Search For Single Production Of A Vector-Like T Quark Decaying To A Z Boson And A Top Quark İn Proton–Proton Collisions At √S=13tev

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Search

for

single

production

of

a

vector-like

T

quark

decaying

to

a

Z

boson

and

a

top

quark

in

proton–proton

collisions

at

√

s

=

13 TeV

.TheCMS Collaboration

CERN,Switzerland

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

Articlehistory:

Received3August2017

Receivedinrevisedform23March2018 Accepted15April2018

Availableonline23April2018 Editor:M.Doser

Keywords:

CMS Physics Vector-likequarks

A search is presented for single production of a vector-like quark (T) decaying to a Z boson and a top quark, with the Z boson decaying leptonically and the top quark decaying hadronically. The search uses data collected by the CMS experiment in proton–proton collisions at a center-of-mass energy of 13 TeV in 2016, corresponding to an integrated luminosity of 35.9 fb−1_{. The presence of forward jets is a particular}

characteristic of single production of vector-like quarks that is used in the analysis. For the first time, different T quark width hypotheses are studied, from negligibly small to 30% of the new particle mass. At the 95% confidence level, the product of cross section and branching fraction is excluded above values in the range 0.26–0.04 pb for T quark masses in the range 0.7–1.7 TeV, assuming a negligible width. A similar sensitivity is observed for widths of up to 30% of the T quark mass. The production of a heavy Zboson decaying to Tt, with T →tZ, is also searched for, and limits on the product of cross section and branching fractions for this process are set between 0.13 and 0.06 pb for Z _{boson masses in the range} from 1.5 to 2.5 TeV.

©2018 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3_.

1. Introduction

A possible extension of the standard model (SM), able to address some of the problems related to the nature of elec-troweaksymmetrybreaking,involvesheavyparticlescalled vector-like quarks (VLQs) [1–5]. Unlike the chiral fermions of the SM, thesenewparticlesdonotobtainmassthroughaYukawacoupling but through a direct mass term of the form mψ ψ. This means that theyare notexcluded by precision SMmeasurements asare fourth-generationchiralquarks [6].

Previous searches forVLQs have beenperformed by both the ATLAS [7–14] andCMS [15–22] Collaborations, aswell asby the D0 [23,24] andCDF [25–30] Collaborations.

We study the single production of vector-like T quarks with charge+2/3 thatdecaytoaZbosonandatquark.Wesearchfor afinalstatewithaZbosondecayingtoelectronsormuons,anda tquark producingjetsviathedecayt→Wb→qqb.Anexample ofaleading-order(LO)Feynmandiagramforthesingleproduction of a T quark in association witheither a b quark, denoted T(b), orat quark,denoted T(t),is showninFig.1 (top).Thethree de-caychannelsoftheTquarkintoSM particlesare bW,tZ,andtH. IftheT isa singletofthe SM,the equivalencetheorem [31]

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

pliesthatthebranchingfractionsforthethreedecaymodesofthe Tquarkareapproximately0.5,0.25,and0.25,respectively.IftheT isadoubletoftheSM,thedecaymodesaretZandtH,eachwith abranchingfractionof0.5.

The T quark could be singly produced inassociation with ei-therat orabquarkandanadditionalquark wouldbe produced intheforwardregionofthedetector.The couplingcoefficientsof the Tquark to SM particles are denotedC(bW) for theT(b) pro-cess,andC(tZ)fortheT(t)process.Theproductioncrosssectionof the Tquark dependson its massandwidth, aswell ason these couplings.The Tquarkcan haveboth left-handed(LH)and right-handed (RH) couplingsto SM particles.Inthe caseofa singletT quark, the RH chiralityis suppressed by a factorproportional to theSM quarkmassdivided bytheTquarkmass.Inthecaseofa doubletTquark,itistheLHchiralitythatissuppressed [32].

The present search is also sensitive to the production of a T quark together with a t quark in the decay of a heavy neutral spin-1 Z boson [33–35]. A LO Feynman diagram for this pro-duction modeis showninFig. 1(bottom).Thischannel was also consideredinRefs. [18,36].

ThissearchfollowsastrategysimilartothatusedbyRef. [18]. However,significantimprovementstothesensitivityofthemethod havebeenmadebyemployingacategorizationbasedonthe pres-enceofforwardjets,andbyanalyzingthemassspectrumof recon-structedTquarkcandidates,mtZ,ineventswherethetquark

prod-https://doi.org/10.1016/j.physletb.2018.04.036

0370-2693/©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

Fig. 1. Leading-orderFeynmandiagramsfortheproductionofasinglevector-likeT quarkanditsdecaytoaZbosonandatquark,eitherinassociationwithabquark oratquark(top),orinthedecayofaZbosontoTt(bottom).

uctsarehighlyLorentz-boostedandthereforearereconstructedas a single, large-radius jet. The presentanalysis alsobenefits from themuchlargerdatasamplerecordedin2016.Thispaperalso in-cludes the first results assuming a Tquark with a nonnegligible decaywidththatvariesbetween10and30%oftheTquarkmass. 2. TheCMSdetector,data,andsimulation

Thegeneral-purpose CMSdetectoroperatesatone ofthefour interactionpointsoftheLHC.Itscentralfeatureisa3.8 T supercon-ductingsolenoidmagnetwithan inner diameterof 6 m.The fol-lowingsubdetectorsarefoundwithinthemagnetvolume:asilicon tracker,a crystalelectromagnetic calorimeter(ECAL), and a brass andscintillator hadron calorimeter (HCAL).Muons are measured ingas-ionizationdetectorsembeddedinthesteelflux-returnyoke outsidethesolenoid. In addition,the CMSdetectorhasextensive forward calorimetry: two steel and quartz-fiber hadron forward calorimeters that extend the HCAL coverage to regions close to thebeampipe,andcoverthepseudorapidityrange3.0<|η|<5.2. A moredetaileddescriptionof theCMSdetector,together witha definitionofthecoordinatesystemandkinematicvariables,canbe foundinRef. [37].

This analysis is based on the data collected by the CMS ex-periment in proton–proton collisions at a center-of-mass energy of13 TeV in 2016, corresponding to an integrated luminosity of 35.9 fb−1.Events witha Zbosondecayingto muonsareselected onlinebyrequiringthepresence ofanisolated muonwith trans-versemomentum pT>24 GeV.EventswiththeZbosondecaying toelectronsareselectedonlineifanelectronisreconstructedwith

pT>115 GeV.Itispossibleto usethisrelatively highpT thresh-oldwithoutlosingsignal efficiency,since theelectronsofinterest arisefromthedecayofaheavyresonance.

Background events are generated using the next-to-LO (NLO) generator MadGraph5_amc@nlo 2.2.2 [38] forZ/γ∗+jets,tt+V, and tZq processes, and the NLO generator powheg 2.0 [39–42]

for tt and single t quark production. They are interfaced with pythia 8.212 [43], with the tune cuetp8m2t4 [44] used for the descriptionofpartonhadronizationandfragmentation.Events for dibosonproductionaregeneratedatLOusing MadGraph 5.2and atNLOwith powheg 2.0.SimulatedeventsarenormalizedtoNLO crosssectionsforallprocessesexceptfortt,singletquark produc-tionanddiboson(WWonly)processes,wherenext-to-NLOvalues areused.

Signal events withthe Tquark produced eitherdirectly or in thedecayofaZ bosonaregeneratedatLOusing MadGraph 5.2 interfaced to pythia 8.212. For the single production of the T quark,differentTquarkwidthhypothesesareconsidered: negligi-blysmallandlargerwidths(10,20,and30%oftheTquarkmass). Spincorrelationsaretreatedinthedecaywith madspin [45].

In the case where the T and Z particles are generated with narrow widths, i.e., negligibly small with respect to the experi-mentalreconstructedmassresolution,TquarkmassesmTbetween 0.7 and 1.7 TeV in steps of 0.1 TeV, and Z masses mZ of 1.5, 2.0, and 2.5 TeV are considered. The singlet T(b) signal process withLHcouplingstoSMparticles,anddoubletT(t)signalprocess withRHcouplings,aregenerated.Theoreticalcrosssectionsforthe narrow-widthTquarkassumptionarelistedinTable1,calculated followingtheprocedures describedinRef. [4],whereasimplified approachisusedtoprovideamodel-independentinterpretationof experimentalresults.ThewidthoftheVLQisnegligiblecompared totheexperimentalmassresolutionforC(bW)andC(tZ)couplings ≤0.5.

SignalsforTquarkswithlargerwidths(10,20,and30%ofthe Tquarkmass) aregeneratedinthesamemassrangebutinsteps of 0.2 TeV. The effect of the finite-width approximation is eval-uated using a modified version of themodel constructed by the authorsofRefs. [5,46,47].Modifications ofthepublished versions werenecessarytoprovideasimulationofthefull2→4process, i.e., pp→Tbq/Ttq→tZbq/tZtq, inthe finite-width hypothesis.It hasbeenverifiedthattheinterferenceofthe2→4processwith theSMbackgroundprocessesisnegligible.

In thegeneral case, the totalproduction cross section fora T quarkwithafinitewidth(FW)canbewrittenas:

σFW(C1,C2,mT,(C1,C2,Ci,mT,mj))= C2₁C2₂σ˜FW(mT,(C1,C2,Ci,mT,mj)) ,

(1)

where(C1,C2,Ci,mT,mj)isthewidthoftheTquark, C1 andC2

are its couplingsto SM quarksandbosons in thespecific single-productionprocessunderconsideration,Ci summarizesother

pos-sible couplings that allow the T to decay to other final states, and the quantities mj represent the masses of the decay

prod-Table 2

Theoreticalreducedcrosssections ˜σFWforsingleproductionofaTquarkwithaboratquark,wheretheT

quarkdecaystotZanditswidthis10,20,and30%ofitsmass,forthebenchmarkmassesconsideredinthe analysis.Thecorrespondingleadingordercrosssectionsσforthespecifiedproductionanddecayareshown inparentheses.

mT[TeV] σ˜FW(σ) for pp→Tbq→tZbq [pb] σ˜FW(σ) for pp→Ttq→tZtq [pb]

10% 20% 30% 10% 20% 30% 0.8 226(0.675) 108(0.650) 70 (0.631) 19 (0.144) 9.3 (0.139) 6.0 (0.135) 1.0 183(0.314) 87(0.299) 55 (0.284) 17 (0.075) 7.9 (0.072) 5.0 (0.069) 1.2 145(0.158) 68(0.149) 43 (0.141) 14 (0.042) 6.4 (0.039) 4.1 (0.037) 1.4 112(0.084) 52(0.079) 33 (0.074) 11 (0.024) 5.0 (0.022) 3.2 (0.021) 1.6 85(0.047) 39(0.043) 29 (0.041) 8.2 (0.014) 3.8 (0.013) 2.4 (0.012)

uctsof the T quark. The σ˜FW is the “reduced crosssection” and it corresponds to the physical cross section after factorizing the productioncrosssectionandthedecaycouplings.Fortheprocess pp→Ttq→tZtq the couplings are C1=C2= (gw/2)C(tZ),while for pp→Tbq→tZbq the couplings are C1= (gw/2)C(bW) and C2= (gw/2)C(tZ). The normalization factor gw/2 hasbeen intro-ducedtoproperlycomparethecouplingsasdefinedinRef. [4] and inEq. (1). InTable2,thevaluesforσ˜FW areshowntogetherwith thecrosssectionsforthesingletT(b)anddoubletT(t)signalsused tointerprettheresults.Thesecrosssectionsarecalculatedby fix-ingthebranchingfractionsoftheTtotheexpectedvaluesinthe narrow-width approximation, as described above and in Ref. [4]. Thischoicecorrespondstodifferentsetsofcouplingsthantheones usedinthenarrowwidthapproximation.

The generated events are passed through a simulation of the CMSdetectorbased on Geant4 [48,49].The numberofadditional interactions in the same or adjacent bunch crossings (pileup) is included in simulation witha distribution ofthe number of ad-ditionalinteractions matchingthat observedin data.Samplesare generated using the nnpdf 3.0 [50] parton distribution function (PDF)sets,matchingtheperturbativeorderusedinsimulation. 3. Objectreconstruction

Primaryverticesarereconstructedusingadeterministic anneal-ingfilteralgorithm [51].Thereconstructedvertexwiththelargest valueofsummedphysics-object p2_T istakentobe theprimarypp interactionvertex.Thephysicsobjectsaretheobjectsreturnedby ajet findingalgorithm [52,53] appliedtoall chargedtracks asso-ciatedwiththevertex, plus thecorresponding associatedmissing transverse momentum. Selected events are required to have this primary vertex within 24 cm of the center of the detectoralong thez-direction,andwithin2 cm inthex– y plane.

Aparticle-flow(PF)algorithm [54] isusedtoidentifyandto re-constructchargedandneutralhadrons, photons,muons,and elec-trons,throughanoptimalcombinationoftheinformationfromthe entiredetector.

Electroncandidates are reconstructed by combiningthe infor-mationfromtheECALandfromthesilicontracker [55].Electrons arethenselectediftheyareisolatedandiftheyhavepT>20 GeV andpseudorapidity |η|<2.5.Additionalrequirementsareapplied totheenergydistributioninthe ECAL,tothe geometrical match-ing ofthetrackerinformation tothe ECALenergycluster, onthe impactparameters of thechargedtracks, andonthe ratioofthe energiesmeasuredintheHCALandtheECALintheregionaround the electron candidate. The leading electron is required to have

pT>120 GeV, in order to be in the region where the trigger is closeto100%efficiency.

Muoncandidatesarereconstructedbycombininginaglobalfit theinformationfromthesilicontrackerandthemuonsystem [56]. Muonsarethenrequiredtobeisolated,tosatisfypT>20 GeV and |η|<2.4,andtopassadditionalidentificationcriteriabasedonthe

trackimpactparameter,thequalityofthetrackreconstruction,and thenumberofhitsrecordedinthetrackerandthemuonsystems. Like the leading electron, the leading muon is required to have

pT>120 GeV.

For both muons and electrons, a lepton isolation variable is usedtoreducebackgroundfromeventsinwhichajetis misidenti-fiedasalepton.ThisvariableisdefinedasthescalarsumofthepT ofthechargedandneutralhadronsandphotonsinaconeofsize R=(η)2_{+ (φ)}2_{aroundtheoriginalleptontrack,corrected} fortheeffectsofpileup [55,56],anddividedbythelepton pT.The conesizeis0.4formuonsand0.3forelectrons.

Jet candidatesare clusteredfromthe PF candidates using the anti-kT clustering algorithm [52] withdistance parameters of0.4 (“AK4jets”) and0.8(“AK8jets”).Thejetenergyscale(JES)is cal-ibratedthroughcorrectionfactorsdependentonthe pT, η,energy density,andareaofthejet.Thejetenergyresolution(JER)forthe simulatedjetsisdegradedtoreproducetheresolutionobservedin data. The AK4jet candidates are required to have pT>20 GeV, |η|<2.4 andtobeseparatedbyR>0.4 fromanidentified lep-ton. The AK8jet candidates are required to have pT>180 GeV, |η|<2.4 and to be separated by R>0.8 from an identified lepton.TheAK8jetsmaybetaggedascomingfromaWboson de-cayingtoqq (denoted“Wjets”)orfroma tquark decayingfully hadronically (“tjets”).FortheWjets, apruningalgorithm [57] is applied.Themassofthejet,afterthepruningisperformed,isused asa discriminantto selectW bosons andreject quarkandgluon jets. ThediscriminationbetweenWjetsandjetsfromquarksand gluons is further improved by requiring the N-subjettiness ratio τ21 tobe lessthan 0.6, where τ21=τ2/τ1 [58], andthe massof theprunedAK8jettobewithintherange65–105 GeV.Ina simi-larway,AK8jetsmaybeidentifiedasarisingfromtheall-jetsfinal stateofatquark.Thesetjetsarerequiredtohave pT>400 GeV, massofthejetreconstructedthroughthemodifiedmassdrop tag-geralgorithm [59,60] between105and220 GeV,and τ32=τ3/τ2 less than 0.81. Finally, AK4 jets may be tagged as arising from a b quark (“b jets”) using the combined secondary vertex algo-rithm [61,62].A“medium”workingpointwithanefficiencyof70% forgenuinebjetsandarejectionof99%oflight-flavorjetsisused, together witha“loose”workingpoint thathasan 85% identifica-tionefficiencyandrejects90%oflight-flavoredjets.Theefficiency foridentifyingW,t,andbjetsinsimulationiscorrectedtomatch theresultsfoundindata.

An interesting feature of the direct production of a single vector-like T quark is the presence of an additional jet that is produced intheforwarddirection. Forwardjetsare reconstructed asAK4 jetsusingthe sameselections andcorrectionsasdefined above,buthave2.4<|η|<5.0 andpT>30 GeV.

4. Eventselection

Eventsarerequiredtohavetwooppositelychargedleptons (ei-thermuonsorelectrons)formingaZbosonwithaninvariantmass

resolved (three AK4 jets are reconstructed). We therefore define teneventcategories,dependingonhowtheZbosonorthetquark candidates are reconstructed andon the number of forwardjets present,assummarizedinTable3.

The hierarchy places the most sensitive categories first. If an eventfallsintotwo ormore categoriesitis assignedonlyto the first.Forcategories1and2,thetquark candidateisgivenbythe t jet; forcategories3–6 it isreconstructed by summing the mo-mentum vectorsofthe W jet andthe bjet; while forcategories 7–10themomentaofthethreejetsaresummed.Ifmorethanone tquarkcandidateisfound,theonewiththelargest pT isselected forthesubsequentrestoration.

Inadditionto requiringaZbosonanda t quarkin theevent, atleast one “medium”b jet has to be present (for the partially merged and the resolved categories, it is the one used to re-constructthe t quark), the two leptons from the Z boson decay haveto be closeto each other (R<0.6–1.4,depending on the category), and the leading lepton (muon or electron) must have

pT>120 GeV.Ifmorethanone mediumbjetispresent,theone givingthelargesttquark pT isselectedforsubsequent reconstruc-tion.Furthermore,intheresolvedcategories,thetwojetswiththe lowestbtaggingdiscriminantofthethreejetsformingthetquark candidateare requiredtohaveadijetinvariantmassmj1,j2 below 200 GeV. All these requirementswere optimized to increase the sensitivityoftheanalysisandaresummarizedinTable3.

The Tquark candidate massmtZ is obtainedby summing the momentaoftheZcandidate,givenbythetwomuonsorthetwo electrons, andthe t quark candidate,reconstructed for the three scenariosasdescribedabove.

5. Backgroundestimate

Inthisanalysis, the signal is searchedfor asan excess inthe mass spectrum of reconstructed T quark candidates, mtZ, which is used asthe discriminating variable. The background is largely dominatedby Z/γ∗+jets events (>80%),with smaller contribu-tions from other sources (tt+V, tZq, tt, single t quark, and VV dibosonproduction,whereVrepresentsaWorZboson).

Thebackgroundisestimatedfromdatainordertoreduce de-pendenceonthesimulation.Thisestimate, whichincorporatesall ofthebackgroundprocessesdescribedabove,isobtainedby mea-suringthemt Z distributioninacontrolregiondefinedbyapplying

theeventselectiondescribedinSection4,butinsteadofrequiring thepresenceofajetpassingthemediumbtaggingrequirements, a vetoisappliedonthepresenceofanyjetpassingthelooseb tag-gingrequirements.This vetoeffectivelyremovesthe signal while leavingasubstantialfractionofthedominantZ+jets background.

quoteduncertaintiesin the backgroundestimates include both statisticalandsystematiccomponents,asdescribedinSection6. Expectedsignalyieldsandtheirrespectiveefficienciesin paren-thesesaregivenfortwobenchmarkmassesandtwovaluesofthe width“”,foraTquarkproducedinassociationwithab,T(b), andaTquarkproducedinassociationwithat,T(t).Thesignal ef-ficienciesarecalculatedforeventswiththeZbosondecayingto electronsormuons.Background,data,andsignalyieldsareshown fortherangeinmtZbetween500and2100 GeV.

Channel 2μ+1 t-jet 2e+1 t-jet Estimated background 37.3±4.6 25.8±4.1 Data events 33 31 T(b), mT=0.8 TeV,0 1.2(0.2%) 0.9(0.1%) T(b), mT=0.8 TeV,=0.3mT 22.9(1%) 17.1(1%) T(t), mT=0.8 TeV,0 1.3(1%) 1.0(1%) T(t), mT=0.8 TeV,=0.3mT 6.3(2%) 5.4(2%) T(b), mT=1.6 TeV,0 2.9(6%) 2.6(6%) T(b), mT=1.6 TeV,=0.3mT 5.3(5%) 4.8(5%) T(t), mT=1.6 TeV,0 0.8(6%) 0.7(6%) T(t), mT=1.6 TeV,=0.3mT 1.5(5%) 1.4(5%)

The background expectation inthe signal region is then esti-matedas:

Nbkg(mtZ)=NCR(mtZ)α(mtZ), (2) where NCR(mtZ) is the number of events found in the data in the control region as a function of mtZ, and α(mtZ) is the ra-tioobtainedfromsimulationofthenumberofbackgroundevents in the signal region to that in the control region, at each value of mtZ. A closure test is performed to validate the method in an independentsignal-freeregion, definedbyconsidering the re-solved categories andinverting thecut on mj1,j2.This region has beenchosen becauseithasa negligiblesignalcontamination and yetit preservesthebackground compositionof thesignal region. Good agreementis foundbetweenthe predictedbackgroundand the observed data in this region, showing the robustness of the backgroundestimationmethod.Furthermoreagoodagreement is alsofoundbetweenthepredictedbackgroundusingthedescribed methodandthepredictedbackgroundfromthesimulatedevents.

Comparisonsbetweenthebackgroundestimatesandthe obser-vations in data in the mtZ distribution are shown in Figs. 2, 3, and4.Thenumberofpredictedbackgroundeventsandthe num-ber of observed events are reported in Tables 4, 5, and 6, to-getherwiththenumberofexpectedsignaleventsfortwoexample masses. The numbers ofobserved events are consistent withSM backgroundpredictions.

Fig. 2. Comparisonbetweenthedata,thebackgroundestimate,andtheexpectedsignalforthe2categorieswheretheTquarkisreconstructedinthefullymergedtopology, foreventswiththeZbosondecayingintomuons(left)andelectrons(right).Thebackgroundcompositionistakenfromsimulation.Theuncertaintiesinthebackground estimateincludebothstatisticalandsystematiccomponents.Theexpectedsignalisshownfortwobenchmarkvaluesofthewidth,foraTquarkproducedinassociationwith ab,T(b):narrow-widthapproximation(NW)and30%oftheTquarkmass.Thelowerpanelineachplotshowstheratioofthedataandthebackgroundestimation,with theshadedbandrepresentingtheuncertaintiesinthebackgroundestimate.TheverticalbarsforthedatapointsshowthePoissonerrorsassociatedwitheachbin,whilethe horizontalbarsindicatethebinwidth.

Table 5

Thenumberofestimatedbackgroundeventscomparedtotheobservednumberofeventsforthefourpartiallymergedcategories. Thequoteduncertaintiesinthebackgroundestimatesincludebothstatisticalandsystematiccomponents,asdescribedinSection6. Expectedsignalyieldsandtheirrespectiveefficienciesinparenthesesaregivenfortwobenchmarkmassesandtwovaluesofthe width“”,foraTquarkproducedinassociationwithab,T(b),andaTquarkproducedinassociationwithat,T(t).Thesignal efficienciesarecalculatedfor eventswith theZboson decayingtoelectronsor muons.Background,data,and signalyieldsare shownfortherangeinmtZbetween500and2100 GeV.

Channel 2μ+1 W-jet+1 b-jet 2e+1 W-jet+1 b-jet 2μ+1 W-jet+1 b-jet 2e+1 W-jet+1 b-jet

N (forward jets)=0 N (forward jets)>0

Estimated background 17.2±2.0 14.5±1.9 8.5±1.8 5.7±1.6 Data events 21 16 3 7 T(b), mT=0.8 TeV,0 2.7 (0.5%) 1.7 (0.3%) 5.4(0.9%) 4.3 (0.7%) T(b), mT=0.8 TeV,=0.3mT 8.2 (0.5%) 5.0 (0.3%) 12.2(0.8%) 9.5 (0.6%) T(t), mT=0.8 TeV,0 0.9 (0.8%) 0.8 (0.7%) 2.0(2%) 1.5 (1%) T(t), mT=0.8 TeV,=0.3mT 2.8 (0.9%) 2.1 (0.6%) 4.7(1%) 3.9 (1%) T(b), mT=1.6 TeV,0 0.2 (0.3%) 0.2 (0.3%) 0.4(0.9%) 0.3 (0.6%) T(b), mT=1.6 TeV,=0.3mT 0.4 (0.4%) 0.3 (0.3%) 0.7(0.7%) 0.6 (0.6%) T(t), mT=1.6 TeV,0 0.1 (0.7%) 0.1 (0.5%) 0.2(1%) 0.2 (1%) T(t), mT=1.6 TeV,=0.3mT 0.2 (0.7%) 0.2 (0.6%) 0.4(1%) 0.4 (1%) Table 6

Thenumberofestimatedbackgroundeventscomparedtotheobservednumberofeventsforthefourresolvedcategories.Thequoted uncertaintiesinthebackgroundestimatesincludebothstatisticalandsystematiccomponents,asdescribedinSection6.Expected signalyieldsandtheirrespectiveefficienciesinparenthesesaregivenfortwobenchmarkmassesandtwovaluesofthewidth“”, foraTquarkproducedinassociationwithab,T(b),andaTquarkproducedinassociationwithat,T(t).Thesignalefficienciesare calculatedforeventswiththeZbosondecayingtoelectronsormuons.Background,data,andsignalyieldsareshownfortherange inmtZbetween500and2100 GeV.

Channel 2μ+1 b-jet+2 jets 2e+1 b-jet+2 jets 2μ+1 b-jet+2 jets 2e+1 b-jet+2 jets

N (forward jets)=0 N (forward jets)>0

Estimated background 315±16 228±13 108.3±7.5 66.2±5.7 Data events 339 239 115 88 T(b), mT=0.8 TeV,0 13.7(2%) 10.0(2%) 25.7(4%) 18.5(3%) T(b), mT=0.8 TeV,=0.3mT 35.9(2%) 29.7(2%) 66.5(4%) 52.7(3%) T(t), mT=0.8 TeV,0 2.5(2%) 2.0(2%) 5.0(5%) 4.0(4%) T(t), mT=0.8 TeV,=0.3mT 8.9(3%) 6.7(2%) 15.8(5%) 12.0(4%) T(b), mT=1.6 TeV,0 1.0(2%) 0.9(2%) 2.5(5%) 2.0(4%) T(b), mT=1.6 TeV,=0.3mT 2.2(2%) 1.9(2%) 4.7(5%) 3.9(4%) T(t), mT=1.6 TeV,0 0.3(3%) 0.3(2%) 0.8(6%) 0.7(5%) T(t), mT=1.6 TeV,=0.3mT 0.8(3%) 0.7(2%) 1.7(6%) 1.5(5%)

Fig. 3. Comparisonbetweenthedata,thebackgroundestimate,andtheexpectedsignalforthe4categorieswheretheTquarkisreconstructedinthepartiallymerged topology,foreventswith theZbosondecayinginto muons(left)andelectrons(right),andzero(atleastone)forwardjetsintheupper(lower)row.Thebackground compositionistakenfromsimulation.Theuncertaintiesinthebackgroundestimateincludebothstatisticalandsystematiccomponents.Theexpectedsignalisshownfor twobenchmarkvaluesofthewidth,foraTquarkproducedinassociationwithab,T(b):narrow-widthapproximation(NW)and30%oftheTquarkmass.Thelowerpanel ineachplotshowstheratioofthedataandthebackgroundestimation,withtheshadedbandrepresentingtheuncertaintiesinthebackgroundestimate.Theverticalbars forthedatapointsshowthePoissonerrorsassociatedwitheachbin,whilethehorizontalbarsindicatethebinwidth.

6. Systematicuncertainties

Systematiceffectshavebeenevaluatedbypropagating the un-certaintiesintheinputquantities.Unlessexplicitlystated,the im-pactoftheseuncertaintiesareevaluatedbothinthenormalization andintheshapeofthedistributionofmtZ.

Five main sources of uncertainty contribute to the estimated background.Thedominantonesarethestatisticaluncertaintiesin thecontrolregionsusedtoestimatethebackground,bothindata, givinganuncertaintyof10–46%dependingonthecategory,andin thesimulation,withanuncertaintyof3–34%.Thesmalldifferences between the observation and the prediction for the closure test describedpreviouslyaretakenassystematicuncertainties(6%).An uncertaintyduetopossiblemismodeling oftheZ+light quarkand Z+b quarkfractionsinthesimulation isevaluated.This system-atic uncertaintyis evaluated by observing the effectof changing theZ+b fractionby10% [63],yieldingacontributiontothe uncer-taintyinthebackgroundestimationofbetween2and4%.Finally,

the uncertainty from the b tagging forthe b, c,and light-flavor jets is evaluated by changing the b tagging corrections by their uncertainties [61,62], yielding a change in the normalization of 2% for the b tagging efficiency and 2% for the misidentification probability. Other systematic uncertainties related to the simula-tionmodeling havebeenstudiedandfoundnegligible,becauseof thedata-drivenmethodusedtoestimatethebackground.

Thesystematicuncertaintyinthesignal isestimatedfromthe corrections applied to the simulation to match distributions in data. The corrections for lepton identification and lepton trigger efficiency are obtained from dedicated analyses, using the “tag-and-probe” method [55,56]. Changing these corrections by their uncertainties providesanestimate oftheuncertaintiesinthe sig-nalyield of3%formuonsandelectrons fora masshypothesis of 1.0 TeV, and 1% for the trigger. The jet four-momenta are var-ied by the JES and JER uncertainties, which provide respective changes in the signal yield of 1% (JES) and0.5% (JER), while for forwardjetsachangeof8% isobserved.ForW andt jettagging,

Fig. 4. Comparisonbetweenthedata,thebackgroundestimate,andtheexpectedsignalforthe4categorieswheretheTquarkisreconstructedintheresolvedtopology,for eventswiththeZbosondecayingintomuons(left)andelectrons(right),andzero(atleastone)forwardjetsintheupper(lower)row.Thebackgroundcompositionistaken fromsimulation.Theuncertaintiesinthebackgroundestimateincludebothstatisticalandsystematiccomponents.Theexpectedsignalisshownfortwobenchmarkvalues ofthewidth,foraTquarkproducedinassociationwithab,T(b):narrow-widthapproximation(NW)and30%oftheTquarkmass.Thelowerpanelineachplotshowsthe ratioofthedataandthebackgroundestimation,withtheshadedbandrepresentingtheuncertaintiesinthebackgroundestimate.Theverticalbarsforthedatapointsshow thePoissonerrorsassociatedwitheachbin,whilethehorizontalbarsindicatethebinwidth.

the same procedure of varying the corrections is applied, yield-ing an uncertainty of 4 and 8%, respectively. The uncertainty in the b tagging efficiency is evaluated, as forthe background; the changein yield ofthe signal is foundto be 2.5%. The uncertain-tiesfromthechoiceofPDFareevaluatedusingthe nnpdf 3.0PDF eigenvectors [64],consideringonlythechangeintheshapeofthe

mtZdistribution.Theuncertaintyinthesimulationofpileupis ob-tainedby changingtheinelasticcross section,whichcontrolsthe averagepileup multiplicity,by 5% [65],resulting inasignal yield uncertaintyof1%.Additionalsourcesofsystematicuncertaintyare theintegrated luminosity(2.5%, normalizationonly) [66] andthe factorizationandrenormalizationscalesusedinsimulation(shape only).

7. Results

Nosignificantdeviationsfromtheexpectedbackgroundare ob-served inanyofthesearch channels.We setupperlimitsonthe

product of the crosssection andbranching fractionof a Tquark decaying to tZ. The exclusionlimits ata confidence level(CL) of 95%are obtainedusingtheasymptoticCLscriterion [67–70],with templatesforbackgroundandsignalgivenbythebinned distribu-tions inFigs. 2, 3,and 4.Systematic uncertainties are treatedas nuisance parameters,assuming a log-normal distribution for nor-malization parameters anda Gaussian distribution forsystematic uncertaintiesthataffectthemtZshape.

In Fig.5,theobserved andexpectedlimitsfromtheten cate-goriesoftheTquarksearchareshowncombinedtogether,forthe singletLHT(b)(left)anddoubletRHT(t)(right)productionmodes. Thetencategorieshavedifferentsensitivitiestodifferentvaluesof

mT, andthefinal resultbenefits fromthisbehavior: the resolved categories drivethe limitat lowmT,the fullymergedcategories, athighervalues,while atintermediatevalues thelimit takes ad-vantageofallthethreetopologies.Limitson σ(pp→Tbq→tZbq) forthesingletLHT(b)excludevaluesgreaterthan 0.26–0.04 pb at

Fig. 5. Observedandexpectedlimitsat95%CLontheproductofthesingleproductioncrosssectionandbranchingfractionforthesingletLHTquarkproducedinassociation withabquark(left)andforthedoubletRHTquarkproducedinassociationwithatquark(right),wheretheTquarkhasanarrowwidthanddecaystotZ.Theinner greenandouteryellowbandsrepresentthe1and2standarddeviationuncertaintiesintheexpectedlimit.Theredlinesindicatetheoreticalcrosssections,ascalculatedat next-to-leadingorderinRef. [4].ThebranchingfractionB(T→tZ)is0.25(0.5)fortheleft(right)plot.(Forinterpretationofthecolorsinthefigure(s),thereaderisreferred tothewebversionofthisarticle.)

Table 7

Observedandexpected95% CLupperlimitonσ(pp→Z)B(Z→Tt)B(T→tZ).The ±1and ±2standard devi-ation(s.d.)expectedlimitsarealsogiven.Thelimitsaregivenin pb.

mZ[TeV] mT[TeV] Observed Expected Expected−1(2)s.d. Expected+1(2)s.d.

1.5 0.7 0.13 0.10 0.07 (0.05) 0.14 (0.19) 1.5 0.9 0.11 0.08 0.06 (0.05) 0.12 (0.16) 1.5 1.2 0.09 0.05 0.04 (0.03) 0.07 (0.10) 2.0 0.9 0.08 0.06 0.04 (0.03) 0.08 (0.11) 2.0 1.2 0.08 0.05 0.04 (0.03) 0.07 (0.09) 2.0 1.5 0.06 0.04 0.03 (0.02) 0.05 (0.07) 2.5 1.2 0.06 0.05 0.03 (0.02) 0.06 (0.09) 2.5 1.5 0.06 0.04 0.03 (0.02) 0.05 (0.07)

95% CL,for massesinthe range 0.7–1.7 TeV. Foran RH T(t) sig-nal,theregionabove 0.14–0.04 pb isexcluded forthesamemass range.Upper limitsare compared withtheoretical cross sections calculatedatNLO inRef. [4].Forthismodel,asingletLHTquark withC(bW)=0.5 isexcluded at95% CL formassesin therange 0.7–1.2 TeV.

InFig.6,theobservedandexpectedupperlimitsat95%CLare shownasafunctionoftheTquarkwidthandTquarkmassinthe rangesfrom10to30%and0.8to1.6 TeV,respectively.Asensitivity similarto that obtainedassuminga narrow-width Tquark is ob-served.Inthiscasetheexperimentalresultsarecomparedwiththe theoreticalcrosssectionscalculatedatLOusingamodifiedversion ofthemodelconstructedbytheauthorsof [5,46,47] andreported inTable2.For thismodel,thedataexclude a singletLHTquark producedin associationwith ab quark, formassesbelowvalues intherange1.34and1.42 TeV dependingonthewidth.Adoublet RHTquarkproducedinassociationwithatquark isexcludedfor massesbelowvaluesintherange0.82and0.94 TeV.

Inadditiontobeingsinglyproduceddirectly,asdiagrammedin Fig.1(top),theTquark mayalsoappearsinglyineventswherea single Z is produced that decaysZ→Tt, asillustrated in Fig.1 (bottom). Observed and expected limits for the production of a T quark via the decay of a Z boson, Z→Tt and T→tZ, are shown in Table 7. We assume negligible widths for both the Z bosonandtheTquark. The productofcross sectionand branch-ingfractions isexcluded above 0.13–0.06 pb, fora Z bosonmass intherangefrom1.5to2.5 TeV andforaTquark massfrom0.7 to1.5 TeV.

8. Summary

Thispaperhaspresentedresultsofasearchforthesingle pro-ductionofaTquarkwithachargeof+2/3,decayingtoaZboson and a t quark. No deviations were observed relative to the ex-pected standard model background. Upper limitson the product of the cross section and branching fraction range between 0.26 and0.04 pb at95%confidencelevelforaleft-handedTquark pro-ducedin associationwitha b quark, T(b), andbetween0.14and 0.04 pb for a right-handed Tquark produced in association with a t quark,T(t), fortherangeofmassesbetween0.7and1.7 TeV. Thisresultwasobtainedunderthehypothesisofanarrow-widthT quark,providinganinterpretationofresultsthroughthesimplified approachofRef. [4].Inthiscase,left-handedTquarksproducedin associationwithabquarkandwithacouplingC(bW)of0.5were excluded formassesinthe range0.7–1.2 TeV.A largegain inthe search sensitivity was found relative to previous results [18] be-causeof improvements introduced inthe analysis aswell asthe increase in the integratedluminosity. The effectof a nonnegligi-blewidthwas also studied;values ofthewidthbetween10 and 30% of the T quark mass were considered, and similar sensitivi-tieswereobserved.Theresultswereinterpretedusingamodified version of the model constructed by the authors of Refs. [5,46, 47], and a left-handed T(b) signal was excluded for masses be-low valuesin the range 1.34–1.42 TeV, depending on the width, while a right-handed T(t) signal was excluded formasses below valuesin therange0.82–0.94 TeV. Finally,theproductionof aZ boson that decays to Tt was excluded for values of the product ofcrosssectionandbranchingfractionsbetween0.13–0.06 pb,for

Fig. 6. Observed(upper)andexpected(lower)limitsat95%CLontheproductofthesingleproductioncrosssectionandbranchingfractionforthesingletLHTquark producedinassociationwithabquark(left)andforthedoubletRHTquarkproducedinassociationwithatquark(right),wheretheTquarkhasawidthfrom10%to30% ofitsmassanddecaystotZ.Thesolidblacklinesindicatetheoreticalcrosssections,ascalculatedatleadingorderusingamodifiedversionofthemodelconstructedbythe authorsofRefs. [5,46,47] andreportedinTable2.Ineachplot,theexcludedregionliestotheleftoftheline,exceptinthelower-leftplotwheretheentireregionshownis excluded.

Z boson and T quark masses in the respective ranges of 1.5 to 2.5 TeV and0.7to1.5 TeV.Theresultspresentedinthispaperare themost-stringentlimitstodateonthesingleproductionofheavy vector-like Tquarks, the first to set limits for a variety of reso-nancewidths,andthemost-stringentlimitsfortheproductionof aZ bosondecayingtoTt.

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,wegratefullyacknowledgethecomputingcentersand 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, and NSFC (China); COLCIEN-CIAS(Colombia);MSESandCSF(Croatia);RPF(Cyprus);SENESCYT (Ecuador); MoER, ERC IUT, and ERDF (Estonia); Academy of Fin-land,MEC,andHIP(Finland);CEAandCNRS/IN2P3(France);BMBF,

DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH (Hun-gary);DAEandDST(India);IPM(Iran);SFI(Ireland);INFN (Italy); MSIPandNRF (RepublicofKorea);LAS(Lithuania);MOE andUM (Malaysia); BUAP,CINVESTAV, CONACYT, LNS,SEP, andUASLP-FAI (Mexico); MBIE (New Zealand); PAEC (Pakistan); MSHE and NSC (Poland);FCT(Portugal);JINR(Dubna);MON,RosAtom,RAS,RFBR andRAEP(Russia);MESTD (Serbia); SEIDI,CPAN,PCTIandFEDER (Spain);SwissFundingAgencies(Switzerland);MST(Taipei); ThEP-Center, IPST, STAR, and NSTDA (Thailand); TUBITAK and TAEK (Turkey);NASU andSFFR (Ukraine);STFC(United Kingdom);DOE andNSF(USA).

Individuals have received support from the Marie-Curie pro-gramandtheEuropeanResearchCouncilandHorizon2020Grant, contract No. 675440 (European Union);the Leventis Foundation; the A. P. Sloan Foundation; the Alexander von Humboldt Foun-dation; theBelgian Federal Science PolicyOffice; the Fonds pour la Formationà laRecherche dansl’Industrie etdansl’Agriculture (FRIA-Belgium); the Agentschap voor Innovatie doorWetenschap en Technologie (IWT-Belgium); the Ministry of Education, Youth and Sports (MEYS) of the Czech Republic; the Council of Sci-ence and Industrial Research, India; the HOMING PLUS program of the Foundation for Polish Science, cofinanced from European Union, Regional DevelopmentFund, theMobilityPlusprogram of theMinistryofScienceandHigherEducation,theNationalScience

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TheCMSCollaboration

A.M. Sirunyan,A. Tumasyan

YerevanPhysicsInstitute,Yerevan,Armenia

W. Adam, F. Ambrogi, E. Asilar,T. Bergauer, J. Brandstetter, E. Brondolin,M. Dragicevic, J. Erö,M. Flechl, M. Friedl,R. Frühwirth1,V.M. Ghete, J. Grossmann, J. Hrubec, M. Jeitler1, A. König, N. Krammer,

I. Krätschmer,D. Liko, T. Madlener,I. Mikulec, E. Pree, D. Rabady, N. Rad, H. Rohringer, J. Schieck1, R. Schöfbeck, M. Spanring, D. Spitzbart,W. Waltenberger, J. Wittmann, C.-E. Wulz1, M. Zarucki

InstitutfürHochenergiephysik,Wien,Austria

V. Chekhovsky, V. Mossolov,J. Suarez Gonzalez

InstituteforNuclearProblems,Minsk,Belarus

E.A. De Wolf,D. Di Croce, X. Janssen, J. Lauwers,H. Van Haevermaet, P. Van Mechelen,N. Van Remortel

UniversiteitAntwerpen,Antwerpen,Belgium

S. Abu Zeid,F. Blekman, J. D’Hondt, I. De Bruyn, J. De Clercq, K. Deroover, G. Flouris, D. Lontkovskyi, S. Lowette,S. Moortgat, L. Moreels, Q. Python, K. Skovpen,S. Tavernier, W. Van Doninck, P. Van Mulders, I. Van Parijs

N. Beliy

UniversitédeMons,Mons,Belgium

W.L. Aldá Júnior, F.L. Alves,G.A. Alves,L. Brito, M. Correa Martins Junior,C. Hensel, A. Moraes,M.E. Pol, P. Rebello Teles

CentroBrasileirodePesquisasFisicas,RiodeJaneiro,Brazil

E. Belchior Batista Das Chagas, W. Carvalho,J. Chinellato3, A. Custódio, E.M. Da Costa, G.G. Da Silveira4, D. De Jesus Damiao,S. Fonseca De Souza, L.M. Huertas Guativa, H. Malbouisson, M. Melo De Almeida, C. Mora Herrera,L. Mundim,H. Nogima, A. Santoro, A. Sznajder,E.J. Tonelli Manganote3,

F. Torres Da Silva De Araujo,A. Vilela Pereira

UniversidadedoEstadodoRiodeJaneiro,RiodeJaneiro,Brazil

S. Ahujaa,C.A. Bernardesa, T.R. Fernandez Perez Tomeia,E.M. Gregoresb,P.G. Mercadanteb, S.F. Novaesa, Sandra S. Padulaa,D. Romero Abadb,J.C. Ruiz Vargasa

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

A. Aleksandrov, R. Hadjiiska,P. Iaydjiev, M. Misheva, M. Rodozov,M. Shopova, S. Stoykova, G. Sultanov

InstituteforNuclearResearchandNuclearEnergyofBulgariaAcademyofSciences,Bulgaria

A. Dimitrov,I. Glushkov, L. Litov, B. Pavlov,P. Petkov

UniversityofSofia,Sofia,Bulgaria

W. Fang5, X. Gao5

BeihangUniversity,Beijing,China

M. Ahmad, J.G. Bian, G.M. Chen,H.S. Chen, M. Chen, Y. Chen, C.H. Jiang, D. Leggat,H. Liao, Z. Liu, F. Romeo,S.M. Shaheen, A. Spiezia,J. Tao, C. Wang, Z. Wang, E. Yazgan, H. Zhang,J. Zhao

InstituteofHighEnergyPhysics,Beijing,China

Y. Ban, G. Chen, Q. Li, S. Liu,Y. Mao, S.J. Qian,D. Wang, Z. Xu

StateKeyLaboratoryofNuclearPhysicsandTechnology,PekingUniversity,Beijing,China

C. Avila,A. Cabrera, L.F. Chaparro Sierra, C. Florez,C.F. González Hernández, J.D. Ruiz Alvarez

B. Courbon, N. Godinovic, D. Lelas,I. Puljak, P.M. Ribeiro Cipriano, T. Sculac

UniversityofSplit,FacultyofElectricalEngineering,MechanicalEngineeringandNavalArchitecture,Split,Croatia

Z. Antunovic, M. Kovac

UniversityofSplit,FacultyofScience,Split,Croatia

V. Brigljevic,D. Ferencek, K. Kadija,B. Mesic, A. Starodumov6, T. Susa

InstituteRudjerBoskovic,Zagreb,Croatia

M.W. Ather,A. Attikis, G. Mavromanolakis, J. Mousa,C. Nicolaou, F. Ptochos, P.A. Razis, H. Rykaczewski

UniversityofCyprus,Nicosia,Cyprus

M. Finger7, M. Finger Jr.7

CharlesUniversity,Prague,CzechRepublic

E. Carrera Jarrin

UniversidadSanFranciscodeQuito,Quito,Ecuador

Y. Assran8,9_{,S. Elgammal}9_{, A. Mahrous}10

AcademyofScientificResearchandTechnologyoftheArabRepublicofEgypt,EgyptianNetworkofHighEnergyPhysics,Cairo,Egypt

R.K. Dewanjee, M. Kadastik, L. Perrini, M. Raidal, A. Tiko, C. Veelken

NationalInstituteofChemicalPhysicsandBiophysics,Tallinn,Estonia

P. Eerola, J. Pekkanen,M. Voutilainen

DepartmentofPhysics,UniversityofHelsinki,Helsinki,Finland

J. Härkönen,T. Järvinen, V. Karimäki, R. Kinnunen, T. Lampén,K. Lassila-Perini, S. Lehti, T. Lindén, P. Luukka, E. Tuominen, J. Tuominiemi,E. Tuovinen

HelsinkiInstituteofPhysics,Helsinki,Finland

J. Talvitie, T. Tuuva

LappeenrantaUniversityofTechnology,Lappeenranta,Finland

M. Besancon, F. Couderc,M. Dejardin, D. Denegri, J.L. Faure, F. Ferri, S. Ganjour, S. Ghosh, A. Givernaud, P. Gras, G. Hamel de Monchenault, P. Jarry,I. Kucher, E. Locci, M. Machet, J. Malcles,G. Negro, J. Rander, A. Rosowsky, M.Ö. Sahin,M. Titov

IRFU,CEA,UniversitéParis-Saclay,Gif-sur-Yvette,France

A. Abdulsalam, I. Antropov, S. Baffioni, F. Beaudette, P. Busson, L. Cadamuro, C. Charlot,

R. Granier de Cassagnac, M. Jo,S. Lisniak, A. Lobanov, J. Martin Blanco, M. Nguyen,C. Ochando,

G. Ortona,P. Paganini, P. Pigard,S. Regnard, R. Salerno,J.B. Sauvan, Y. Sirois, A.G. Stahl Leiton, T. Strebler, Y. Yilmaz,A. Zabi, A. Zghiche

LaboratoireLeprince-Ringuet,Ecolepolytechnique,CNRS/IN2P3,UniversitéParis-Saclay,Palaiseau,France

J.-L. Agram11, J. Andrea, D. Bloch,J.-M. Brom, M. Buttignol,E.C. Chabert, N. Chanon, C. Collard, E. Conte11, X. Coubez, J.-C. Fontaine11,D. Gelé, U. Goerlach,M. Jansová, A.-C. Le Bihan, N. Tonon, P. Van Hove

C. Autermann,S. Beranek, L. Feld, M.K. Kiesel, K. Klein, M. Lipinski, M. Preuten,C. Schomakers, J. Schulz, T. Verlage

RWTHAachenUniversity,I.PhysikalischesInstitut,Aachen,Germany

A. Albert, E. Dietz-Laursonn,D. Duchardt, M. Endres, M. Erdmann,S. Erdweg, T. Esch, R. Fischer, A. Güth, M. Hamer,T. Hebbeker,C. Heidemann, K. Hoepfner, S. Knutzen,M. Merschmeyer, A. Meyer,P. Millet, S. Mukherjee,M. Olschewski, K. Padeken,T. Pook, M. Radziej, H. Reithler,M. Rieger, F. Scheuch, D. Teyssier,S. Thüer

RWTHAachenUniversity,III.PhysikalischesInstitutA,Aachen,Germany

G. Flügge,B. Kargoll, T. Kress,A. Künsken, J. Lingemann, T. Müller, A. Nehrkorn, A. Nowack,C. Pistone, O. Pooth,A. Stahl13

RWTHAachenUniversity,III.PhysikalischesInstitutB,Aachen,Germany

M. Aldaya Martin,T. Arndt, C. Asawatangtrakuldee, K. Beernaert,O. Behnke, U. Behrens, A. Bermúdez Martínez, A.A. Bin Anuar, K. Borras14,V. Botta, A. Campbell,P. Connor,

C. Contreras-Campana, F. Costanza, C. Diez Pardos,G. Eckerlin, D. Eckstein, T. Eichhorn, E. Eren, E. Gallo15, J. Garay Garcia, A. Geiser, A. Gizhko, J.M. Grados Luyando, A. Grohsjean, P. Gunnellini, M. Guthoff,A. Harb, J. Hauk, M. Hempel16,H. Jung, A. Kalogeropoulos,M. Kasemann, J. Keaveney, C. Kleinwort,I. Korol, D. Krücker,W. Lange, A. Lelek, T. Lenz,J. Leonard, K. Lipka,W. Lohmann16, R. Mankel, I.-A. Melzer-Pellmann,A.B. Meyer, G. Mittag, J. Mnich, A. Mussgiller,E. Ntomari, D. Pitzl, A. Raspereza,B. Roland, M. Savitskyi,P. Saxena, R. Shevchenko,S. Spannagel, N. Stefaniuk,

G.P. Van Onsem,R. Walsh, Y. Wen,K. Wichmann, C. Wissing, O. Zenaiev

DeutschesElektronen-Synchrotron,Hamburg,Germany

S. Bein,V. Blobel, M. Centis Vignali, T. Dreyer, E. Garutti, D. Gonzalez, J. Haller, A. Hinzmann, M. Hoffmann,A. Karavdina, R. Klanner, R. Kogler,N. Kovalchuk, S. Kurz, T. Lapsien, I. Marchesini, D. Marconi,M. Meyer, M. Niedziela, D. Nowatschin, F. Pantaleo13,T. Peiffer, A. Perieanu, C. Scharf, P. Schleper, A. Schmidt, S. Schumann,J. Schwandt,J. Sonneveld, H. Stadie,G. Steinbrück, F.M. Stober, M. Stöver,H. Tholen, D. Troendle, E. Usai, L. Vanelderen,A. Vanhoefer, B. Vormwald

UniversityofHamburg,Hamburg,Germany

M. Akbiyik, C. Barth,S. Baur, E. Butz, R. Caspart,T. Chwalek, F. Colombo, W. De Boer,A. Dierlamm, B. Freund,R. Friese, M. Giffels, A. Gilbert, D. Haitz,F. Hartmann13,S.M. Heindl, U. Husemann,F. Kassel13, S. Kudella, H. Mildner,M.U. Mozer, Th. Müller, M. Plagge, G. Quast, K. Rabbertz,M. Schröder, I. Shvetsov, G. Sieber,H.J. Simonis, R. Ulrich, S. Wayand,M. Weber, T. Weiler, S. Williamson,C. Wöhrmann, R. Wolf

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

InstituteofNuclearandParticlePhysics(INPP),NCSRDemokritos,AghiaParaskevi,Greece

G. Karathanasis,S. Kesisoglou, A. Panagiotou, N. Saoulidou

NationalandKapodistrianUniversityofAthens,Athens,Greece

I. Evangelou, C. Foudas,P. Kokkas, S. Mallios, N. Manthos, I. Papadopoulos,E. Paradas, J. Strologas, F.A. Triantis

UniversityofIoánnina,Ioánnina,Greece

M. Csanad, N. Filipovic,G. Pasztor

MTA-ELTELendületCMSParticleandNuclearPhysicsGroup,EötvösLorándUniversity,Budapest,Hungary

G. Bencze,C. Hajdu, D. Horvath17,Á. Hunyadi, F. Sikler,V. Veszpremi, G. Vesztergombi18,A.J. Zsigmond

WignerResearchCentreforPhysics,Budapest,Hungary

N. Beni, S. Czellar, J. Karancsi19,A. Makovec, J. Molnar,Z. Szillasi

InstituteofNuclearResearchATOMKI,Debrecen,Hungary

M. Bartók18,P. Raics, Z.L. Trocsanyi, B. Ujvari

InstituteofPhysics,UniversityofDebrecen,Debrecen,Hungary

S. Choudhury, J.R. Komaragiri

IndianInstituteofScience(IISc),Bangalore,India

S. Bahinipati20, S. Bhowmik, P. Mal, K. Mandal, A. Nayak21, D.K. Sahoo20, N. Sahoo, S.K. Swain

NationalInstituteofScienceEducationandResearch,Bhubaneswar,India

S. Bansal,S.B. Beri, V. Bhatnagar, R. Chawla,N. Dhingra, A.K. Kalsi, A. Kaur, M. Kaur, R. Kumar,P. Kumari, A. Mehta,J.B. Singh, G. Walia

PanjabUniversity,Chandigarh,India

Ashok Kumar, Aashaq Shah, A. Bhardwaj,S. Chauhan,B.C. Choudhary, R.B. Garg, S. Keshri,A. Kumar, S. Malhotra, M. Naimuddin,K. Ranjan, R. Sharma, V. Sharma

UniversityofDelhi,Delhi,India

R. Bhardwaj,R. Bhattacharya, S. Bhattacharya, U. Bhawandeep, S. Dey,S. Dutt, S. Dutta, S. Ghosh, N. Majumdar, A. Modak, K. Mondal, S. Mukhopadhyay, S. Nandan, A. Purohit,A. Roy, D. Roy, S. Roy Chowdhury, S. Sarkar, M. Sharan, S. Thakur

SahaInstituteofNuclearPhysics,HBNI,Kolkata,India

P.K. Behera

IndianInstituteofTechnologyMadras,Madras,India

R. Chudasama, D. Dutta, V. Jha,V. Kumar, A.K. Mohanty13,P.K. Netrakanti,L.M. Pant, P. Shukla,A. Topkar

BhabhaAtomicResearchCentre,Mumbai,India

T. Aziz, S. Dugad,B. Mahakud, S. Mitra, G.B. Mohanty, N. Sur, B. Sutar

M. Abbresciaa,b, C. Calabriaa,b, C. Caputoa,b, A. Colaleoa,D. Creanzaa,c, L. Cristellaa,b,N. De Filippisa,c, M. De Palmaa,b, F. Erricoa,b,L. Fiorea,G. Iasellia,c,S. Lezkia,b, G. Maggia,c, M. Maggia,G. Minielloa,b, S. Mya,b, S. Nuzzoa,b, A. Pompilia,b,G. Pugliesea,c,R. Radognaa,b,A. Ranieria,G. Selvaggia,b,

A. Sharmaa, L. Silvestrisa,13,R. Vendittia,P. Verwilligena a_{INFNSezionediBari,Bari,Italy}

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

G. Abbiendia,C. Battilanaa,b,D. Bonacorsia,b, S. Braibant-Giacomellia,b, R. Campaninia,b,P. Capiluppia,b, A. Castroa,b,F.R. Cavalloa,S.S. Chhibraa, 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, A. Montanaria, F.L. Navarriaa,b_{,A. Perrotta}a_{, A.M. Rossi}a,b_{,T. Rovelli}a,b_{, G.P. Siroli}a,b_{,N. Tosi}a

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

S. Albergoa,b, S. Costaa,b, A. Di Mattiaa,F. Giordanoa,b, R. Potenzaa,b,A. Tricomia,b, C. Tuvea,b a_{INFNSezionediCatania,Catania,Italy}

b_{UniversitàdiCatania,Catania,Italy}

G. Barbaglia,K. Chatterjeea,b,V. Ciullia,b,C. Civininia,R. D’Alessandroa,b, E. Focardia,b, P. Lenzia,b, M. Meschinia, S. Paolettia,L. Russoa,27,G. Sguazzonia,D. Stroma,L. Viliania,b,13

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

L. Benussi,S. Bianco, F. Fabbri, D. Piccolo,F. Primavera13

INFNLaboratoriNazionalidiFrascati,Frascati,Italy

V. Calvellia,b, F. Ferroa, L. Panizzi,E. Robuttia, S. Tosia,b a_{INFNSezionediGenova,Genova,Italy}

b_{UniversitàdiGenova,Genova,Italy}

L. Brianzaa,b,F. Brivioa,b, V. Cirioloa,b, M.E. Dinardoa,b,S. Fiorendia,b, S. Gennaia,A. Ghezzia,b, P. Govonia,b, M. Malbertia,b,S. Malvezzia,R.A. Manzonia,b, D. Menascea,L. Moronia,M. Paganonia,b, K. Pauwelsa,b, D. Pedrinia,S. Pigazzinia,b,28, S. Ragazzia,b,T. Tabarelli de Fatisa,b

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

S. Buontempoa, N. Cavalloa,c, S. Di Guidaa,d,13, F. Fabozzia,c,F. Fiengaa,b, A.O.M. Iorioa,b, W.A. Khana, L. Listaa,S. Meolaa,d,13,P. Paoluccia,13,C. Sciaccaa,b,F. Thyssena

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

P. Azzia,13, N. Bacchettaa, L. Benatoa,b,D. Biselloa,b, A. Bolettia,b,R. Carlina,b,

A. Carvalho Antunes De Oliveiraa,b, M. Dall’Ossoa,b,P. De Castro Manzanoa, T. Dorigoa,U. Dossellia, F. Gasparinia,b, U. Gasparinia,b, A. Gozzelinoa,S. Lacapraraa,M. Margonia,b,A.T. Meneguzzoa,b,

F. Montecassianoa, D. Pantanoa,N. Pozzobona,b,P. Ronchesea,b, R. Rossina,b, E. Torassaa,M. Zanettia,b, P. Zottoa,b, G. Zumerlea,b

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

A. Braghieria,A. Magnania,b_{, P. Montagna}a,b_{,S.P. Ratti}a,b_{, V. Re}a_{, M. Ressegotti, C. Riccardi}a,b_,

P. Salvinia, I. Vaia,b, P. Vituloa,b a_{INFNSezionediPavia,Pavia,Italy}

b_{UniversitàdiPavia,Pavia,Italy}

L. Alunni Solestizia,b,M. Biasinia,b, G.M. Bileia,C. Cecchia,b, D. Ciangottinia,b,L. Fanòa,b, P. Laricciaa,b, R. Leonardia,b,E. Manonia,G. Mantovania,b,V. Mariania,b, M. Menichellia, A. Rossia,b,A. Santocchiaa,b, D. Spigaa

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

K. Androsova,P. Azzurria,13,G. Bagliesia,J. Bernardinia,T. Boccalia,L. Borrello, R. Castaldia, M.A. Cioccia,b, R. Dell’Orsoa,G. Fedia, L. Gianninia,c, A. Giassia, M.T. Grippoa,27,F. Ligabuea,c, T. Lomtadzea, E. Mancaa,c,G. Mandorlia,c,L. Martinia,b, A. Messineoa,b, F. Pallaa,A. Rizzia,b, A. Savoy-Navarroa,29, P. Spagnoloa, R. Tenchinia,G. Tonellia,b, A. Venturia,P.G. Verdinia a_{INFNSezionediPisa,Pisa,Italy}

b_{UniversitàdiPisa,Pisa,Italy}

c_{ScuolaNormaleSuperiorediPisa,Pisa,Italy}

L. Baronea,b,F. Cavallaria, M. Cipriania,b,N. Dacia, D. Del Rea,b,13, M. Diemoza,S. Gellia,b, E. Longoa,b, F. Margarolia,b,B. Marzocchia,b, P. Meridiania,G. Organtinia,b,R. Paramattia,b,F. Preiatoa,b,

S. Rahatloua,b,C. Rovellia,F. Santanastasioa,b a_{INFNSezionediRoma,Rome,Italy}

b_{SapienzaUniversitàdiRoma,Rome,Italy}

N. Amapanea,b, R. Arcidiaconoa,c,S. Argiroa,b,M. Arneodoa,c,N. Bartosika,R. Bellana,b, C. Biinoa, N. Cartigliaa, F. Cennaa,b,M. Costaa,b, R. Covarellia,b,A. Deganoa,b,N. Demariaa, B. Kiania,b,

C. Mariottia, S. Masellia, E. Migliorea,b,V. Monacoa,b,E. Monteila,b,M. Montenoa, M.M. Obertinoa,b, L. Pachera,b, N. Pastronea,M. Pelliccionia,G.L. Pinna Angionia,b,F. Raveraa,b, A. Romeroa,b,M. Ruspaa,c, R. Sacchia,b, K. Shchelinaa,b, V. Solaa, A. Solanoa,b,A. Staianoa, P. Traczyka,b

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

c_{UniversitàdelPiemonteOrientale,Novara,Italy}

S. Belfortea,M. Casarsaa, F. Cossuttia, G. Della Riccaa,b,A. Zanettia a_{INFNSezionediTrieste,Trieste,Italy}

b_{UniversitàdiTrieste,Trieste,Italy}

D.H. Kim,G.N. Kim, M.S. Kim,J. Lee, S. Lee,S.W. Lee, C.S. Moon, Y.D. Oh, S. Sekmen,D.C. Son, Y.C. Yang

H.D. Yoo,G.B. Yu

SeoulNationalUniversity,Seoul,RepublicofKorea

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

UniversityofSeoul,Seoul,RepublicofKorea

Y. Choi,C. Hwang, J. Lee,I. Yu

SungkyunkwanUniversity,Suwon,RepublicofKorea

V. Dudenas, A. Juodagalvis,J. Vaitkus

VilniusUniversity,Vilnius,Lithuania

I. Ahmed,Z.A. Ibrahim, M.A.B. Md Ali30,F. Mohamad Idris31,W.A.T. Wan Abdullah, M.N. Yusli, Z. Zolkapli

NationalCentreforParticlePhysics,UniversitiMalaya,KualaLumpur,Malaysia

R. Reyes-Almanza,G. Ramirez-Sanchez, M.C. Duran-Osuna, H. Castilla-Valdez, E. De La Cruz-Burelo, I. Heredia-De La Cruz32, R.I. Rabadan-Trejo,R. Lopez-Fernandez, J. Mejia Guisao, A. Sanchez-Hernandez

CentrodeInvestigacionydeEstudiosAvanzadosdelIPN,MexicoCity,Mexico

S. Carrillo Moreno, C. Oropeza Barrera, F. Vazquez Valencia

UniversidadIberoamericana,MexicoCity,Mexico

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

BenemeritaUniversidadAutonomadePuebla,Puebla,Mexico

A. Morelos Pineda

UniversidadAutónomadeSanLuisPotosí,SanLuisPotosí,Mexico

D. Krofcheck

UniversityofAuckland,Auckland,NewZealand

P.H. Butler

UniversityofCanterbury,Christchurch,NewZealand

A. Ahmad, M. Ahmad, Q. Hassan,H.R. Hoorani, A. Saddique, M.A. Shah,M. Shoaib, M. Waqas