Measurement of the t(t)over-bar production cross section in pp collisions at root s=8 TeV in dilepton final states containing one tau lepton

UNIVERSIDADE ESTADUAL DE CAMPINAS SISTEMA DE BIBLIOTECAS DA UNICAMP

REPOSITÓRIO DA PRODUÇÃO CIENTIFICA E INTELECTUAL DA UNICAMP

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DOI: 10.1016/j.physletb.2014.10.032

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DIRETORIA DE TRATAMENTO DA INFORMAÇÃO Cidade Universitária Zeferino Vaz Barão Geraldo

CEP 13083-970 – Campinas SP Fone: (19) 3521-6493 http://www.repositorio.unicamp.br

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Measurement

of

the

tt production

cross

section

in

pp

collisions

at

√

s

=

8 TeV in

dilepton

final

states

containing

one

τ

lepton

.CMSCollaboration

CERN,Switzerland

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

Articlehistory:

Received24July2014

Receivedinrevisedform9October2014 Accepted10October2014

Availableonline16October2014 Editor:M.Doser Keywords: CMS Physics Topquark Tau

The top-quark pair production cross section is measured in final states with one electron or muon and onehadronicallydecayingτ leptonfromtheprocesstt_{→ (}ν)(τ ντ)bb,where=e,μ.Thedata samplecorrespondstoanintegratedluminosityof19.6 fb−1_{collectedwiththeCMSdetectorinproton–}

protoncollisionsat√s=8 TeV.Themeasuredcrosssectionσtt=257±3 (stat)±24 (syst)±7 (lumi) pb,

assumingatop-quarkmassof172.5 GeV,isconsistentwiththestandardmodelprediction.

©2014TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/3.0/).FundedbySCOAP3.

1. Introduction

TopquarksattheCERNLHCaremostlyproducedinpairswith the subsequent decays tt→W+bW−b. The decay modes of the twoW bosonsdeterminethe eventsignature. Thedileptondecay channel corresponds to the casein which both W bosons decay intoleptons,wherethe termleptonusually refersto electrons or muons, as studied in Refs. [1,2]. In this letter we measure the productioncross section oftop-quark pairsby considering dilep-ton decays where one W boson promptly decays into ν, with

=e or μ,andtheotherdecaysinto τ ντ ,tt→ (ν)(τ ντ)bb.The expectedfractionoftheseeventsis4/81ofalltt decays.The τ lep-tonis identifiedby meansof itshadronicdecayproducts, witha branchingfractionB(τ→hadrons+ντ)65%,toproducea nar-rowjet withasmall numberofcharged hadrons,denoted as τh.

Thecrosssectionismeasuredbycountingthenumberofτh+X

eventsconsistent withoriginating from tt production, after sub-tractingthecontributionsfromotherprocesses,andcorrectingfor theefficiency ofthe eventselection. A similar method was used inppcollisionsatacentre-of-massenergyof√s=7 TeV[3].This “τ dilepton” channel isof particularinterest because it isa nat-uralbackgroundprocess tothe search fora chargedHiggsboson [4,5]withamasssmallerthanthatofthetopquark.Inthiscase, the production chain tt→H+bW−b, with H+→τ+ντ (or the

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

corresponding charge-conjugate particles) could give rise to dif-ferences with respect to the standard model (SM) prediction of the number of tt events with a τ lepton [6]. The present mea-surementisbasedondatacollectedbytheCMSexperimentinpp collisionsat√s=8 TeV correspondingtoanintegratedluminosity of19.6 fb−1.Therelativeaccuracyofthismeasurement improves overprevious results[7–11],thankstothe inclusionofadditional dataandimprovedanalysistechniques.

TheCMSdetectorisbrieflyintroducedinSection2,followedby detailsofthesimulatedsamplesinSection3,andabrief descrip-tion oftheeventreconstruction andeventselection inSection 4. Thedescriptionsofthebackgrounddeterminationandthe system-atic uncertainties aregiven inSections5and 6, respectively.The measurement of the cross section is discussed in Section 7, and theresultsaresummarisedinSection8.

2. TheCMSdetector

Thecentral featureoftheCMSapparatusisasuperconducting solenoid of 6 m internal diameter and 13 m in length, provid-ingamagneticfieldof3.8 T.Withinthesuperconductingsolenoid volume are a silicon pixel and strip tracker, a lead tungstate crystalelectromagneticcalorimeter,andabrass/scintillatorhadron calorimeter,each composed of abarrel andtwo endcapsections. The calorimetry provides high-resolution energy and direction measurementsofelectronsandhadronicjets.Muonsareidentified using gas-ionisation detectors embedded in the steel flux-return http://dx.doi.org/10.1016/j.physletb.2014.10.032

0370-2693/©2014TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/3.0/).Fundedby SCOAP3_.

yokeoutsidethesolenoid. Extensive forwardcalorimetry comple-ments thecoverage providedby the barrelandendcapdetectors. TheCMSexperimentusesaright-handedcoordinatesystem,with theoriginatthenominalinteractionpoint,the x axispointingto the centre of the LHC ring, the y axis pointing up (perpendic-ular to the LHC plane), and the z axis along the anticlockwise-beamdirection. The polarangle θ ismeasured fromthe positive z axis andthe azimuthal angle ϕ is measured inthe x– y plane. Charged particle trajectories are measured covering 0<ϕ≤2π inazimuth and|η|<2.5, where thepseudorapidity η is defined as η= −ln[tan(θ/2)]. The detector is nearly hermetic, allowing for energybalance measurements in the plane transverse to the beamdirections.Atwo-leveltriggersystemselectsthemost inter-esting proton–protoncollision events forusein physics analyses. A moredetaileddescriptionoftheCMSdetectorcanbefound else-where[12].

3. Dataandsimulationsamples

Events are selected online by a trigger requiring a single isolated electron (muon) with transverse momentum pT> 27

(24)GeV and|η|<2.5(2.1).

Thismeasurementmakesuseofsimulatedsamplesoftt events as well as other processes that mimic the τh decay signature.

These samples are used to optimise the event selection, to cal-culate the acceptancefor tt events, andto estimate some ofthe backgroundsintheanalysis.

The signal acceptance and tt dilepton background are evalu-ated using a version of MadGraph which includes the effects of spincorrelations[13,14].Thenumberofexpectedtt eventsis es-timated with the next-to-next-to-leading-order (NNLO) SM cross sectionof251.7₋+6_8..3₆ (scale)±6.5 (PDF) pb[15–19]foratop-quark massof172.5 GeV,wherethefirstuncertaintyisdueto renormali-sationandfactorisationscales,andthesecondisduetothechoice ofparton distribution functions(PDFs). The generated eventsare subsequentlyprocessedwith pythia 6.426[20]whichperformsthe hadronisation ofpartons.Softradiation ismatched tothe contri-butionsfromdirectemissionsaccountedforinthematrix-element calculationsusingthekT-MLMapproach[21].The τ leptondecays

are simulatedusing tauola 27.121.5 [22], whichaccountsforthe τ-leptonpolarisation.

ThesamplescontainingW + jetandZ+ jeteventsare simu-latedusingthe MadGraph 5.1.3.30eventgenerator[23].The elec-troweakproduction ofsingle top quarksis considered asa back-groundprocessandissimulatedwith powheg 1.0,r1380[24–28]. ThedibosonproductionprocessesWW,WZ,andZZ aregenerated with pythia 6.424.Ineachcase,the pythia parametersforthe un-derlying eventare setaccordingtotheZ2*tune [29],whichuses theCTEQ6LPDFs[30].

Simulated events are processed using the full CMS detector simulation basedon Geant4 [31,32], followed by a detailed trig-geremulationandeventreconstruction.Forbothsignaland back-groundevents,additionalpp interactions (pileup)in thesame or nearby bunch crossings are simulated with pythia and superim-posedonthehardcollision,usingapileupmultiplicitydistribution thatreflectstheluminosityprofileoftheanalyseddata.

4. Eventselection

Events arereconstructed withtheparticle-flow (PF)algorithm [33,34], which combines information from all sub-detectors to identify and reconstruct individual electrons, muons, photons, chargedandneutralhadrons. Theprimary collisionvertexis cho-sen as the reconstructed vertex with the largest p2

T of the

associated tracks.Electronsareidentified withamultivariate dis-criminantcombiningseveralquantitiesdescribingthetrackquality, theshapeoftheenergydepositsintheelectromagnetic calorime-ter, andthe compatibility ofthe measurements fromthe tracker and the electromagnetic calorimeter [35], and are reconstructed withanaverageefficiencyofapproximately95%.Muonsare iden-tified with additional requirements on the quality of the track reconstructionandonthenumberofmeasurementsinthetracker andthe muon systems [36],and arereconstructed withan aver-ageefficiencyofapproximately96%.Chargedandneutralparticles provide the input to the anti-kT jet clustering algorithm with a

distanceparameter of0.5[37]. Thejet momentumis determined fromthevector sumofparticle momentainthejet.Afterjet en-ergies are corrected for additional pileup contributions and for detectoreffects,theyarefoundinsimulationstobewithin5–10% of the actual jet momentum [38]. The missingtransverse energy Emiss_T is calculated as the magnitude of the vector sum of mo-menta fromall reconstructed particles inthe plane transverse to thebeam.

Inaddition,higher-levelobservablessuchasb-tagging discrim-inatorsandleptonisolationvariablesareused.Theleptonrelative isolation is defined as the transverse energy contributions de-posited by charged hadrons (ET,ch), neutral hadrons (ET,nh), and

photons (ET,ph) in a cone of radius R=



(ϕ)2_{+ (η)}2_=0.4

centred on the lepton candidate track, relative to the lepton’s transverse momentum (pT), Irel= (ET,ch+ET,nh+ET,ph)/pT. An

electron(muon)candidateisconsideredtobenon-isolatedandis rejectedifIrel>0.1 (>0.12).

Thehadronicproductsofthe τ-leptondecayarereconstructed using a jet as the initial seed, and are then classified as hav-ing one or three charged hadrons with the “hadron-plus-strips” algorithm[39,40].Inthe“hadron-plus-strips”algorithm, calorime-ter energy deposits clusteredalong strips inthe ϕ direction are used forneutralpion identification.Then, thedecaymodes, four-momenta, andisolation quantities ofthe τh are determined,and

thefollowingcategoriesareconsidered:singlehadron,hadronplus a strip, hadron plus two strips, and three hadrons. These cate-goriestogetherencompassapproximately95%ofhadronic τ-lepton decays. The sum of the charged hadron chargesprovides the τh

charge. The τh-jet momentum isrequired tomatch thedirection

of the original jet within a maximumdistance R=0.1.Isolation criteriarequirethat there beno additionalchargedhadronswith pT >1.0 GeV or photons with transverse energy ET>1.5 GeV

within a cone ofsize R=0.5 around thedirectionof the τh jet.

Electronsandmuonsmisidentifiedas τh aresuppressedusing

al-gorithmsthatcombineinformationfromthetracker,calorimeters, and muon detectors [12]. The τh identification efficiency is

de-finedastheratioofthenumberofselected τhcandidatesdivided

by the number ofhadronic τ-leptondecays in tt events;the ra-tio depends on pT and η of the τh, and is on average 50% for

pτh

T >20 GeV,witha probability ofapproximately 1%for generic

jetstobemisidentifiedasa τhjet.

Thecombinedsecondaryvertex(CSV)algorithm[41]isusedto identifyjetsoriginatingfromthehadronisationofbquarks.The al-gorithm combinestheinformationabouttrackimpactparameters andsecondaryverticeswithinjetsintoalikelihooddiscriminantto provide separationbetweenb jetsandjetsoriginatingfromlight quarks, gluons,or charmquarks.The output ofthisCSV discrim-inant has values between zero and one; a jet with a CSV value above a certain threshold isreferred to asbeing “b tagged”. We chooseaworkingpointwheretheb-taggingefficiencyis approxi-mately60%,asmeasuredinadatasampleofeventsenrichedwith jetsfromsemileptonicb-hadrondecays.Themisidentificationrate of light-flavourjetsis estimatedfrominclusivejet studies andis measuredtobeabout0.1%forjetswithpT>30 GeV.

Fig. 1. Theb-taggedjet multiplicity afterthe fullevent selection.Thesimulated contributionsarenormalisedtotheSMpredictedvalues.Thehatchedareashows thetotaluncertainty.

Eventsarepreselectedbyrequiringexactlyoneisolatedelectron (muon) withtransverse momentum pT>35(30)GeV and |η|<

2.5 (2.1),atleast twojetswith pT>30 GeV, andoneadditional

jetwith pT>20 GeV.Theselectedjetsmustbe within|η|<2.4.

Theelectronormuon isrequiredtobeseparatedfromanyjetin the(η, ϕ)plane byadistanceR>0.4.Events withanyadditional looselyisolated, Irel<0.2, electron (muon) of pT>15 (10)GeV

arerejected.FurthereventselectionrequirementsincludeEmiss T >

40 GeV andonlyone τhwith pT>20 GeV and |η|<2.4.The τh

andthe lepton are required to have electric charges of opposite sign(OS). At least one of the jetsis requiredto be identified as originatingfromb-quarkhadronisation(b-tagged).

Fig. 1shows,forthesumoftheeτhand μτhfinalstates,a

com-parison betweendata andsimulation ofthe number ofb-tagged jetsineachevent N_b-tag after allthe selectioncriteriahavebeen applied. The distributions of the τh pT and EmissT after the final

eventselectionare showninthetopandbottompanelsofFig. 2, respectively. The distributions show agreement between the ob-servednumbersofeventsandtheexpectednumbersofsignaland backgroundeventsobtainedfromthesimulateddistributions nor-malisedtotheintegratedluminosityoftheselecteddatasample.

Following the final selection, additional kinematic features of thett events are studied toevaluate theagreement betweenthe observeddataandthepredictedsumofsignalandbackground.For eachevent,twoinvariantmasscombinationsarereconstructedby pairing the τh with thetwo candidateb-jets: (1) inevents with

two or more b-tagged jets, the two combinations are based on thetwo b-taggedjetswiththehighestvalueofthediscriminator; (2) ineventswithoneb-taggedjet,thisisusedforthefirst com-bination,while thenon-b-tagged jet withthehighest pT isused

toformthesecondcombination.Forthetwocombinations,the in-variantmasswiththelowestvalueisshowninFig. 3(top),forthe eτhand μτhchannelscombined.

Foreachevent,thetop-quarkmassmtopisreconstructedusing

theKINb algorithm [42,43].Due tothe multipleneutrinos inthe event,thereconstructionofmtopleadstoanunderconstrained

sys-tem.TheKINbalgorithmappliesconstraintsontheWbosonmass, themassdifference betweenthetopandanti-topquark, andthe longitudinalmomentumofthett system.Foreachevent,solutions tothekinematicequationsareevaluated,varyingthejetmomenta andthedirectionofEmiss

T withintheirresolutions.Foreachsetof

variations and each lepton–jet combination, the kinematic equa-tions allow up to four solutions; the one with the lowest tt in-variantmass isaccepted ifthemass difference betweenthe two

Fig. 2. Distributionoftheτh pT (top)and EmissT (bottom)afterthefullevent

se-lection,fortheeτh andμτh channelscombined.Thesimulatedcontributionsare

normalisedtotheSMpredictedvalues.Thehatchedareashowsthe total uncer-tainty.Thelastbinsincludetheoverflowevents.

top quarksislessthan3 GeV.Foreach event,the accepted solu-tions corresponding to the two possible lepton–jet combinations are countedandthecombinationwiththe largestnumberof so-lutions ischosen andmtop is obtainedby fittingthepeak ofthis

distribution.Theeventsinwhichsolutionsarefoundareshownin Fig. 3 (bottom).Dataare inagreement withtheexpectedsumof signalandbackgroundevents.

5. Backgroundestimate

The main background (misidentified τh) comes from events

with one lepton (electron or muon), significant E_Tmiss, and three or more jets, where one jet is misidentified asa τh jet [6].The

dominant source is tt lepton + jet events. The misidentified τh

backgroundaccounts alsoforevents withW bosons produced in association withjets, eithergenuineW + jetor single-top-quark production,andforQCDmultijetevents.Inordertoestimate this background from data, the misidentification probability w(jet→ τh)isparameterisedasafunctionofthejetpT, η,andwidth(Rjet).

ThequantityRjetisdefinedas

 σ2

η+σϕ ,2 where ση (σϕ )expresses theextentin η(ϕ)ofthejetcluster[38].

Theprobability w(jet→τh)isevaluatedfromtwocontrol

sam-ples:

• wW+jets:fromaW + jeteventsample,selectedby requiring

one isolated muon with pT>20 GeV and |η|<2.1, and at

Fig. 3. (Top)Minimuminvariantmassreconstructedbypairingtheτh witheither

ab-taggedjetorwiththehighest pT non-b-taggedjet,asdescribedinthetext.

(Bottom)Distributionofthereconstructedtop-quarkmassmtopfortheτh

candi-dateeventsafterthefulleventselection.Data(points)arecomparedwiththesum ofsignalandbackgroundyields,fortheeτhandμτhchannelscombined.The

sim-ulatedcontributionsarenormalisedtotheSMpredictedvalues.Thehatchedarea showsthetotaluncertainty.Thelastbinsincludetheoverflowevents.

• wQCD:fromaQCDmultijetsample,triggered byone jetwith

pT>40 GeV,selectedbyrequiringeventstohaveatleasttwo

jetswith pT>20 GeV and|η|<2.4,wherethetriggeringjet

isremovedfromthemisidentificationratecalculationtoavoid atriggerbias.

Bothprobabilitiesareevaluated insimulatedeventsaswell asin data,withgoodagreementfoundbetweentheresultsfrom simu-lationanddata[39].

Thenumberofeventscontainingmisidentified τhcandidatesis

thendeterminedas Nmisid= M  i m  j w_ij(jet→τ)−Nother, (1)

where j is thejet indexofevent i,andm is thenumberof jets in each event and M is the total number of events. The quan-tity Nother _{is the expected 20% contamination from signal and}

otherprocessestothemisidentifiedbackgroundasestimatedfrom simulated samples. The value of Nother is evaluated by applying theproceduredescribedabovetosimulatedeventsofZ/γ∗→τ τ, single-top-quark production, dibosonproduction, andthe tt pro-cessesincludedinthemisidentified τhbackgroundestimation.

JetsinQCDmultijeteventsoriginatemainlyfromgluons,while inW +jeteventstheyarepredominantlyfromquarks.Thequark

Table 1

Listofsystematicuncertaintiesinthecrosssectionmeasurement,andtheir combi-nation.Leptonreconstructionuncertaintiesareuncorrelated,whileallother uncer-taintiesareassumed100%correlated.

Source Uncertainty [%] eτh μτh Combined Experimental uncertainties: τhjet identification 6.0 6.0 6.0 τhmisidentification background 4.3 4.3 4.3 τhenergy scale 2.4 2.5 2.5

b-jet tagging, jet misidentification 1.6 1.6 1.6 jet energy scale, jet energy resolution, Emiss

T 1.9 1.9 1.9

lepton reconstruction 0.8 0.6 0.5 other backgrounds 0.6 0.7 0.7 luminosity 2.6 2.6 2.6

Theoretical uncertainties:

matrix element–parton shower matching 1.7 1.3 1.5 factorisation/renormalisation scale 2.9 2.9 2.9

generator 1.5 1.5 1.5

hadronisation 1.7 1.7 1.7 top-quark pTmodelling 0.7 0.5 0.6

parton distribution functions 0.8 0.7 0.7 total systematic uncertainty 9.6 9.5 9.5

andgluoncompositioninthemisidentified τheventsliesbetween

these twocontrol samples. As wQCD<wW+jets,the actual Nmisid

value isunder- (over-)estimatedbyapplyingthe wQCD(wW+jets)

probability. Wedetermine fromdata theratefor the misidentifi-cation of a jet to be identified asa τh,and fromsimulation the

quark/gluon composition in the W + jet and multijet samples. Fromthesequantitieswederivethefollowingcombination: 

Nmisid=SFW+jet×NWmisid+jet+SFQCD×NmisidQCD , (2)

wherethemisidentificationrates,extractedfromthedatacontrol samplesdiscussedabove, arecombinedwiththescale factorsSFs determined fromthe set ofequationsdescribing the quark/gluon composition of the samples: SFQCD=0.83 and SFW+jet=0.17.

ThecorrespondingsystematicuncertaintyisobtainedfromEq.(2) by weighting the relative deviations of Nmisid

W+jet and NmisidQCD from

Nmisid_{ with the related scale factors. This results in an}

uncer-taintyof7%forbotheτhand μτhchannels.

The efficiencyof the OS requirement εOS is determined from

simulated lepton + jet tt events and is applied in order to ob-tain the misidentified τh background after the final event

selec-tion Nmisid_OS ,where Nmisid_OS =εOS·Nmisid.We find values of εOS=

0.729±0.002 (stat)±0.004 (syst) fortheeτhselectionand εOS=

0.731±0.002 (stat)±0.003 (syst) forthe μτhselection,whereall

sources ofsystematic uncertainty are accounted forin the mod-ellingofthesimulatedtt lepton+jetevents.

6. Systematicuncertainties

Several sources of systematic uncertainty are considered and listedinTable 1.Theyarerelatedbothtothesignalreconstruction efficiency,backgrounddetermination,andluminositymeasurement (Experimental uncertainties) and to the theoretical assumptions onthe tt production(Theoreticaluncertainties).InTable 1andin whatfollows,relativevaluesrefertothecrosssectionuncertainty unlessexplicitlystatedotherwise.

6.1. Experimentaluncertainties

Regarding the τh reconstruction, the uncertainty associated

with the identification efficiency amounts to 6%, while the con-tribution relative tothe τh jet energyscaleis 2.4%(2.5%) forthe

by 3%[39,40].Theuncertaintyinthe τhidentificationefficiency

in-cludestheuncertaintyinchargedeterminationwhichisestimated to be smaller than 1%. The uncertainty related to the misidenti-fied τhbackgroundprocess,discussedinSection5,isobtainedby

propagatingthe7%uncertaintyonNmisid_{tothecrosssection}

de-terminationandresultsin4.3%forbothchannels. Italsoincludes theuncertaintyintheOSefficiencydetermination.

Thereconstructionofalightflavourjetasabquarkisdefined asmistagging.Theuncertaintyduetob(mis)taggingisestimated to reflect the data-to-simulation scale factors and corresponding uncertaintiesforb-taggingandmistaggingefficiencies [41].When propagatedtothecrosssectionmeasurement,theyamountto1.6% forbotheτhand μτhchannels.

Thejetenergyscale(JES)uncertaintyisestimated[38]by vary-ingthejetenergywithin thepT- and η-dependentJES

uncertain-tiesperjet,andtakingintoaccount theuncertaintyduetopileup andpartonflavour.Thejetenergyresolution(JER)isestimatedby smearingthejetenergyinsimulationwithinthe η-dependentJER uncertaintiesperjet.TheJESandJERuncertaintiesarepropagated inordertoestimatetheuncertaintyofthe Emiss_T scale.Inaddition, modellingoftheEmiss_T component,whichisnotclusteredinjets,is alsoconsidered.Theresultinguncertaintyfrompropagatingthese effectstothe crosssectionmeasurement is1.9%forboth theeτh

and μτhchannels.

Uncertaintiesduetotrigger,leptonidentification,isolation,and leptonenergyscalearecalculatedfromindependentsampleswith a“tag-and-probe”method[35,36],andyield0.8%(0.6%)fortheeτh

(μτh)channel.

Anoverall 0.6%(0.7%)uncertaintyfortheeτh(μτh)channelis

dueto other minorbackgrounds, accountingforthe uncertainties relatedtothetheoreticalcrosssections,JES,andb-tagginginthese simulatedsamples,andthe→τh (=e, μ)misidentificationin

theZ/γ∗→ +_−_{andtt dileptonprocesses.}

Finally, the integrated luminosity is known with 2.6% accu-racy[44].

6.2.Theoreticaluncertainties

Thetheoreticaluncertaintyduetothematrixelement(ME)and partonshower(PS)matchingisestimatedbyvaryingupanddown byafactoroftwothethresholdbetweenjetproductionattheME level and via PS, andit results in 1.7% (1.3%) for the eτh (μτh)

channel.

Themodellinguncertaintyinthesignal acceptanceduetothe factorisation and renormalisation scale choices is estimated by varyingthemsimultaneouslyupanddownbyafactoroftwofrom thenominal value equal to the Q2 inthe event, withan uncer-taintyof2.9%foundforbothchannels.

Theuncertaintyduetothechoiceofthegeneratorisestimated astherelativedifferencebetweentheacceptancesevaluatedwith MadGraph and powheg[24–26,45] after the full event selection andresultsin1.5%.Inasimilarway,theuncertaintyinthe hadro-nisationschemeisevaluatedfromtherelativedifferencesbetween the acceptances from powheg + pythia_{and powheg} + herwig samples,estimated prior to the b-tagging or τh jet requirement,

resultingina1.7%uncertainty.

Weconsiderthe uncertaintyrelatedtothetop-quark pT scale

modellingby varying the top-quark pT spectrum and evaluating

the change in the signal acceptance, resulting in 0.6%, and the uncertaintyrelated to the PDF variations following the PDF4LHC prescriptions[17],resultingin0.7%.

7.Crosssectionmeasurement

The number of expected signal and background events as well as the number of observed events after all selections are

Table 2

Number of expectedevents for signal (assuming mtop=172.5 GeV) and

back-grounds.Thebackgroundfrommisidentifiedτhisestimatedfromdata,whilethe

otherbackgroundsareestimatedfromsimulation.Statisticalandsystematic uncer-taintiesareshown.

Source eτh μτh

misidentifiedτh 1341±3±94 1653±3±116

tt→ (ν)(ν)bb 55±1±3 68±2±4

Z/γ∗→ee,μμ 11±5±5 12±5±5 Z/γ∗→τ τ 85±14±8 166±20±18 single top quark 104±7±9 133±8±10 dibosons 15±1±1 19±1±1 total expected background 1611±17±95 2051±22±118 expected signal yield 2134±9±170 2632±11±212

data 3779 4767

summarised inTable 2.Thestatisticalandsystematicuncertainties arealsoshown. The tt productioncrosssectionmeasured from τ dileptoneventsis σtt= (N−B)/(L·Atot),whereN isthenumber

ofobservedcandidateevents,B istheestimateofthebackground and L is the integrated luminosity. The total acceptance Atot is

theproductofthebranchingfractions,geometricalandkinematic acceptance, trigger, lepton identification, and the overall recon-structionefficiency.Itisevaluated withrespecttotheinclusivett sample.AftertheOSrequirementandassumingatop-quarkmass mtop=172.5 GeV,weobtain:

Atot(eτh)=0.04333±0.00017 (stat)±0.00300 (syst)%;

Atot(μτh)=0.05370±0.00021 (stat)±0.00376 (syst)%.

Thestatisticaluncertaintiesareduetothelimitednumberof sim-ulated events and the systematic uncertainties are estimated by accountingforallsourceslistedinTable 1.Thestatisticaland sys-tematicuncertainties listedinTable 2 arepropagated tothefinal crosssectionmeasurements:

σ_tt(eτh)=255±4 (stat)±24 (syst)±7 (lumi) pb; σ_tt(μτh)=258±4 (stat)±24 (syst)±7 (lumi) pb.

TheBLUE method[46]isusedtocombinethecrosssection mea-surementsin theeτhand μτh channels,yielding weights of0.47

and0.53,respectively.Leptonreconstruction uncertaintiesare un-correlated, while all other uncertainties are assumed 100% cor-related. With this methodwe obtain a combined resultof σtt=

257±3 (stat)±24 (syst)±7 (lumi) pb, in agreement with the NNLO expectation of 251.7+₋6_8..₆3 (scales)±6.5 (PDF)pb. Follow-ing the most recent conventions for the treatment of PDF and scaleuncertaintiesthesamecalculationyields252.9₋+₈6._.4₆ (scale)±

11.7 (PDF+αS)pb [15–19]. The dependence on the top-quark

masshasbeenstudiedfortherange160–185 GeVandiswell de-scribedbyalinearvariation.Ifweadjustourresulttothecurrent world averagevalue of173.3 GeV [47],we obtaina crosssection thatislowerby3.1 pb.

8. Summary

Ameasurementofthett productioncrosssectioninthe chan-nel tt→ (ν)(τ ντ)bb is presented, where  is an electron or a muon, andthe τ lepton is reconstructed through its hadronic decays.The data sample corresponds to an integratedluminosity of 19.6 fb−1 _{collected inproton–protoncollisions at}√_{s=8 TeV.}

Events are selected by requiring the presence of one isolated electron or muon, two or more jets (at least one of which is b-tagged), significant missing transverse energy, and one τ. The

largest backgroundcontribution is estimatedfromdata and con-sists of tt events with one W boson decaying into jets, where one jet is misidentified as a τ. The measured cross section is σtt=257±3 (stat)±24 (syst)±7 (lumi) pb foratop-quarkmass

of172.5 GeV.Thismeasurementimprovesoverpreviousresultsin thisdecaychannel,anditisingoodagreementwiththestandard modelexpectationandothermeasurementsofthett crosssection atthesame centre-of-massenergy.

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 effectivelythe computinginfrastructureessential to ouranalyses. Finally, we acknowledge the enduring support for the construc-tionandoperation oftheLHCandthe CMSdetectorprovidedby 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); NRF and WCU (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 Alfred P. Sloan Foundation; the Alexander von Humboldt-Stiftung; the Belgian Federal Science Policy Office; the Fonds pour la Formation à la Recherchedansl’Industrieetdansl’Agriculture(FRIA-Belgium);the AgentschapvoorInnovatiedoorWetenschapenTechnologie (IWT-Belgium);the Ministry ofEducation,Youth andSports (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); theConsorzio per la Fisica (Trieste);MIUR project20108T4XTM(Italy); the Thalisand Aristeia programmescofinanced 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. Frühwirth1, 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

InstitutfürHochenergiephysikderOeAW,Wien,Austria

V. Mossolov, N. Shumeiko, J. Suarez Gonzalez

NationalCentreforParticleandHighEnergyPhysics,Minsk,Belarus

S. Alderweireldt, M. Bansal, S. Bansal, T. Cornelis, E.A. De Wolf, X. Janssen, A. Knutsson, S. Luyckx, S. Ochesanu, 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

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. Dograa, T.R. Fernandez Perez Tomeia, E.M. Gregoresb, P.G. Mercadanteb, 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

J.G. Bian, G.M. Chen, H.S. Chen, M. Chen, R. Du, C.H. Jiang, S. Liang, R. Plestina7, J. Tao, X. Wang, Z. Wang

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

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

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. Conte14, J.-C. Fontaine14, D. Gelé, U. Goerlach, C. Goetzmann, A.-C. Le Bihan, P. Van Hove

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

M. Ata, 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, G. Hellwig, 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, 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

InstitutfürExperimentelleKernphysik,Karlsruhe,Germany

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. 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

S.K. Swain

NationalInstituteofScienceEducationandResearch,Bhubaneswar,India

S.B. Beri, V. Bhatnagar, N. Dhingra, 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. Fahim23, R. Goldouzian, A. Jafari, M. Khakzad, M. Mohammadi Najafabadi, M. Naseri, S. Paktinat Mehdiabadi, B. Safarzadeh24, M. Zeinali

InstituteforResearchinFundamentalSciences(IPM),Tehran,Iran

M. Felcini, M. Grunewald

UniversityCollegeDublin,Dublin,Ireland

M. Abbresciaa,b, L. Barbonea,b, C. Calabriaa,b, S.S. Chhibraa,b, A. Colaleoa, D. Creanzaa,c,

N. De Filippisa,c, M. De Palmaa,b, L. Fiorea, G. Iasellia,c, G. Maggia,c, M. Maggia, S. Mya,c, S. Nuzzoa,b, A. Pompilia,b, G. Pugliesea,c, R. Radognaa,b,2, G. Selvaggia,b, L. Silvestrisa,2, G. Singha,b, R. Vendittia,b, P. Verwilligena, G. Zitoa

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

G. Abbiendia, A.C. Benvenutia, D. Bonacorsia,b, S. Braibant-Giacomellia,b, L. Brigliadoria,b, R. Campaninia,b, P. Capiluppia,b, A. Castroa,b, F.R. Cavalloa, G. Codispotia,b, M. Cuffiania,b,

G.M. Dallavallea, F. Fabbria, A. Fanfania,b, D. Fasanellaa,b, P. Giacomellia, C. Grandia, L. Guiduccia,b, S. Marcellinia, G. Masettia,2, A. Montanaria, F.L. Navarriaa,b, A. Perrottaa, F. Primaveraa,b, A.M. Rossia,b, T. Rovellia,b, G.P. Sirolia,b, N. Tosia,b, R. Travaglinia,b

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

S. Albergoa,b, G. Cappelloa, M. Chiorbolia,b, S. Costaa,b, F. Giordanoa,2, R. Potenzaa,b, A. Tricomia,b, C. Tuvea,b

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

G. Barbaglia, V. Ciullia,b, C. Civininia, R. D’Alessandroa,b, E. Focardia,b, E. Galloa, S. Gonzia,b, V. Goria,b,2, P. Lenzia,b, M. Meschinia, S. Paolettia, G. Sguazzonia, A. Tropianoa,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. Robuttia, S. Tosia,b

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

M.E. Dinardoa,b, P. Dinia, S. Fiorendia,b,2, S. Gennaia,2, R. Gerosa2, A. Ghezzia,b, P. Govonia,b,

M.T. Lucchinia,b,2, S. Malvezzia, R.A. Manzonia,b, A. Martellia,b, B. Marzocchi, D. Menascea, L. Moronia, M. Paganonia,b, S. Ragazzia,b, N. Redaellia, T. Tabarelli de Fatisa,b

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

S. Buontempoa, N. Cavalloa,c, S. Di Guidaa,d,2, F. Fabozzia,c, A.O.M. Iorioa,b, L. Listaa, S. Meolaa,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. Bacchettaa, D. Biselloa,b, A. Brancaa,b, R. Carlina,b, P. Checchiaa, M. Dall’Ossoa,b, T. Dorigoa, M. Galantia,b, F. Gasparinia,b, U. Gasparinia,b, P. Giubilatoa,b, A. Gozzelinoa, K. Kanishcheva,c,

S. Lacapraraa, M. Margonia,b, A.T. Meneguzzoa,b, M. Passaseoa, J. Pazzinia,b, N. Pozzobona,b, P. Ronchesea,b, F. Simonettoa,b, E. Torassaa, M. Tosia,b, P. Zottoa,b, A. Zucchettaa,b, G. Zumerlea,b

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

M. Gabusia,b, S.P. Rattia,b, C. Riccardia,b, P. Salvinia, 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, M. Menichellia, F. Romeoa,b, A. Sahaa, A. Santocchiaa,b, A. Spieziaa,b,2

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

K. Androsova,25, P. Azzurria, G. Bagliesia, J. Bernardinia, T. Boccalia, G. Broccoloa,c, R. Castaldia,

M.A. Cioccia,25, R. Dell’Orsoa, S. Donatoa,c, F. Fioria,c, L. Foàa,c, A. Giassia, M.T. Grippoa,25, F. Ligabuea,c, T. Lomtadzea, L. Martinia,b, A. Messineoa,b, C.S. Moona,26, F. Pallaa,2, A. Rizzia,b, A. Savoy-Navarroa,27, A.T. Serbana, P. Spagnoloa, P. Squillaciotia,25, R. Tenchinia, G. Tonellia,b, A. Venturia, P.G. Verdinia, C. Vernieria,c,2

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

c_{ScuolaNormaleSuperiorediPisa,Pisa,Italy}

L. Baronea,b, F. Cavallaria, G. D’imperioa,b, D. Del Rea,b, M. Diemoza, M. Grassia,b, C. Jordaa, E. Longoa,b, F. Margarolia,b, P. Meridiania, F. Michelia,b,2, S. Nourbakhsha,b, G. Organtinia,b, R. Paramattia, S. Rahatloua,b, C. Rovellia, F. Santanastasioa,b, L. Soffia,b,2, P. Traczyka,b

a_{INFNSezionediRoma,Roma,Italy} b_{UniversitàdiRoma,Roma,Italy}

N. Amapanea,b, R. Arcidiaconoa,c, S. Argiroa,b,2, M. Arneodoa,c, R. Bellana,b, C. Biinoa, N. Cartigliaa, S. Casassoa,b,2, M. Costaa,b, A. Deganoa,b, N. Demariaa, L. Fincoa,b, C. Mariottia, S. Masellia,

E. Migliorea,b, V. Monacoa,b, M. Musicha, M.M. Obertinoa,c,2, G. Ortonaa,b, L. Pachera,b, N. Pastronea, M. Pelliccionia, G.L. Pinna Angionia,b, A. Potenzaa,b, A. Romeroa,b, M. Ruspaa,c, R. Sacchia,b,

A. Solanoa,b, A. Staianoa, U. Tamponia

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

c_{UniversitàdelPiemonteOrientale(Novara),Torino,Italy}

S. Belfortea, V. Candelisea,b, M. Casarsaa, F. Cossuttia, G. Della Riccaa,b, B. Gobboa, C. La Licataa,b, M. Maronea,b, D. Montaninoa,b, A. Schizzia,b,2, T. Umera,b, A. Zanettia

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

T.J. Kim

ChonbukNationalUniversity,Chonju,RepublicofKorea

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

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

Y. Choi, Y.K. Choi, J. Goh, D. Kim, E. Kwon, J. Lee, H. Seo, I. Yu

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

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

I. Golutvin, V. Karjavin, V. Konoplyanikov, V. Korenkov, G. Kozlov, A. Lanev, A. Malakhov, V. Matveev29, V.V. Mitsyn, P. Moisenz, V. Palichik, V. Perelygin, S. Shmatov, S. Shulha, N. Skatchkov, V. Smirnov, E. Tikhonenko, 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

PetersburgNuclearPhysicsInstitute,Gatchina(St.Petersburg),Russia

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, M. Dubinin31, L. Dudko, A. Ershov, A. Gribushin, V. Klyukhin, O. Kodolova, I. Lokhtin, S. Obraztsov, M. Perfilov, S. Petrushanko, V. Savrin

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

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, 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, P. Silva, M. Simon, P. Sphicas35, D. Spiga, J. Steggemann, B. Stieger, M. Stoye, D. Treille, A. Tsirou, G.I. Veres17, J.R. Vlimant, N. Wardle, H.K. Wöhri, H. Wollny, W.D. Zeuner

CERN,EuropeanOrganizationforNuclearResearch,Geneva,Switzerland

W. Bertl, K. Deiters, W. Erdmann, R. Horisberger, Q. Ingram, H.C. Kaestli, D. Kotlinski, U. Langenegger, D. Renker, T. Rohe

PaulScherrerInstitut,Villigen,Switzerland

F. Bachmair, L. Bäni, L. Bianchini, P. Bortignon, M.A. Buchmann, B. Casal, N. Chanon, A. Deisher,

G. Dissertori, M. Dittmar, M. Donegà, M. Dünser, P. Eller, C. Grab, D. Hits, W. Lustermann, B. Mangano, A.C. Marini, P. Martinez Ruiz del Arbol, D. Meister, N. Mohr, C. Nägeli36, F. Nessi-Tedaldi, F. Pandolfi, F. Pauss, M. Peruzzi, M. Quittnat, L. Rebane, M. Rossini, A. Starodumov37, M. Takahashi, K. Theofilatos, R. Wallny, H.A. Weber

InstituteforParticlePhysics,ETHZurich,Zurich,Switzerland

C. Amsler38, M.F. Canelli, V. Chiochia, A. De Cosa, A. Hinzmann, T. Hreus, B. Kilminster, C. Lange, B. Millan Mejias, J. Ngadiuba, P. Robmann, F.J. Ronga, S. Taroni, M. Verzetti, Y. Yang

UniversitätZürich,Zurich,Switzerland

M. Cardaci, K.H. Chen, C. Ferro, C.M. Kuo, W. Lin, Y.J. Lu, R. Volpe, S.S. Yu

P. Chang, Y.H. Chang, Y.W. Chang, Y. Chao, K.F. Chen, P.H. Chen, C. Dietz, U. Grundler, W.-S. Hou, K.Y. Kao, Y.J. Lei, Y.F. Liu, R.-S. Lu, D. Majumder, E. Petrakou, Y.M. Tzeng, R. Wilken

NationalTaiwanUniversity(NTU),Taipei,Taiwan

B. Asavapibhop, N. Srimanobhas, N. Suwonjandee

ChulalongkornUniversity,FacultyofScience,DepartmentofPhysics,Bangkok,Thailand

A. Adiguzel, M.N. Bakirci39, S. Cerci40, C. Dozen, I. Dumanoglu, E. Eskut, S. Girgis, G. Gokbulut, E. Gurpinar, I. Hos, E.E. Kangal, A. Kayis Topaksu, G. Onengut41, K. Ozdemir, S. Ozturk39, A. Polatoz, K. Sogut42, D. Sunar Cerci40, B. Tali40, H. Topakli39, M. Vergili

CukurovaUniversity,Adana,Turkey

I.V. Akin, B. Bilin, S. Bilmis, H. Gamsizkan, G. Karapinar43, K. Ocalan, S. Sekmen, U.E. Surat, M. Yalvac, M. Zeyrek

MiddleEastTechnicalUniversity,PhysicsDepartment,Ankara,Turkey

E. Gülmez, B. Isildak44, M. Kaya45, O. Kaya46

BogaziciUniversity,Istanbul,Turkey

H. Bahtiyar47, E. Barlas, K. Cankocak, F.I. Vardarlı, M. Yücel

IstanbulTechnicalUniversity,Istanbul,Turkey

L. Levchuk, P. Sorokin

NationalScientificCenter,KharkovInstituteofPhysicsandTechnology,Kharkov,Ukraine

J.J. Brooke, E. Clement, D. Cussans, H. Flacher, R. Frazier, J. Goldstein, M. Grimes, G.P. Heath, H.F. Heath, J. Jacob, L. Kreczko, C. Lucas, Z. Meng, D.M. Newbold48, S. Paramesvaran, A. Poll, S. Senkin, V.J. Smith, T. Williams

UniversityofBristol,Bristol,UnitedKingdom

K.W. Bell, A. Belyaev49, C. Brew, R.M. Brown, D.J.A. Cockerill, J.A. Coughlan, K. Harder, S. Harper, E. Olaiya, D. Petyt, C.H. Shepherd-Themistocleous, A. Thea, I.R. Tomalin, W.J. Womersley, S.D. Worm

RutherfordAppletonLaboratory,Didcot,UnitedKingdom

M. Baber, R. Bainbridge, O. Buchmuller, D. Burton, D. Colling, N. Cripps, M. Cutajar, P. Dauncey, G. Davies, M. Della Negra, P. Dunne, W. Ferguson, J. Fulcher, D. Futyan, A. Gilbert, G. Hall, G. Iles, M. Jarvis, G. Karapostoli, M. Kenzie, R. Lane, R. Lucas48, L. Lyons, A.-M. Magnan, S. Malik, B. Mathias, J. Nash, A. Nikitenko37, J. Pela, M. Pesaresi, K. Petridis, D.M. Raymond, S. Rogerson, A. Rose, C. Seez, P. Sharp†, A. Tapper, M. Vazquez Acosta, T. Virdee

ImperialCollege,London,UnitedKingdom

J.E. Cole, P.R. Hobson, A. Khan, P. Kyberd, D. Leggat, D. Leslie, W. Martin, I.D. Reid, P. Symonds, L. Teodorescu, M. Turner

BrunelUniversity,Uxbridge,UnitedKingdom

J. Dittmann, K. Hatakeyama, A. Kasmi, H. Liu, T. Scarborough

BaylorUniversity,Waco,USA

O. Charaf, S.I. Cooper, C. Henderson, P. Rumerio