Search for W′ → tb decays in the hadronic final state using pp collisions at √s = 13 TeV with the ATLAS detector

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Search for W′ → tb decays in the hadronic final state using pp collisions at √s = 13 TeV with the ATLAS detector

ATLAS Collaboration AKILLI, Ece (Collab.), et al.


A search for W′ -boson production in the W′→tb¯→qq¯′bb¯ decay channel is presented using 36.1fb−1 of 13 TeV proton–proton collision data collected by the ATLAS detector at the Large Hadron Collider in 2015 and 2016. The search is interpreted in terms of both a left-handed and a right-handed chiral W′ boson within the mass range 1–5 TeV. Identification of the hadronically decaying top quark is performed using jet substructure tagging techniques based on a shower deconstruction algorithm. No significant deviation from the Standard Model prediction is observed and the results are expressed as upper limits on the W′→tb¯ production cross-section times branching ratio as a function of the W′ -boson mass. These limits exclude W′ bosons with right-handed couplings with masses below 3.0 TeV and W′ bosons with left-handed couplings with masses below 2.9 TeV, at the 95% confidence level.

ATLAS Collaboration, AKILLI, Ece (Collab.), et al . Search for W′ → tb decays in the hadronic final state using pp collisions at √s = 13 TeV with the ATLAS detector. Physics Letters. B , 2018

DOI : 10.1016/j.physletb.2018.03.036

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Contents lists available atScienceDirect

Physics Letters B

Search for W

tb decays in the hadronic final state using pp collisions at √

s = 13 TeV with the ATLAS detector


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



Receivedinrevisedform13March2018 Accepted13March2018

Availableonline4April2018 Editor:W.-D.Schlatter

AsearchforW-bosonproductionintheWtb¯qq¯bb¯ decaychannelispresentedusing36.1 fb1 of13 TeVproton–protoncollisiondatacollectedbytheATLASdetectorattheLargeHadronColliderin 2015and2016.Thesearchisinterpretedintermsofbothaleft-handedandaright-handedchiralW bosonwithinthemassrange1–5TeV.Identificationofthehadronicallydecayingtopquarkisperformed using jetsubstructuretaggingtechniques basedonashower deconstruction algorithm. Nosignificant deviationfromtheStandardModelpredictionisobservedandtheresultsareexpressedasupperlimits onthe Wtb¯ productioncross-sectiontimesbranching ratioas afunctionof the W-boson mass.

TheselimitsexcludeWbosonswithright-handedcouplingswithmassesbelow3.0 TeVandWbosons withleft-handedcouplingswithmassesbelow2.9 TeV,atthe95%confidencelevel.

©2018TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (

1. Introduction

Severaltheories beyondthe Standard Model (SM) involve en- hancedsymmetriesthatpredictnewgauge bosons,usuallycalled Wor Zbosons.TheWbosonisthemediatorofanewcharged vector current that can be massive enough to decay into a top quark anda b-quark (as in Fig. 1). Many models such as those withextradimensions[1],strongdynamics[2–5],compositeHiggs [6], or the Little Higgs [7,8] predict new vector charged-current interactions,some withpreferential couplings toquarks orthird- generation particles [6,9–12]. Due to the large mass of the top quark,itsinteractions decouplefromtherestofthephenomenol- ogyin many theories beyond the SM. An effective Lagrangian is used to capture the relevant phenomenology of the Sequential StandardModel(SSM)[13] Wtb¯ signal[14,15],whichhasthe samecouplingstrengthtofermionsastheSMW bosonbuthigher mass.

Searchesfora W bosondecayingintotb,¯1 classifiedaseither leptonic or hadronic according to the decay products of the W bosonoriginatingfromthetopquark,wereperformedattheTeva- tron[16,17] andtheLargeHadronCollider(LHC)infinalstatesthat includeleptons[18–21] orthatarefullyhadronic[22].Thespecific searchfora W bosondecayingintotb¯ allowsforaright-handed W boson(WR) in models in which the right-handedneutrino’s mass isassumed to be much higher than that of the W boson

1 Forsimplicity, the notation “tb”¯ is used todenote the finalstate for both W+tb¯andW→ ¯tbdecays.

Fig. 1.FeynmandiagramforW-bosonproductionwithdecayintotb¯andahadron- icallydecayingtopquark.

(mνR >mW), which the leptonic decay mode cannot access. In such a model,the branchingratio fora WR bosondecaying into tb¯ isO(10%)higherrelativetothatforaWL bosondecayinginto tb¯ sinceaWL bosoncanalsodecaytoaleptonandneutrino.Lim- its on a SSMleft-handed W boson (WL) decaying into alepton andaneutrinohavebeensetpreviously[23,24].Previoussearches intheall-hadronicfinal stateexclude WR bosonswithmassesup to2 TeV,setatthe95% confidencelevel(CL)using20.3fb1 ofpp collisiondataatacentre-of-massenergy(

s)of8 TeV [22].Are- centsearchbytheCMSCollaborationinthelepton+jetsfinalstate excludes WR-bosonmasses up to 3.6TeV using 35.9fb1 of pp collisiondatacollectedat

s=13 TeV [18].

Thisanalysissearchesfora W bosondecaying intotb¯ witha massin therangeof 1–5TeV,inthe invariant massspectrum of the top quark and bottom quark (mtb) reconstructed in the fully hadronicchannel.Thisincludesa WR bosonthat isnot kinemati- callyallowedtodecayintoaleptonandneutrinoanda WL boson that candecayinto quarksorleptons. Thelarge W massresults

0370-2693/©2018TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense( SCOAP3.


ina top quark and ab-quark that have hightransverse momen- tum (pT).2 The decay products of the top quark become more collimated asthe top-quark pT increases, andtheir showerspar- tiallyoverlap[25].Thishigh-pTtopology isreferred toasboosted.

Theboostedtop-quarkdecayis reconstructedasa single jet.The shower deconstruction(SD) algorithm[26,27] is employed to se- lect, or tag, jets from boosted top-quark decays. A signal would be reconstructedasalocalised excessin themtb distribution ris- ingabovethesmoothlyfallingbackgroundoriginatingmostlyfrom jetscreatedbythestronginteractiondescribedbyquantumchro- modynamics (QCD). This analysis represents an improvement on theprevious ATLAS analysisinthis channel [22] dueto a higher centre-of-mass energy, higher integrated luminosity, and better top-taggingtechniques,understanding ofsystematicuncertainties, andstatisticaltreatment.

2. ATLASdetector

The ATLAS detector [28] at the LHC covers almost the entire solid angle around the collision point. Charged particles in the pseudorapidity range |η|<2.5 are reconstructed with the inner detector (ID), which consists of several layers of semiconductor detectors (pixel and strip) and a straw-tube transition-radiation tracker,the latter covering |η|<2.0. The high-granularity silicon pixel detector provides four measurements per track; the clos- est layer to the interaction point is known as the insertable B- layer (IBL) [29]. The IBL was added in 2014 and provides high- resolution hits at small radius to improve the tracking perfor- mance. The ID isimmersed in a 2 T magneticfield provided by a superconducting solenoid. The solenoid is surrounded by elec- tromagneticandhadroniccalorimeters,anda muonspectrometer incorporatingthreelarge superconductingtoroid magnetsystems.


Electromagneticcalorimetry isperformedwithbarrelandendcap high-granularitylead/liquid-argon(LAr)electromagneticcalorime- ters,within the region |η|<3.2. There is an additional thinLAr presamplercovering|η|<1.8,tocorrectforenergylossinmate- rialupstreamofthecalorimeters.For|η|<2.5,theLArcalorime- ters are divided into three layers in depth. Hadronic calorimetry is performed with a steel/scintillator-tile calorimeter, segmented intothree barrelstructureswithin |η|<1.7,andtwo copper/LAr hadronicendcapcalorimeters,which covertheregion1.5<|η|<

3.2. The forward solid angle up to |η|=4.9 is covered by cop- per/LAr and tungsten/LAr calorimeter modules, which are opti- misedforenergymeasurementsofelectrons/photonsandhadrons, respectively.Themuonspectrometer(MS)comprisesseparatetrig- gerandhigh-precisiontrackingchambersthatmeasurethedeflec- tion ofmuons in a magnetic field generated by superconducting air-coretoroids.TheATLASdetectorusesatieredtriggersystemto selectinteresting events.Thefirstlevelisimplementedincustom electronicsandreducesthe eventratefromtheLHCcrossing fre- quencyof40MHztoadesignvalueof100kHz.Thesecondlevelis implementedinsoftwarerunningona general-purposeprocessor farmwhichprocessestheeventsandreducestherateofrecorded eventsto1kHz[30].

2 ATLASusesaright-handedcoordinatesystemwithitsoriginatthenominalin- teractionpoint(IP)inthecentreofthedetectorandthez-axisalongthebeampipe.

Thex-axispointsfromtheIPtothecentreoftheLHCring,andthey-axispoints upwards.Cylindricalcoordinates(r,φ)areusedinthe transverseplane,φ being theazimuthalanglearoundthez-axis.Thepseudorapidityisdefinedintermsof thepolarangleθasη= −ln tan(θ/2).Angularseparationismeasuredinunitsof


(η)2+(φ)2,whereηandφaretheseparationsinηand φ.Mo- mentuminthetransverseplaneisdenotedbypT.

3. Dataandsimulationsamples

This analysis uses data from proton–proton (pp) collisions at

s=13 TeV collectedwiththeATLAS detectorin2015and2016 that satisfy a numberof criteriatoensure that theATLAS detec- tor wasin goodoperatingcondition. Theamountofdata usedin thisanalysiscorrespondstoanintegratedluminosityof36.1 fb1. The average number of pp interactions delivered per LHC bunch crossingwas23.7.

MonteCarlo(MC)eventgeneratorswere usedtosimulatesig- nalandbackgroundevents.Signal eventsweregeneratedatlead- ingorder(LO)inQCDbyMadGraph5_aMC@NLO v2.2.3[31],using a chiral W-boson model in which the coupling strength of the Wbosonto theright- andleft-handedfermionsarethesameas thoseoftheSM W bosontoleft-handedfermions.The WL boson can decay into all left-handed fermions, but the WR boson can decayonlyintoright-handedquarksastheright-handedneutrino is assumed to be more massive than the WR boson. MadGraph was used to simulatethetop-quark andW-bosondecays,taking spincorrelationsintoaccount.Pythia v8.186[32] wasusedforthe modelling of the parton shower, fragmentation andthe underly- ing event.TheNNPDF23LOpartondistributionsfunction(PDF)set [33] and theA14setoftuned parameters[34] wereusedforthe eventgeneration.Allsimulatedsampleswererescaledtonext-to- leading-order (NLO) calculations using NLO/LO K-factors ranging from1.3to1.4,dependingonthemassandhandednessoftheW boson,calculatedwithZtop[15].ThewidthoftheMadGraphsim- ulated W bosonissettotheNLO Ztopwidthcalculation,O(3%) of its mass. Signal samples with gauge-boson massesbetween 1 and3 TeVwere generated in250 GeVsteps, andbetween3and 5 TeVin500 GeVsteps.

The dominantSM backgroundprocess ismulti-jet production.

Inordertoreducethedependenceonthemodellingofthesimu- lation adata-driven methodis implementedasdescribed inSec- tion 5. Correctionsin thismethod areestimated usingQCD dijet simulationproduced atLObyPythia v8.186. Uncertaintiesinthis methodareobtainedusingsimulatedQCDdijeteventsproducedat LObyHerwig++v2.7.1[35] andSherpa v2.1.1[36],andatNLO by Powheg-Box v2[37,38] witheitherPythia8orHerwig+Jimmy[39]

for the parton shower, fragmentation and the underlying event simulation (referred to as Powheg+Pythia and Powheg+Herwig, respectively). Vector bosons (W/Z) produced inassociation with jetsareincludedinthedata-drivenapproach.Theseprocessesare expected to contribute lessthan 1% of themulti-jet background.

ThisW/Z+jetspredictionischeckedusingeventssimulatedwith theSherpa v2.2.1[36] generatorandtheCT10PDFset[40].

Top-quarkpairproductionisanimportantbackgroundwithan inclusive cross-section of σt¯t=832+4651 pb for a top-quark mass of 172.5 GeV as obtained from calculations accurate to next- to-next-to-leading order and next-to-next-to-leading logarithms (NNLO+NNLL)inQCDwithTop++2.0[41–47].Simulatedtop-quark pairprocesseswereproducedusingtheNLOPowheg-Box v2gen- erator withthe CT10PDF. The partonshower, fragmentationand theunderlyingeventwereaddedusingPythiav6.42[48] withthe Perugia2012setoftunedparameters[49].Toincreasethenumber ofsimulatedeventsathighmass,sampleswere producedbinned int¯tmass.InterferenceandbackgroundcontributionsfromtheSM s-channelsingle-topprocessarefoundtobenegligibleandarenot consideredfurtherinthisanalysis.

Thegenerationofthesimulatedeventsamplesincludestheef- fectofmultiple ppinteractionsperbunchcrossing,aswell asthe effect on the detector response due to interactions from bunch crossingsbefore orafter theone containing the hard interaction.

ForallMadGraph,Powheg,PythiaandHerwigsamples,theEvt- Gen v1.2.0 program [50] was used for the bottom and charm


Geant4-basedATLAS detectorsimulation[51,52] andwererecon- structedwiththesamealgorithmsasthedataevents.

4. Eventreconstructionandshowerdeconstruction 4.1.Eventreconstruction

Thisanalysisrelies onthe reconstruction andidentificationof jetsinitiated by the top- and bottom-quark daughters ofthe W boson.Jetsare builtfromtopologicallyrelatedenergydepositions inthecalorimeterswiththeanti-kt algorithm[53] usingtheFastJet package [54].Two radiusparameters are used forjet reconstruc- tion: asmall radius (small-R) of0.4and alarge radius (large-R) of 1.0. The momenta of both the small-R and large-R jets are corrected for energylosses in passive material and for the non- compensating response of the calorimeter [55]. Small-R jets are alsocorrectedfortheaverageadditionalenergyduetopile-upin- teractions[56].Energydepositionsfrompile-upareremovedfrom large-R jetsusing the trimming algorithm [57]: the constituents ofthe large-R jet are reclusteredusing the kt jet algorithm [58, 59] with R=0.2.Constituentjetscontributinglessthan5% ofthe large-R jet’s pT are removed. The remaining energy depositions areusedtocalculatethetrimmed-jetkinematicsandsubstructure properties.In order to improve on the angular resolution of the calorimeter,themassofalarge-R jetiscomputedusingacombi- nationofcalorimeterandtrackinginformation[60].

Small-R jetsareusedtoidentifythejetscompatiblewithorig- inatingfromab-quark createdeitherdirectlyfromtheW boson orfromthetop-quarkdecay.Onlysmall-R jetswith pT>25 GeV and|η|<2.5 (in orderto be within the coverage of theID) are consideredinthisanalysis.Additional pTrequirementsareapplied toenhancethesensitivityofthesearch (seeSection5).Toreduce thenumber ofsmall-R jetsoriginatingfrompile-up interactions, alikelihood discriminant, basedon trackandvertex information, isusedtodeterminewhethertheprimary vertex3 istheoriginof thecharged-particletracksassociatedwithajetcandidateandre- jects jets originatingfrom pile-up interactions [61]. This is done only for small-R jets with pT<60 GeV and |η|<2.4. Small-R jetswhichoriginatefromb-quarksareidentifiedusingamultivari- ateb-taggingalgorithm[62,63].Severalobservables,suchasthose basedonthelonglifetimeofb-hadronsandtheb- toc-hadronde- caytopology,areusedasalgorithminputstodiscriminatebetween b-jets,c-jetsandotherjets.Theb-taggingrequirementcorrespond- ing to an efficiency of 77% to identify b-jets with pT>20 GeV, as determined from a sample of simulated tt¯ events, is found to be optimal for the statistical significance of this search. This 77% working point (WP) provides rejection factors against light- flavour/gluon jets and c-jets of 134 and 6 respectively [63,64].

Jetsidentifiedthiswayarereferred toasb-taggedjets. Sincethe b-taggingfactorsaremeasured ina differentpT region,an uncer- taintyisassignedtotheextrapolationofthemeasurementtothe highpTregionofinterest.

Eventswithreconstructedelectrons[65] ormuons[66] areve- toed in order to ensure statistical independence of this analysis fromanalyses usingthe leptonicdecayofthe W bosonfromthe top quark [19]. Electrons and muons with transverse momenta above25 GeV andselected withcriteriasimilarto those usedin Ref. [67] areconsideredforthisveto.

3 Collisionverticesareformedfromtrackswith pT>400 MeV.Ifaneventcon- tainsmorethanonevertexcandidate,theonewiththehighest

p2Tofitsassoci- atedtracksisselectedastheprimaryvertex.

The SD algorithm can be used to identify the jets compati- ble withthe hadronicdecayof a W/Z boson, Higgsboson, ora top quark aswell asto discriminate between quark- and gluon- initiated jets. In this analysis, an SD-algorithm-based tagger (SD tagger) is used to identify jets originating from the top quark.

TheSDtaggercalculateslikelihoodsthat agivenlarge-R jetorigi- natesfromahadronictop-quarkdecayorfromahigh-momentum light quarkorgluon. Theconstituentsof thetrimmedlarge-R jet areusedtobuild exclusivesubjets[54],andthefour-momentaof these subjets serve as inputsto the SD algorithm. Thesesubjets are usedassubstitutesforindividual quarksandgluonsoriginat- ingfromthehardscatter.Alikelihoodweightiscalculatedforeach possibleshowerhistorythat canleadtotheobservedsubjetcon- figuration.Thisstep isanalogoustorunninga partonshowerMC generator in reverse, where emission and decay probabilities at each vertex, colour connections, andkinematic requirements are considered. For each shower history, the assignedweight is pro- portionaltotheprobabilitythattheassumedinitialparticlegener- atesthefinalconfiguration,takingintoaccounttheSMamplitude fortheunderlyinghard processandtheSudakov formfactorsfor the parton shower. A variable called χSD is defined as the ratio of the sum of the signal-hypothesis weights to the sum of the background-hypothesisweights.Foraset{pki}ofN observedsub- jetfour-momenta,wherei∈ [1,N],thevalueof χSD isgivenby:


permP({pki}|top-quark jet)

permP({pki}|gluon/light-quark jet),

where P({pki}|top-quark jet) isbuiltusingtheweightsforthehy- pothesis that a signal process leads to the observed subjet con- figuration{pki}and P({pki}|gluon/light-quark jet)isbuiltusingthe weightsforthehypothesis thatabackgroundprocessleads tothe observedsubjetconfiguration.The

perm notation representsthe sumover all the shower histories inwhich signal processeslead to the subjet configuration. The large-R jet is tagged as a top- quark jetif χSD islargerthan agivenvalue,whichisadjusted to achievethedesiredtaggingefficiency.Thereisaninternalmecha- nismintheSDalgorithmtosuppresspile-upcontributionstothe jets,throughtheapplicationofadditionalweightsinthelikelihood ratio,whichcontaintheprobabilitythatasubsetofthesubjetsdid notoriginatefromthehardinteractionbutfrompile-up[68].

TheSDalgorithmselectseventsthatarekinematicallycompat- ible witha hadronictop-quarkdecay.The followingrequirements are made to optimise the algorithm to achieve a balance be- tween goodtop-quarkjet signalselection efficiencyandrejection of gluon/light-quarkjet backgrounds: the large-R jet hasat least threesubjets; twoormoresubjets musthaveacombinedinvari- ant mass ina 60.3–100.3 GeV windowcentred on the W-boson mass;andatleastonemoresubjetcanbeaddedtoobtainatotal massina132–212GeVwindowcentredonthetop-quarkmass.

TheSDtaggerwasoptimisedforthisanalysissothatitismore efficientfortop-quarkjetsignalselectionandgluon/light-quarkjet backgroundrejectionforpT>800 GeVcomparedtotheversionof the SD taggerfirst studied by the ATLAS Collaboration [25]. This is done bybuilding subjetsobtainedby using an exclusivekt al- gorithm [54].First, thekt algorithmwith R=1.0 is runover the large-R jet constituents and the kt reclusteringis stopped if the splitting scale [69] is larger than 15 GeV. Oncethe kt recluster- ing is stopped the reclusteredprotojets are used assubjets. The choiceofa15 GeV requirementisbasedontheexpecteddiscrim- inationbetweensignalandbackgroundevents.ThesixhighestpT



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