Article
Reference
Search for vector-boson resonances decaying to a top quark and bottom quark in the lepton plus jets final state in pp collisions at √s =
13 TeV with the ATLAS detector
ATLAS Collaboration AKILLI, Ece (Collab.), et al .
Abstract
A search for new charged massive gauge bosons, W′ , is performed with the ATLAS detector at the LHC. Data were collected in proton–proton collisions at a center-of-mass energy of s=13 TeV and correspond to an integrated luminosity of 36.1 fb−1 . This analysis searches for W′ bosons in the W′→tb¯ decay channel in final states with an electron or muon plus jets. The search covers resonance masses between 0.5 and 5.0 TeV and considers right-handed W′
bosons. No significant deviation from the Standard Model (SM) expectation is observed and upper limits are set on the W′→tb¯ cross section times branching ratio and the W′ boson effective couplings as a function of the W′ boson mass. For right-handed W′ bosons with coupling to the SM particles equal to the SM weak coupling constant, masses below 3.15 TeV are excluded at the 95% confidence level. This search is also combined with a previously published ATLAS result for W′→tb¯ in the fully hadronic final state. Using the combined searches, right-handed W′ bosons with masses below 3.25 TeV are excluded at the 95%
confidence level.
ATLAS Collaboration, AKILLI, Ece (Collab.), et al . Search for vector-boson resonances
decaying to a top quark and bottom quark in the lepton plus jets final state in pp collisions at √s
= 13 TeV with the ATLAS detector. Physics Letters. B , 2019, vol. 788, p. 347-370
DOI : 10.1016/j.physletb.2018.11.032
Available at:
http://archive-ouverte.unige.ch/unige:111792
Disclaimer: layout of this document may differ from the published version.
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Physics Letters B
www.elsevier.com/locate/physletb
Search for vector-boson resonances decaying to a top quark and bottom quark in the lepton plus jets final state in pp collisions at
√ s = 13 TeV with the ATLAS detector
.TheATLASCollaboration
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received27July2018
Receivedinrevisedform15October2018 Accepted3November2018
Availableonline22November2018 Editor: W.-D.Schlatter
A search for new charged massive gauge bosons, W, is performed with the ATLAS detector at the LHC. Datawere collectedinproton–proton collisions atacenter-of-massenergy of√
s=13 TeV and correspond to an integrated luminosity of 36.1 fb−1. This analysis searches for W bosons in the W→tb¯ decaychannelinfinalstateswithanelectronormuonplusjets.Thesearchcoversresonance massesbetween0.5and 5.0 TeV andconsidersright-handedW bosons.Nosignificantdeviationfrom theStandardModel(SM)expectationisobservedandupperlimitsaresetontheW→tb¯crosssection timesbranchingratio andthe W bosoneffective couplingsasafunctionofthe W bosonmass.For right-handed W bosonswith couplingto the SMparticlesequal tothe SMweak coupling constant, massesbelow3.15 TeV are excludedatthe95%confidencelevel.Thissearchisalsocombinedwitha previously publishedATLAS result for W→tb¯ in the fully hadronicfinal state. Usingthe combined searches,right-handedW bosonswithmassesbelow3.25 TeV areexcludedatthe95%confidencelevel.
©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
ManyapproachestotheoriesbeyondtheStandardModel(SM) introducenew chargedvector currents mediated by heavy gauge bosons, usually referred to as W. For example, the W boson can appear in theories with universal extra dimensions, such as Kaluza–Klein excitations of the SM W boson [1–3], or in mod- els that extend fundamental symmetries ofthe SM and propose amassiveright-handedcounterparttothe W boson [4–6].Little- Higgs [7] and composite-Higgs [8,9] theories also predict a W boson.ThesearchforaWbosondecayingintoatopquarkanda b-quark (illustratedinFig.1) exploresmodelspotentiallyinacces- sibletosearchesforaWbosondecayingintoleptons [10–15].
Forinstance,intheright-handedsector, the W boson cannot decayintoachargedleptonandahypotheticalright-handedneu- trino if the latter has a mass greater than the W boson mass (mixingbetweenW andSM W bosonsisusually constrainedto besmallfromexperimentaldata [16]).Also,inseveraltheoriesbe- yondtheSMtheWbosonisexpectedtocouplemorestronglyto thethird generationofquarksthantothefirstandsecond gener- ations [17,18].Searchesfora W bosondecayingintothetb¯ final
E-mailaddress:atlas.publications@cern.ch.
state1 havebeenperformedattheTevatron [19,20] intheleptonic top-quark decaychannel and atthe Large Hadron Collider (LHC) inboththeleptonic [21–25] andfullyhadronic [26,27] finalstates, andthemostrecentresultsexcluderight-handedWbosonswith massesupto about3.6 TeV atthe 95%confidencelevel.A previ- ousATLASsearchintheleptonicchannel [24] usingproton–proton (pp) collisions ata center-of-mass energyof √
s=8 TeV yielded a lower limit of 1.92 TeV on the mass of W boson with right- handed couplings.More recently,the CMSCollaboration reported results usinga 13 TeV pp dataset of 35.9 fb−1 [25], yielding a lowerlimit of3.6 TeV onthemassofright-handed W bosons.A search by the ATLAS Collaboration inthe fullyhadronicdecay of the tb¯ final state using 36.1 fb−1 of 13 TeV data yieldedlower limitsonthemassofright-handedW bosonsat3.0 TeV [27]. In each oftheseanalyses,the couplingstrengthofthe W boson to right-handed particleswas assumed tobe equal tothe SM weak couplingconstant.
ThisLetterpresentsasearchforWbosonsusingdatacollected during the period 2015–2016 by the ATLAS detector [28] at the LHC,correspondingtoanintegratedluminosityof36.1 fb−1 from pp collisionsat acenter-of-mass energyof13 TeV.The search is performedintheWR →tb¯→νbb¯decaychannel,wherethelep-
1 Thenotation“tb”¯ isusedtodescribeboththeW +→tb¯andW −→ ¯tbpro- cesses.
https://doi.org/10.1016/j.physletb.2018.11.032
0370-2693/©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
Table 1
Eventgeneratorsusedforthesimulationofthesignalandbackgroundprocesses.ThePS/Hadcolumnde- scribestheprogramusedforpartonshowerandhadronization.
Process Generator PS/Had MC Tune PDF
WR MadGraph5_aMC@NLO Pythia8 A14 NNPDF23LO
t¯t Powheg-Box Pythia6 Perugia 2012 NLO CT10
Single-topt-channel Powheg-Box Pythia6 Perugia 2012 NLO CT10
Single-topW+t Powheg-Box Pythia6 Perugia 2012 NLO CT10
Single-tops-channel Powheg-Box Pythia6 Perugia 2012 NLO CT10
W,Z+ jets Sherpa2.2.1 Sherpa2.2.1 Default NLO CT10
W W,W Z,Z Z Powheg-Box Pythia8 AZNLO LO CTEQ6L1
Fig. 1.Feynmandiagramfor Wbosonproductionfromquark–antiquarkannihila- tionwiththesubsequentdecayintotb¯andaleptonicallydecayingtopquark.
ton,,iseitheran electronora muon.Right-handed W bosons, denotedWR ,aresearchedforinthemassrangeof0.5to5.0 TeV.
AgeneralLorentz-invariantLagrangianisusedtodescribethecou- plingsoftheWR bosontofermionsasafunction ofitsmass [29, 30]. The mass of the right-handed neutrino is supposed to be largerthanthemassoftheWR boson,thusnon-hadronicdecaysof theWR bosonhaveanegligiblebranchingfraction.Inthisweakly coupledmodel,theresultingbranching fractionofthe WR tothe tb¯ finalstateincreasesasafunctionofmassfrom29.9%at0.5 TeV to33.3%at5 TeV.
2. ATLASdetector
The ATLAS detectorat the LHC covers almost the entiresolid anglearoundthecollisionpoint.2 Chargedparticlesinthepseudo- rapidityrange|η|<2.5 arereconstructed withtheinner detector (ID), which consists of several layers of semiconductor detectors (pixelandstrip)andastraw-tubetransition–radiation tracker,the latter extending to |η|=2.0. The high-granularity silicon pixel detectorprovides four measurements per track;the closest layer to the interaction point is known as the insertable B-layer [31, 32], which was added in 2014and provides high-resolution hits at small radius to improve the tracking performance. The ID is immersed in a 2 T magnetic field provided by a superconduct- ing solenoid. The solenoidis surrounded by electromagnetic and hadronic calorimeters, and a muon spectrometer incorporating three large superconducting air–core toroid magnet systems.The calorimetersystemcoversthepseudorapidityrange|η|<4.9.Elec- tromagnetic calorimetry is provided by barrel and endcap high- granularity lead/liquid-argon (LAr) electromagnetic calorimeters, within the region |η|<3.2. There is an additional thinLAr pre- sampler covering |η|<1.8 to correct forenergy loss inmaterial upstream of the calorimeters.For |η|<2.5, the LAr calorimeters
2 ATLASusesaright-handed coordinatesystemwith itsoriginat thenominal interactionpointinthecenter ofthedetectorandthez-axisalongthebeampipe.
Thex-axispointsfromtheinteractionpointtothecenter oftheLHCring,andthe y-axispointsupward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane, φbeingtheazimuthalanglearoundthebeampipe.Thepseudorapidityisdefined intermsofthepolarangleθasη= −ln tan(θ/2).Observableslabeled “transverse”
areprojectedintothex–yplaneandangulardistanceismeasuredinunitsofR= (η)2+(φ)2.
aredividedintothreelayersindepth.Hadroniccalorimetryispro- videdby a steel/scintillator-tilecalorimeter,segmentedinto three barrel structures within |η|<1.7, and two copper/LAr hadronic endcap calorimeters,which coverthe region 1.5<|η|<3.2. The forwardsolid angleout to|η|=4.9 iscoveredbycopper/LArand tungsten/LAr calorimetermodules, which are optimized forelec- tromagnetic and hadronicmeasurements, respectively. The muon spectrometer comprisesseparate triggerandhigh-precisiontrack- ingchambersthatmeasurethedeflectionofmuonsinamagnetic fieldgeneratedbythethreetoroidmagnetsystems.TheATLASde- tector selects eventsusing a tiered trigger system [33]. The first levelisimplementedincustomelectronicsandreducestheevent ratefromtheLHCcrossingfrequencyof40MHztoadesignvalue of100kHz.The secondlevelisimplementedinsoftwarerunning onacommodityPCfarmwhichprocessestheeventsandreduces therateofrecordedeventsto1.0kHz.
3. Dataandsimulatedsamples
This analysis uses 36.1±0.8 fb−1 of pp collisions data at
√s=13 TeV recordedusingsingle-electronandsingle-muontrig- gers. Additional data-quality requirements are also imposed, and thesearedetailedinSection 4.During 2015thiscorrespondedto 3.2 fb−1 withanaverageof13.4interactions perbunchcrossing.
The2016data-takingperiodcorrespondsto32.9 fb−1withanav- erageof25.1interactionsperbunchcrossing.
The WR boson search isperformed inthe semileptonicdecay channel,wheretheWR decaysintoatopquarkandab-quark,the top quark decaysintoa W bosonandab-quark, andthe W bo- son decays in turn into a lepton and a neutrino. The final-state signature thereforeconsistsoftwob-quarks,one chargedlepton3 andaneutrino, whichisundetectedandresultsinmissingtrans- verse momentum, ETmiss.The dominant backgroundprocesses for thissignaturearethereforetheproductionofW/Z+jets(jetsaris- ing fromlight and heavy partons),electroweak single top quarks (t-channel, W tands-channel),t¯t pairsanddibosons(W W,W Z, and Z Z).Aninstrumental backgroundduetomultijetproduction, where ahadronicjet ismisidentified asalepton, isalsopresent.
MonteCarlo(MC)simulatedeventsareusedtomodeltheWR sig- nal andall the SM background processes, withthe exception of the multijetbackground prediction, which is derived using data.
TheMCgeneratorprogramsandconfigurationsaresummarizedin Table1,anddescribedingreaterdetailinthetextbelow.
Simulated signal events were generatedat leading order(LO) byMadGraph5_aMC@NLOv2.2.3 [34–37] usingachiralWR boson model in which the couplings to the right-handed fermions are likethoseintheSM.MadGraph5_aMC@NLOisalsousedtomodel thedecaysofthetop quark,takingspincorrelationsintoaccount.
3 The analysis selects electrons or muons, while the simulation includes τ-leptons.Thustheeventyieldincludesasmallcontributionduetoleptonicde- caysofτ-leptons.
Pythia8v8.186 [38] wasusedforpartonshoweringandhadroniza- tion, wherein the NNPDF23LO [39] parton distribution functions (PDF)oftheprotonandasetoftunedparameters calledtheA14 Pythiatune [40] were used.Allsamplesofsimulatedeventswere rescaledtonext-to-leading-order(NLO) calculationsusingNLO/LO K-factorsrangingfrom 1.1to1.4, decreasing asa function ofthe massofthe WR boson,calculatedwithZTOP[30].Signalsamples weregenerated between0.5and3 TeV insteps of250 GeV, and between3and5 TeV instepsof500 GeV.
Thebenchmarksignal modelusedforthiswork nominallyas- sumesthattheWR bosoncouplingstrengthtofermions, g,isthe sameasforthe W boson: g=g,whereg istheSMSU(2)L cou- pling.The couplingof left chiralfermions tothe WR is assumed tobe zero.Thetotal widthofthe WR boson increasesfrom2to 130 GeV formasses between0.5 and5 TeV [29] for g=g and scalesasthe squareof theratio g/g. Inorderto explorethe al- lowedrangeofthe WR coupling g, sampleswerealsogenerated forvaluesof g/g upto5.0, forseveral WR bosonmasshypothe- ses,allowingtheeffectofincreasedWR widthtoalsobeincluded.
Simulated top-quark pair and single-top-quark processes (t- channel,s-channelandW t)wereproducedusingtheNLOPowheg- Box[41,42] generatorwiththeCT10PDF [43].Thepartonshower and the underlying event were added using Pythia v6.42 [44]
with the Perugia 2012 tune [45]. The top-quark pair produc- tion MC sample is normalized to an inclusive cross section of
σt¯t=832+−4651 pb for a top-quarkmass of 172.5 GeV as obtained from next-to-NLO (NNLO) plus next-to-next-to-leading-logarithm (NNLL)QCDcalculationswiththeTop++2.0program [46–52].
The background contributions from W and Z boson produc- tion in association with jets were simulated using the Sherpa v2.2.1 [53] generator. Matrix elements were calculated for up to twopartonsatNLO andfourpartons atLOandmergedwiththe SherpapartonshowerusingtheME+PS@NLOprescription [54–56].
TheW/Z+jetssamplesarenormalizedtotheinclusiveNNLOcross sectionscalculatedwithFEWZ [57,58].
Theproductionofvector-bosonpairs (W W, W Z or Z Z) with at least one charged lepton in the final state was simulated by thePowheg-Box generatorin combinationwithPythia8 andthe leading-orderCTEQ6L1PDF [59].Thenon-perturbativeeffectswere modeled withtheAZNLOsetoftunedparameters [60].
ForallMadGraphandPowhegsamples,theEvtGenv1.2.0pro- gram [61] wasusedforthebottomandcharmhadrondecays.
Allsimulated eventsamplesinclude theeffect ofmultiple pp interactionsinthesameandneighboring bunchcrossings(pile-up) byoverlaying,oneachsimulatedsignalorbackgroundevent,sim- ulatedminimum-biaseventsgenerated usingPythia8,the A2set oftunedparameters [62] andtheMSTW2008LOPDFset [63].
Simulatedsamples were processed through the Geant4-based ATLAS detectorsimulationorthrough a fastersimulation making useof parameterized showers in the calorimeters [64,65]. Simu- latedevents were then processed using the same reconstruction algorithmsandanalysischainasusedfordata.
4. Eventselectionandbackgroundestimation
This search makes use of the reconstruction of multi-particle vertices,theidentificationandthekinematicpropertiesof recon- structedelectrons, muons, jets, andthe determinationofmissing transversemomentum.
Collisionverticesarereconstructedfromatleasttwo IDtracks withtransversemomentum pT>400 MeV. Theprimary vertexis selectedastheonewiththehighest
pT2,calculatedconsidering allassociatedtracks.
Electrons are reconstructed from ID tracks that are matched tonoise-suppressedtopologicalclustersofenergydepositions [66]
inthe electromagneticcalorimeter.Theclustersare reconstructed using the standard ATLAS sliding-window algorithm, which clus- ters calorimeter cells within fixed-size rectangles [67]. Electron candidates are requiredto satisfy criteriaforthe electromagnetic showershape,trackquality,andtrack–clustermatching;thesecri- teria areappliedusinga likelihood-basedapproach.Electroncan- didatesmustmeetthe“Tight”workingpointrequirementsdefined inRef. [68] andare furtherrequiredto have pT > 25 GeV anda pseudorapidityofthecalorimeterclusterposition,|ηcluster|,smaller than2.47. Events withelectrons falling inthecalorimeterbarrel–
endcaptransitionregion,1.37<|ηcluster|<1.52,whichhaslimited instrumentation,arerejected.
Muons are identified by matching tracks found in the ID to either full tracks or track segments reconstructed in the muon spectrometer(“combinedmuons”),orbystand-alonetracksinthe muon spectrometer [69]. They are requiredto pass identification requirementsbasedonqualitycriteriaappliedtotheIDandmuon spectrometer tracks. Muon candidates must meet the “Medium”
identificationworkingpointrequirementsdefinedinRef. [69],have atransversemomentum pT >25 GeV,andsatisfy|η|<2.5.
Electron andmuon candidates must further satisfy additional isolationcriteriathatimproverejectionofcandidatesarisingfrom sourcesotherthanpromptW/Z bosondecays(e.g.hadronsmim- ickinganelectronsignature,heavy-flavor hadrondecaysorphoton conversions).Muonsarerequiredtobeisolatedusingtherequire- mentthatthescalarsumofthe pT ofthetracksinavariable-size conearoundthemuondirection(excludingthetrackidentifiedas the muon) be less than 6% of the transverse momentum of the muon. The trackisolation cone size is givenby the minimum of R=10 GeV/pμ
T andR=0.3.Electronsarealsorequiredtobe isolatedusingthesametrack-basedvariableasformuons, except thatthemaximumR inthiscaseis0.2.Forthepurposeofmul- tijet background estimation (see Section 5) electrons and muons satisfyingaloosersetofidentificationcriteria,inparticularwith- outanisolationrequirement,arealsoconsidered.
Jetsarereconstructedfromtopologicalcalorimeterclustersus- ingtheanti-kt algorithm [70] witharadiusparameterof R=0.4, and must satisfy pT > 25 GeV and |η| < 2.5. To suppress jets originating from in-time pile-up interactions, jets in the range pT<60 GeV and |η|<2.4 are required to pass the jet vertex tagger [71] selection, which has an efficiency of about 90% for jetsoriginatingfromthe primaryvertex. Theclosest jetsoverlap- ping with selectedelectron candidates within a cone ofsize R equal to 0.2 are removed from events, as the jet and the elec- tron very likely correspond to the same reconstructed object. If a remaining jet with pT > 25 GeV isfound close toan electron withinaconeofsizeR=0.4,thentheelectroncandidateisdis- carded.SelectedmuoncandidatesnearjetsthatsatisfyR(muon, jet)<0.04+10 GeV/pμ
T are rejectedifthejet hasatleastthree tracksoriginatingfromtheprimaryvertex.Anyjetswithlessthan threetracksthatoverlapwithamuonarerejected.
Theidentificationofjetsoriginatingfromthe hadronizationof b-quarks(“b-tagging”)isbasedonpropertiesspecifictob-hadrons, such aslong lifetimeandlargemass. Suchjetsare identified us- ing the multivariate MV2c10 b-tagging algorithm [72,73], which makes useof informationaboutthejet kinematic properties,the characteristicsoftrackswithinjets,andthepresenceofdisplaced secondary vertices. The algorithm is used at the 77% efficiency working point and provides a rejection factor of 134 (6.21) for jetsoriginatingfromlight-quarksorgluons(charmquarks),asde- termined in simulatedt¯t events.Jets satisfyingthese criteriaare referredtoas“b-tagged”jets.
The presence of neutrinos can be inferred from an apparent momentumimbalanceinthetransverseplane.Themissingtrans- versemomentum(EmissT )iscalculatedasthemodulusoftheneg-
