Search for charged Higgs bosons produced in association with a top quark and decaying via H± → τν using pp collision data recorded at √s=13 TeV by the ATLAS detector

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Physics Letters B

www.elsevier.com/locate/physletb

Search for charged Higgs bosons produced in association with a top quark and decaying via H

→ τ ν using pp collision data recorded at

√ s = ^{13 TeV by the ATLAS detector}

.TheATLASCollaboration^

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

Articlehistory:

Received31March2016

Receivedinrevisedform2June2016 Accepted9June2016

Availableonline15June2016 Editor:W.-D.Schlatter

ChargedHiggsbosonsproducedinassociationwithasingle topquarkanddecayingvia H^±→τ ν are searchedforwiththeATLASexperimentattheLHC,usingproton–protoncollisiondataat√

s=^{13 TeV} corresponding toanintegratedluminosityof3.2 fb⁻¹.Thefinalstateischaracterisedbythe presence ofahadronicτ decayandmissingtransversemomentum,aswellasahadronicallydecayingtopquark, resultingintheabsenceofhigh-transverse-momentumelectronsandmuons. Thedataarefoundtobe consistentwiththeexpectedbackgroundfromStandardModel processes.Astatistical analysisleadsto 95% confidence-level upper limits onthe production cross sectiontimesbranching fraction, σ(pp→ [^b]^{t H±})×^BR(H^±→τ ν),between1.9 pband 15 fb,forchargedHiggsbosonmassesrangingfrom200 to2000 GeV.Theexclusionlimitsforthissearchsurpassthoseobtainedwiththeproton–protoncollision datarecordedat√

s=^{8 TeV.}

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

1. Introduction

FollowingthediscoveryofaneutralscalarparticleattheLarge Hadron Collider (LHC) in 2012 [1,2], an important question is whether this new particle is the Higgs boson of the Standard Model (SM) or part of an extended Higgs sector. Charged Higgs bosons¹ appear in several non-minimal scalar sectors, where a second doublet [3] or triplets [4–8] are added to the SM Higgs doublet. The observationof a charged Higgsboson would there- foreclearlyindicatephysicsbeyondtheSM.

The ATLAS and CMS collaborations have searched for light chargedHiggsbosons,producedintop-quarkdecays,usingproton–

proton(pp)collisionsat√

s=^{7–8 TeV inthe} τ ν[9–13]andcs[14, 15] decay modes. Using data collected at √

s=^{8 TeV, charged} Higgsbosons heavier than the top quark were also searchedfor, using final states originating from both the τ ν and tb decay modes [11,13,16]. Vector-boson-fusion H⁺ production was also searchedforbyATLASusingthe W Z finalstate[17].Noevidence ofachargedHiggsbosonwasfoundinanyofthesesearches.

Form_H+ greater than the top-quark mass mtop, which is the mass range ofinterest in thispaper, the main production mode

 E-mailaddress:[email protected].

1 Inthe following,charged Higgs bosons aredenoted H⁺, with the charge- conjugateH⁻ alwaysimplied.Similarly,genericsymbolsareusedfortheirdecay products.

Fig. 1. Leading-orderFeynmandiagramsfortheproductionofachargedHiggsboson withamassmH+>mtop,inassociationwithasingletopquark(leftinthe4FS,and rightinthe5FS).

of a charged Higgs boson at the LHC is expected to be in as- sociation with a top quark [18–20]. The corresponding Feynman diagramsareshowninFig. 1.Whencalculatingthecorresponding crosssectioninafour-flavourscheme(4FS),b-quarksaredynami- callyproduced,whereasinafive-flavourscheme(5FS),theb-quark is alsoconsidered asan active flavour in the proton.For model- dependentinterpretations,4FSand5FScrosssectionsareaveraged accordingtoRef.[21].Intwo-Higgs-doubletmodels(2HDMs),the productionanddecayofthechargedHiggsbosonalsodependon the parameter tanβ, defined as the ratio of the vacuum expec- tationvalues ofthe two Higgsdoublets, andthe mixingangle α

betweentheCP-evenHiggsbosons.Inthealignment limit,where cos(β−α)^0,thedecayH+→τ νcanhaveasubstantialbranch- http://dx.doi.org/10.1016/j.physletb.2016.06.017

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

ing fraction. In a type-II 2HDM, even when the decay H⁺→^tb dominates,thebranchingfractionBR(H⁺→τ ν)canreach10–15%

atlargevaluesoftanβ [22].

Thispaper describesa search forchargedHiggs bosons in pp collisions at √

s=13 TeV using the ATLAS experiment. The pro- ductionofachargedHiggsbosoninassociation witha singletop quarkanditsdecayvia H⁺→τ ν areexploredinthemassrange 200to2000 GeV,extendingby1000 GeV themassrangeconsid- eredbytheATLAScollaborationduringRun1oftheLHC.Thefinal state ischaracterisedby the presenceof a hadronic τ decayand missingtransversemomentumarisingfromtheH⁺decay,aswell as a fully hadronic top-quark decay, resulting in the absence of high-transverse-momentumelectronsandmuons. TheSM predic- tioniscomparedtothedata,andresultsforthe signalcrosssec- tiontimesbranchingfraction σ(pp→ [^b]^{t H±})×^BR(H^±→τ ν)are presented,together withan interpretation in the hMSSM bench- mark scenario [23,24], in which the light CP-even Higgs boson massm_h isset to 125 GeV,without choosing explicitlythe soft- supersymmetry-breakingparameters.

2. Dataandsimulatedevents

The ATLASexperiment [25] consistsofan inner detectorwith coverageinpseudorapidity² upto|η|=².5,surroundedbyathin 2 Tsuperconducting solenoid, a calorimetersystemextendingup to |η|=⁴.9 and a muon spectrometer extending up to |η|=².7 that measures the deflection ofmuon trajectoriesin the field of threesuperconducting toroid magnets. Theinnermost pixellayer, theinsertableB-layer(IBL),was addedbetweenthefirstandsec- ond runsof theLHC, around a new,narrower andthinner beam pipe [26]. A two-level trigger system is used to select events of interest[27].Theintegratedluminosity,consideringonlythedata- takingperiods of2015in whichall relevant detectorsubsystems were operational, is 3.2 fb⁻¹ and has an uncertainty of 5%. It is derived following a methodology similar to that detailed in Ref. [28], from a calibration of the luminosity scale using x– y beam-separationscansperformedinAugust2015.

Simulatedevents of H⁺ production inassociation witha sin- gle top quark are generated in the 4FS at the next-to-leading order (NLO) with MadGraph5_aMC@NLO v.2.2.2 [29] using the NNPDF23LO[30] partondistributionfunction(PDF)set,interfaced to Pythia v8.186 [31] with the A14 set of tuned parameters (tune)[32]fortheunderlyingevent.

Themainbackgroundsaretheproductionoft¯t pairs,singletop quarks, W+^{jets, Z}/γ^∗+^{jets and} electroweak gauge boson pairs (W W/W Z/Z Z ), as well as multi-jet events. For the generation oft¯t pairsandsingle top quarks inthe W t- and s-channels,the Powheg-Boxv2 [33,34]generator withthe CT10 [35,36]PDF set inthematrix-elementcalculationsisused.Electroweak t-channel single-top-quark events are generated using the Powheg-Box v1 generator.Thisgeneratorusesthe4FSfortheNLOmatrix-element calculationstogether withthe fixed four-flavour PDF set CT10F4.

For this process, the top quark is decayed using MadSpin [37], therebypreservingallspincorrelations.Forallbackgroundsabove, the parton shower, the fragmentation and the underlying event are simulated using Pythia v6.428 [38] with the CTEQ6L1 [39]

PDF set and the corresponding Perugia 2012 (P2012) tune [40].

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

Thex-axispointsfromtheIPtothecentreoftheLHCring,andthey-axispoints upward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbeingthe azimuthalanglearoundthez-axis.Thepseudorapidityisdefinedintermsofthe polarangleθasη= −^{ln tan}(θ/2).

The top-quark mass is set to 172.5 GeV for all relevant back- ground and signal samples. The t¯t cross section is calculated at next-to-next-to-leadingorder(NNLO),includingsoft-gluonresum- mation to the next-to-next-to-leading logarithmic (NNLL) order, with Top++ v2.0 [41–47]. The single-top-quark samples are nor- malised to the approximate NNLO cross sections [48–50]. Events containing a W or Z boson with associated jets are simulated using MadGraph5_aMC@NLO v.2.2.2 at LO with the NNPDF23LO PDF set, interfaced to Pythia v8.186 with the A14 underlying- eventtune.ThesesamplesarenormalisedtotheNNLO crosssec- tionscalculatedwith FEWZ[51–53].Finally,dibosonprocessesare simulated using the Powheg-Box v2 generator interfaced to the Pythia v8.186 parton shower model. The CT10 NLO set is used as the PDF forthe hard-scatter process, while the CTEQ6L1 PDF set is used for the parton shower. The non-perturbative effects aremodelledusingtheAZNLOtune[54].Thedibosonsamplesare normalisedtotheir NLOcrosssections,ascomputedbytheevent generator.

Wheneverapplicable, Photos++ v3.52[55]isemployedforpho- tonradiationfromchargedleptons,and EvtGen v1.2.0[56]isused to simulate b- and c-hadron decays. Multiple overlaid pp colli- sions (pile-up, with 14 collisions per bunch-crossing onaverage) are simulatedwiththesoftQCDprocessesof Pythia v8.186using the MSTW2008LO [57–59] PDF set and the A2 underlying-event tune [60]. Allsimulated signal and backgroundsamples are pro- cessed through a simulation [61] of the detector geometry and response using Geant4 [62]. Finally, they are processed through thesamereconstructionsoftwareasthedata.

Inthefollowing,thebackgroundsarecategorisedbasedonthe type ofreconstructedobjectsidentifiedasthevisibledecayprod- ucts³ofthehadronicallydecaying τ candidate(τhad-vis).Onlysim- ulated eventshaving a truehadronically decaying τ atgenerator level(τ_had)orwithachargedlepton(electronormuon)misidenti- fiedasa τhad-vis arekept.Backgroundsarisingfromajetmisiden- tifiedasa τhad-visareestimatedwithadata-drivenmethod.

3. Objectreconstructionandidentification

In the ATLAS experiment, hadronic jets are reconstructed from energydeposits in the calorimeters,using the anti-kt algo- rithm [63,64] witha radius parameter R=⁰.4.In the following, jets are required to have a transverse momentum pT>25 GeV and|η_|<2.5.Amulti-variatetechnique(JetVertexTagger)relying on jet energy andtracking variables to determine the likelihood thatagivenjetoriginatesfrompile-up[65]isappliedtojetswith p_T<50 GeV and|η|<2.4.Jetsarisingfromb-hadron decaysare identified by using an algorithm that combines impact parame- ter information with the explicit identification of secondary and tertiary verticeswithin thejetintoa b-taggingscore [66,67].The minimalrequirementimposedontheb-taggingscoreinthisanal- ysiscorrespondstoa70%efficiencytotagab-quark-initiatedjetin t¯t events,withrejectionratesof400forlight-quark-initiated jets, 27for τhad-initiatedjetsand8forc-quark-initiatedjets,enhanced with respect to Run 1 by the use of the IBL and an improved algorithm. The taggingefficiencies fromsimulation are corrected basedontheresultsofflavour-taggingcalibrationsperformedwith data[68].

Candidatesforidentificationas τhad-visarisefromjetsthathave p_T>10 GeV and forwhich one orthree charged-particle tracks are found within a cone of size⁴ R=⁰.2 around the axis of

3 Thisreferstoallτdecayproductsexcepttheneutrinos.

4 R=

(η)²+ (φ)^2,whereηand φaredifferencesinpseudorapidity andazimuthalangle,respectively.

the τhad-vis candidate[69,70].Theseobjectsarefurtherrequiredto haveavisibletransverse momentum(pτ_T)of atleast 40 GeVand tobewithin |η|<2.3.The outputofboosteddecisiontree(BDT) algorithms [71]is usedin orderto distinguish τhad-vis candidates from jets not initiated by hadronically decaying τ-leptons. This is done separately for decays withone orthree charged-particle tracks,andforvaryingvaluesoftheidentificationefficiency.Inthis analysis,a workingpointcorresponding to a55% (40%)efficiency fortheidentificationof1-prong(3-prong) τhad-vis objectsisused, withrejectionratesofO(¹⁰²)forjets.

Inthis analysis, eventswith isolated electron ormuon candi- dates witha transverse energyor momentum above 20 GeV are rejected. Electron candidates [72] are reconstructed fromenergy deposits (clusters) in the electromagnetic calorimeter, associated with a reconstructed track in the inner detector. The pseudora- pidity range for the electromagnetic clusters covers the fiducial volumeofthe detector,|η_|<2.47 (thetransitionregion between the barrel and end-cap calorimeters, 1.37<|η|<1.52, is ex- cluded).Qualityrequirementsontheelectromagneticclustersand thetracks,aswellasisolationrequirementsinaconearoundthe electron candidatebased on its transverse energy andthe track- inginformation,arethenappliedinordertoreducecontamination fromjets. Themuoncandidatesare reconstructedfromtrackseg- mentsinthemuonspectrometer, andmatchedwithtracksfound intheinner detectorwithin |η|<2.5[73].The finalmuoncandi- datesarerefittedusingthecompletetrackinformationfromboth detector systems. They must fulfil quality requirements includ- ing a pT-dependent track-based isolation requirement in a cone of variable size around the muon, which has good performance underhighpile-up conditionsand/or when amuon is closeto a jet.

When objects overlap geometrically, the following procedures are applied, in this order. Every τhad-vis candidate that overlaps withaloosely identifiedelectron ormuon, withina cone ofsize R of0.4or0.2,respectively,isremoved.Then,reconstructedjets are discarded if an electron ora τhad−vis candidatefulfilling the selectioncriteriaaboveisfoundwithinaconeofsizeR=⁰.2.

ThemagnitudeE^miss_T ofthemissingtransversemomentum[74]

isreconstructed fromthe negative vector sumoftransverse mo- menta of reconstructed andfully calibrated objects (collected in the hard term), as well as from reconstructed tracks associated withthehard-scatter vertexwhichare notinthehardterm(col- lectedinthesoftterm).Inordertomitigatetheeffectsofpile-up, theE^miss_T isrefinedbyusingobject-levelcorrectionsfortheidenti- fiedelectrons,muons,jetsand τhad-viscandidatesinthehardterm.

As the soft term contains only tracks associated with the hard- scattervertex,itisrobustagainstpile-up.

4. Eventselectionandbackgroundmodelling

Charged Higgsbosons are searched forin the topology pp→ [^b]^{t H+}→ [^b](^{j jb})(τhadν).Events collected using an E^miss_T trigger with a threshold at 70 GeV are considered. After ensuring that no jets are consistent with having originated from instrumental effectsornon-collisionbackground,eacheventisrequiredtocon- tainone τhad-vis withpτ_T>40 GeV (onlythehighest-pτ_T candidate mustfulfiltheidentificationcriteriadescribedinSection3),three ormorejetswithp_T>25 GeV, ofwhichatleastone isb-tagged, noelectronormuonwithatransverseenergyormomentumabove 20 GeV,andtohave E^miss_T >150 GeV.Fortheselectedevents,the transversemassmTofthe τhad-vis andE^miss_T systemisdefinedas:

m_T=

2p^τ_TE^miss_T (1−^cosφ_τ,miss), (1) where φ_τ,miss is the azimuthal angle betweenthe τhad-vis and thedirectionofthe missingtransversemomentum. Thisdiscrim-

inating variable takes values lower than the W boson mass for W→τ ν decays in background events and lower than the H⁺ mass for signal events, in the absence ofdetector resolution ef- fects. Arequirementofm_T>50 GeV isapplied inorderto reject events with mismeasured E^miss_T , where τhad-vis is nearly aligned withthedirectionofthemissingtransversemomentum.

The E^miss_T triggerefficiencyismeasuredindataandthenused toreweightthesimulatedevents,ratherthanrelyingonthe E^miss_T triggerinthe simulatedsamples.Thismeasurement isperformed in a control region of the data that is orthogonal to the signal region described above, while retaining as many similarities as possible. For this purpose, events passing a single-electron trig- gerwitha transverseenergy thresholdat24 GeV areconsidered andrequiredtocontainexactlyoneelectronmatchedtothecorre- spondingtriggerobject,exactlyone τhad-vis andtwoormorejets, ofwhichatleastoneisb-tagged.Boththeelectronandthe τhad-vis fulfil loose identification criteria in order to improve the statis- ticalprecision, withlittle impact on themeasured E^miss_T turn-on curve.

The “jet→τhad-vis” background includes multi-jet events and other processes where a quark- or gluon-initiated jet is recon- structed and selected as the τhad-vis candidate. This background is estimated with a data-driven method that relies on the mea- surement of the rate at which jets are misidentified as τhad-vis, hereafter referred to as the fake factor (FF). For this purpose, a controlregionpopulatedprimarilywithmisidentified τhad-vis can- didatesisdefinedbyusingthe samerequirementsasforthesig- nal region, except that E_T^miss<80 GeV and that the number of b-taggedjets iszero. The FF is definedasthe ratio ofthe num- berofmisidentified τhad-viscandidatesfulfillingthenominalobject selectiontothenumberofmisidentified τhad-viscandidatessatisfy- ingan“anti-τ_had-vis”selection.Thisanti-τ_had-visselectionisdefined by inverting the τhad-vis identification criteria while maintaining a loose requirement on the BDT output score,which selectsthe samekindofobjectsmimicking τhad-vis candidatesasthosefulfill- ing the identification criteria. In order to account for differences betweengluon-,light-quark- andb-quark-initiatedjets,FFsarepa- rameterisedasfunctionsofpT,thetypeof τhaddecayviathemea- surednumberofchargedandneutralparticles(π⁰)[70],andthe b-taggingscore,asillustratedinFig. 2.Foreachtypeof τhaddecay, the threshold value for theb-tagging score of the τhad-vis candi- dateisoptimisedtokeepenough entriesineach ofthetwo bins, belowandabovethethreshold.Aftercorrectingfor τhad-vis candi- dates not fulfillingthe identification criteriabutmatching a true

τhad atgeneratorlevel,thenumberofeventswithamisidentified

τhad-vis inthesignalregion(Nτhad-vis

fakes )isderivedfromthesubsetof anti-τhad-viscandidatesasfollows:

N_fakes^τhad-vis=

i

N_{anti−τhad-vis}(i)×^FF(i), (2)

wherethe indexi referstoeach bininterms of p_T,type of τhad decayandb-taggingscore,inwhichtheFFisevaluated.

Backgroundsarisingfromeventsinwhichanelectronormuon ismisidentified asa τhad-vis onlycontribute atthe levelof 5% to thetotalbackground,withmisidentifiedmuonscontributingabout one order of magnitude less than misidentified electrons. These backgroundsare estimatedwithsimulationandincludecontribu- tionsfromtt,¯ single-top-quark,W/Z+^{jets anddibosonprocesses.}

If an electron is misidentified as a τhad-vis,a correction factor is applied totheeventinorder toaccount forthemisidentification ratemeasuredina Z→^{ee controlregionindata,whereoneelec-} tron is reconstructed asa τhad-vis. Charged leptons from in-flight decays inmulti-jet events are accountedfor in themisidentified jet→τhad-visbackgroundestimate.