_, M.T. Lucchinia,b,2_{, S. Malvezzi}a_{, R.A. Manzoni}a,b_{,A. Martelli}a,b_{, B. Marzocchi}a,b_{, D. Menasce}a_,

L. Moronia, M. Paganonia,b_{,D. Pedrini}a_{, S. Ragazzi}a,b_{, N. Redaelli}a_{,T. Tabarelli de Fatis}a,b a_{INFNSezionediMilano-Bicocca,Milano,Italy}

b_{UniversitàdiMilano-Bicocca,Milano,Italy}

S. Buontempoa_{, N. Cavallo}a,c_{,S. Di Guida}a,d,2_{, F. Fabozzi}a,c_{,A.O.M. Iorio}a,b_{, L. Lista}a_{,S. Meola}a,d,2_, M. Merolaa, P. Paoluccia,2

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

P. Azzia_{, N. Bacchetta}a_{, D. Bisello}a,b_{, A. Branca}a,b_{,R. Carlin}a,b_{,P. Checchia}a_{, M. Dall’Osso}a,b_{, T. Dorigo}a_,

F. Fanzagoa,M. Galantia,b_{, F. Gasparini}a,b_{, U. Gasparini}a,b_{, F. Gonella}a_{,A. Gozzelino}a_{,K. Kanishchev}a,c_, S. Lacapraraa_{,M. Margoni}a,b_{,A.T. Meneguzzo}a,b_{, J. Pazzini}a,b_{, N. Pozzobon}a,b_{,P. Ronchese}a,b_,

F. Simonettoa,b_{,E. Torassa}a_{,M. Tosi}a,b_{,P. Zotto}a,b_{,A. Zucchetta}a,b_{,G. Zumerle}a,b a_{INFNSezionediPadova,Padova,Italy}

b_{UniversitàdiPadova,Padova,Italy} c_{UniversitàdiTrento(Trento),Padova,Italy}

M. Gabusia,b_{,S.P. Ratti}a,b_{, C. Riccardi}a,b_{,P. Salvini}a_{,P. Vitulo}a,b

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

M. Biasinia,b_{, G.M. Bilei}a_{,D. Ciangottini}a,b_{,L. Fanò}a,b_{,P. Lariccia}a,b_{, G. Mantovani}a,b_{, M. Menichelli}a_,

F. Romeoa,b_{, A. Saha}a_{, A. Santocchia}a,b_{,A. Spiezia}a,b,2 a_{INFNSezionediPerugia,Perugia,Italy}

b_{UniversitàdiPerugia,Perugia,Italy}

K. Androsova,25_{,P. Azzurri}a_{,G. Bagliesi}a_{,J. Bernardini}a_{,T. Boccali}a_{,G. Broccolo}a,c_{,R. Castaldi}a_,

M.A. Cioccia,25_{, R. Dell’Orso}a_{,S. Donato}a,c_{,F. Fiori}a,c_{,L. Foà}a,c_{,A. Giassi}a_{, M.T. Grippo}a,25_{, F. Ligabue}a,c_, T. Lomtadzea_{, L. Martini}a,b_{, A. Messineo}a,b_{,C.S. Moon}a,26_{, F. Palla}a,2_{, A. Rizzi}a,b_{, A. Savoy-Navarro}a,27_, A.T. Serbana,P. Spagnoloa,P. Squillaciotia,25_{,R. Tenchini}a_{, G. Tonelli}a,b_{,A. Venturi}a_{, P.G. Verdini}a_, C. Vernieria,c,2

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

c_{ScuolaNormaleSuperiorediPisa,Pisa,Italy}

L. Baronea,b_{,F. Cavallari}a_{, G. D’imperio}a,b_{,D. Del Re}a,b_{,M. Diemoz}a_{, M. Grassi}a,b_{,C. Jorda}a_, E. Longoa,b_{, F. Margaroli}a,b_{,P. Meridiani}a_{,F. Micheli}a,b,2_{, S. Nourbakhsh}a,b_{,G. Organtini}a,b_,

R. Paramattia,S. Rahatloua,b_{,C. Rovelli}a_{, F. Santanastasio}a,b_{, L. Soffi}a,b,2_{,P. Traczyk}a,b a_{INFNSezionediRoma,Roma,Italy}

b_{UniversitàdiRoma,Roma,Italy}

N. Amapanea,b_{, R. Arcidiacono}a,c_{,S. Argiro}a,b,2_{, M. Arneodo}a,c_{, R. Bellan}a,b_{, C. Biino}a_{,N. Cartiglia}a_, S. Casassoa,b,2_{, M. Costa}a,b_{, A. Degano}a,b_{,N. Demaria}a_{,L. Finco}a,b_{,C. Mariotti}a_{, S. Maselli}a_,

E. Migliorea,b_{,V. Monaco}a,b_{,M. Musich}a_{,M.M. Obertino}a,c,2_{, G. Ortona}a,b_{,L. Pacher}a,b_{,N. Pastrone}a_,

M. Pelliccionia,G.L. Pinna Angionia,b_{, A. Potenza}a,b_{,A. Romero}a,b_{, M. Ruspa}a,c_{,R. Sacchi}a,b_, A. Solanoa,b_{, A. Staiano}a_{, P.P. Trapani}a,b

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

c_{UniversitàdelPiemonteOrientale(Novara),Torino,Italy}

S. Belfortea,V. Candelisea,b_{,M. Casarsa}a_{, F. Cossutti}a_{,G. Della Ricca}a,b_{, B. Gobbo}a_{, C. La Licata}a,b_, M. Maronea,b_{,D. Montanino}a,b_{,A. Schizzi}a,b,2_{,T. Umer}a,b_{, A. Zanetti}a

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

S. Chang, A. Kropivnitskaya,S.K. Nam

KangwonNationalUniversity,Chunchon,RepublicofKorea

D.H. Kim,G.N. Kim, M.S. Kim,D.J. Kong, S. Lee, Y.D. Oh,H. Park, A. Sakharov, D.C. Son

KyungpookNationalUniversity,Daegu,RepublicofKorea

T.J. Kim

ChonbukNationalUniversity,Jeonju,RepublicofKorea

J.Y. Kim, S. Song

ChonnamNationalUniversity,InstituteforUniverseandElementaryParticles,Kwangju,RepublicofKorea

S. Choi, D. Gyun,B. Hong, M. Jo,H. Kim, Y. Kim,B. Lee, K.S. Lee, S.K. Park,Y. Roh

KoreaUniversity,Seoul,RepublicofKorea

M. Choi,J.H. Kim, I.C. Park, S. Park,G. Ryu, M.S. Ryu

UniversityofSeoul,Seoul,RepublicofKorea

SungkyunkwanUniversity,Suwon,RepublicofKorea

A. Juodagalvis

VilniusUniversity,Vilnius,Lithuania

J.R. Komaragiri, M.A.B. Md Ali

NationalCentreforParticlePhysics,UniversitiMalaya,KualaLumpur,Malaysia

H. Castilla-Valdez,E. De La Cruz-Burelo, I. Heredia-de La Cruz28_{,R. Lopez-Fernandez,}

A. Sanchez-Hernandez

CentrodeInvestigacionydeEstudiosAvanzadosdelIPN,MexicoCity,Mexico

S. Carrillo Moreno, F. Vazquez Valencia

UniversidadIberoamericana,MexicoCity,Mexico

I. Pedraza, H.A. Salazar Ibarguen

BenemeritaUniversidadAutonomadePuebla,Puebla,Mexico

E. Casimiro Linares, A. Morelos Pineda

UniversidadAutónomadeSanLuisPotosí,SanLuisPotosí,Mexico

D. Krofcheck

UniversityofAuckland,Auckland,NewZealand

P.H. Butler,S. Reucroft

UniversityofCanterbury,Christchurch,NewZealand

A. Ahmad, M. Ahmad, Q. Hassan,H.R. Hoorani, S. Khalid, W.A. Khan, T. Khurshid,M.A. Shah, M. Shoaib

NationalCentreforPhysics,Quaid-I-AzamUniversity,Islamabad,Pakistan

H. Bialkowska,M. Bluj, B. Boimska,T. Frueboes, M. Górski, M. Kazana, K. Nawrocki, K. Romanowska-Rybinska,M. Szleper, P. Zalewski

NationalCentreforNuclearResearch,Swierk,Poland

G. Brona,K. Bunkowski, M. Cwiok,W. Dominik,K. Doroba, A. Kalinowski,M. Konecki, J. Krolikowski, M. Misiura, M. Olszewski, W. Wolszczak

InstituteofExperimentalPhysics,FacultyofPhysics,UniversityofWarsaw,Warsaw,Poland

P. Bargassa,C. Beirão Da Cruz E Silva, P. Faccioli, P.G. Ferreira Parracho,M. Gallinaro, F. Nguyen, J. Rodrigues Antunes,J. Seixas, J. Varela, P. Vischia

LaboratóriodeInstrumentaçãoeFísicaExperimentaldePartículas,Lisboa,Portugal

S. Afanasiev,P. Bunin,M. Gavrilenko,I. Golutvin, I. Gorbunov, A. Kamenev,V. Karjavin, V. Konoplyanikov, A. Lanev,A. Malakhov,V. Matveev29, P. Moisenz, V. Palichik,V. Perelygin, S. Shmatov, N. Skatchkov, V. Smirnov,A. Zarubin

JointInstituteforNuclearResearch,Dubna,Russia

V. Golovtsov,Y. Ivanov, V. Kim30,P. Levchenko, V. Murzin,V. Oreshkin, I. Smirnov, V. Sulimov,L. Uvarov, S. Vavilov, A. Vorobyev,An. Vorobyev

Yu. Andreev,A. Dermenev, S. Gninenko, N. Golubev, M. Kirsanov,N. Krasnikov, A. Pashenkov, D. Tlisov, A. Toropin

InstituteforNuclearResearch,Moscow,Russia

V. Epshteyn, V. Gavrilov, N. Lychkovskaya,V. Popov, G. Safronov, S. Semenov, A. Spiridonov, V. Stolin, E. Vlasov, A. Zhokin

InstituteforTheoreticalandExperimentalPhysics,Moscow,Russia

V. Andreev,M. Azarkin, I. Dremin, M. Kirakosyan, A. Leonidov, G. Mesyats, S.V. Rusakov,A. Vinogradov

P.N.LebedevPhysicalInstitute,Moscow,Russia

A. Belyaev,E. Boos, V. Bunichev, M. Dubinin31, L. Dudko,A. Gribushin, V. Klyukhin, O. Kodolova, I. Lokhtin,S. Obraztsov, M. Perfilov, V. Savrin, A. Snigirev

SkobeltsynInstituteofNuclearPhysics,LomonosovMoscowStateUniversity,Moscow,Russia

I. Azhgirey,I. Bayshev,S. Bitioukov, V. Kachanov, A. Kalinin, D. Konstantinov, V. Krychkine, V. Petrov, R. Ryutin, A. Sobol, L. Tourtchanovitch,S. Troshin, N. Tyurin, A. Uzunian,A. Volkov

StateResearchCenterofRussianFederation,InstituteforHighEnergyPhysics,Protvino,Russia

P. Adzic32_{, M. Ekmedzic,J. Milosevic,V. Rekovic} UniversityofBelgrade,FacultyofPhysicsandVincaInstituteofNuclearSciences,Belgrade,Serbia

J. Alcaraz Maestre, C. Battilana,E. Calvo, M. Cerrada,M. Chamizo Llatas, N. Colino, B. De La Cruz, A. Delgado Peris,D. Domínguez Vázquez, A. Escalante Del Valle, C. Fernandez Bedoya,

J.P. Fernández Ramos,J. Flix, M.C. Fouz,P. Garcia-Abia,O. Gonzalez Lopez, S. Goy Lopez, J.M. Hernandez, M.I. Josa,G. Merino, E. Navarro De Martino,A. Pérez-Calero Yzquierdo, J. Puerta Pelayo,

A. Quintario Olmeda, I. Redondo,L. Romero,M.S. Soares

CentrodeInvestigacionesEnergéticasMedioambientalesyTecnológicas(CIEMAT),Madrid,Spain

C. Albajar, J.F. de Trocóniz, M. Missiroli, D. Moran

UniversidadAutónomadeMadrid,Madrid,Spain

H. Brun, J. Cuevas,J. Fernandez Menendez, S. Folgueras, I. Gonzalez Caballero, L. Lloret Iglesias

UniversidaddeOviedo,Oviedo,Spain

J.A. Brochero Cifuentes, I.J. Cabrillo, A. Calderon, J. Duarte Campderros,M. Fernandez, G. Gomez, A. Graziano, A. Lopez Virto,J. Marco, R. Marco,C. Martinez Rivero, F. Matorras, F.J. Munoz Sanchez, J. Piedra Gomez, T. Rodrigo,A.Y. Rodríguez-Marrero,A. Ruiz-Jimeno, L. Scodellaro,I. Vila,

R. Vilar Cortabitarte

InstitutodeFísicadeCantabria(IFCA),CSIC-UniversidaddeCantabria,Santander,Spain

D. Abbaneo, E. Auffray, G. Auzinger, M. Bachtis,P. Baillon, A.H. Ball, D. Barney, A. Benaglia, J. Bendavid, L. Benhabib,J.F. Benitez, C. Bernet7_{,G. Bianchi, P. Bloch, A. Bocci, A. Bonato, O. Bondu,C. Botta,}

H. Breuker, T. Camporesi, G. Cerminara, S. Colafranceschi33,M. D’Alfonso, D. d’Enterria, A. Dabrowski, A. David,F. De Guio, A. De Roeck, S. De Visscher, M. Dobson, M. Dordevic, B. Dorney, N. Dupont-Sagorin, A. Elliott-Peisert, J. Eugster,G. Franzoni, W. Funk, D. Gigi, K. Gill,D. Giordano, M. Girone, F. Glege,

R. Guida, S. Gundacker, M. Guthoff,J. Hammer, M. Hansen, P. Harris,J. Hegeman, V. Innocente, P. Janot, K. Kousouris,K. Krajczar, P. Lecoq,C. Lourenço, N. Magini, L. Malgeri,M. Mannelli, J. Marrouche,

L. Masetti, F. Meijers, S. Mersi,E. Meschi, F. Moortgat, S. Morovic, M. Mulders, P. Musella, L. Orsini, L. Pape,E. Perez, L. Perrozzi,A. Petrilli, G. Petrucciani, A. Pfeiffer,M. Pierini, M. Pimiä, D. Piparo, M. Plagge, A. Racz,G. Rolandi34, M. Rovere,H. Sakulin, C. Schäfer, C. Schwick,A. Sharma, P. Siegrist,