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Search for W′→tb¯ in the lepton plus jets final state in proton–proton collisions at a centre-of-mass energy of √s=8 TeV with the ATLAS detector

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collisions at a centre-of-mass energy of √s=8 TeV with the ATLAS detector

ATLAS Collaboration

ALEXANDRE, Gauthier (Collab.), et al .

Abstract

A search for new charged massive gauge bosons, called W′ , is performed with the ATLAS detector at the LHC, in proton–proton collisions at a centre-of-mass energy of s=8 TeV , using a dataset corresponding to an integrated luminosity of 20.3 fb−1 . This analysis searches for W′ bosons in the W′→tb¯ decay channel in final states with electrons or muons, using a multivariate method based on boosted decision trees. The search covers masses between 0.5 and 3.0 TeV, for right-handed or left-handed W′ bosons. No significant deviation from the Standard Model expectation is observed and limits are set on the W′→tb¯ cross-section times branching ratio and on the W′ -boson effective couplings as a function of the W′ -boson mass using the CLs procedure. For a left-handed (right-handed) W′ boson, masses below 1.70 (1.92) TeV are excluded at 95% confidence level.

ATLAS Collaboration, ALEXANDRE, Gauthier (Collab.), et al . Search for W′→tb¯ in the lepton plus jets final state in proton–proton collisions at a centre-of-mass energy of √s=8 TeV with the ATLAS detector. Physics Letters. B , 2015, vol. 743, p. 235-255

DOI : 10.1016/j.physletb.2015.02.051

Available at:

http://archive-ouverte.unige.ch/unige:55733

Disclaimer: layout of this document may differ from the published version.

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Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Search for W

t b in ¯ the lepton plus jets final state in proton–proton collisions at a centre-of-mass energy of √

s = 8 TeV with the ATLAS detector

.ATLASCollaboration

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

Articlehistory:

Received15October2014

Receivedinrevisedform17February2015 Accepted23February2015

Availableonline25February2015 Editor:W.-D.Schlatter

Asearchfornewchargedmassivegaugebosons,calledW,isperformedwiththeATLASdetectoratthe LHC,inproton–protoncollisionsatacentre-of-massenergyof

s=8 TeV,usingadatasetcorresponding toan integratedluminosity of20.3 fb1.Thisanalysissearches for W bosonsinthe Wtb decay¯ channelinfinalstateswithelectronsormuons,usingamultivariatemethodbasedonboosteddecision trees.Thesearchcoversmassesbetween0.5and3.0 TeV,forright-handedorleft-handedWbosons.No significantdeviationfromtheStandardModelexpectationisobservedandlimitsaresetontheW tb cross-section¯ times branchingratio and on the W-boson effective couplingsas a functionof the W-bosonmassusingtheCLs procedure.Foraleft-handed(right-handed)W boson,massesbelow1.70 (1.92) TeV areexcludedat95%confidencelevel.

PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.

1. Introduction

ManyapproachestotheoriesbeyondtheStandardModel(SM) introducenew chargedvector currents mediated by heavy gauge bosons, usually called W. For example, the W boson can ap- pearintheorieswithuniversalextradimensions,such asKaluza–

Klein excitations of the SM W boson [1–3], or in theories that extend fundamental symmetries of the SM and propose a mas- siveright-handed counterpartto the W boson[4–6]. LittleHiggs theories [7] also predict a W boson. The search for a W bo- son decaying to a top quark anda b-quark exploresmodels po- tentially inaccessible to searches for a W boson decaying into leptons [8–11]. For instance, in the right-handed sector, the W bosoncannot decaytoachargedlepton andaright-handedneu- trino if the latter has a mass greater than the W-boson mass.

Also,inseveraltheoriesbeyondtheSMthe Wbosonisexpected to be coupled more strongly to the third generation of quarks than to the first and second generations [12,13]. Searches for a W boson decaying to the tb final¯ state1 have been performed

E-mailaddress:atlas.publications@cern.ch.

1 Forsimplicity,the notation“tb”¯ isusedtodescribeboththe W +tb and¯ W→ ¯tb processes.

at the Tevatron [14,15] in the leptonic top-quark decay chan- nel and at the Large Hadron Collider (LHC) in both the leptonic [16–18]andfullyhadronic[19]finalstates,excludingright-handed W bosons with massesup to 2.05 TeV at 95% confidence level (CL).

This Letter presents a search for W bosons using data col- lected in 2012 by the ATLAS detector [20] at the LHC, corre- sponding to an integrated luminosity of 20.3 fb1 from proton–

proton (pp) collisions at a centre-of-mass energy of 8 TeV. The searchisperformedinthe Wtb¯νbb decay¯ channel,where the lepton, , is either an electron or a muon, using a mul- tivariate method based on boosted decision trees. Right-handed and left-handed W bosons, denoted WR and WL, respectively, are searched for in the mass range of 0.5 to 3.0 TeV. A gen- eralLorentz-invariantLagrangianisusedtodescribethecouplings of the W boson to fermions for various W-boson masses [21, 22]. The mass of the right-handed neutrino is assumed to be larger than the mass of the W boson [23], thus allowing only hadronic decays of the WR boson. In the case of a WL boson, leptonic decays are allowed and, since the signal has the same event signature asSM s-channel single top-quark production,an interference termbetween thesetwo processesis taken into ac- count[24].

http://dx.doi.org/10.1016/j.physletb.2015.02.051

0370-2693/PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.

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toroids provide the magnetic field for the muon spectrometer.

Threestationsofprecision drifttubes andcathode-stripchambers providean accuratemeasurementofthemuon trackcurvaturein theregion|η|<2.7.Resistive-plateandthin-gapchambersprovide muontriggeringcapabilityupto|η|=2.4.

3. Dataandsimulationsamples

Thedatausedforthisanalysiswererecordedusingunprescaled single-electron and single-muon triggers. After stringent data- quality requirements, the amount of data corresponds to an in- tegratedluminosityof20.3±0.6 fb1 [25].

The WR and WL signals are generated withMadGraph5 [26]

using FeynRules [27,28] and the CTEQ6L1 [29] parton distribu- tionfunction(PDF)set.Pythia8[30]isusedforpartonshowering andhadronisation. Simulated samplesare normalised to next-to- leading order (NLO) QCD calculations [22] using K -factors rang- ing from1.15to 1.35depending on the massandhandedness of the W boson. The model assumes that the W-boson coupling strength to quarks, g, is the same as forthe W boson: gR=0 and gL=g (gR=g and gL=0) for left-handed (right-handed) W bosons, where g is the SM SU(2)L coupling. The total width oftheleft-handed(right-handed) W bosonincreasesfrom17to 104 GeV (12 to 78 GeV) for masses between 0.5 and 3.0 TeV, where the decay to leptons is (is not) allowed [21]. In order to accountforthe effectoftheinterferencebetween WL-bosonand s-channelsingle top-quarkproductiondedicated ppWL/W tb¯νbb samples¯ are simulated,usingMadGraph5,andassum- ing a destructive interferenceterm[24].In addition, samplesare generatedforvaluesof g/g upto5.0,forseveralW-boson(left- andright-handed)masshypotheses.

Top-quarkpair(tt)¯ andsingletop-quarks-channelandW t pro- cesses are simulated with the Powheg [31,32] generator, which uses a NLO QCD matrix element with the CT10 PDFs [29]. The parton shower and the underlying event are simulated using Pythia v6.4[33].The t-channel single-top-quark process is mod- elled using the AcerMC v3.8 [34] generator with the CTEQ6L1 PDFsand Pythia v6.4. The t¯t cross-section is calculatedat next- to-next-to-leadingorder(NNLO)inQCDincludingresummationof next-to-next-to-leadinglogarithmicsoftgluontermswithtop++2.0 [35–41]. The single top-quark cross-sections are obtained from

2 ATLASusesaright-handed coordinatesystemwith itsoriginat thenominal interactionpointinthecentreofthedetectorandthez-axisalongthebeampipe.

Thex-axispointsfromtheinteractionpointtothecentreoftheLHCring,andthe y-axispointsupward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane, φbeingtheazimuthalanglearoundthebeampipe.Thepseudorapidityisdefined intermsofthepolarangleθasη= −ln tan(θ/2).Observableslabelled“transverse”

areprojectedintothex– y plane.

previousbunch-crossings(in-timeandout-of-timepileup)andare re-weighted tomatchtheconditionsofthedatasample (20.7in- teractionsperbunchcrossingonaverage).

4. Objectandeventselections

The search for Wtb events¯ relies on the measurement of the following objects: electrons, muons, jets, and the missing transverse momentum.Electrons areidentified asenergyclusters intheelectromagneticcalorimetermatchedtoreconstructedtracks intheinnerdetector[52,53].Electroncandidatesarerequiredtobe isolated,usingafixedcone-sizeisolationcriterion[54],fromother objectsintheeventandfromhadronicactivity,toreducethecon- taminationfrommis-reconstructedhadrons,electronsfromheavy- flavour decaysand photon conversions. Electrons are required to have transverse momentum, pT, above 30 GeV and |η|<2.47 with a veto on the barrel-endcap transition region in the range 1.37<|η|<1.52.

Muonsareidentifiedusingthemuonspectrometerandthein- ner detector [55]. A variable cone-size isolation criterion [54,56]

isappliedtoreducethecontributionofmuonsfromheavy-flavour decays. Muon candidates are required to have pT>30 GeV and

|η|<2.5.

Jets are reconstructed usingthe anti-kt algorithm [57] with a radius parameter R=0.4,usingtopologicalenergyclustersasin- puts [58,59]. Jets are calibrated using energy- and η-dependent correction factors derived fromsimulation andwithresidualcor- rectionsfrominsitumeasurements [60].Jetsarerequiredtohave pT>25 GeV and |η|<2.5.Tosuppressjetsfromin-time pileup, at least 50% of the scalar pT sum of the tracks associated with a jet is required to be from tracks associated with the primary vertex [61]. This requirement, called the “jet vertex fraction” re- quirement,isappliedonlyforjetswithpT<50 GeV and|η|<2.4.

The identification of jets originating from the hadronisation of b-quarks(“b-tagging”)isbasedonpropertiesspecifictob-hadrons, such aslonglifetimeandlargemass.Thisanalysisusesa neural- network-basedcombinationofseveralhigh-performanceb-tagging algorithms[62].Thealgorithmhasanefficiencyof70%(20%,0.7%) forjetsoriginatingfromb-quarks(c-quarks,light-quark/gluon)as obtainedfromsimulatedtt events.¯

The missing transverse momentum, EmissT , is the modulus of the vector sumof thetransverse momentum incalorimetercells associated with topological clusters, and is further refined with object-levelcorrectionsfromidentifiedelectrons, muons,andjets [63,64].ThisanalysisrequireseventstohaveEmissT >35 GeV tore- ducethemultijetbackground.

Candidate events are requiredto have exactlyone lepton and twoorthreejetswithexactlytwoofthemidentifiedasoriginating from a b-quark (denoted 2-tag events). The multijet background

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contribution is further reduced by imposing a requirement on the sum of the W -boson transverse mass,3 mT(W), and EmissT : mT(W)+EmissT >60 GeV.Eventswithexactlytwo(three)jetspass- ingalltheaboveselectionsdefinethe2-jet(3-jet)channel.

Thesignal regionis definedbyselecting eventswherethere- constructed invariant mass of the tb system,¯ mtb¯ (see definition below),islargerthan330 GeV.Theacceptancetimesefficiencyfor theWtb process¯ intheleptonplusjetsfinalstateis5.5%,2.2%

and2.1%(4.9%,2.2%,2.3%)fora WR(WL)bosonwithamassof1, 2or3 TeV,respectively.ThedropinacceptanceforhighW-boson massesisduetotheleptonfailingtheisolationcriterionandtothe decreaseoftheb-taggingefficiency.Acontrolregionisdefinedby invertingtherequirementonthetb invariant¯ mass,mtb¯<330 GeV, and is used to derive the normalisation of the W +jets back- ground.

The method used to reconstruct the invariant mass of the tb system¯ in the selected sample proceeds as follows.The four- momentumofthetopquark isreconstructedby addingthefour- momentaofthe W bosonandofthe b-taggedjet that givesthe reconstructed invariant top-quarkmass closest to the value used forgeneration (172.5 GeV). Thereafter, thisb-tagged jet iscalled the“top-jet”and isassumedto be theb-jet fromthe top quark.

In this calculation the transverse momentum of the neutrino is givenbythe x- and y-componentsofthe EmissT vector, whilethe unmeasuredz-component of theneutrino momentumis inferred by imposing a W -boson mass constrainton the lepton–neutrino system[65].Thefour-momentumofthetb system¯ isthenrecon- structedby addingthefour-momenta ofthetop quark tothat of theremainingb-taggedjet.

5. Backgroundestimation

Thett,¯ single-top-quark,dibosonandZ+jets backgroundsare modelledusingthesimulationandarescaledtothetheorypredic- tionsoftheinclusivecross-sections.

Thebackgroundoriginatingfrommultijetevents,whereajetis misidentifiedasaleptonoranon-promptleptonappearsisolated (bothreferredtoasa“fake”lepton),isestimateddirectlyfromdata usingthematrixmethod[54].Theshapeandnormalisationofthe multijetbackgroundaredeterminedinboththeelectronandmuon channelsusingthismethod.

TheW+jets backgroundisalsomodelledusingthesimulation, butinthecaseofthe2-jetchanneltheeventyieldforthisprocess isderivedfromdatatoimprovethemodellinginthischannel.The numberof W+jets events is estimatedin the 2-jet control re- gionasthenumberofdataeventsobservedaftersubtractionofall non-W+jets background sources described above. This estimate isthenextrapolatedtothe2-jet signalregion usingtheW+jets simulation.Forthe3-jetchanneltheW+jets backgroundisscaled tothetheoryprediction.

6. Analysis

The analysisstrategy relies on a multivariate approach, based ontheboosteddecisiontree(BDT)methodusingtheframeworkof TMVA[66],toenhancetheseparationbetweenthesignalandthe background.ForeachjetmultiplicityandW-bosonhandedness,a separateBDTistrainedinthesignalregion.Forthebackground,a mixtureoftop-quark, W/Z+jets,dibosonandmultijetssamples, all weighted according to their relative abundances, is used. The W-bosonsample usedas signal in the BDT training andtesting

3 Defined as mT(W)=

(pT()+EmissT )2(px()+Emissx )2(py()+Emissy )2, whereEmissx andEmissy arethex- andy-componentsoftheEmissT vector.

phasesischosenatamassof1.75 TeV sincethisgivesthebestex- pected exclusionlimit onthe W-bosonmass,compared toBDTs trained withother W-boson mass samples.Thischoice also en- suresverygoodseparationbetweentheBDTshapesofsignaland backgroundforW-bosonmassesof1 TeV andabove.Thisanaly- sis isthussensitive tothepresenceofa signalover awide mass range.

Ten (eleven) variables with significant separation power are identified in the 2-jet (3-jet) samples for the W-boson search.

TheseareusedasinputstotheBDTs.Thelistofvariableschanges slightlydependingonthechiralityofthesignal.

A set offive variables is common to all fourBDTs. Two vari- ables,mt¯bandthetransversemomentumofthereconstructedtop quark, pT(t), provide the best separation poweramong all those considered and are shown in Fig. 1. The other three common variables are: the angularseparation4 betweenthe jet associated withtheb-jetoriginatingfromthe Wbosonandthetop-jet(de- noted bt), R(b,bt);the transverseenergyof thetop-jet, ET(bt), andtheaplanarity.5

In addition, for the 2-jet channel, the following variables are used: theangularseparationbetweenthe top-jetandthe W bo- son, R(bt,W),andtheηbetweenthe leptonandthetop-jet, η(,bt). Forthe caseofthe right-handed W-boson search the followingvariablesare alsoused:thesphericity;theangularsep- arationbetweentheleptonandtheb-jetoriginatingfromtheW boson, R(,b); the transverse momentum of thelepton, pT(). Fortheleft-handedW-bosonsearch,threedifferentvariablesare chosen:theanglebetweenthetop-jetandthemissingtransverse momentum, φ (bt,EmissT ); the ratio of the transverse momenta of the top-jet and of the b-jet originating from the W boson, pT(bt)/pT(b),andmT(W).

Foreventswiththree jets, thefollowing variablesare used in addition to the commonset ofvariables: R(,bt); the spheric- ity; pT(b); the invariant mass of the three jets m(b,bt,j). Two more variables are used, for the right-handed case only: pT() and R(b,W), and for the left-handed case: φ (bt,EmissT ) and pT(bt)/pT(b).

Fig. 2showstheexpectedBDToutputdistributions,normalised tounity, inthesignal regionfortheelectron andmuon channels combined, forseveral simulated right-handed W-boson samples andfortheexpectedbackground.

7. Systematicuncertainties

Systematicuncertaintiescanaffecttheshapeandnormalisation oftheBDT outputdistributions.Theyare splitintothecategories describedbelow.

Objectmodelling: Themain uncertainty inthis category isdue to uncertaintiesonb-taggingefficiencyandmistaggingrates[68,69].

The resulting uncertainty on the event yield is 6% for the total backgroundcontributionand8–30%forthesignal.Thelargeuncer- taintiesonthe signalratesareduetoadditionalb-tagginguncer- taintiesforjetswith pT above300 GeV.Theseuncertaintiesrange from3% forb-jetswith pT of50 GeV upto15% at500 GeV and 35%above.Theimpactissizeableforthesignalwherehigh-pTjets stemfromtheW-bosondecay,inparticularwhenthe W-boson massisabove1 TeV.Thejetenergyscaleuncertaintydependson thepTand ηofthereconstructedjetandincludestheuncertainty on the b-jet energy scale. It results in an uncertainty on event yieldsof1–6%forthesignaland1–4%forthebackground,depend- ingonthechannel.Thesystematicuncertaintyassociatedwiththe

4 DefinedasR=

(φ)2+(η)2.

5 Aplanarityandsphericityareeventshapevariablescalculatedfromthespheric- itytensoroftheleptonandjetmomenta[67].

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