Search for single production of a vector-like quark via a heavy gluon in the 4b final state with the ATLAS detector in $\mathit{pp}$ collisions at $\sqrt{s}=8$ TeV

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Physics Letters B

www.elsevier.com/locate/physletb

Search for single production of a vector-like quark via a heavy gluon in the 4b final state with the ATLAS detector in pp collisions at

√ s = ^{8 TeV}

.ATLASCollaboration^

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

Articlehistory:

Received22February2016

Receivedinrevisedform14April2016 Accepted29April2016

Availableonline3May2016 Editor: W.-D.Schlatter

A searchis performedfor theprocess pp→^G∗→^BHb¯/ ¯BHb→^Hbb¯→^bbb¯ b,¯ predicted incomposite Higgsscenarios,whereG^∗isaheavycolouroctetvectorresonanceandBHavector-likequarkofcharge

−¹/3.Thedatawereobtainedfrompp collisionsatacentre-of-massenergyof8 TeVcorrespondingtoan integratedluminosityof19.5 fb⁻¹,recordedbytheATLASdetectorattheLHC.Thelargestbackground, multijetproduction,isestimatedusingadata-drivenmethod.Nosignificantexcessofeventswithrespect to Standard Model predictions is observed, and upper limits on the production cross section times branching ratio are set. Comparisons to the predictions from aspecific benchmark model are made, resultinginlowermasslimitsinthetwo-dimensionalmassplaneofmG^∗vs.mB_H.

©^{2016TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Composite Higgs [1–4] models interpret the Higgs boson dis- covered at the Large Hadron Collider (LHC) [5] as a pseudo- Goldstone boson resulting from spontaneous symmetry breaking inanewstronglycoupledsector,thus addressingthenaturalness problem,theextremefinetuning requiredintheStandardModel (SM)tocancelquadraticallydivergentradiative correctionstothe Higgsbosonmass.Agenericpredictionofthesemodelsistheexis- tenceofmassivevector-likequarks(VLQ).TheseVLQsareexpected to mix mainly withthe third family of quarks of the SM [6–8], leadingtopartialcompositeness.Colouroctetresonances(massive gluons)alsooccurnaturallyinthesemodels[6,7,9,10].

Searchesfor vector-like quarks inthe ATLAS andCMS experi- ments,in both thepair andsingle productionprocesses [11–22], constraintheir massto beabove 700–900 GeV.This analysisisa search for single production of a vector-like quark BH of charge

−¹/3 via the s-channel exchange of a heavy colour octet vec- tor resonance G^∗, using data recorded by the ATLAS detector at theLHC.Thesearch isperformedfortheprocessof Hb_{b produc-}¯ tionthrough pp→^G∗→^BHb¯/ ¯B_Hb→^Hbb¯→^bbb¯ b (see¯ Fig. 1),¹ based on Ref. [23] and using the benchmark model of Ref. [9].

Thissimplified minimal composite Higgsmodel has a composite sector with a global SU(3)c×^SU(2)L×^SU(2)R×^U(1)Y symme- tryandanelementarysectorwhichcontainstheSMparticlesbut

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

1 Chargeconjugatestatesareimpliedinthefollowingtext.

Fig. 1. Feynman diagram of the signal process qq¯→^G∗→^BHb¯→^Hbb¯→^bbb¯b.¯

not the Higgs boson. Physical states of the composite sector in- clude the heavy gluon G^∗, a composite Higgs boson and heavy vector-like quarks of charge 5/3, 2/3, −¹/3 and −⁴/3. Among theseheavyquarks,thereisonesingletofcharge2/3 whichmixes withtheright-handedtopquarkoftheSMwithan angleθt_R,and similarlyone singletofcharge−¹/3 whichmixeswiththeright- handed bottom quark ofthe SM with an angleθb_R. Aftermixing between the gluons from the elementary and composite sectors by an angle θs,the physical state ofthe heavy gluon has a cou- pling g_ccosθs tocomposite states,where g_c=^gs/sinθs andg_s is thecouplingoftheSM gluon.Theother parametersofthemodel are the composite fermion masses, assumed to be universal, the heavy gluonmassmG^∗ andtwoYukawacouplings YT andYB.In alargepartoftheparameterspace,thelightestofthenewheavy quarksis B_H,of charge−¹/3,andinthismodelit decaysexclu- sively to Hb. InRef. [23], the conditionm_BH =^mG^∗/2 is applied, withtheresultthatpairproductionoftheheavypartnersiskine- maticallyforbidden andthe widthof G^∗ is consequently not too large.Inthesearchpresentedhere,thephasespaceisextendedto mBH≥^mG^∗/2.WhenmBH<mG^∗/2,presentresultsonpairproduc- http://dx.doi.org/10.1016/j.physletb.2016.04.061

0370-2693/©^{2016TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

tionofvector-likequarkscanberecastinamodelwithamassive colouroctet[24].

ForhighmassesoftheG^∗andB_H resonances,theHiggsboson is highly boosted and the decayproducts are reconstructed in a singlelarge-radius(large-R)jetinthedetector,whereas forlower masses the four b-quarks are reconstructed as separate small- radiusjets.Theanalysisusestwosetsofselectioncriteriatotarget thesetwocases.

2. TheATLASdetector

TheATLAS detector,² locatedattheLHC,is describedindetail in Ref. [25]. It covers nearly the full solid angle around the col- lision point. The inner detectoris surroundedby a solenoid that produces a 2 T axial magnetic field. The tracks of charged par- ticles are reconstructed with a high-granularity silicon pixel and microstripdetectorfor|η_|<2.5.Astraw-tubetransitionradiation detector extends the tracking to larger radii and provides elec- tron/piondiscrimination.Theelectromagneticcalorimeterconsists of a barrel andend-cap lead/liquid-argon (LAr)sections with an accordiongeometrycovering|η_|<3.2,precededbyathinpresam- pler,covering|η|<1.8,whichallowscorrectionsforfluctuationsin upstreamenergylosses.Acopper/LArelectromagneticcalorimeter covers the very forwardangles. Hadronic calorimetry is installed in the barrel region, |η|<1.7, using steel as the absorber and scintillatortilesastheactivematerial.Intheendcaps,copper/LAr calorimeterscover1.5<|η_|<3.2 followedbyaforwardcalorime- terbasedontungstenabsorbersinLArassensitivemedium,upto

|η|=⁴.9.Surroundingthehadroniccalorimetersarelargetoroidal magnetswhosemagneticfieldsdeflectthe trajectoriesofcharged particles exiting the barrel and end-cap calorimeters. The muon spectrometer usesmonitored drift tubes fortracking in|η|<2.7 withcathodestripchambersintheinnermoststationfor|η_|>2.0.

A dedicated muon trigger is provided by resistive plate cham- bersinthebarrelandthin-gapchambersintheend-cap,covering

|η|<2.4.

A three-leveltrigger system, consistingof a hardware Level-1 triggerandtwosoftware-basedtriggerlevelsreducetheeventrate toberecordedtolessthanabout400 Hz.

3. Dataandsimulation

Datausedinthisanalysiscorrespondtoanintegratedluminos- ityof19.5 fb⁻¹ of pp collisionscollected attheLHCata centre- of-massenergyof√

s=^{8 TeV, withall theessential elements of} theATLASdetectorfullyoperationalandstable.

Simulated signal and background samples are produced by MonteCarlo(MC) eventgeneratorsandpassedthrough a Geant4 [26]simulationoftheATLASdetector[27].Additionaleventsfrom thesameandneighbouringbunchcrossings(pile-up)areincluded by adding simulated diffractive and non-diffractive pp collisions tohard-scatteringevents.Thepile-uprateisreweightedinaccor- dancewiththeluminosity profileofthe recordeddata.Allsimu- latedeventsarethenreconstructedusingthesamereconstruction softwareasthedata.

Signal samples basedon the modeldiscussed in Ref. [23] are generated with MadGraph5_aMC@NLO [28], using CTEQ6L1 [29]

2 ATLASexperimentusesaright-handedcoordinatesystemwithitsoriginatthe nominalinteractionpointinthecentreofthedetector andthe z-axisalongthe beampipe.Thex-axispointsfromtheIPtothe centreoftheLHCring,andthe y-axispointsupward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane, φbeing theazimuthalanglearoundthe z-axis.Thepseudorapidityisdefinedin termsofthepolarangleθasη= −^{ln tan}(θ/2).Thedistanceinη–φspaceisre- ferredtoasR=

(η)²+ (φ)^2.

partondistribution functions(PDFs), inthemass regionmG^∗/2≤ mBH <mG^∗, with 1 TeV<mG^∗<3 TeV, in steps of 250 GeV in mG^∗ and in steps of 125 GeVin mBH. The Higgs boson mass is setto126 GeVanditsbranchingratioBR

 H→^bb¯



to56.1%[30].

The parametersofthemodelare setasinRef.[23]: gc=^3,YT= Y_B=^3,sinθtR=^sinθbR=⁰.6.

The event selection requires at least two b-jets in the final state. Multijetevents from stronginteractions have a large cross section andarethedominantbackground.Due tothelargenum- ber ofevents requiredto simulatethisbackgroundandthe diffi- cultyofmodellingitaccurately,itisevaluatedusingadata-driven method, as described in Section 6. Other background contribu- tions includetop-pair andsingle-top-quark production,generated with Powheg-Box [31–33] interfaced to Pythia [34] using CT10 PDFs[35].Thet¯t sampleisnormalised tothetheoretical calcula- tion performedatnext-to-next-to-leading order(NNLO)including resummation of next-to-next-to-leading logarithmic (NNLL) soft gluon terms with Top++2.0 [36,37], givingan inclusivecross sec- tion of 253⁺₋¹³₁₅ pb [38]. Samplesoftt¯+^{Z and t}t¯+^{H events are} generatedwith Pythia andCTEQ6L1PDFs.The Sherpa[39]gener- ator,withCT10PDFs,isusedtosimulateW/Z+jets sampleswith leptonic decayofthevectorbosons. Sherpa isalsousedtogener- ateZ+jets events,withZ→^bb,¯ wheretheextrajetsareproduced inclusively. Contributions from diboson backgrounds—W W , W Z and Z Z —areestimatedtobenegligible.

4. Objectreconstruction

The finalstate consistsoffourjetsfromb-quarks(b-jets),two ofwhichcomefromtheHiggsbosondecay.IftheHiggsbosonis sufficientlyboosted,havingatransversemomentum pT^{300 GeV,} thetwob-jetsmaybemergedintoasinglejetwithalargeradius parameter (large-R jet)andthereforetwodifferentjet definitions areused.

Jetswithsmaller radiusparameter, orsmall-R jets,arerecon- structed fromcalibratedcalorimeterenergyclusters[40,41]using theanti-kt algorithm[42] withadistanceparameter R=⁰.4.The highpTthresholdusedintheeventselectionensuresthatthecon- tamination of jets from pile-up is small. To ensure high-quality reconstruction ofcentral jetswhile rejecting mostjets not com- ing from hard-scatteringevents,criteria asdescribed inRef. [43]

areapplied.Jetsarecorrectedforpile-upbyajet-areasubtraction method andcalibratedby a jetenergyscale factor[44].Theyare requiredtohavepT>50 GeV and|η|<2.5.

Small-R jetsareidentifiedascontainingab-hadron(b-tagged) byamultivariatealgorithm[45].Thisalgorithmwasconfiguredto give a b-taggingefficiency of 70% insimulated t¯t events,with a mistag probabilityofabout1% forgluonandlight-quarkjetsand ofabout20%forc-quark-initiatedjets.The b-taggingefficiencyin simulated eventsiscorrected to account fordifferencesobserved betweendataandsimulation.

Large-R jetsarereconstructedusingtheanti-kt algorithmwith R=¹.0. Jet trimming [46,47] is applied to reduce the contam- ination from pile-up and underlying-event activity: subjets are formedusingthekt algorithm[48]with R=0.3 and subjetswith p_T(subjet)/p_T(jet)<5% areremoved.

Leptonsarevetoedinthisanalysistoreducebackgroundinvolv- ing leptonically decaying vector bosons. Electroncandidates with pT>7 GeV are identified in the range |η_|<2.47 from energy clustersintheelectromagneticcalorimeter,matchedto atrackin the inner detector. Requirements of ‘medium’ quality, asdefined in Ref. [49],are applied together with two isolation criteria: the scalar sum of the transverse momentum (energy) within a ra- dius R=⁰.2 aroundthe electroncandidatehas tobe lessthan

Signalregiondefinitions:category1(2)referstothecasewherethenext-to-leading-pT(leading-pT)jetnotassociatedwiththeHiggsbosonisassumedtobefromtheBH decay.

Category 1 Category 2

SR1 SR2 SR3 SR4 SR5

Lower cut on reconstructed mG^∗ and mBH [TeV] (1.0,0.5) (1.3,0.5) (0.8,0.5) (1.5,0.5) (1.8,1.0)

15% (14%) of the electron pT(ET). Muons with pT>7 GeV and

|η_|<2.4 arereconstructedfrommatchedtracksinthemuonspec- trometer and the inner detector. Quality criteria are applied, as describedinRef.[50],andanisolationrequirementisapplied:the scalarsumofthetransversemomentumoftrackswithin aradius

R=⁰.2 aroundthe muoncandidatehasto belessthan 10%of themuon pT.

5. Eventselection

Because of the very high hadronic background at the LHC, it isnot possibletohave adequateMonte Carlostatisticsformulti- jetevents.Theuncertainties inthequality ofsimulationofb-jets at high-pT can also be large. For these reasons, for each mass pair 

mG^∗,mBH

 being tested, a data-driven technique was used to evaluate the expected background, as described in Section 6.

Thetechnique requires thatwe define control regions orthogonal tothesignal regions. Ablind analysisisperformed, inwhichthe backgroundisfirstevaluatedwithoutinitialknowledgeofthedata inthe signal regions. In order to test the large numberof mass pairhypotheses,allsignalregioncutsareappliedexcepttheHiggs masswindowwhichisblindedwhenevaluatingthebackgroundin thesignalregions.

5.1.Eventpreselection

Events in the signal region are first preselected according to thefollowingcriteria(seeendofSection5.2forthesignalregion definition).

•^{Theymustsatisfya}combinationofsixtriggersrequiringmul- tiplejetsandb-jetsforvariouspTthresholds,whereb-jetsare identifiedbyadedicatedonlineb-taggingalgorithm.Thiscom- binationoftriggersis>99% efficientforsignaleventspassing the offlineselection, acrossthe BH and G^∗ massrangescon- sideredinthisanalysis.

•^{Theyarevetoedifthey contain}reconstructedisolated leptons (e orμ)inordertoreducethecontributionfromW/Z+^jets andt¯t backgrounds.

•^{At least three} small-R b-tagged jets must be present in the signalregion.

•^{TheinvariantmassofthesystemcomposedofallselectedR}= 0.4 jetsisrequiredtobegreaterthan600 GeV.

Twoeventtopologiesare consideredforthesignal,depending on theboostoftheHiggsboson.HighlyboostedHiggsbosonsarere- constructedusing large-R jets asdescribed in Section 4and this topologycorresponds to themerged scenario(see Section 5.2).If no large-R jet is found, an attempt is made to reconstruct the Higgsboson from two small-R jets (see Section 5.3). The accep- tance times reconstruction efficiency for the combined yields of thetwotopologiesvariesfrom5%to20%dependingonthemasses oftheG^∗andB_H.

5.2.Mergedselection

Thesignal regionforthemergedcaseconsistsofthefollowing requirements.

• ^Alarge-R jet must be presentwith pT>300 GeV and |η_|<

2.0 andmass in therange [90,140] GeV. The mass window wasoptimisedbasedonthesignalsensitivity.Ifmorethanone suchlarge-R jetispresent,theHiggscandidateischosentobe theone withmassclosest to126 GeV. Atleastone b-tagged jetmustbematchedtoitwithinadistanceR=¹.0.

• ^{Theremustbeatleasttwoadditionalb-taggedjetsseparated} fromtheHiggsbosoncandidate,R(H,j) >1.4.Thetwowith thehighest pT areused toreconstructtheG^∗ and BH candi- dates.

Oncethe Higgsboson candidatehasbeen identifiedasabove, there remains an ambiguity in assigning the other jets to the vector-like quark BH. The four-momentum of the BH candidate is reconstructed asthe four-momentum sumof the Higgsboson candidate and either the next-to-leading-pT (category 1) or the leading-pT (category2) b-jet away fromit,depending on theas- sumedmassdifferencebetweenG^∗andBH.ForlargeG^∗–BH mass difference, the BH andb-quark fromG^∗ splitting havehigh mo- mentum and therefore the jet fromthe subsequent B_H decay is likely to be the next-to-leading jet. For a small mass difference the opposite is truesince in thislattercase the BH decayprod- ucts are more boosted than the G^∗ splitting products. For each

mG^∗,mBH

pair,thecategorywhichhasthehigherprobabilitythat the correct pairing is formed is chosen, based on the simulated signal events. Finally, the G^∗ four-momentum is reconstructed asthe four-momentum sum ofthe Higgs boson jet andthe two leading-pTb-jetsnotmatchedtotheHiggsbosoncandidate.

Differentsignalregionsaredefinedforthedifferent

mG^∗,mBH

 masspairhypotheses.Theyarecharacterisedbythechoiceofcat- egorydefinedaboveaswellasbylowercutsonthereconstructed massesofG^∗andB_H candidates.Fiveinclusivesignalregionswere defined,withtheminimummassoftheG^∗candidaterangingfrom 0.8to1.8 TeVandoftheBH candidatefrom0.5to1 TeV;theseare showninTable 1.No uppercut ontheresonancemasseswasset sincethemultijetbackgrounddistributionfallsrapidlyandtheres- onancewidthsbecomelargerforhighmasses.Foreachmasspair considered,the signalregionthat givesthemaximumsignal sen- sitivity, theratio ofthe expectednumber ofsignal events to the squarerootofthenumberofbackgroundevents,ischosen.

5.3. Resolvedselection

Eventsintheresolved signalregion arerequiredtosatisfythe followingcriteria.

• ^{In order to be able to later combine the results with the} mergedchannel,eventsarerequiredtofail themergedselec- tioncriteria.

• Eventsarerequiredtohaveexactlyfoursmall-R jetswithpT>

50 GeV and |η|<2.5,withat leastthreeof thesejetsbeing b-tagged.TheHiggsbosoncandidateisreconstructedusingthe twojetswithinvariantmassnearestto126 GeV.Theinvariant mass isrequired tobe in theinterval [⁹⁰,140] ^{GeV and the} transversemomentumofthedijetsystemp_T(j j)>200 GeV.

The four-momentum of the BH candidate is reconstructed from the four-momentum sum of the Higgs candidate and either the