Search for a CP-odd Higgs boson decaying to Zh in pp collisions at √s=8 TeV with the ATLAS detector

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Reference

Search for a CP-odd Higgs boson decaying to Zh in pp collisions at

√s=8 TeV with the ATLAS detector

ATLAS Collaboration

ALEXANDRE, Gauthier (Collab.), et al.

Abstract

A search for a heavy, CP-odd Higgs boson, A , decaying into a Z boson and a 125 GeV Higgs boson, h , with the ATLAS detector at the LHC is presented. The search uses proton–proton collision data at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb −1 . Decays of CP-even h bosons to ττ or bb pairs with the Z boson decaying to electron or muon pairs are considered, as well as h→bb decays with the Z boson decaying to neutrinos. No evidence for the production of an A boson in these channels is found and the 95% confidence level upper limits derived for σ(gg→A)×BR(A→Zh)×BR(h→ff¯) are 0.098–0.013 pb for f=τ and 0.57–0.014 pb for f=b in a range of mA=220–1000 GeV . The results are combined and interpreted in the context of two-Higgs-doublet models.

ATLAS Collaboration, ALEXANDRE, Gauthier (Collab.), et al . Search for a CP-odd Higgs boson decaying to Zh in pp collisions at √s=8 TeV with the ATLAS detector. Physics Letters. B , 2015, vol. 744, p. 163-183

DOI : 10.1016/j.physletb.2015.03.054

Available at:

http://archive-ouverte.unige.ch/unige:55725

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Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Search for a CP-odd Higgs boson decaying to Zh in pp collisions at √

s = ^{8 TeV with} ^the ^ATLAS ^detector

.ATLASCollaboration

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

Articlehistory:

Received16February2015

Receivedinrevisedform19March2015 Accepted24March2015

Availableonline28March2015 Editor:W.-D.Schlatter Keywords:

BSMHiggsboson ATLAS

A searchfor aheavy, CP-oddHiggs boson, A,decaying intoa Z bosonand a 125 GeVHiggsboson, h, withthe ATLAS detector atthe LHCis presented. The searchusesproton–proton collisiondata at a centre-of-massenergy of 8 TeV corresponding to anintegrated luminosity of 20.3 fb⁻¹. Decays of CP-evenhbosonstoτ τ orbbpairswiththeZbosondecayingtoelectronormuonpairsareconsidered, as wellash→^bb^decays^with^the ^Z ^boson^decaying^to^neutrinos.^Noêvidence ^for^the^productionôf an Abosoninthesechannelsisfoundandthe95%confidencelevelupperlimits derivedforσ(gg→ A)×^BR(A→^Zh)×^BR(h→^f¯f)are0.098–0.013 pbfor f=τ and0.57–0.014 pbfor f =^bⁱⁿâ^range ofmA=220–1000 GeV.Theresultsare combinedandinterpretedinthecontextoftwo-Higgs-doublet models.

PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

1. Introduction

Afterthediscovery ofaHiggsboson attheLHCin2012[1,2], one of the most important remaining questions is whether the newly discovered particle is part of an extended scalar sector.

A CP-odd Higgs boson, A, appears in many models with an extended scalar sector, e.g. in the case of the two-Higgs-doublet model(2HDM)[3].

The addition of a second Higgs doublet leads to ﬁve Higgs bosonsaftertheelectroweaksymmetrybreaking.Thephenomenol- ogy of such a model is very rich and depends on the vacuum expectationvaluesoftheHiggsdoublets, theCPpropertiesofthe Higgspotential andthevalues ofits parameters andtheYukawa couplingsof the Higgs doublets withthe fermions. In general, it is possible to accommodate in the model a Higgs boson com- patible to the one discovered atthe LHC. In the casewhere the Higgspotential of the 2HDMis CP-conserving, the Higgs bosons afterelectroweaksymmetrybreakingare twoCP-even(hand H), one CP-odd(A) and two charged (H^±) Higgs bosons. Manythe- ories beyond the Standard Model (SM) include a second Higgs doublet,such asthe minimal supersymmetric SM (MSSM) [4–8], axionmodels(e.g.Ref.[9])andbaryogenesismodels(e.g.Ref.[10]).

SearchesforaCP-oddHiggsbosonarereportedinRefs.[11–14].

InthisLetter,asearchforaheavyCP-oddHiggsbosondecaying intoa Z boson andthe ∼¹²⁵ ^GeV^Higgs ^boson, ^h, ^is ^described.

E-mailaddress:[email protected].

The A→^Zh ^decay^rate ^can ^be ^dominant^for ^part ^of ^the ^2HDM parameterspace,especiallyforan Abosonmass,mA,belowthet¯t threshold.Inthiscase, the A boson isproducedmainlyviagluon fusionanditsnaturalwidthistypicallysmall:A/mAO(1%).

ThesearchisperformedformAintherange220to1000 GeV, reconstructing¹ Z → decays (where =^e,μ) withh→^bb ^or h→τ τ,aswellasZ→ννwithh→^bb.^The^selected^h^boson^de- caymodesprovidehighbranchingratiosandthepossibilitytofully reconstruct theHiggs boson decay kinematics.The reconstructed invariantmass(ortransversemass)ofthe Zhpair,employing the measuredvalueofthehbosonmass,mh,toimproveitsresolution, isusedtosearchforasignal.

2. Dataandsimulatedsamples

ThedatausedinthissearchwererecordedwiththeATLASde- tector in proton–proton collisions at a centre-of-mass energy of 8 TeV. The ATLAS detector is described in detail elsewhere [15].

The integrated luminosity of the data sample, selecting only pe- riodswhere all relevantdetectorsubsystems were operational, is 20.3±⁰.6 fb⁻¹ [16]. The data used in the τ τ and bb ﬁ- nal states were collected using a combinationof single-electron, single-muon,dielectron(ee)anddimuon(μμ)triggers.Depending

1 ThroughoutthisLetter,thenotationh→bb,h→τ τ, Z→ννand Z→is usedforh→bb,¯ h→τ⁺τ⁻,Z→νν¯andZ→⁺⁻,respectively.

http://dx.doi.org/10.1016/j.physletb.2015.03.054

0370-2693/PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

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ZWW^(∗), ZZZ^(∗)) andtop pair production inassociation witha Z bosonare generatedwithMadGraph5.Finally,the productionof theSMHiggsbosoninassociationwitha Z bosonisconsideredas abackgroundinthissearch.ItissimulatedusingPYTHIA8.

The CTEQ6L1 [25] set of parton distribution functions was used forsamplesgenerated withMadGraph5 andPYTHIA8. The CT10[26]setwasusedfortheothersamples.

All generated samples are passed through the GEANT4-based [27]detectorsimulationoftheATLASdetector[28].Thesimulated eventsareoverlaidwithminimum-biasevents,toaccount forthe effectofmultipleinteractionsoccurringinthesameandneighbor- ing bunchcrossings(“pile-up”).Theeventsarereweightedsothat theaveragenumberofinteractionsperbunchcrossingagreeswith thedata.

The background estimation in this search for most processes is basedon data driven techniques,butin some cases onlysim- ulatedsamples are used.In that case, thesimulated samplesare normalizedusingtheoreticalcrosssectioncalculations.Inparticu- lar,for dibosonproduction bothqq¯ [29] and gg [30,31] initiated processesare included.Triboson productionfollowsRef. [32] and toppairproductioninassociationwithaZ bosonfollowsRefs.[33, 34].SMHiggsbosonproductioninassociationwithaZ bosonuses acalculationdescribedinRef.[35].

3. Objectreconstruction

Electronsareidentiﬁedfromenergyclustersintheelectromag- neticcalorimeterthatarematchedto tracksinthe innerdetector [36].Electronsarerequiredtohave|η_|<2.47 andpT>7 GeV.Iso- lationrequirements,deﬁnedintermsofthecalorimetricenergyor thepToftrackswithinconesaroundtheobject,aswellasquality requirementsareappliedtodistinguishelectronsfromjets.

Muons are reconstructed by matching tracks reconstructed in theinner detectorto tracksortracksegments inthemuon spectrometer systems [37]. The muon acceptance is extended to the region 2.5<|η|<2.7, which is outside the inner detector cov- erage,using onlytracks reconstructed inthe forwardpart ofthe muon detector. Muons used forthis search must have |η_|<2.7, pT>6 GeV andarealsorequiredtopassisolationrequirements.

Jets are reconstructed using the anti-k_t algorithm [38] with radius parameter R=⁰.4 and pT>20 GeV (pT>30 GeV) for

|η_|<2.5 (2.5<|η_|<4.5). Low-pT jetsfrompile-up are rejected

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

Thex-axispointsfromtheIPtothecentreoftheLHCring,andthey-axispoints upward.Cylindricalcoordinates(r,φ)areusedinthetransverseplane,φbeingthe azimuthalanglearoundthebeampipe.Thepseudorapidityisdeﬁnedintermsof thepolarangleθasη= −^{ln tan}(θ/2).Transversemomentaarecomputedfromthe three-momenta,p,aspT= |^p|^sinθ.

tified ashadronicτ decaysare applied.In thisanalysis,two τhad identificationselectionsareused—“loose”and“medium”—with efficienciesofabout65%and55%,respectively.

The missing transverse momentum (E^miss_T ) iscomputed using fullycalibratedandreconstructedphysics objects,aswellasclus- tersofcalorimeter-cellenergydepositsthatarenotassociatedwith anyobject[41].Inaddition,atrack-basedmissingtransversemo- mentum (p^miss_T ) is calculated asthe negative vector sum of the transverse momentaof trackswith|η|<2.4 and associatedwith theprimaryvertex.

4. SearchforA→^{Z h}^with^h→τ τ

In thesearch for A→^Zh→τ τ, threechannelsare considered, distinguishedbythewaythe τ τ pairdecays:two τ leptons decaying hadronically (τ_hadτ_had), one leptonic and one hadronic decay (τlepτhad) and, ﬁnally, two leptonic decays (τlepτlep). Elec- trons in the τhadτhad and τlepτhad channels are rejected in the transitionregion betweenthe barrelandend-capof thedetector (1.37<|η_|<1.52). Muons inthe τhadτhad andτlepτhad channels areconsideredonlyfor|η|<2.5.

The resolutionofthereconstructed A bosonmassisimproved usingamass-differencevariable,

m^rec_A =^mτ τ−^m−^mτ τ+^mZ+^mh,

wheremZ isthemassofthe Z boson,mh=^{125 GeV is}^the^mass oftheCP-evenHiggsboson,m istheinvariant massofthetwo leptons associated with the Z boson decay, and mτ τ denotes the τ τ invariant mass. The value of mτ τ, the invariant mass of the τ’s,is estimatedwiththe MissingMass Calculator(MMC) [42]. The mass resolutionfor all τ τ channelsranges from 3% at mA=220 GeV to5%atmA=1 TeV.

4.1. τhadτhad

Eventsintheτhadτhadchannelarerequiredtocontainexactly twoopposite-signleptons(eeor μμ)andexactlytwoopposite- sign τhad.The pT requirementsfortheseobjectsare pT>26 GeV (15 GeV) for the leading (subleading) electron, p_T>25–36 GeV (10 GeV) for the leading (subleading) muon, depending on the trigger, and pT>35 GeV (20 GeV) for the leading (subleading)

τhad candidates. The τhad candidates are required to satisfy the

“loose” τhadidentiﬁcationcriterion.Inaddition,theee/μμinvari- ant mass and the τ τ invariant mass have to lie in the ranges 80<m<100 GeV and 75<mτ τ<175 GeV.Finally, the p_T of thepair, p_T^Z,isrequiredtobe:

3 Theprimary vertexistaken tobethereconstructedvertexwith thehighest p²_Toftheassociatedtracks.

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p_T^Z>

_{125 GeV}

,ifm^rec_A >400 GeV 0.64×^m^rec_A −^{131 GeV},otherwise.

This requirement maximizes the sensitivity over the whole ex- ploredA massrange.Intheregionofp_T^Z>125 GeV,thereislittle background present, so tightening the requirement results in no additionalincreaseinsensitivity.Thetotalacceptancetimesselec- tioneﬃciencyvariesfrom6.2%,formA=^{220 GeV,}^to^around^18%

forthehighest Abosonmassesconsidered.

The dominant background for this channel originates from events where one or both of the τhad’s is a misidentiﬁed jet (“fake-τhadbackground”).Thisbackgroundisdominatedby Z+^jets events, withsmall contributions from dibosons and events with topquarks,anditisestimatedusingatemplatemethod.Theshape of the fake-τhad background is taken from a control region (the

“templateregion”)thatcontainseventssatisfyingalltheτhadτhad selection criteria apart from the requirements for an opposite- sign τhadτhadpairandthe τhadidentiﬁcationcriteria.Thefake-τhad backgroundisnormalizedbyusingtwoadditionalcontrolregions.

Theﬁrst region,“A”,contains eventsthat satisfy thesignal selec- tioncriteria,withtheexceptionthatthemτ τ constraintisinverted, i.e.mτ τ<75 GeV ormτ τ>175 GeV.Thesecondregion,“B”,con- tainseventsthatsatisfyallthetemplateselectioncriteria,withthe exceptionthatthemτ τ constraintisinverted,asintheregion“A”

deﬁnition.Theratio ofthe numberofevents in“A” tothe num- berofeventsin“B”isusedtoscalethetemplateregioneventsin ordertoobtainthenormalizationofthefake-τhadbackground.

Inadditiontothefake-τhadbackground,therearealsocontribu- tionsfrombackgroundswithrealτhadτhadobjects intheevent.

These backgrounds come primarily from Z Z^(∗) production.⁴ SM HiggsbosonproductioninassociationwithaZbosonisestimated usingsimulation,andcontributes17%ofthetotalbackground.

4.2.τlepτhad

Eventsintheτlepτhadchannelarerequiredtocontainexactly threelightleptons, μμμ,eμμ,eeμoreee,andexactlyone τhad. The pT requirementsfortheseobjectsare pT>26 GeV (15 GeV) forthe leading(remaining) electron(s), p_T>25–36 GeV (10 GeV) for the leading (remaining) muon(s), depending on the trigger, and pT>20 GeV for the τ_had. Subsequently, all the possible pairsthat arecomposed ofopposite-sign,same-ﬂavor leptonsare selected. From these pairs, the pair that has the invariant mass closesttom_Z isconsideredtobetheleptonpairfromthe Z boson decay.Thethirdlightleptonisconsideredtobetheleptonic τ decay,anditisusedalongwiththe τhad todeﬁnethe τlepτhadpair.

Thislightleptonisrequiredtohaveopposite-signchargewithre- spect to the τhad.In addition, the τhad is required to satisfy the

“medium”τhadidentiﬁcationrequirement, andm andmτ τ have tolieintheranges80<m<100 GeV and75<mτ τ<175 GeV.

Thetotal acceptancetimesselection efficiencyvariesfrom6%for m_A=^{220 GeV,}^toâround^17%^for^the^highestÂ^boson^masses^considered.

Abouthalfofthetotalbackgroundforthischannelcomesfrom eventswherethe τhadand/orthelightleptonisamisidentiﬁedjet (“fake-τ/background”).Thisbackgroundisdominatedbydiboson and Z+jets eventsandit isestimatedusingatemplatemethod.

The shape of the fake-τ/ background is taken from a control region (the “template region”) that contains events satisfying all τlepτhad selection criteria, apart from requiring “medium” τhad identiﬁcation criterion and opposite-sign charge for the τlepτhad pair. The fake-τ/ background is normalized by using two addi-

4 ThenotationZ Z^(∗)isusedheretoincludeZ Z,Z Zand Zγ.

tionalcontrolregions,deﬁnedsimilarlytothoseintheτhadτhad channel.

Theother halfofthebackgroundcomes fromeventswithreal τlepτhad objects in theevent. These backgrounds comeprimar- ilyfromZ Z⁽^∗⁾production.Thereisalsoasmall(11%)contribution fromtheSM Higgsbosonproduction inassociationwitha Z boson,whichisestimatedusingsimulation.

4.3. τlepτlep

Eventsintheτlepτlep channelarerequiredtocontainatleast four leptons, which formone same-flavor and opposite-sign pair consistent with the Z mass (80<m<100 GeV), andeither a same-flavor ordifferent-flavor pair withan invariant massrecon- structed withthe MMC algorithm, consistent with a decay from theCP-evenHiggsboson (90<mτ τ<190 GeV). Onemuon isal- lowedtobe reconstructedintheforwardregion (2.5<|η_|<2.7) of the muon spectrometer, orto be identified inthe calorimeter with pT>15 GeV and|η_|<0.1[37].The highest-pT lepton must satisfy p_T>20 GeV, and the second (third) lepton in p_T order must satisfy pT>15 GeV (pT>10 GeV). Among all the possible leptonquadrupletsinaneventtheoneminimizingthesumofthe massdifferenceswithrespecttoboththe Z andhbosonsischo- sen.

Twodifferentanalysiscategoriesaredeﬁnedbasedonthelep- tonﬂavors intheHiggsbosondecay:ee or μμ(SF),andeμ(DF).

The expected background is very different in the two cases. For theSFchannel,thebackgroundisdominatedby Z Z⁽^∗⁾ production withZ→êe/μμdecays.FortheDFchannel,themainbackground is fromthe Z Z^(∗) process through the Z→τ_lepτ_lep decay chain, butother backgroundsarealso important.The signal-to-noise ra- tiointheSF categoryisimprovedbyusinga setofrequirements specificallytargetedtosuppressthemain Z Z^(∗)background.First, avetoonthe on-shellproductionof Z bosonpairsis introduced, requiringtheinvariantmassoftheh bosonleptonstolie outside the Zpeak:mh<80 GeV ormh>100 GeV.Backgroundeventsare characterized by low missing transversemomentum andare fur- therrejectedbyrequiringE_T^miss>30 GeV,andtheazimuthalangle betweenthe E^miss_T directionandthe Z bosontransversemomen- tumtobegreater than π/2.Furthermore,a requirementthat the highest-pT lepton ofthe pairassociated withthehboson has pT>15 GeV is applied, since it is found to be effective against backgroundsfrom Z+jets production.Thetotalacceptancetimes selectionefficiencyvariesfrom6.5%(1.5%)forDF(SF)channelfor mA=^{220 GeV,}^toâround^20%^for^both^channels^for^the^highest Â bosonmassesconsidered.

Thesubleadingcontributionstothebackgroundarefromdibo- sonandtribosonproduction,t¯tproductioninassociationwitha Z boson,andSM Higgs bosonproduction.All thesearedetermined fromsimulationandamounttoabout95%(65%)ofthetotalback- groundintheSF(DF)category.Theotherbackgroundeventshave atleastoneleptonwhichisamisidentiﬁedjetoraleptonfroma heavy-ﬂavor quarkdecayandaredominatedby Z +^jets^production,withasmallercontributionfromtop-quarkproduction.These backgrounds are estimated using a control region where one or bothoftheleptonsinthepairassociatedwiththeh→τlepτlep decayfailtosatisfy theisolation criteria.Aftersubtractionofgen- uine sources of four-lepton eventsusing simulation,the dataare extrapolatedtotheisolatedsignal regionusingnormalizationfac- torsderivedfromsimulatedsamples.

4.4. Systematicuncertaintiesandresults

Themostimportantsystematicuncertaintyforthebackgrounds withrealτ τ objectsinthe τlepτhadand τlepτlepchannelscomes