Search for top-squark pairs decaying into Higgs or Z bosons in pp collisions at √s=8 TeV

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

.

CMS

Collaboration



CERN,Switzerland

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

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-squarkmasseigenstatet˜2decayingtoalightertop-squarkmasseigenstate

˜

t1 viaeither˜t2→Ht˜1 or˜t2→Z˜t1,followed inbothcases by˜t1→t

χ

˜10,where

χ

˜ 0

1 is anundetected, stable,lightestsupersymmetricparticle.Theinterpretation isperformedinthe regionwhere themass differencebetweenthe˜t1and

χ

˜₁0statesisapproximatelyequaltothetop-quarkmass(m_˜t1−m˜χ10mt),

whichisnotprobedbysearchesfordirectt˜1squarkpairproduction.Theanalysisexcludestopsquarks withmassesm_˜t2<575 GeV andm˜t1<400 GeV ata95%confidencelevel.

©2014TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (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 bosonmass cal-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

→

t

χ

˜

10

→

bW

χ

˜

10,

ortoa bottom quark anda chargino,

˜

t1

→

b

χ

˜

₁+

→

bW

χ

˜

₁0.These

two decaymodes are expectedto have large branching fractions

 _{For correspondence please use e-mail address:}

cms-publication-committee-chair@cern.ch.

ifkinematicallyallowed.Thelightestneutralino,

χ

˜

0

1,isthelightest

SUSYparticle(LSP)intheR-parityconservingmodelsconsidered; theexperimentalsignatureofsuchaparticleismissingtransverse energy(Emiss_T ).

Searchesfortop-squarkpairproductionarechallengingbecause thecrosssection isapproximatelysixtimessmallerthanthat for top–antitop quark pair (tt) productionif m_˜t1

∼

mt anddecreases rapidlywithincreasingtop-squarkmass[20].Whenthemass dif-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 Emiss_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

→

t

χ

˜

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 difficultto distin-guishkinematicallybetween

˜

t1 pairproductionandthedominant http://dx.doi.org/10.1016/j.physletb.2014.07.053

0370-2693/©2014TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/3.0/).Fundedby SCOAP3_.

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

−

m_χ˜0

1



mt by focusingon signaturesofttHH, ttHZ, andttZZ with Emiss

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 t

χ

˜

10, as shown

in Fig. 1. Other decay modes such as

˜

t1

→

b

χ

˜

1+

→

bW

χ

˜

10 are

largelycovered form_˜t

1

−

mχ˜10



mt by existinganalyses [19].The

final states pursued in this search can arise in other scenarios, such as

˜

t1

→

t

χ

˜

₂0,with

χ

˜

₂0

→

H

χ

˜

₁0 or

χ

˜

₂0

→

Z

χ

˜

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 model consid-ered,

˜

t2 is assumedalways to decayto

˜

t1 in association withan

H orZ boson, such that the sum ofthe two branching fractions is

B(˜

t2

→

H

˜

t1

)

+

B(˜

t2

→

Z

˜

t1

)

=

100%.Otherpossibledecaymodes

are

˜

t2

→

t

χ

˜

10 and

˜

t2

→

b

χ

˜

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 experiment at the LHC during 2012, correspondingtoanintegratedluminosityof19.5 fb−1_.The

analy-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,oratleastthree lep-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

→ 

ν



νbb decay

modes, where q denotes a quark jet. The sensitivitytothesignalarisesbothfromeventswithadditionalb quarksin the final state (mainly fromH

→

bb), andfrom events withadditionalleptonsfromH orZ bosondecays.

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

modelofthepairproductionofa heaviertop-squarkmass eigen-state

˜

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,withthe pos-itive y axispointingupwards.Theazimuthal angle

φ

ismeasured aroundthebeamaxisinradiansandthepolarangle

θ

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, re-spectively.Thehadroniccalorimetersextendupto

|

η

|

≈

5, improv-ing momentum balancemeasurements intheplane transverse to thebeamdirection.Theonlinetriggersystemthatselectscollision events ofinterest is basedon two stages: a first-level hardware-basedselectionandasecondsetofrequirementsimplementedin software.

3. Eventsamples,objectselection,andeventsimulation

The data used for this search were collected with a high transverse-momentum(pT)electron(e)ormuon(μ)single-lepton

trigger, whichrequiresatleastone electronwith pT

>

27 GeV or muonwithpT

>

24 GeV.Thetriggerefficiencies,asmeasuredwith asampleofZ

→ 

+



−events,varybetween85%and97%for elec-trons, andbetween80% and95% formuons, depending onthe

η

and pT valuesoftheleptons. Eventswere alsocollectedwiththe ee,eμ,and

μμ

double-leptontriggers,whichrequireatleastone e orone

μ

withpT

>

17 GeV andanotherwithpT

>

8 GeV.Events are alsoacquiredwith adouble-lepton triggertargetinglower-pT

leptons, requiring pT

>

8 GeV, butwith an additionalonline se-lection of HT

≡ Σ

jet

|

pjet_T

|

>

175 GeV, considering only jets with pT

>

40 GeV inthesum.Theefficienciesliebetween90%and95% for the trigger targetinglower-pT leptons, and between80% and

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

η

andpT valuesofthelower-pT lepton.Forselectionswithmore

thantwoleptons,thetriggersarefullyefficient.

Eventsarereconstructedofflineusingtheparticle-flow(PF) al-gorithm [26,27]. Electroncandidates are reconstructed by associ-atingtrackswithenergyclustersintheelectromagnetic calorime-ter[28,29]. Muon candidates are reconstructed by combining in-formation from the tracker and the muon detectors [30]. Signal leptons are produced in the decays of W and Z bosons. In or-der 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 arerequiredtohavea trans-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 lepton can-didates are required to be isolated from hadronicactivity in the event. This is achieved by imposing a maximum allowed value on the quantity psum_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 psum_T

<

0

.

15pT.

Forthelower lepton-multiplicityselections,the isolation require-mentistighter(seeRefs. [19]and[31] fordetails).The surround-inghadronicactivityiscorrectedfortheenergycontributionfrom additionalproton–protoninteractionsintheevent(pileup),as de-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 energy 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,definedsuchthattheyhavetagging efficien-ciesof70% or80–85%,andmisidentificationratesforlight-flavor jetslessthan2% or10%, respectively[36].The Emiss_T iscalculated asthemagnitudeofthevectorsumofthetransversemomentaof allPF candidates,incorporating jetenergy corrections[37]. Qual-ityrequirements areappliedto removeasmallfractionofevents in which detector effects such as electronic noise can affect the

Emiss_T reconstruction. Events are required to have Emiss_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] pro-gram. The SUSY particle decays are simulated with pythia with a uniform amplitude over phase space, so that all decays are isotropic[40].Thefirsttwodecaymodesconsidered(see

Fig. 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˜t1. 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 addi-tional 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. Allthebackground sam-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,and effi-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 definitionsoftheSRsaresummarizedin

Table 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 isappliedto fur-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 pT

>

25 GeV and

|

η

|

<

2

.

1. Events withan indication ofan

additional lepton, either an isolated track [31] or a hadronically decaying

τ

-lepton candidate

τ

h [64–66], are rejectedin order to

reducethebackgroundfromtt eventsinwhichbothWbosons de-cayleptonically. Inthedouble-leptonchannel,eventsarerequired tocontainexactlytwochargedleptons(ee,eμ,or

μμ

),eachwith

pT

>

20 GeV and

|

η

|

<

2

.

4.Inthiscase,eventswithanadditionale or

μ

withpT

>

10 GeV arerejected.Anyelectroncandidateinthe region 1

.

44

<

|

η

|

<

1

.

57, a less well-instrumented transition re-gionbetweenthebarrelandendcapregionsofthecalorimeter,is

Table 1

SummaryoftheSRdefinitionsforthedifferentselections,specifiedbyrowsinthetable.TheSRscorrespondtoallpossiblecombinationsofrequirementsineachrow,where differentregionsforthekinematicvariablesareseparatedbycommas.FortheeventcategorywithtwoSSleptons,twoselectionsinleptonpTareused(lowandhigh),as explainedinthetext.Thereare96SRsintotal.

N Veto Nb jets Njets EmissT [GeV] Additional requirements [GeV]

1 track orτh =3 ≥5 ≥50 mT>150

≥4 ≥4 mT>120

2 OS extra e/μ =3 ≥5 ≥50 Nbb=1 with 100≤mbb≤150 or Nbb≥2

≥4 ≥4

2 SS extra e/μ =1 [2,3],≥4 [50,120],≥120 for low (high) pT: 250(200)≤HT≤400, HT≥400

≥2

≥3 – =1 [2,3],≥4 [50,100],[100,200],≥200 for on/off-Z: 60≤HT≤200, HT≥200

=2

≥3 ≥3

excludedintheeventselectionsincestandard electron identifica-tioncapabilitiesarenot optimal.Jetsarerequiredtobe separated fromthecandidateleptonsby

R

>

0

.

4.

Intheseeventcategories,atypicaltt backgroundeventhastwo b jetsinthefinalstate,whilesignal eventscould haveuptofour additionalb jets,twofromeachH decay.Therequirementofmore thantwob jetsgreatlysuppressesthett backgroundcontribution. Foreventswithexactlythreeb jets,thejet pTthresholdappliedis 40 GeV;foreventswithatleastfourb jets,thethresholdis low-eredto30 GeV. Inboth cases,the mediumworkingpoint ofthe b-jet taggerisused(seeSection3).Tofurtherreducethett back-groundcontributioninthesamplewithexactlythreeb jets,events are requiredto contain two additional jetswith pT

>

30 GeV, at

leastoneofwhichmustsatisfytheloosebutnotthemedium cri-teriaoftheb-jettagger.Signaleventscanhavelargejetandb-jet multiplicities, while in the case of the tt background, additional jets are needed to satisfy this selection criterion. To reduce the contributionofjetsfrompileupintheevent,arequirementis ap-plied on a multivariate discriminating variable that incorporates themultiplicity ofobjects clusteredinthe jet,the jetshape, and the compatibilityof thecharged constituentsof the jet withthe primaryinteractionvertex[67].

Besides the requirements listed above, the analysis in the single-lepton channel selects events with large transverse mass of the

(,

ν

)

system, definedas mT

≡



2p

TpνT

[

1

−

cos

(φ



− φ

ν

)

]

,

wherethe pToftheselectedleptonisusedandthe

(

x

,

y

)

compo-nentsoftheneutrinomomentumareequatedtothecorresponding

Emiss_T components.ForeventsinwhichtheEmiss_T arisesfroma sin-gleneutrinofromaW bosondecay,thisvariablehasa kinematic endpointmT

≈

mW,wheremW istheW bosonmass.The require-ment oflarge mT (mT

>

150 GeV for eventswith three b jets or mT

>

120 GeV foreventswithatleastfourb jets)providesstrong suppressionofthesemileptonictt background.

The study of the OS dilepton channel uses information from pairs of b jets (ignoring their charge) to identify pairs consis-tent with H

→

bb decay:

Rbb

≤

2π

/

3, mbb

/

[

pbbT

Rbb

]

≤

0

.

65,

and

|

ybb

|

≤

1

.

2, wherethe rapidity isdefinedas y

≡

12ln

[(

E

+

pz

)/(

E

−

pz

)

]

,withpz denotingthecomponentofthemomentum

along the beamaxis.Only b jetssatisfying the medium working point of the tagger are used to form bb combinations. Different b-jetpairs arenot allowedto haveb-jetsincommon. Wedenote thenumberofselectedb-jet pairsasNbb andtheinvariantmass

ofa pairasmbb.Events are requiredto haveeither Nbb

=

1 and 100

≤

mbb

≤

150 GeV,orelse Nbb

≥

2.Forthesignalmodelsof

in-terest,particularlythe

˜

t2

→

H

˜

t1 decaymode,theSRswithlargest

b-jetmultiplicity(

≥

4 b jets)havethehighestsensitivity.

4.2. EventcategorywithtwoSSleptons

TheeventcategorywithtwoSSleptonstargetssignatureswith multiple sources of leptons. Standard model processes withtwo SS leptons are extremely rare. The analysis for this event cate-gorycloselyfollowsthatdescribedinRef.[31].Theonlydifference is the addition of a veto on events containing a third lepton, to removetheoverlapwiththe3



eventcategory.TheseSRsalso re-covereventswiththreeleptonsinwhichoneofthethreeleptons falls outside the detector acceptance or fails the selection crite-ria. Multiple SRsare defined forthe SS eventcategory based on the jet and b-jet multiplicities, E_Tmiss, and HT, and on whether theleptons satisfy pT

>

10 GeV (low-pT analysis) orpT

>

20 GeV

(high-pT analysis).Theleptons mustappearwithin

|

η

|

<

2

.

4.The

jet pT thresholdapplied is 40 GeV. The low andhigh lepton pT

samples,whichpartiallyoverlap,targetcomplementarysignatures. Thelow-pTsampleextendsthesensitivitytosignatureswith

com-pressedSUSYspectra,whilethehigh-pT analysistargetsscenarios

with leptons produced via on-shell W and Z bosons. Only the high-pT analysisisusedto targetthe signalsexplicitlystudiedin

thisletter,whilethelow-pT analysisisincludedforsensitivityto

othernewphysicsscenarios.

4.3. Eventcategorywithatleastthreeleptons

Theeventcategorywithatleastthreeleptonsandatleastone b jet is sensitive to all of the processes shown in Fig. 1. These processescontainmanysourcesofleptons,suchasZ bosonsfrom the top-squark decays,and

τ

leptons and W andZ bosons from theH bosondecays.Eventhoughsignaturesgivingrisetothreeor moreleptons havesmallproductionrates,thiseventcategoryhas goodsensitivitybecausethebackgroundsarestronglysuppressed. The dataset is acquired using the double-lepton triggers. Events are selected offline by requiringatleast threee or

μ

candidates with pT

>

10 GeV, includingat leastone with pT

>

20 GeV,and

|

η

|

<

2

.

4.Eventswithtwoleptonsofopposite-signchargewithan invariantmassbelow12 GeVareremovedfromthesampleto re-ducethecontributionofleptonsoriginatingfromlow-massbound states.

Eventsarerequiredtohaveatleasttwojetswith pT

>

30 GeV andatleastone b jetsatisfyingthemediumworkingpointofthe tagger. Leptonswithin

R

<

0

.

4 of a b-quark jet arenot consid-ered isolated and are merged with the b jet. This requirement imposes an additional isolation criterion forleptons and reduces thedominantbackground,tt production,by25–40%dependingon theSR,comparedtothecasewheresuchanobjectisreconstructed asaleptonratherthanab jet.Theefficiencyforsignal leptonsis reducedby 1%.Theremaining SM backgroundinthe

≥

3



event

CMS Collaboration / Physics Letters B 736 (2014) 371–397 375

category from WZ

+

jets production is highly suppressed by the b-jetrequirement.

Thisthree-lepton event sample is divided into several SRs by imposingrequirementsonthejetandb jetmultiplicity,Emiss_T ,and thehadronicactivityintheevent, asgivenbythekinematic vari-able HT, considering jets with pT

>

30 GeV in the sum. Finally, eventsare classifiedaseither “on-Z” ifthere isa pairof leptons with the same flavor and opposite charge that has an invariant masswithin15 GeVofthenominalZ bosonmass,or“off-Z”ifno suchpairexistsoriftheinvariantmassliesoutsidethisrange.

TheseparationofeventsintotheseSRsimprovesthesensitivity ofthesearch.Forthesignal modelsofinterest,theSRswithlarge b jetmultiplicity (thosedesignated Nb jets

=

2 andNb jets

≥

3) and

thatwithbothhighEmiss

T andhighHTprovidethegreatest

sensi-tivity.Theon-Z regionsarethemostsensitive,whenthedecayto anon-shell Z bosoniskinematically allowedforthe

˜

t2

→

Z

˜

t1

de-caymode.Conversely,theoff-Z regionshavemoresensitivitywhen on-shellZ bosondecaysarenotkinematicallyallowedandforthe

˜

t2

→

H

˜

t1decaymode.

5. Backgroundestimation

Themain backgroundarisesfromSM tt events,which usually havetwo b jets andat mosttwo leptons from W bosondecays. Thus, tt events canonly satisfy theselection criteriaif accompa-niedby sourcesofadditional b jetsorleptons.Such backgrounds are estimatedusing control samplesin data,as described below. This method greatly reduces the dependence of the background predictionontheaccuratemodelinginsimulation,theknowledge ofthe inclusive tt production cross-section, the measurement of theintegratedluminosity,andtheaccuracyoftheobject-selection efficiencydetermination.

Additional backgrounds arise from processes involving one or moreW andZ bosons,althoughthesecontributionsaresuppressed bythe b-jetrequirements. Finally,alleventcategorieshave back-groundsfromrareSM processes,suchasttZ andttW production, whosecrosssectionshavenot beenprecisely measured[68].The predictionforthesecontributionsisderived fromsimulation,and asystematicuncertaintyof50%isassignedtoaccountforthe un-certaintyintheNLOcalculationsoftheirdifferentialcrosssections. Theremainderofthissectiondescribesthebackgroundpredictions foreachofthespecificeventcategories.

5.1.BackgroundsineventcategorieswithasingleleptonortwoOS leptons

For the single-lepton or two-OS-lepton event categories, the dominantbackgroundisfromtt events(85–95%ofthetotal).These eventscanhavethreeormoreb jetsifthett pairisaccompanied byadditionaljetsthatmaybemistaggedinthecaseoflight-parton jetsorthatmaycontaingenuineb jetsfromgluondecaystoabb pairs.Inthe caseofsemileptonictt events, thereare small addi-tionalcontributionsfromW

→

cs decays,withacharm-quark jet misidentifiedas ab jet, andfromthe rareW

→

cb decay mode. In the case of dileptonic tt events,

τ

leptons from the second W boson decay that are misidentified as b jets also contribute. Scalefactors,definedastheratiooftheyieldindatatotheyield in simulation,are used to normalize the background predictions from simulation. For each SR, the corresponding scale factor is derived froma control region enhanced inbackground tt events. Thesecontrol regions are definedby 50

≤

mT

≤

100 GeV forthe single-leptonselectionsandbyeitherNbb

=

0 orNbb

=

1,with ei-thermbb

≤

100 GeV ormbb

≥

150 GeV,for theOS-dilepton case. The contributionfromnon-tt events isevaluatedfromsimulation

and subtracted from the data before deriving the normalization. Toreducethecontributionfroma possiblesignalinthesecontrol regions, the samples are restrictedto events withlow jet multi-plicity:for thethree-b-jet category, only eventswithexactly five jets are used, andfor the category withfour b jets, only events withexactlyfourjetsareused.Thedominantsourceofuncertainty forthe backgroundprediction arisesfromthe limitednumber of events inthe control samples (15–35% on the totalbackground). The tt backgroundpredictionalsodependsonthe ratioofevents inthesignal andcontrolregions,whichisevaluatedfrom simula-tionandvalidatedusingtt-dominatedcontrolsamplesobtainedby selectingeventswithfewerthanthreebjets,asdescribedbelow.

Inthe single-leptonchannel,the modelingofthe high-mT tail

iscritical forthebackgroundestimation.Genuine semileptonic tt events have an endpoint at mT

≈

mW, with Emiss_T resolution ef-fects primarily responsible for populating the mT

>

mW tail.The effect of Emiss_T resolution on the mT tails is investigated by se-lecting events withone or two b jets and by varying the num-ber ofadditionaljets. The comparisonofsimulationwithdata in the mT tailregion is usedto extract scale factors and

uncertain-ties for the semileptonic tt prediction. The scale factors are in the range 1.1–1.2, depending on the mT requirement, with cor-responding uncertainties of 5–10%. The semileptonic background contributes 50–60% of the total background in the single-lepton SRs.Events fromgenuinedileptonic tt eventscan alsosatisfy the single-leptoneventselectionifthesecondleptonisnotidentified orisnotisolatedandcangiverisetolargevaluesofEmiss_T andmT

duetothepresenceoftwoneutrinos.Thistt

→ 

+

jets contribu-tion constitutes

∼

30–40% of thetotal backgroundandisderived from simulation, with scale factors consistent with unity, as de-terminedfromcomparisonofdatawithsimulationinthedilepton controlregions.

InthechannelswithtwoOSleptons,themostimportantissues for the background prediction are related to the construction of b-jet pairs(see Section4.1forthefulllistofrequirements). Mod-eling of the emission of additional radiation leading to jets and gluonsplittingtobb pairs,andofeffectssuchas

τ

-lepton mistag-ging,c-quark-jetmistagging,andb-jetidentificationefficiency,can affectthembb variable.Themodelingoftheseeffectsisvalidated

using the statistically precise single-lepton control sample with 50

≤

mT

≤

100 GeV, in which the mbb distributions in data and

simulation are compared as a function of the b-jet multiplicity. Theratioofthenumberofeventssatisfyingthe Nbbandmbb re-quirementsthatdefinethesignalandcontrolregionsis compared indataandsimulation.This studyis usedtoderive scale factors, whicharefoundtobeconsistentwithunity,anduncertainties cor-respondingto20–30%ofthetotalbackgrounduncertainty.

5.2. BackgroundsintheeventcategorywithtwoSSleptons

FortheSRswithtwoSSleptons,thebackgroundestimatesand uncertainties are derived following the procedures described in Ref. [31]. There are three main categories of backgrounds. Non-prompt leptons are produced fromheavy-flavor decays, misiden-tified hadrons, muons from the decay-in-flight of light mesons, andelectrons fromunidentified photon conversions. Charge mis-identification arises mainly from electrons that undergo severe bremsstrahlunginthetrackermaterial,leadingtoa misreconstruc-tion of the charge sign. Finally, rare SM processes yielding two genuine SS leptons (typicallya tt pair in association with an H, W,orZ boson)cancontribute significantly,especiallyinSRswith tight selection requirements. Backgrounds from non-prompt lep-tonsandrareSMprocessesdominate,eachcontributing20–80%of thetotal,whilechargemisidentificationcontributes1–5%.

The background from non-prompt leptons is evaluated using theeventyieldinacontrolsampleinwhichthesameanalysis se-lectionsareapplied,exceptthereisatleastoneleptonthatpasses a loose lepton selection but fails the full set of tight identifica-tion and isolation requirements. This observed yield is corrected by a “tight-to-loose” ratio, the probability that a loosely identi-fied non-prompt lepton also passes the full set of requirements. Thiscorrection factoris inturn measuredin a controlsample of QCD multijeteventsenrichedin non-promptleptons. Theratiois obtained as a function of lepton pT and

η

. The event kinemat-icsandthevarioussourcesofnon-promptleptonsaredifferentin the QCD multijet sample, where the tight-to-loose ratio is mea-sured,andthe signal sample, where it isapplied. This givesrise toasystematicuncertaintyinthenon-promptlepton background estimate.The chargemisidentificationbackgroundisobtained us-ingasampleofOSee andeμeventsthatsatisfythefullkinematic selectionweightedbythepT- and

η

-dependentprobabilityof elec-tronchargemisassignment.Thesystematicuncertaintyofthetotal backgroundpredictionisdominatedbytheuncertaintiesfromrare SMprocessesandfromeventswithajetmisidentifiedasaprompt lepton(30–50%ofthetotalbackground).

5.3. Backgroundsintheeventcategorywithatleastthreeleptons

ForSRs withatleast threeleptons, there are two main types ofbackgrounds.Intheoff-Z SRs,thebackgroundwithtwoprompt leptonsandanadditionalobjectmisidentifiedasapromptlepton dominates, comprising50–90% of the total. In the on-Z SRs, the dominantbackgroundistypicallyfromSMprocesseswithatleast three genuine prompt leptons, corresponding to 60–100% of the total.

The backgroundsources withtwo prompt leptons fromW or Z bosondecay anda third objectmisidentified as a prompt lep-ton are predominantly fromtt production, although the Z

+

jets andWW

+

jets processes also contribute. The procedure to esti-matethisbackgroundcontributionfollowscloselythatusedforthe analysisofeventswithtwo SSleptons [31].The probabilityfora looselyidentifiedleptontosatisfythefullsetofselection require-mentsisappliedtoasampleof

≥

3



events,inwhichtheisolation requirement on one of the leptons is removed, providing an es-timate ofthe background contributionfrom non-prompt leptons. A systematic uncertainty of 30% is derived for this background based on studies of the method in simulation. This uncertainty accounts for the difference in the pT spectrum of b jets in the control sample, where the probability is measured, compared to thespectrum in thesignal sample, where it isapplied. This sys-tematicuncertaintydominatesthe uncertaintyinthe background predictionintheSRswithlooserkinematicrequirements.SRswith tightkinematicrequirementsalsohaveasignificantstatistical un-certainty duetothe size ofthesample usedto derivethis back-ground estimate. These are the dominant sources of uncertainty in the backgrounds in the off-Z signal regions, corresponding to 20–90%uncertaintyonthetotalbackground.

The background contribution from events with two vector bosonsthatproducethreegenuinepromptisolatedleptons,mainly WZ

+

jets and ZZ

+

jets events,is estimatedfromsimulation and isvalidatedbycomparingdataandsimulationincontrolsamples in which the full selection is applied and the b-jet requirement isinverted. Acontrol sample enhanced inthe WZ backgroundis obtainedby selectingeventswiththreehigh-pT leptons. Onepair

of leptons is required to form a Z

→ 

+



− candidate. The third lepton is combinedwith the Emiss_T vector, and thissystem is re-quired toform a W boson candidate(50

<

Emiss_T

<

100 GeV and 50

<

mT

<

120 GeV).Asecondcontrolsample,enhancedintheZZ background,isobtainedbyselectingeventswithfourleptons and

Emiss_T

<

50 GeV.Two leptons are requiredto form a Z candidate. Scale factors are derived based on the comparison of data and simulation in these control samples. The scale factors are found to beunityand0.9fortheWZ and ZZ backgrounds,respectively. The systematicuncertaintyforthedibosonbackgroundisderived based on these comparisons,which are limited by the statistical precision ofthecontrol samples.A50%uncertaintyisassignedto accountforpossiblemismodelingofadditionalpartonsrequiredto satisfytheb-jetrequirement.

6. Results

The results of the search are shown in Tables 2–4, and in

Figs. 2–4,wherethebackgroundpredictionsarebrokendowninto thevariouscomponents.

For the event selections with one lepton, Fig. 2 (top) shows a comparison ofthe mT distribution in data andsimulation. The

sample at low mT is enhanced in semileptonic tt events and is

usedasacontrolsampletoderivethenormalizationforthis back-groundcontribution.Asshownin

Fig. 2

(top),thebackgroundsin theSRaremainlysemileptonicanddileptonictt events.

FortheSRswithtwoOSleptons,

Fig. 2

(bottom)showsa com-parisonofthembbdistributionindataandsimulation.Thesample

in the region outside the mbb signal window is used to derive

thenormalizationforthett

→ 

+

jets backgroundpredictionfor events withthree b jets.In thecase ofeventswith atleastfour bjets,multiplebb pairsarepossible.Thecontrolregionisnot in-dicated in Fig. 2(bottomright) sincethe mbb requirementis not

appliedwhenNbb

≥

2.

The dominant background in the SRs is from tt

→ 

+

jets events. The results for the SRs with one lepton or two OS lep-tonsaresummarizedin

Table 2

.Thepredictedandobservedyields agree within 1.4 standard deviations of local significance [69], giventhestatisticaluncertaintyinthepredictedyields.

Fig. 3 shows a comparison of data and the predicted back-groundsforeventswithtwoSSleptonssatisfyingamoreinclusive selection, which is enhanced in SM processes: at least two jets, moderate HT (

>

250 GeV inthelow-pT analysisand

>

80 GeV in

the high-pT analysis), and moderate ETmiss (

>

30 GeV for events

with HT

<

500 GeV; otherwise, there is no Emiss_T requirement). This sample serves to validate the methods used to predict the backgrounds in the SRs, which are defined by applying require-mentsontheselectionobservablesshown:thejetmultiplicity,the b-jet multiplicity, and Emiss_T ,aswell as HT.The amount of back-ground varies strongly among the signal regions; some of them includingtensofbackgroundeventswhileothershaveessentially none.TherelativecontributionfromrareSMprocessesincreasesas therequirementsaretightened.Asshownin

Table 3

,theSM back-groundpredictions andobservationsin theSRs are inagreement forboththehigh-pT andlow-pT selections.

Finally,fortheeventsamplewithatleastthreeleptons,

Fig. 4

shows a comparison of data and the predicted backgrounds for the jetandb-jet multiplicitiesandforthe Emiss

T distribution.The

dominantbackgroundisfromprocesseswithtwopromptleptons and additional non-prompt leptons, mainly due to tt events, al-thoughinthecaseoftheon-Z selection,backgroundsourceswith Z bosons also contribute significantly. The results of the search, summarized in Table 4, demonstrate agreement between back-groundpredictionsandobservationsforalltheSRsconsidered.

In summary,the data yields are found to be consistent with thebackgroundpredictionsacrossall eventcategoriesandSRs.Of the 96SRs,the largestdiscrepancycorresponds toa 1.6standard deviationexcessoflocalsignificance(30eventscomparedto16

±

5 expected, see

Table 4

),computedfollowing therecommendations

CMS Collaboration / Physics Letters B 736 (2014) 371–397 377

Fig. 2. ComparisonofthemT distributionsfor eventswithonelepton(toprow)andmbb distributionsforeventswithtwoOSleptons (bottomrow)indataandMC simulationsatisfyingthe3b (left)and≥4b (right)SRrequirements.Theverticaldashedlinesindicatethecorrespondingsignalregionrequirement.Thesemileptonictt and dileptonictt componentsrepresentsimulatedeventscharacterizedbythepresenceofoneortwoW bosonsdecayingtoe,μorτ.Theyieldsofthett simulatedsamples areadjustedsothatthetotalSMpredictionisnormalizedtothedatainthesamplesobtainedbyinvertingtheSRrequirements.Thedistributionforthemodel˜t2→H˜t1 wherem˜t2=450 GeV andm˜t1=200 GeV isdisplayedontopofthebackgrounds.Thelastbincontainstheoverflowevents.Theuncertaintiesinthebackgroundpredictions

arederivedforthetotalyieldsinthesignalregionsandarelistedinTable 2.

Table 2

SelectionwithoneleptonortwoOSleptons:backgroundpredictionsandobserveddatayields.Theuncertaintiesinthetotalbackgroundpredictionsincludeboththe statisticalandsystematiccomponents.

Nb jets Njets EmissT [GeV] 1high mT 2 OSand bb requirement

Bkg. Obs. Bkg. Obs.

=3 ≥5 ≥50 10.0±1.8 14 8.4±2.7 15

≥4 ≥4 27±6 31 11±5 3

ofRef. [69]. Thus, no indication of top-squark pairproduction is observed.

7.Interpretation

The results are used to set upper limitson the cross section timesbranchingfractionforpairproductionof

˜

t2 squarksforthe

decaymodesshownin

Fig. 1

.Theupperlimitsarecalculatedata 95%confidencelevel(CL)usingtheLHC-styleCLSmethod[70–72].

The exclusioncurves onparticle massesat 95% CL are evaluated froma comparisonofthecrosssectionupperlimitsandthe the-oretical signal cross section predictions. As explained below, the resultsfromthevariousSRsarecombinedinthelimit-setting pro-cedureinordertoimprovethesensitivityofthesearch.

Thelimitcalculationonthecrosssectiontimesbranching frac-tiondependsonthesignalselectionefficiencyandthebackground

estimates.TheSRswithatleastthreeleptonshavethehighest ex-pectedsensitivitybecauseofthesmalllevelofSMbackground.For SRswithatleastthree leptons,theoff-Z SRs with HT

>

200 GeV are used for the ttHH interpretation, while both the off-Z and on-Z SRswith HT

>

200 GeV areusedforthettZZ interpretation. The totalsignal acceptanceforallSRs withatleastthreeleptons varies from around 0.4–0.5% for the ttHH signal, to 1.2–1.5% for the ttZZ signal. The acceptance for the most sensitive SR alone is around

∼

0

.

1% for ttHH and approximately three times larger forttZZ.Thisdifferenceinacceptanceisduetothelargerleptonic branching fraction for Z bosondecays compared to H boson de-cays.TheSRswithlowerleptonmultiplicitiesalsohavesensitivity tothettHH signal.AllSRsofthehigh-pT SSdileptonanalysisare

usedinthelimit setting.Whileonly thehigh-pT resultsare used

in the interpretation presented in thisletter, the low-pT

Fig. 3. DataandpredictedSMbackgroundfortheeventsamplewithtwoSSleptonsasafunctionofnumberofb jets,numberofjets,andEmiss

T foreventssatisfyingthe high-pT(toprow)orthelow-pT(bottomrow)selection.Theshadedbandscorrespondtothetotalestimateduncertaintyinthebackgroundprediction.Thedistributionfor themodel˜t2→H˜t1wherem˜t2=400 GeV andmt˜1=200 GeV isdisplayedontopofthebackgrounds.Thelastbininthehistogramsincludesoverflowevents.

Table 3

SSdileptoneventcategory:predictedtotalbackgroundandobserveddatayieldsasafunctionofthejetmultiplicity,b-jetmultiplicity,EmissT ,andHTrequirements,forthe low-pTandhigh-pTregions.Theuncertaintiesinthetotalbackgroundpredictionscontainthestatisticalandsystematiccomponents.

Selection Low-pT High-pT

Nb jets Njets Emiss_T [GeV] HT∈ [250,400]GeV HT≥400 GeV HT∈ [200,400]GeV HT≥400 GeV

Bkg. Obs. Bkg. Obs. Bkg. Obs. Bkg. Obs.

=1 2–3 50–120 29±12 39 5.6±2.0 5 31±12 27 3.4±1.2 5 ≥120 11±4 8 4.9±1.8 5 9.0±3.2 9 3.5±1.3 2 ≥4 50–120 15±6 15 10±4 6 9.2±3.4 6 5.4±2.0 2 ≥120 3.9±1.5 3 6.1±2.2 10 2.6±1.0 3 3.5±1.3 6 ≥2 2–3 50–120 6.6±2.4 10 1.3±0.5 1 6.0±2.1 11 0.78±0.34 1 ≥120 2.4±0.9 1 1.2±0.5 2 2.4±0.9 3 0.8±0.4 1 ≥4 50–120 6.5±2.5 5 4.0±1.5 11 3.4±1.3 2 2.3±1.0 7 ≥120 1.8±0.7 0 3.1±1.2 3 1.1±0.5 0 2.0±0.8 2

interpretations.InSRswithtwoSSleptons,theoverallacceptance for ttHH events is 0.3–0.5%, where the mostsensitive signal re-gions contribute

∼

0

.

15%. In the case of SRs withone lepton or two OSleptons, the acceptancefor ttHH events is approximately 0.2–0.4%. Theacceptances forthe single-leptonanddileptonfinal statesare slightly lower for thettZZ signal. Because ofthe large branching fraction forthe H

→

bb decay mode, SRs with higher b-jet multiplicity requirements dominatethe expected sensitivity forscenarioswithH bosons. SRswithlowb-jet multiplicitiesare mostsensitiveforscenarioswithZ bosons.

The systematic uncertainties, listed in Table 5, are evaluated for the signal selection efficiency in every SRand for every sig-nal point separately.The total uncertainty inthe signal selection efficiency is in the 9–30% range. The dominant source of uncer-tainty depends on the SR and decay mode considered. An im-portant source of uncertainty arises from the estimation of the triggerandleptonidentificationefficiencies,whicharederived us-ingZ

→ 

+



−samplesandcontribute6–13%.Theuncertaintydue to the knowledge of the energy scale of hadronic jetsincreases

with tighter kinematic requirements and corresponds to an un-certainty of 1–15%.The uncertaintydueto the knowledge ofthe b jet identification performance dependsontheeventproperties, such as the jet flavor and pT value, and gives rise to an uncer-tainty of2–20%.Forsmallerdifferencesbetweenthem_˜t2 andm_˜t1

mass values,uncertainties in the modeling of initial-state radia-tion (ISR)become important.Theuncertaintyrelatedto thePDFs ontheacceptanceisdeterminedusingthePDF4LHC recommenda-tions [73]andcontributes2–5%.Thecorrespondinguncertaintyin the signal selection efficiencyis of 3–15%, increasing for smaller

m_˜t2–m_˜t1 massdifferences. The systematicuncertainties, including their correlations, are treated consistently in the different analy-ses. Thecorrelationsbetweenthe differentanalyses havea small impactonthecombinedresult.

Fig. 5 (left) shows the 95% CL upper limits on the cross sec-tion times branching fraction in the m_˜t

1 versus m˜t2 plane for

the (a)

˜

t2

→

H

˜

t1 and (b)

˜

t2

→

Z

˜

t1 decay modes. The contour

boundstheexcludedregionintheplaneassuming theNLO

+

NLL cross section calculation in thedecoupling limit for all the SUSY

CMS Collaboration / Physics Letters B 736 (2014) 371–397 379

Fig. 4. DataandpredictedSMbackgroundfortheeventsamplewithatleastthreeleptonsasafunctionofnumberofb jets,numberofjets,andEmiss

T foreventsthatdonot contain(off-Z),toprow,orcontain(on-Z),bottomrow,anOSsame-flavorpairthatisaZ bosoncandidate.Theshadedbandscorrespondtothetotalestimateduncertainty inthebackgroundprediction.Thedistributions forthemodels˜t2→H˜t1and˜t2→Z˜t1aredisplayedontopofthebackgroundsinthetopandbottomrowsrespectively.The top-squarkmassesarem˜t1=200 GeV andm˜t2= (450,600)GeV forthe(H,Z)channel.Thelastbininthehistogramsincludesoverflowevents.

Table 4

Predictedtotalbackgroundandobserveddatayieldsasafunctionofthejetmultiplicity,b-jetmultiplicity,Emiss

T ,andHTrequirements,foreventswithatleastthreeleptons, with(on-Z)andwithout(off-Z)aZ bosoncandidatepresent.Theuncertaintiesinthetotalbackgroundpredictionsincludeboththestatisticalandsystematiccomponents.

Selection Off-Z On-Z

Nb jets Njets Emiss_T [GeV] HT∈ [60,200]GeV HT≥200 GeV HT∈ [60,200]GeV HT≥200 GeV

Bkg. Obs. Bkg. Obs. Bkg. Obs. Bkg. Obs.

=1 2–3 50–100 34±7 36 11.2±2.5 9 16±5 30 10±4 13 100–200 12.2±2.7 13 9.1±2.1 6 5.3±1.8 6 5.9±2.1 3 ≥200 0.33±0.22 0 1.2±0.5 0 0.37±0.23 0 0.9±0.4 0 ≥4 50–100 0.9±0.4 2 5.4±1.3 3 0.11±0.13 1 5.0±2.0 4 100–200 0.10±0.12 0 3.6±1.0 3 0.08±0.12 0 3.0±1.3 5 ≥200 0.0±0.1 0 0.76±0.35 0 0.02±0.10 0 0.56±0.32 1 =2 2–3 50–100 4.9±1.2 7 3.9±1.2 7 2.4±0.9 5 2.5±1.1 2 100–200 2.3±0.7 1 1.9±0.7 0 1.3±0.5 1 1.4±0.6 1 ≥200 0.22±0.21 1 0.14±0.14 0 0.12±0.13 0 0.43±0.26 0 ≥4 50–100 0.03±0.11 0 2.8±0.9 1 0.20±0.17 1 2.9±1.3 1 100–200 0.05±0.11 0 1.7±0.6 0 0.10±0.13 0 1.7±0.8 0 ≥200 0.0±0.1 0 0.38±0.21 0 0.0±0.1 0 0.29±0.19 0 ≥3 ≥3 50–100 0.0±0.1 0 0.56±0.27 1 0.0±0.1 0 0.18±0.15 0 100–200 0.02±0.11 0 0.18±0.14 0 0.0±0.1 0 0.25±0.17 0 ≥200 0.0±0.1 0 0.2±0.2 0 0.0±0.1 0 0.02±0.10 0

sparticles not included in the model. The results are presented assuming a branching fraction of100% to each decay mode. The 95%CL expected (thickdashed)and observed(solid black)limits areobtainedincludingall uncertaintieswiththeexception ofthe theoreticaluncertaintyinthesignal productioncross section.The expectedlimitisdefinedasthemedianoftheupper-limit distribu-tionobtainedusingpseudo-experimentsandthelikelihoodmodel considered. The bands around the expected limit correspond to the impact of experimental uncertainties, andthe bands around the observed limit indicate the change for a

±

1 standard devi-ation (σ) variation in the theoretical cross section (mainly due

to uncertaintiesin therenormalization/factorization scales andin the knowledgeof thePDFs).In the

˜

t2

→

H

˜

t1 decay mode,taking

a

−

1σ theory lower bound on signal cross sections,a

˜

t2 squark

withm_˜t2



525 GeV isexcluded at a95% CL for

˜

t1 squarks with m_˜t

1



300 GeV.Similarly,inthe

˜

t2

→

Z

˜

t1 decaymode,a

˜

t2 squark

with m_˜t2



575 GeV is excluded at 95% CL for

˜

t1 squark with m_˜t

1



400 GeV.

Forthepure

˜

t2

→

H

˜

t1decay(Fig. 5upperright),theSRswithat

leastthreeleptons,noZ

→ 

+



− candidates,andlarge b-jet mul-tiplicitiesarethemostsensitive.Nevertheless,theSRswithlower lepton multiplicities (one lepton or two leptons) havesignificant

Table 5

Relativesystematicuncertainties(inpercent)inthesignalyieldsforthedifferenteventselections:onelepton(1),twoOSleptons(2OS),twoSSleptons(2SS),andat leastthreeleptons(≥3).TherangeindicatesthevariationinthesystematicuncertaintyforthedifferentdecaychannelsandSRsconsidered.

Source 1[%] 2 OS[%] 2 SS[%] ≥3[%]

Luminosity[74] 2.6

Pileup modeling <5

Trigger efficiency 3 6 6 5

Lepton identification and isolation efficiency 5 10 10 12

Jet energy scale modeling 1–3 1–3 1–10 5–15

b-jet identification[36] 3–5 3–5 2–10 5–20

ISR modeling[19] 3–5 3–5 3–15 3–15

PDFs 5 5 2 4

Total 9–11 14–15 14–23 15–30

Fig. 5. InterpretationoftheresultsinSUSYsimplifiedmodelparameterspace,m˜t1 vs. m˜t2,withtheneutralinomassconstrainedbytherelationmt˜1−m˜χ10=175 GeV.

Theshadedmaps(plotsontheleft)showtheupperlimit(95%CL)onthecrosssectiontimesbranchingfractionateachpointinthem˜t1 vs. m˜t2 planefortheprocess

pp→ ˜t2˜t∗2,with˜t2→Ht˜1,˜t1→tχ˜10(upperplots)and˜t2→Z˜t1,˜t1→tχ˜10(lowerplots).Intheseplots,theresultsfromallchannelsarecombined.Theexcludedregioninthe m˜t1 vs. m˜t2 parameterspaceisobtainedbycomparingthecrosssectiontimesbranchingfractionupperlimitateachmodelpointwiththecorrespondingNLO+NLLcross

sectionfortheprocess,assumingthat(a)B(˜t2→H˜t1)=100% or(b) thatB(˜t2→Z˜t1)=100%.Thesolid(dashed)curvesdefinetheboundaryoftheobserved(expected) excludedregion.The±1standarddeviation(σ)bandsareindicatedbythefinercontours.Thefiguresontherightshowtheobserved(expected)exclusioncontours,which areindicatedbythesolid(dashed)curvesforthecontributingchannels.Asindicatedinthelegendsoftheright-handfigures,thethinnercurvesshowtheresultsfromeach ofthecontributingchannels,whilethethickercurveshowstheircombination.Thefoureventcategoriesforthe˜t2→H˜t1studyareshownintheupperplots,whiletheon-Z andoff-Z categoriesforeventswithatleastthreeleptonsareshowninthelowerplots.(Forinterpretationofthereferencestocolorinthisfigure,thereaderisreferredto thewebversionofthisarticle.)

expected sensitivity in the

˜

t2

→

H

˜

t1 decay mode. Including the

final states with lower lepton multiplicities in the combination lowersthecross sectionupperlimit resultsby 15–20%compared tothethree-leptonresultsalone.Therefore,all leptonmultiplicity categoriesareusedintheinterpretationofthettHH signal.

InthecaseofthesignalswithZ bosons(Fig. 5lowerright),the SRswithatleastthreeleptonscompletelydominatetheexpected

sensitivity. The different SRs withat least three leptons provide sensitivitytodifferenttypesofsignals. Inparticular,off-Z SRsare sensitive totheregion ofparameterspaceinwhichtheZ bosons are off-shell,m_˜t2

−

m_t˜1

<

mZ,whiletheon-Z regionsprovide sen-sitivityto signalswithlarger massdifferences. OnlytheSRswith atleastthreeleptonsareusedintheinterpretationofthettZZ sig-nal.

CMS Collaboration / Physics Letters B 736 (2014) 371–397 381

Fig. 6. Upperlimitsonthecrosssectionfor˜t2pairproductionfordifferent branch-ingfractionsof˜t2→H˜t1and˜t2→Z˜t1,assumingthatB(˜t2→H˜t1)+ B(˜t2→Z˜t1)= 100%.The˜t1squarkisassumedtoalwaysdecaytoatopquarkandaneutralinoχ˜10 withm˜t1−m˜χ10=mt.Thedecay˜t2→H˜t1 isonlyconsideredwhentheH boson

productioniskinematicallyallowed,m˜t2−m˜t1>mH.

Mixed-decay scenarios, with non-zero branching fractions for theZ andH decaymodes, arealsoconsidered,assumingtheseto be the only decay modespossible. Fig. 6 showsthe correspond-inglimitsasafunctionoftherelativebranching fractionoftheZ andH decaymodes.Thescenariowiththeleastexpected sensitiv-ityiswhere the H bosondecaymode dominates, whilethe best expected sensitivity is achieved when the Z boson decay mode dominates.

Thecrosssectionupperlimitsareobtainedneglectingthe con-tribution of direct

˜

t1 squark pair production, which can satisfy

the selection criteria for the single-lepton or OS-lepton SRs if a light-partonjet ismisidentifiedasab jetorifthereisadditional radiationleading togenuine b jets.Includingdirect

˜

t1 squarkpair

production in the single-lepton or two OS lepton SRs typically lowersthecrosssectionlimitbyafewpercent,withthemost pro-nounceddifferencesoccurringatlarger

˜

t2 mass.Thecontribution

inthecaseofeventswithtwoSSleptonsoratleastthreeleptons issmalldueto thelow probability ofmisidentifying non-prompt leptons.Since the signature withthree leptons hasthe best sen-sitivityoverall,theimpactonthecombinedlimitismuchsmaller thantheuncertaintyintheproductioncrosssection.

8. Summary

Thisletterpresentsresultsofasearchforthepairproductionof theheaviertop-squarkmass eigenstate

˜

t2 decayingtothe lighter

eigenstate

˜

t1, producing a signature of a top–antitop quark pair

inassociationwithHiggsorZ bosons. The analysisexploresfinal stateswithexactlyoneleptonandatleastthreeidentified bottom-quarkjets(b jets),withexactlytwoleptonsofoppositechargeand atleast three b jets,with exactly two same-sign leptons andat leastone b jet,andwith threeormore leptons andatleast one b jet, whereby “lepton” we mean an electron ormuon. No sig-nificantexcess event yield above standard model expectations is observed. The results are used to exclude a range of

˜

t2 masses

belowapproximately575 GeV for

˜

t1 massesbelowapproximately

400 GeV.Theinterpretationassumesthatthe

˜

t1squarkalways

de-caystot

χ

˜

0

1 andthatm˜t1

−

mχ˜10



mt,wherethe

χ

˜

0

1 particle

repre-sentsastable, weaklyinteractinglightestsupersymmetricparticle neutralino whose signature inthe detectoris missing transverse

energy. Thisregion ofphase space isnot probed by searches for direct

˜

t1 squarkpairproduction.

Acknowledgements

WecongratulateourcolleaguesintheCERNaccelerator depart-ments for the excellent performance of the LHC and thank the technicalandadministrative staffsatCERN andatother CMS in-stitutes for their contributions to the success of the CMS effort. Inaddition,wegratefullyacknowledgethecomputingcenters and personneloftheWorldwideLHCComputingGridfordeliveringso effectivelythe computinginfrastructureessential to ouranalyses. Finally, we acknowledge the enduring support for the construc-tionandoperation oftheLHC andtheCMSdetectorprovidedby thefollowingfundingagencies:BMWFWandFWF(Austria);FNRS and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES(Bulgaria);CERN;CAS,MoST,andNSFC(China);COLCIENCIAS (Colombia);MSESandCSF(Croatia);RPF(Cyprus);MoER,ERCIUT andERDF(Estonia);Academy ofFinland,MEC,andHIP (Finland); CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH (Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); NRF and WCU (Republic of Korea); LAS (Lithuania); MOE and UM (Malaysia); CINVESTAV, CONACYT,SEP,andUASLP-FAI(Mexico);MBIE(NewZealand);PAEC (Pakistan);MSHEandNSC (Poland);FCT(Portugal);JINR(Dubna); MON,RosAtom,RASandRFBR(Russia);MESTD(Serbia);SEIDIand CPAN(Spain);SwissFundingAgencies(Switzerland);MST(Taipei); ThEPCenter,IPST, STARandNSTDA(Thailand);TUBITAKandTAEK (Turkey);NASUandSFFR(Ukraine); STFC(United Kingdom);DOE andNSF(USA).

Individuals have received support from the Marie-Curie pro-grammeand theEuropean Research CouncilandEPLANET (Euro-peanUnion);theLeventisFoundation;theAlfredP.Sloan Founda-tion; the Alexander von Humboldt Foundation; the Belgian Fed-eral Science Policy Office; the Fonds pour la Formation à la Recherchedansl’Industrieetdansl’Agriculture(FRIA-Belgium);the AgentschapvoorInnovatiedoorWetenschapenTechnologie (IWT-Belgium); theMinistry ofEducation, YouthandSports (MEYS) of theCzechRepublic;theCouncilofScienceandIndustrialResearch, India; the Compagnia di San Paolo (Torino); the HOMING PLUS programmeofFoundationForPolishScience,cofinancedbyEU, Re-gionalDevelopmentFund;andtheThalisandAristeiaprogrammes cofinancedbyEU-ESFandtheGreekNSRF.

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