Physics Letters B

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Physics Letters B 736 (2014) 371–397

Contents lists available atScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Search for top-squark pairs decaying into Higgs or Z bosons in pp collisions at √

s = ^{8 TeV}

.CMSCollaboration

CERN,Switzerland

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

Articlehistory:

Received15May2014

Receivedinrevisedform10July2014 Accepted28July2014

Availableonline1August2014 Editor:M.Doser

Keywords:

CMS SUSY Stop Higgs

A search for supersymmetry throughthe direct pairproduction oftop squarks,with Higgs(H) or Z bosonsinthedecaychain,isperformedusingadatasampleofproton–protoncollisionsat√

s=^{8 TeV} collectedin2012withtheCMSdetectorattheLHC.Thesamplecorrespondstoanintegratedluminosity of19.5 fb⁻¹.Thesearchisperformedusingaselectionofeventscontainingleptonsand bottom-quark jets. Noevidence forasigniﬁcantexcessofevents overthestandard model backgroundprediction is observed. The results are interpreted in the context ofsimpliﬁed supersymmetric models with pair productionofaheaviertop-squarkmasseigenstate˜t2decayingtoalightertop-squarkmasseigenstate

˜

t1 viaeither˜t2→^H˜t1 or˜t2→^Z˜t1,followed inbothcases by˜t1→^tχ˜₁⁰,whereχ˜₁⁰ is anundetected, stable,lightestsupersymmetricparticle.Theinterpretation isperformedinthe regionwhere themass differencebetweenthe˜t1andχ˜₁⁰statesisapproximatelyequaltothetop-quarkmass(m_˜_t₁−^m_˜χ₁⁰^m^t^), whichisnotprobedbysearchesfordirect˜t1squarkpairproduction.Theanalysisexcludestopsquarks withmassesm_˜_t₂<575 GeV andm_˜_t₁<400 GeV ata95%conﬁdencelevel.

©²⁰¹⁴^TheÂuthors.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (http://creativecommons.org/licenses/by/3.0/).FundedbySCOAP³.

1. Introduction

Supersymmetry(SUSY)withR-parityconservation[1]isan ex- tension to the standard model (SM) that provides a candidate particle for dark matter and addresses the hierarchy problem [2–7].Thehierarchyproblemoriginatesinthespin-zeronatureof theHiggs (H) boson, whose massis subject todivergences from higher-ordercorrections. The leading divergent contributionfrom SM particles arisesfrom the H bosoncoupling to the top quark.

SUSYprovidesapossiblemeanstostabilizetheH bosonmasscal- culation,through the addition ofcontributions from a scalar top quark(top-squark)witha massnot toodifferentfromthatofthe topquark [8–12].Searches fordirect top-squarkproductionfrom theATLAS[13–18]andCompactMuonSolenoid(CMS)[19]Collab- orationsattheLargeHadronCollider(LHC)atCERNhavefocused mainly on the simplest scenario, in which only the lighter top- squark mass eigenstate, ˜t₁, is accessible at current LHC collision energies. In these searches, the top-squark decaymodes consid- eredarethosetoatopquarkandaneutralino,˜t₁→^tχ˜₁⁰→^bWχ˜₁⁰, ortoa bottom quark anda chargino,˜t1→^bχ˜₁⁺→^bWχ˜₁⁰.These two decaymodes are expectedto have large branching fractions

For correspondence please use e-mail address: cms-publication-committee- [email protected].

ifkinematicallyallowed.Thelightestneutralino,χ˜₁⁰,isthelightest SUSYparticle(LSP)intheR-parityconservingmodelsconsidered;

theexperimentalsignatureofsuchaparticleismissingtransverse energy(E^miss_T ).

Searchesfortop-squarkpairproductionarechallengingbecause thecrosssection isapproximatelysixtimessmallerthanthat for top–antitop quark pair (tt) productionif m_˜_t₁∼^mt anddecreases rapidlywithincreasingtop-squarkmass[20].Whenthemassdif- ference between the top-squark and the χ˜₁⁰ is large, top-squark production can be distinguished from tt production, as the for- mer is typically characterized by events withextreme kinematic features, especially large E^miss_T . This strategy is beingpursued in existing searches and has sensitivityto top-squark massesup to about 650 GeV for low χ˜₁⁰ masses [13–19]. The sensitivity of searchesfordirect top-squarkpairproductionis,however, signif- icantly reduced in the ˜t₁→^tχ˜₁⁰ decay mode for the region of SUSY parameterspace inwhichm_˜_t₁−^m_χ_˜⁰

1 ^mt.For example,in Ref. [19],theregion |^m˜t1−^m_χ_˜⁰

1 −^mt|^{20 GeV is} unexplored.In thisregion,themomentum ofthedaughterneutralinointherest frame of the decaying ˜t₁ is small, and it is exactly zero in the limit m_˜_t₁−^m_χ_˜⁰

1 =^mt. Asa result, the E^miss_T fromthevector sum ofthetransversemomentaofthetwoneutralinosistypicallyalso smallinthe laboratoryframe. Itthen becomes diﬃculttodistin- guishkinematicallybetween˜t1 pairproductionandthedominant http://dx.doi.org/10.1016/j.physletb.2014.07.053

0370-2693/©²⁰¹⁴^TheÂuthors.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/3.0/).Fundedby SCOAP³.

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Fig. 1. Diagramsfortheproductionoftheheaviertop-squark(˜t2)pairsfollowedbythedecays˜t2→^H˜t1or˜t2→^Z˜t1with˜t1→^tχ˜₁⁰.Thesymbol*denoteschargeconjugation.

background,whicharisesfromtt production.Thisregionofphase spacecan be explored usingevents withtopologiesthat are dis- tinctfromthett background.Anexampleisgluinopairproduction whereeachgluinodecaystoa topsquarkandatopquark, giving risetoasignaturewithfourtopquarksintheﬁnalstate[21,22].

This analysis targets the region of phase space where m_˜_t₁− mχ_˜₁⁰^mt by focusingon signaturesofttHH, ttHZ, andttZZ with E^miss_T . These ﬁnal states can arise from the pair production of the heavier top-squark mass eigenstate ˜t2. There are two non- degenerate top-squark mass eigenstates (˜t₂ and ˜t₁) due to the mixing of the SUSY partners ˜tL and ˜tR of the right- and left- handed top quarks. The ˜t2 decays to ˜t1 and an H or Z boson, and the ˜t1 is subsequently assumed to decay to tχ˜₁⁰, as shown in Fig. 1. Other decay modes such as ˜t₁→^bχ˜₁⁺→^bWχ˜₁⁰ are largelycovered form_˜_t

1−^m_χ_˜⁰

1 ^mt by existinganalyses [19].The final states pursued in this search can arise in other scenarios, such as˜t1→^tχ˜₂⁰,withχ˜₂⁰_→Hχ˜₁⁰ orχ˜₂⁰_→Zχ˜₁⁰.Theanalysis is also sensitive to a range of models in which the LSP is a grav- itino[23,24].The relativebranching fractionsformodeswiththe H and Z bosons are model dependent, so it is useful to search forbothdecaymodessimultaneously.Inthesignal modelconsid- ered, ˜t2 is assumedalways to decayto ˜t1 in association withan H orZ boson, such that the sum ofthe two branching fractions isB(˜t2→^H˜t1)+B(˜t2→^Z˜t1)=^100%.Ôther^possible^decay^modes are ˜t₂→^tχ˜₁⁰ and ˜t₂→^bχ˜₁⁺. These alternative decaymodes are not considered here, since they give rise to final states that are coveredbyexistingsearchesfordirecttop-squarkpairproduction [13–19].

Theresultsarebasedonproton–protoncollisiondatacollected at √

s=^{8 TeV by} ^the ^CMS êxperiment ât ^the ^LHC ^during ^2012, correspondingtoanintegratedluminosityof19.5 fb⁻¹.Theanaly- sispresentedheresearchesfor˜t2productioninasampleofevents withchargedleptons,denotedby(electronsormuons),andjets identifiedasoriginatingfrombottomquarks(b jets).Thefourmain searchchannelscontaineitherexactlyonelepton,twoleptonswith opposite-sign(OS)charge andno otherleptons, twoleptons with same-sign(SS) chargeandnoother leptons,oratleastthreelep- tons(3).ThechannelswithoneleptonortwoOSleptonsrequire atleastthreeb jets,whilethechannelswithtwoSSleptonsor3 requireatleastoneb jet.Theserequirementssuppressbackground contributions from tt pair production, which has two b quarks and either one lepton or two OS leptons fromthe tt→ νqqbb or tt→ ννbb decay modes, where q denotes a quark jet. The sensitivitytothesignalarisesbothfromeventswithadditionalb quarksin the final state (mainly fromH→^bb), ând^from êvents withadditionalleptonsfromH orZ bosondecays.

Thisletterisorganizedasfollows:Section2brieﬂy introduces theCMSdetector,whileSection3presentstheeventsamplesand theobjectselectionsused. Section 4describesthesignal regions, andSection5detailsthebackgroundestimationmethods.Theex- perimentalresultsarepresentedinSection 6,andinSection7we discusstheinterpretationoftheresultsinthecontextofthesignal

modelofthepairproductionofa heaviertop-squarkmasseigenstate˜t₂ decayingtoalightertop-squarkmasseigenstate˜t₁. 2. TheCMSdetector

The CMSdetector [25] comprisesa silicon trackersurrounded by a lead-tungstate crystal electromagnetic calorimeter and a brass-scintillatorhadroniccalorimeter,asuperconductingsolenoid supplying a3.8 T magneticfield tothe detectorsenclosed, anda muonsystem.Thesilicontrackersystemconsistsofpixelandstrip detectors,whichmeasurethetrajectoriesofchargedparticles.En- ergy measurements of electrons, photons, and hadronic jets are provided by the electromagnetic andhadroniccalorimeters. Each ofthesesystemsincludesboth central(barrel) andforward(end- cap) subsystems. These detectors operate in the axial magnetic field ofthesolenoid,whilemuonsareidentified ingas-ionization detectors that are embedded in the steelflux-return yokeof the solenoid.

The CMS experiment uses a right-handed coordinate system withthe originatthenominalpp interactionpoint atthecenter ofthedetector.Thepositivex axisisdeﬁnedbythedirectionfrom theinteraction pointtothecenteroftheLHC ring,withthepos- itive y axispointingupwards.Theazimuthal angleφ ismeasured aroundthebeamaxisinradiansandthepolarangleθ ismeasured fromthe z axis pointinginthedirectionofthe counterclockwise LHCbeam.Thepseudorapidityisdeﬁnedas η≡ −^ln[^tan(θ/2)]^.

The silicontracker,themuon system, andtheelectromagnetic calorimetercovertheregions|η_|<2.4,|η_|<2.4,and|η_|<2.5,respectively.Thehadroniccalorimetersextendupto|η|≈^5,^improv- ing momentum balancemeasurements intheplane transverse to thebeamdirection.Theonlinetriggersystemthatselectscollision events ofinterest is basedon two stages: a ﬁrst-levelhardware- basedselectionandasecondsetofrequirementsimplementedin software.

3. Eventsamples,objectselection,andeventsimulation

The data used for this search were collected with a high transverse-momentum(p_T)electron(e)ormuon(μ)single-lepton trigger, whichrequiresatleastone electronwith pT>27 GeV or muonwithpT>24 GeV.Thetriggereﬃciencies,asmeasuredwith asampleofZ→ ⁺⁻events,varybetween85%and97%forelec- trons, andbetween80% and95% formuons, depending onthe η

and pT valuesoftheleptons. Eventswere alsocollectedwiththe ee,eμ,and μμdouble-leptontriggers,whichrequireatleastone e orone μwithpT>17 GeV andanotherwithpT>8 GeV.Events are alsoacquiredwith adouble-lepton triggertargetinglower-pT leptons, requiring pT>8 GeV, butwith an additionalonline se- lection of HT≡ Σjet|^p^jet_T |>175 GeV, considering only jets with pT>40 GeV inthesum.Theeﬃcienciesliebetween90%and95%

for the trigger targetinglower-pT leptons, and between80% and 95% forthetriggertargetinghigher-p_T leptons, depending onthe

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CMS Collaboration / Physics Letters B 736 (2014) 371–397 373

ηandpT valuesofthelower-pT lepton.Forselectionswithmore thantwoleptons,thetriggersarefullyeﬃcient.

Eventsarereconstructedofflineusingtheparticle-flow(PF)algorithm [26,27]. Electroncandidates are reconstructed by associ- atingtrackswithenergyclustersintheelectromagneticcalorime- ter[28,29]. Muon candidates are reconstructed by combiningin- formation from the tracker and the muon detectors [30]. Signal leptons are produced in the decays of W and Z bosons. In order to distinguish these leptons from those produced in the de- caysofheavy-flavor hadrons,allleptoncandidatesarerequiredto be consistent withoriginating from the primary interaction ver- tex, chosen as the vertexwith the highest sumof the p²_T of its constituenttracks.Inparticularthey arerequiredtohaveatrans- verse impact parameter with respect to thisvertex smaller than 0.2 mm.Atighterrequirementisusedfortheeventcategorywith two SS leptons (see Ref. [31]). Furthermore, since misidentified lepton candidates arising from background sources, such as the decaysofhadrons,are typicallyembedded injets, all leptoncan- didates are required to be isolated from hadronicactivity in the event. This is achieved by imposing a maximum allowed value on the quantity p^sum_T , defined as the scalar sum of the pT val- uesofchargedandneutralhadronsandphotonswithinaconeof radius R≡

(η)²+ (φ)²=⁰.3 around thelepton candidate momentum direction at the origin. For the event category with atleastthreeleptons,theisolation requirementis p^sum_T <0.15pT. Forthelower lepton-multiplicityselections,the isolation require- mentistighter(seeRefs. [19]and[31] fordetails).Thesurround- inghadronicactivityiscorrectedfortheenergycontributionfrom additionalproton–protoninteractionsintheevent(pileup),asde- scribedinRef.[32].

Jetsare reconstructed from particle-flowcandidates using the anti-kT clustering algorithm [33] with a distance parameter of 0.5. Their energies are corrected forresidual non-uniformity and non-linearity of the detector response using corrections derived fromexclusive dijetand γ/Z+^{jet data} ^[34]. ^The ênergy ^contri- butionfrompileupisestimatedusingthejetareamethodforeach event [35] and is subtracted from the jet pT. Only high-pT jets inthecentral calorimeter|η_|<2.4 areconsidered.Jetsconsistent with the decay of heavy-flavor hadrons are identified using the combinedsecondaryvertexb-taggingalgorithmatthemediumor looseworkingpoints,definedsuchthattheyhavetaggingefficien- ciesof70% or80–85%,andmisidentificationratesforlight-flavor jetslessthan2% or10%, respectively[36].The E^miss_T iscalculated asthemagnitudeofthevectorsumofthetransversemomentaof allPF candidates,incorporating jetenergy corrections[37]. Qual- ityrequirements areappliedto removeasmallfractionofevents in which detector effects such as electronic noise can affect the E^miss_T reconstruction. Events are required to have E^miss_T >50 GeV toreducebackgroundcontributionsfromsourceswithasingle W bosonandfromjetproductionviaQCDprocesses.

Simulatedeventsamples are usedto studythe characteristics ofthe signal and to calculate its acceptance, aswell asfor part oftheSM backgroundestimation.Pairproductionoft˜2 squarksis described by the MadGraph 5.1.3.30 [38] program, including up totwo additionalpartons atthe matrixelement level,which are matchedtothepartonshoweringfromthe pythia 6.424[39] program. The SUSY particle decays are simulated with pythia with a uniform amplitude over phase space, so that all decays are isotropic[40].Theﬁrsttwodecaymodesconsidered(seeFig. 1)are assumedtohaveabranchingfractionofunitywhensettinglimits onSUSYparticlemasses. TheHiggsbosonmassissetto125 GeV [41], and its branching fractions are set according to the corre- sponding expectations from the SM [42]. For each decay mode, a gridofsignal eventsisgeneratedasa functionofthetwo top- squarkmasses m_˜_t

2 andm_˜_t

1. The ˜t₁ squarkis forced to decay to

atopquark andaneutralinoLSPassuming m_˜_t₁−^m_χ_˜⁰

1=^{175 GeV.}

The top-quarkmassisset to175 GeV.The signaleventratesare normalized to cross sections calculated at next-to-leading order (NLO) in the strong coupling constant, including the resumma- tionofsoft-gluonemissionatnext-to-leading-logarithmicaccuracy (NLO+^NLL)^[43–48].

The SM background processes considered are the production of tt; tt in association with a boson (H, W, Z, γ^∗); W, Z, and

γ^∗+^jets; ^triboson; ^diboson; ^single-top ^quark ⁱⁿ ^the ^s, ^t, ^and tW channels; and single-top quark in association with an additional quark anda Z boson. These processes are generated with the MadGraph, powheg-box 1.0 [49,50], or mc@nlo 2.0.0 beta3 [51,52] programs, using the CT10 [53] (powheg), CTEQ6M [54]

(mc@nlo), and CTEQ6L1 [54] (MadGraph) parton distribution functions (PDFs). SM background event rates are normalized to cross sections [51,52,55–60] calculated at next-to-next-to-leading orderwhenavailable, otherwiseatNLO. Allthebackgroundsam- ples are processed with the full simulation of the CMS detector based on Geant4 [61], while thegenerated signal samples usea fast simulation [62]. The fast simulation is validated against the full simulationforthe variablesrelevant forthissearch,andeﬃ- ciencycorrectionsbasedon dataareapplied [63].The simulation is generated with inelastic collisions superimposed on the hard- scattering event. Events are weighted so that the distribution of thenumberofinelasticcollisionsperbunchcrossingmatchesthat indata.

4. Eventcategoriesandsignalregions

Thesearchiscarriedout throughcomparisonsofthedataand SM background yields in disjoint signal regions (SRs) targeting the SUSY processes shown in Fig. 1, while suppressing the con- tributionsfromSMbackgrounds,predominantlytt production.The definitionsoftheSRsaresummarizedinTable 1,andaredetailed inthefollowingsubsections.Eventsareclassifiedaccordingtothe lepton multiplicity andcharge requirementson the leptons. Four maineventcategoriesareconsidered.Thefirsttwoincludeevents withone leptonortwo OSleptons. Sincetheselepton signatures also arise inthe decays oftop–antitop quark pairs, requirements ofatleastthreeb jetsare usedtosuppressthisbackground.The other two categories are eventswith exactly two SSleptons and events with three or more leptons, which do not typically arise intt events.A requirementofatleastoneb jet isappliedtofur- ther suppress the contribution frombackgrounds fromW and Z bosons.Leptonvetoesareusedtoensurethatthefourmainevent categoriesdonotoverlap.

4.1. Eventcategorieswithasingleleptonortwoopposite-signleptons

The eventcategories with one lepton or two OS leptons, ac- companiedineithercasebyatleastthreeb jets,targetsignatures withH bosons,whichhavelargebranchingfractionforH→^bb.^In thesingle-leptonchannel,eventsarerequiredtohaveexactlyone electron with pT>30 GeV and |η_|<1.44 or exactly one muon with p_T>25 GeV and |η|<2.1. Events withan indication ofan additional lepton, either an isolated track [31] or a hadronically decaying τ-lepton candidate τh [64–66], are rejectedin order to reducethebackgroundfromtt eventsinwhichbothWbosonsde- cayleptonically. Inthedouble-leptonchannel,eventsarerequired tocontainexactlytwochargedleptons(ee,eμ,or μμ),eachwith pT>20 GeV and|η_|<2.4.Inthiscase,eventswithanadditionale or μwithpT>10 GeV arerejected.Anyelectroncandidateinthe region 1.44<|η_|<1.57, a less well-instrumented transition re- gionbetweenthebarrelandendcapregionsofthecalorimeter,is

Physics Letters B