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

(1)

Article

Reference

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

ATLAS Collaboration

ANCU, Lucian Stefan (Collab.), et al .

Abstract

Charged Higgs bosons produced in association with a single top quark and decaying via H±→τν are searched for with the ATLAS experiment at the LHC, using proton–proton collision data at s=13 TeV corresponding to an integrated luminosity of 3.2 fb−1 . The final state is characterised by the presence of a hadronic τ decay and missing transverse momentum, as well as a hadronically decaying top quark, resulting in the absence of high-transverse-momentum electrons and muons. The data are found to be consistent with the expected background from Standard Model processes. A statistical analysis leads to 95%

confidence-level upper limits on the production cross section times branching fraction, σ(pp→[b]tH±)×BR(H±→τν) , between 1.9 pb and 15 fb, for charged Higgs boson masses ranging from 200 to 2000 GeV. The exclusion limits for this search surpass those obtained with the proton–proton collision data recorded at s=8 TeV .

ATLAS Collaboration, ANCU, Lucian Stefan (Collab.), et al . 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. Physics Letters. B , 2016, vol. 759, p. 555-574

DOI : 10.1016/j.physletb.2016.06.017

Available at:

http://archive-ouverte.unige.ch/unige:84915

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

1 / 1

(2)

Contents lists available atScienceDirect

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.}

©²⁰¹⁶^TheÂuthor(s).^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (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 in}^the 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:atlas.publications@cern.ch.

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 association 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,^the^decay^H⁺→τ νcanhaveasubstantialbranch- http://dx.doi.org/10.1016/j.physletb.2016.06.017

0370-2693/©²⁰¹⁶^TheÂuthor(s).^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

(3)

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 hadronicτ 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 single 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¯tpairs,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 pairs andsingle 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 background 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- tionscalculatedwithFEWZ[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,andEvtGenv1.2.0[56]isused to simulate b- and c-hadron decays. Multiple overlaid pp collisions (pile-up, with 14 collisions per bunch-crossing onaverage) are simulatedwiththesoftQCDprocessesofPythiav8.186using the MSTW2008LO [57–59] PDF set and the A2 underlying-event tune [60]. Allsimulated signal and backgroundsamples are processed 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 algorithm [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 parameter 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,whereηand φaredifferencesinpseudorapidity andazimuthalangle,respectively.

(4)

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 candidates 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 selectioncriteriaaboveisfoundwithinaconeofsizeR=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−^cosφ_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 effects. 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 reconstructed 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 candidates not fulfillingthe identification criteriabutmatching a true

τhad atgeneratorlevel,thenumberofeventswithamisidentiﬁed

τhad-vis inthesignalregion(Nτhad-vis

fakes )isderivedfromthesubsetof anti-τhad-viscandidatesasfollows:

N_fakes^τ^had-vis=

i

N_anti₋_τ_had-vis(i)×^FF(i), (2)

wherethe indexi refers toeach 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 and}^diboson^processes.

If an electron is misidentified as a τhad-vis,a correction factor is applied totheeventinorder toaccount forthemisidentification ratemeasuredina Z→êe^control^regionⁱⁿ^data,^whereôneêlec- tron is reconstructed asa τhad-vis. Charged leptons from in-flight decays inmulti-jet events are accountedfor in themisidentified jet→τhad-visbackgroundestimate.