Search for the associated production of the Higgs boson with a top quark pair in multilepton final states with the ATLAS detector

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Search

for

the

associated

production

of

the

Higgs

boson

with

a

top

quark

pair

in

multilepton

final

states

with

the

ATLAS

detector

.

ATLAS

Collaboration



a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received19June2015

Receivedinrevisedform29July2015 Accepted31July2015

Availableonline5August2015 Editor:W.-D.Schlatter

A search for the associated production of the Higgs boson with a top quark pair is performed in multileptonfinalstatesusing20.3 fb−1_{ofproton–protoncollisiondatarecordedbytheATLASexperiment} at √s=8 TeV at the LargeHadron Collider. Five final states, targeting the decays H→W W∗,

τ τ,

and Z Z∗,are examinedfor the presenceof theStandard Model (SM)Higgs boson:twosame-charge lightleptons (e or

μ)

withoutahadronically decaying

τ

lepton; threelightleptons;twosame-charge lightleptons withahadronically decaying

τ

lepton; fourlightleptons;and one lightleptonandtwo hadronically decaying

τ

leptons. No significant excess of events is observed above the background expectation.Thebestfitforthe

t

¯t H production crosssection,assumingaHiggsbosonmassof125 GeV, is2.1+₋1.4_1.2timestheSMexpectation,andtheobserved(expected)upperlimitatthe95%confidencelevel is4.7(2.4)timestheSMrate.The

p-value

forcompatibilitywiththebackground-onlyhypothesisis1.8σ; theexpectationinthepresenceofaStandardModelsignalis0.9σ.

©2015CERNforthebenefitoftheATLASCollaboration.PublishedbyElsevierB.V.Thisisanopen accessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

The discovery of a new particle H with a mass of about 125 GeVinsearchesfortheStandardModel(SM)

[1–3]

Higgs bo-son [4–7] at the LHC was reported by the ATLAS [8] and CMS

[9] Collaborations in July 2012. The particle has been observed in the decays H

→

γ γ

[10,11], H

→

Z Z∗

→

4



[12,13], and

H

→

W W∗

→ 

ν



ν

[14,15],andevidence hasbeen reportedfor

H

→

τ τ

[16,17], consistent with the rates expected for the SM Higgsboson.

TheobservationoftheprocessinwhichtheHiggsbosonis pro-ducedinassociationwithapairoftopquarks(t

¯

t H )wouldpermit adirectmeasurementofthetopquark–Higgsboson Yukawa cou-plingin aprocess that is tree-levelatthe lowestorder, whichis otherwise accessible primarily through loop effects. Having both thetree- and loop-level measurements wouldallow disambigua-tion of newphysics effects that could affectthe two differently, such asdimension-six operators contributing to the gg H vertex.

This letter describes a search for the SM Higgs boson in the

t¯t H production mode in multilepton final states. The five final statesconsideredare:twosame-charge-signlightleptons(e or

μ

) withnoadditionalhadronicallydecaying

τ

lepton;threelight lep-tons;twosame-sign lightleptons withonehadronically decaying

τ

lepton;fourlightleptons;andonelightleptonwithtwo hadron-ically decaying

τ

candidates. Thesechannels are sensitive to the Higgs decays H

→

W W∗,

τ τ

, and Z Z∗ produced in association

 _{E-mailaddress:atlas.publications@cern.ch.}

with a top quark pair decaying to one ortwo leptons. Asimilar searchhasbeenperformedbytheCMSCollaboration[18].

Theselectionsofthissearcharedesignedtoavoidoverlapwith ATLASsearchesfortt H in

¯

H

→

bb

¯

[19]andH

→

γ γ

[20]decays. Themainbackgroundstothesignal arisefromt¯t productionwith additionaljetsandnon-promptleptons,associatedproductionofa top quark pair anda vector boson W or Z (collectively denoted

t¯t V ),andother processeswheretheelectronchargeisincorrectly measured or wherequark or gluon jetsare incorrectly identified as

τ

candidates.

2. ATLASdetectoranddataset

The features of the ATLAS detector [21] mostrelevant to this analysisare briefly summarized here. Thedetector consistsofan inner tracking detector system surrounded by a superconduct-ing solenoid, electromagnetic and hadronic calorimeters, and a muonspectrometer.Chargedparticlesinthepseudorapidity1_range

|

η

| <

2

.

5 arereconstructedwiththeinnertrackingdetector,which is immersedin a2 T magnetic field parallel to the detectoraxis

1 _{TheATLASexperimentusesaright-handedcoordinatesystemwithitsoriginat}

thenominalinteractionpoint(IP)inthecentreofthedetector,andthez-axisalong thebeam line.Thex-axispoints fromtheIPtothecentreoftheLHCring,and they-axispointsupwards.Cylindricalcoordinates

(

r, φ)areusedinthetransverse plane,

φ

beingtheazimuthalanglearoundthez-axis.Observableslabelled “trans-verse”areprojectedontothe x– y plane.Thepseudorapidity isdefinedinterms ofthepolarangle

θ

asη= −ln tanθ/2.Thetransversemomentumisdefinedas

pT=psinθ=p/ coshη,andthetransverseenergyEThasananalogousdefinition. http://dx.doi.org/10.1016/j.physletb.2015.07.079

0370-2693/©2015CERNforthebenefitoftheATLASCollaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

andconsistsofpixelandstripsemiconductordetectorsaswellasa straw-tubetransitionradiationtracker.Thesolenoidissurrounded byacalorimetersystemcovering

|

η

|

<

4

.

9,whichprovides three-dimensional reconstruction ofparticle showers. Lead/liquid-argon (LAr) sampling technology is used for the electromagnetic com-ponent. Iron/scintillator-tile sampling calorimeters are used for the hadroniccomponentfor

|

η

|

<

1

.

7, andcopper/LAr and tung-sten/LArtechnologyis usedfor

|

η

|

>

1

.

5.Outside thecalorimeter system, air-core toroids provide a magnetic field for the muon spectrometer.Three stationsofprecision drifttubes and cathode-stripchambersprovideameasurementofthemuontrackposition andcurvatureintheregion

|

η

|

<

2

.

7.Resistive-plateandthin-gap chambersprovidemuontriggeringcapabilityupto

|

η

|

=

2

.

4.

This search uses data collected by the ATLAS experiment in 2012 ata centre-of-mass energy of

√

s

=

8 TeV. All events con-sidered were recorded while the detector and trigger systems were fullyfunctional; theintegratedluminosity of thisdatasetis 20.3 fb−1.

3. Crosssectionsforsignalandbackgroundprocesses

The cross section for the production of t¯t H in pp collisions

has been calculated at next-to-leading order (NLO) in quantum chromodynamics (QCD) [22–26]. Uncertainties on the cross sec-tionareevaluatedbyvaryingtherenormalizationandfactorization scales by factors of two and by varying the input parton dis-tribution functions (PDF) of the proton. A Higgs boson mass of

mH

=

125 GeV is assumed;thisgivesapredictedt¯t H production crosssectionat

√

s

=

8 TeV of129₋+5₁₂(scale)

±

10(PDF)fb

[27]

. ThisassumedHiggs bosonmass isconsistent withthecombined ATLASandCMSmeasurement[28].

In this letter the associated production of single top quarks with a Higgs boson is considered a background process and set to the Standard Model rate. The production of t Hqb and t H W

istakenintoaccount. IntheStandardModel theserates arevery smallcomparedtot¯t H production. Theseprocessesare simulated with the same parameters as used by the ATLAS t¯t H , H

→

γ γ

search [20]. The cross sections for both are computedusing the MG5_aMC@NLO generator

[29]

atNLOinQCD.Fort Hqb,the renor-malizationandfactorizationscalesaresetto75 GeVandthe pro-cessiscomputed inthe four-flavourscheme, yielding

σ

(

t Hqb

)

=

17

.

2+₋0₁._.8₄ (scale)₋+₀1._.2₉ (PDF)fb.Fort H W ,dynamicfactorizationand renormalizationscales are used, andthe process is computedin the five-flavour scheme; the result is

σ

(

t H W

)

=

4

.

7+₋0₀._.4₃ (scale)

+0.8

−0.6(PDF)fb.Theinterferenceoft H W productionwithtt H ,

¯

which

appearsat NLOfort H W in diagramswithan additionalb-quark

inthefinalstate,isnotconsidered.

The production of t¯t W and tt

¯

(

Z

/

γ

∗

)

→

t¯t



+



− yield multi-lepton final states with b-quarks and are major backgrounds to the t¯t H signal. For simplicity of notation the latter process is referred to as t¯t Z throughout this letter with off-shell Z and

photon componentsalso includedexcept wherenotedotherwise. The t¯t W process includes both t¯t W+ and t¯t W− components. Next-to-leading-order cross sections are used for t¯t W [30] and

t¯t Z [31]. The MG5_aMC@NLO generatoris usedto reproducethe QCDscaleuncertaintiesofthesecalculationsanddetermine uncer-tainties due to the PDF. For t¯t W production the value 232

±

28 (scale)

±

18(PDF)fbisused,andfort¯t Z production2 _thevalueis

206

±

23 (scale)

±

18 (PDF)fb.

2 _{TheNLOcrosssection isonlyevaluatedfor tt Z production¯ with on-shellZ.}

Thecrosssectionobtainedfort¯t(Z/γ∗)productionincludingoff-shellZ/γ∗ con-tributionsinaleading-ordersimulationisscaledbyaK -factorof1.35obtainedas theratioofNLOandLOon-shellcrosssections.The K -factordiffersfromthatof Ref.[31]duetoadifferentchoiceofPDF.

The associated productionof a single top quark and a Z

bo-son is a subleading background for the most sensitive channels. The cross section has been calculated at NLO for the t- and s-channels [32]. The resulting values used in this work are 160

±

7(scale)

±

11(PDF)fbfort Z and76

±

4(scale)

±

5(PDF)fb fort Z .

¯

The crosssection fortheproductionoft W Z iscomputed atleading order(LO) usingthe MadGraph v5 generator [33]and foundtobe4.1fb.

Thecrosssectionforinclusiveproductionofvectorbosonpairs

W W , W Z ,and Z Z iscomputedusingMCFM

[34]

.Contributions fromvirtualphotonsandoff-shell Z bosonsareincluded.The un-certainties on the acceptance for these processes in the signal regions (which favour productionwithadditional b- or c-quarks)

dominate over the inclusive cross-section uncertainty (see Sec-tion7.2)andsothelatterisneglectedintheanalysis.

The inclusive t¯t cross section is calculated at next-to-next-to-leading order (NNLO) in QCD which includes resummation of next-to-next-to-leadinglogarithmic (NNLL)softgluontermsusing Top++ _{[35], yielding 253}+13

−15 pb for

√

s

=

8 TeV. The single-top-quark samples arenormalized tothe approximateNNLO theoret-ical cross sections [36–38] using the MSTW2008 [39] NNLO PDF set.Theproductionof Z

→ 

+



−

+

jets andW

→ 

ν

+

jets is nor-malizedusingNNLOcrosssectionsascomputedbyFEWZ[40].

4. Eventgeneration

Theeventgeneratorconfigurationsusedforsimulatingthe sig-nal and main backgroundprocesses are shown in Table 1. Addi-tionalinformationisgivenbelow.

The t¯t H signal eventsimulationsamplescontainall Higgs bo-son decays with branching fractions set to values computed at NNLO inQCD

[26,66–69]

.The factorization(

μ

F) and

renormaliza-tion(

μ

R) scalesaresettomt

+

mH

/

2.Higgsbosonandtopquark massesof125 and172

.

5 GeV, respectively,are used.These sam-plesarethesameasthoseusedbyotherATLASt¯t H searches

[19,

20].

ProductionofsingletopquarkswithHiggsbosonsissimulated as follows.For t Hqb,events are generated atleading orderwith MadGraphinthefour-flavourscheme.Fort H W ,eventsare gener-atedatNLOwith MG5_aMC@NLO inthefive-flavourscheme.Higgs bosonandtopquarkmassesaresetasfort¯t H production.

The main irreducible backgrounds are production oft¯t W and t¯t Z

(

t¯t V

)

.Forthet¯t W process,eventsaregeneratedatleading or-der withzero,one, ortwo extra partonsin thefinal state, while fort¯t Z zerooroneextrapartonisgenerated.Theimportant con-tributionfromoff-shell

γ

∗

/

Z

→ 

+



−isincluded.Thet Z process

issimulatedwiththesamesetup,withoutextrapartons.

For diboson processes,the full matrixelement for



+



− pro-duction, including

γ

∗ andoff-shell Z contributions, is used. The Sherpaqq and

¯

qg samples includediagramswithadditional par-tons in the final state atthe matrix-element (ME) level, and in-cludeb- andc-quarkmasseffects. Sherpa wasfoundtohavebetter agreementwithdatathan Powheg forW Z ,whilethe Sherpa and Powheg_{descriptionsof Z Z productionaresimilar.}

A t¯t

+

jets samplegenerated withthe Powheg NLO generator

[61] isused; the top quark massis set to 172

.

5 GeV. Small cor-rections to the t¯t system and top quark pT spectra are applied

based on discrepancies in differential distributions observed be-tweendataandsimulationat7 TeV[70].Double-countingbetween thet¯t andW t singletopproductionfinalstatesiseliminatedusing thediagram-removalmethod[71].

Samples of Z

→ 

+



−

+

jets and W

→ 

ν

+

jets events are generated with up to five additional partons using the Alp-gen v_{2.14[65] leading order(LO) generator.Samples are merged} withmatrixelement-partonshoweroverlapsremoved usingMLM

Table 1

Configurationsusedforeventgenerationofsignalandbackgroundprocesses.Ifonlyonepartondistributionfunctionisshown,thesameoneisusedforboththematrix element(ME)andpartonshowergenerators;iftwoareshown,thefirstisusedforthematrixelementcalculationandthesecondforthepartonshower.“Tune”refersto theunderlying-eventtuneofthepartonshowergenerator.“Pythia 6”referstoversion6.425;“Pythia 8”referstoversion8.1;“Herwig++”referstoversion2.6;“MadGraph” referstoversion5;“Alpgen”referstoversion2.14;“Sherpa”referstoversion1.4;“gg2ZZ”referstoversion2.0.

Process ME generator Parton shower PDF Tune

tt H¯ HELAC-Oneloop[41,42] Pythia8[43] CT10[44]/CTEQ6L1[45,46] AU2[47]

+Powheg-BOX[48–50]

t Hqb MadGraph[33] Pythia8 CT10 AU2

t H W MG5_aMC@NLO[29] Herwig++[51] CT10/MRST LO**[52] UE-EE-4[53]

tt W¯ + ≤2 partons MadGraph Pythia6[54] CTEQ6L1 AUET2B[55]

tt¯(Z/γ∗)+ ≤1 parton MadGraph Pythia6 CTEQ6L1 AUET2B

t(Z/γ∗) MadGraph Pythia6 CTEQ6L1 AUET2B

qq¯,qg→W W,W Z Sherpa[56] Sherpa CT10 Sherpadefault

qq→qqW W , qqW Z , qq Z Z Sherpa Sherpa CT10 Sherpadefault

qq¯,qg→Z Z Powheg-BOX[57] Pythia8 CT10 AU2

gg→Z Z gg2ZZ[58] Herwig[59] CT10 AUET2[60]

tt¯ Powheg-BOX[61] Pythia6 CT10/CTEQ6L1 Perugia2011C[62]

s-, t-channel, W t single top Powheg-BOX[63,64] Pythia6 CT10/CTEQ6L1 Perugia2011C

Z→ +−+ ≤5 partons Alpgen[65] Pythia6 CTEQ6L1 Perugia2011C

W→ ν+ ≤5 partons Alpgen Pythia6 CTEQ6L1 Perugia2011C

matching[72].Productionofb- and c-quarksisalsocomputedat matrix-elementlevel,andoverlapsbetweenMEandpartonshower productionarehandledbyseparatingthekinematicregimesbased ontheangularseparationofadditionalheavy partons.The result-ing“light”and“heavy”flavoursamplesarenormalizedby compar-ingtheresultingb-taggedjetspectrawithdata.

Allsimulated sampleswith Pythia 6 and Herwig [59] parton showering use Photos 2.15 [73] to model photon radiation and Tauola_1.20[74]for

τ

_{decays.The Herwig++ samplesmodel} pho-ton radiation with Photos but use the internal

τ

decay model. Samplesusing Pythia 8.1and Sherpa usethosegenerators’internal

τ

leptondecayandphotonradiationgenerators.For Herwig sam-ples,multiplepartoninteractionsaremodelledwith Jimmy[75].

Showered andhadronized events are passed through simula-tions of the ATLAS detector (either full GEANT4 [76] simulation or a hybrid simulation with parameterized calorimeter showers and GEANT4 simulation of the tracking systems [77,78]). Addi-tional minimum-bias pp interactions (pileup) are modelled with the Pythia 8.1generatorwiththe MSTW2008LOPDF setandthe A2tune [79]. Theyare addedto the signal andbackground sim-ulatedeventsaccordingto the luminosity profileof therecorded data,withadditionaloverallscalingtoachieveagoodmatchto ob-servedcalorimetry andtrackingvariables.The contributionsfrom pileup interactions both within the same bunch crossing as the hard-scatteringprocess and in neighbouring bunch crossings are includedinthesimulation.

5. Objectselection

Electron candidates are reconstructed from energy clusters in the electromagnetic calorimeterassociated with reconstructed tracksintheinner detector.Theyare requiredtohave

|

η

cluster

|

<

2

.

47.Candidatesinthetransitionregion1

.

37

<

|

η

cluster

|

<

1

.

52

be-tween sections of the electromagnetic calorimeter are excluded. A multivariate discriminant based on shower shape and track informationisused todistinguish electrons fromhadronic show-ers [80,81]. Only electron candidates with transverse energy ET

greater than 10 GeV are considered. To reduce the background fromnon-promptelectrons,i.e. fromdecaysofhadrons(including heavy flavour) produced in jets, electron candidates are required tobe isolated. Twoisolation variables,based oncalorimetricand trackingvariables,arecomputed.The first(Econe_T ) isbasedonthe sumof transverse energies ofcalorimeter cells within a cone of radius



R

≡



(φ)

2

_{+ (}

_η

₎

2

₌

₀

_.

_{2 around the electron}

candi-datedirection. Thisenergysumexcludescellsassociatedwiththe

electron and is corrected for leakage from the electromagnetic shower and ambient energy in the event. The second (pcone_T ) is definedbasedontrackswith pT

>

1 GeV withinacone ofradius



R

=

0

.

2 aroundthe electron candidate. Both isolation energies are separately required to be lessthan 0

.

05

×

ET. The

longitudi-nal impact parameter of the electron track with respect to the selectedeventprimaryvertex, multipliedby thesineofthepolar angle,

|

z0sin

θ

|

,isrequired tobe lessthan1 mm. The transverse

impactparameterdividedbytheestimateduncertaintyonits mea-surement,

|

d0

|/

σ

(

d0

)

,mustbe lessthan4.Iftwoelectrons closer

than



R

=

0

.

1 are selected, only the one with the higher pT is

considered. An electron is rejectedif, after passing all the above selections,itlieswithin



R

=

0

.

1 ofaselectedmuon.

Muon candidatesare reconstructed bycombininginner detec-tor tracks withtrack segments or full tracks in the muon spec-trometer [82]. Only candidates with

|

η

|

<

2

.

5 and pT

>

10 GeV

are kept. Additionally, muonsare required to be separated by at least



R

>

0

.

04

+ (

10 GeV

)/

pT,μ fromanyselectedjets(see

be-low fordetails onjetreconstruction andselection).The cutvalue is optimizedto maximize the acceptanceforprompt muonsat a fixed rejection factor for non-prompt andfake muon candidates. Furthermore, muons must satisfy similar Econe

T and pTcone

isola-tion criteria asfor electrons, with both required to be lessthan 0

.

10

×

pT.Thevalueof

|

z0sin

θ

|

isrequiredtobelessthan1mm,

while

|

d0

|/

σ

(

d0

)

mustbelessthan3.

Hadronicallydecaying

τ

candidates(

τ

had)arereconstructed

us-ingclustersintheelectromagneticandhadroniccalorimeters.The

τ

candidates are required to have pT greater than 25 GeV and

|

η

|

<

2

.

47. The number of charged tracks associated with the

τ

candidatesisrequiredtobeoneorthreeandthecharge ofthe

τ

candidates, determined from the associated tracks, must be

±

1. The

τ

identification uses calorimeter cluster and tracking-based variables, combinedusing a boosteddecisiontree(BDT)

[83]

.An additionalBDT whichusescombinedcalorimeterandtrack quan-tities is employed to reject electrons reconstructed asone-prong hadronicallydecaying

τ

leptons.

Jetsare reconstructedfromcalibrated topologicalclusters[21]

built from energy deposits in the calorimeters, using the anti-kt algorithm [84–86] with a radius parameter R

=

0

.

4. Prior to jet finding, a local cluster calibration scheme [87,88] is ap-plied to correct the topological cluster energies for the effects of non-compensating calorimeterresponse, inactive material and out-of-cluster leakage. The jets are calibrated using energy and

η

-dependent calibration factors, derived from simulations, to the meanenergyofstableparticles insidethejets. Additional

correc-tions to account for the difference between simulation anddata are derived fromin-situ techniques[89,90]. Afterenergy calibra-tion,jetsarerequiredtohavepT

>

25 GeV and

|

η

|

<

2

.

5.

Toreducethecontamination fromjetsoriginatingin pp

inter-actionswithinthesamebunchcrossing(pileup),thescalarsumof the pT oftracks matchedtothe jetandoriginatingfromthe

pri-maryvertexmust beatleast 50%ofthe scalarsumofthe pT of

alltracksmatchedto thejet.Thiscriterionisonlyappliedto jets withpT

<

50 GeV (thosemostlikelytooriginatefrompileup)and

|

η

|

<

2

.

4 (toavoidinefficiencyattheedgeoftrackingacceptance). The calorimeter energy deposits from electrons are typically alsoreconstructed asjets;in orderto eliminatedouble counting, anyjetswithin



R

=

0

.

3 ofaselectedelectronarenotconsidered. Jetscontaining b-hadronsareidentified(b-tagged)via a multi-variatediscriminant[91]that combinesinformationfromthe im-pactparameters ofdisplacedtrackswithtopologicalpropertiesof secondaryandtertiarydecayverticesreconstructedwithinthejet. The working point used for this search corresponds to approxi-mately70%efficiencytotagab-hadronjet,withalight-jetmistag rateof

≈

1% and a charm-jetrejectionfactorof5,asdetermined forb-tagged jetswith pT of 20–100 GeV and

|

η

|

<

2

.

5 in

simu-latedt¯t events.Toavoidinefficienciesassociatedwiththeedgeof the tracking coverage, only jetswith

|

η

|

<

2

.

4 are considered as possible b-tagged jetsin this analysis. The efficiency andmistag ratesoftheb-taggingalgorithmaremeasuredindata

[91,92]

and correctionfactorsareappliedtothesimulatedevents.

6. Eventselectionandclassification

All events considered in this analysis are required to pass single-lepton (e or

μ

) triggers. These achieve their maximal plateauefficiencyforlepton pT

>

25 GeV.

This analysis primarily targets the H

→

W W∗ and

τ τ

decay modes.Consideringthe decayofthett system

¯

aswell, theset¯t H

eventscontaineitherW W W W bb or

¯

τ τ

W W b¯b.Thestrategyisto targetfinalstatesthatcannotbeproducedint¯t decayalone—i.e., threeormoreleptons,ortwosame-signleptons—thus suppress-ingwhatwouldotherwisebethelargestsinglebackground.

Theanalysiscategoriesareclassifiedbythenumberoflight lep-tons and hadronic

τ

decay candidates. The leptons are selected usingthecriteriadescribedearlier.Eventsareinitiallyclassifiedby countingthe numberoflight leptons with pT

>

10 GeV. At least

onelightleptonisrequiredtomatchaleptonselectedbythe trig-gersystem. After initial sorting into analysiscategories, in some casesthelepton selectioncriteriaare tightenedby raisingthe pT

threshold,tighteningisolation selectionsorrestrictingtheallowed

|

η

|

range,asexplainedinthefollowingper-categorydescriptions. Theanalysisincludes fivedistinctcategories: twosame-sign light leptonswithno

τ

had

(

2



0

τ

had

)

,threelightleptons(3



),two

same-signlight leptons andone

τ

had (2



1

τ

had), four light leptons (4



),

andone lightlepton andtwo

τ

had (1



2

τ

had). The categorieswith

τ

had candidates targetthe H

→

τ τ

decay;the othersare

primar-ily sensitive to H

→

W W∗ witha very small contribution from

H

→

Z Z∗.ThecontributionstoeachcategoryfromdifferentHiggs bosondecaymodesare showninTable 2.Theseselectioncriteria ensurethataneventcanonlycontribute toasinglecategory.The contaminationfromgluonfusion,vectorbosonfusion,and associ-atedV H productionmechanismsfortheHiggsbosonispredicted to be negligible. Summed over all categories, the total expected numberof reconstructed signal events assuming Standard Model

t¯t H productionis10.2,correspondingto0.40%ofallproducedt¯t H

events.Thedetailedcriteriaforeachcategoryaredescribedbelow.

Table 2

Fractionoftheexpectedtt H signal¯ arisingfromdifferentHiggsbosondecaymodes ineachanalysiscategory.Thesix20τhadcategoriesarecombinedtogether,asare

the two4categories.Thedecayscontributingtothe“other”columnare domi-nantlyH→μμandH→bb.¯ Rowsmaynotaddto100%duetorounding.

Category Higgs boson decay mode

W W∗ τ τ Z Z∗ Other 20τhad 80% 15% 3% 2% 3 74% 15% 7% 4% 21τhad 35% 62% 2% 1% 4 69% 14% 14% 4% 12τhad 4% 93% 0% 3% 6.1. 2



0

τ

hadcategories

Selected events are required to includeexactly two light lep-tons, which must have the same charge. Events with

τ

had

can-didates are vetoed. To reduce the background from non-prompt leptons,theleading(subleading)leptonisrequiredtosatisfy pT

>

25

(

20

)

GeV,and themuon isolation requirements are tightened to Econe_T

/

pT

<

0

.

05 and pTcone

/

pT

<

0

.

05.The angularacceptance

of electroncandidates isrestricted to

|

η

|

<

1

.

37 in order to sup-presst¯t backgroundeventswherethesignoftheelectroncharge is misreconstructed,asthe chargemisidentificationrateincreases athighpseudorapidity.

In order to suppressthe lower-multiplicity tt

¯

+

jets and t¯t W

backgrounds,eventsmustincludeatleastfourreconstructedjets. In order to suppress diboson and single-boson backgrounds, at leastone ofthesejetsmust beb-tagged.The selectedeventsare separatedby lepton flavour (e±e±,e±

μ

±,and

μ

±

μ

±) and num-ber ofjets(exactly four jets,at leastfivejets) intosixcategories withdifferentsignal-to-backgroundratio,resultinginhigher over-allsensitivitytothet¯t H signal.

6.2. 3



category

Selectedeventsare requiredto includeexactly threelight lep-tons with total charge equal to

±

1. Candidate events arising from non-prompt leptons overwhelmingly originate as opposite-signdileptoneventswithoneadditionalnon-promptlepton.As a result, thenon-promptlepton isgenerallyoneofthetwoleptons withthesamecharge.Toreduce thesebackgrounds,ahigher mo-mentumthresholdpT

>

20 GeV isappliedtothetwoleptonswith

thesamecharge.No requirementsareimposedonthenumberof

τ

had candidates.In order to suppressthet¯t

+

jets andtt V back-

¯

grounds,selectedeventsarerequiredtoincludeeitheratleastfour jetsofwhichatleastonemustbeb-tagged,orexactlythreejetsof whichatleasttwoareb-tagged.Tosuppressthett Z background,

¯

eventsthatcontainanopposite-signsame-flavourleptonpairwith thedileptoninvariant masswithin 10 GeVofthe Z massare ve-toed.Eventscontaininganopposite-signleptonpairwithinvariant massbelow12 GeV arealsoremovedtosuppressbackgroundfrom resonancesthatdecaytolightleptons.

6.3. 2



1

τ

hadcategory

Selected events are required to include exactly two light leptons, with the same charge and leading (subleading) pT

>

25

(

15

)

GeV, and exactly one hadronic

τ

candidate. The recon-structedchargeofthe

τ

had candidatehastobeoppositetothatof

thelightleptons.Inordertoreducet¯t

+

jets andt¯t V backgrounds,

events must include atleast four reconstructed jets. In order to suppress diboson and single-boson backgrounds, at least one jet must be b-tagged.To suppressthe Z

→ 

+



−

+

jets background, eventswithdielectroninvariantmasswithin10 GeVoftheZ mass

Fig. 1. Thespectrumofthenumberofjetsexpectedandobservedinthett Z (left)¯ andt¯tW (right)validationregions(VR).Thehatchedbandrepresentsthetotaluncertainty onthebackgroundpredictionineachbin.The“non-prompt”backgroundsarethosewithaleptonarisingfromahadrondecayorfromaphotonconversionindetector material.Rareprocessesincludet Z ,t¯tW W ,triboson,t¯ttt,¯andt H production.Theoverlaidredlinecorrespondstothett H signal¯ predictedbytheSM.(Forinterpretationof thereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

6.4.4



categories

Selectedevents are requiredto include exactlyfour light lep-tons with total charge equal to zero and leading (subleading)

pT

>

25

(

15

)

GeV. No requirements are applied on the number

of

τ

hadcandidates.Inordertosuppressthett

¯

+

jets andt¯t V

back-grounds,the selectedevents are requiredto includeatleast two jetsofwhichatleastone mustbe b-tagged.Tosuppressthett Z

¯

background,eventsthatcontainanopposite-signsame-flavour lep-tonpairwithdileptoninvariantmasswithin10 GeVoftheZ mass

are vetoed. In order to suppress background contributions from resonances that decay to light leptons, all opposite-sign same-flavourleptonpairsarerequiredtohaveadileptoninvariantmass greater than 10 GeV. The four-lepton invariant mass is required tobebetween100and500 GeV,whichgiveshighacceptancefor

t¯t H , H

→

W W∗

→ 

ν



ν

, butrejects Z

→

4



and high-masstt Z

¯

events.Selectedeventsare separatedby thepresence orabsence ofasame-flavour,opposite-signleptonpairintotwocategories, re-ferredtorespectivelyastheZ -enrichedandZ -depletedcategories. Inbothcasesthe Z massvetoisapplied,butbackgroundeventsin theZ -enrichedcategorycanarisefromoff-shellZ and

γ

∗

→ 

+

_

−

processeswhileinthe Z -depletedcategorythesebackgroundsare absent.

6.5.1



2

τ

hadcategory

Selectedeventsarerequiredtoincludeexactlyonelightlepton withpT

>

25 GeV andexactlytwohadronic

τ

candidates.The

τ

had

candidates must have opposite charge. In order to suppress the

t¯t

+

jets andt¯t V backgrounds,eventsmustincludeatleastthree reconstructedjets. Inorderto suppressdibosonandsingle-boson backgrounds,atleastoneofthe jetsmustbeb-tagged.Thisfinal stateis primarilysensitive to H

→

τ

+

τ

− decays,allowing useof the invariant mass of the visible decay products of the

τ

had

τ

had

system(mvis) asa signal discriminant.Signal eventsare required

tosatisfy60

<

mvis

<

120 GeV.

7. Backgroundestimation

Important irreducible backgrounds include tt V and

¯

diboson productionandare estimatedfromMC simulation.Validation re-gions enriched in these backgrounds are used to verify proper modellingofdatabysimulation.Reduciblebackgroundsaredueto non-prompt lepton production and electron charge mis-identification, andareestimatedfromdata,withinput from sim-ulation in some categories. In the 1



2

τ

had category the primary

concernisfake

τ

hadcandidates,whicharemodelledusing

simula-tionandvalidatedagainstadata-drivenestimate.

7.1. t¯t V andt Z

The primary backgrounds withprompt leptons stemfrom the production of t¯t W and t¯t Z . The tt W background

¯

tends to have lower jet multiplicity than the signal and so the leading contri-bution comes from eventswith additional high-pT jets; it is the

majortt V contribution

¯

inthe 2



0

τ

had categoriesandcomparable

to t¯t Z in the 2



1

τ

had category. Thet¯t Z process hassimilar

mul-tiplicity to the t¯t H signal but can only contribute to the signal categorieswhen the Z boson decaysleptonically, so the on-shell contributioncanberemovedbyvetoingeventswithopposite-sign dilepton pairs with invariant mass near the Z pole. This is the larger of the two t¯t V contributions for the 3



, 4



, and 1



2

τ

had

categories.Thet Z processmakesasubleadingcontributiontoboth channels.Avalidationregionisusedtoverifythemodellingoft¯t Z

usingon-shell Z decays. Agreementis seen within thelarge sta-tistical uncertainty. No region of equivalent purityand statistical powerexistsforttW production;

¯

neverthelesstheexpectationsare cross-checked witha validation region defined with the 2



0

τ

had

selectionexceptwithtwoormoreb-taggedjetsandeithertwoor threejets,wherethet¯t W purityis

≈

30%,andarefoundtobe con-sistent within uncertainties. Thespectra ofthenumberof jetsin thesevalidationregionsare shownin

Fig. 1

.

Uncertaintieson the t¯t V background contributions arise from boththeoverallcrosssectionuncertainties(seeSection3)andthe acceptance uncertainties. The latter are estimated by comparing particle-level samples after showering produced by three differ-ent pairs of generators: a) the nominal MadGraph LO merged

Table 3

Expectedandobservedyieldsineachchannel.UncertaintiesshownarethesuminquadratureofsystematicuncertaintiesandMonteCarlosimulationstatisticaluncertainties. “Non-prompt”includesthemisidentifiedτhadbackgroundtothe12τhad category.Rareprocesses(t Z ,t¯tW W ,tribosonproduction,ttt¯¯t,t H )arenotshownasaseparate

columnbutareincludedinthetotalexpectedbackgroundestimate.

Category q mis-id Non-prompt tt W¯ t¯t Z Diboson Expected bkg. t¯t H(μ=1) Observed

ee+ ≥5 j 1.1±0.5 2.3±1.2 1.4±0.4 0.98±0.26 0.47±0.29 6.5±1.8 0.73±0.14 10 eμ+ ≥5 j 0.85±0.35 6.7±2.4 4.8±1.2 2.1±0.5 0.38±0.30 15±3 2.13±0.41 22 μμ+ ≥5 j – 2.9±1.4 3.8±0.9 0.95±0.25 0.69±0.39 8.6±2.2 1.41±0.28 11 ee+4 j 1.8±0.7 3.4±1.7 2.0±0.4 0.75±0.20 0.74±0.42 9.1±2.1 0.44±0.06 9 eμ+4 j 1.4±0.6 12±4 6.2±1.0 1.5±0.3 1.9±1.0 24±5 1.16±0.14 26 μμ+4 j – 6.3±2.6 4.7±0.9 0.80±0.22 0.53±0.30 12.7±2.9 0.74±0.10 20 3 – 3.2±0.7 2.3±0.7 3.9±0.8 0.86±0.55 11.4±2.3 2.34±0.35 18 21τhad – 0.4+₋00..46 0.38±0.12 0.37±0.08 0.12±0.11 1.4±0.6 0.47±0.08 1 12τhad – 15±5 0.17±0.06 0.37±0.09 0.41±0.42 16±5 0.68±0.13 10 4Z -enr. – 10−3 _3×10−3 _{0.43±0.12 0.05±0.02 0.55±0.15 0.17±0.02 1} 4Z -dep. – 10−4 _10−3 _{0.002±0.002 2×10}−5 _{0.007±0.005 0.025±0.003 0}

sample versus an equivalent LO merged sample generated with Sherpa 2.1.1, to account for ME-parton shower matching effects; b)theLOmerged Sherpa sampleversusa Sherpa+OpenLoops[93]

NLO sample, tocompare LOmerged andNLO acceptance;andc) MG5_aMC@NLO with Pythia 8 parton shower versus Herwig++ partonshower,tocomparepT-orderedversusangular-ordered

par-ton showers. Eachofthesevariations isinput independentlyinto the final fit. When summed in quadrature they have an im-pactof5–23%depending onthe categoryandbackgroundsource (t

¯

t W versus t¯t Z ). Uncertainties arising from changes in the ac-ceptance dueto the choice of QCD scale and PDF are also eval-uated; these have an impact of 1.3–6.7% for scale and 0.9–4.8% for PDF.

7.2. Otherpromptleptoncontributions

Other backgrounds with prompt leptons arise from multibo-son processes (W Z , Z Z ,and triboson production) in association with heavy-flavour jets, or with a misidentified light-flavour jet. ThemainprocessaffectingthefinalresultisW Z

+

jets.Validation regionswiththreeleptonsincludingaZ candidateandeitherzero orone b-taggedjet are studied. Thenumber ofjetsin W Z

+

0b events is reproduced well in the highly populated bins (upto 4 jets),leading to theconclusion that thejet radiation spectrumis wellmodelled.Thedominantuncertaintyonthepredictioninthe signalregion isexpectedtoarise fromthe W Z

+

b crosssection. Dataconstrain thiscomponentwithroughly 100%uncertainty.As aresulta 100%uncertaintyisassignedto the W Z

+

b cross sec-tion, givinga 50% uncertainty on the total W Z yield, correlated across categories. The cross sections for production of W W

+

b

and Z Z

+

b arealsoassigned50%uncertainties;thesehave negli-gibleimpactonthefinalresult.

7.3. Chargesignmisidentification

The process e±

→

e±

γ

→

e±e+e− occurring in detector ma-terial can result in an electron produced with nearly the same momentum as the parent electron but with opposite charge. In these cases the observed electron has opposite charge to that ofthe primary electron (charge mis-id).The analogousprocesses

μ

±

→

μ

±e+e− and

μ

±

→

μ

±

μ

+

μ

−havenegligible ratesforthe selected events.The t¯t and Z

/

γ

∗

→ 

+

_

−

₊

_{jets events that}

un-dergo this process contribute to 2



0

τ

had in the ee and e

μ

cate-gories. As electrons pass through more material at high

|

η

|

, the chargemis-idrateincreasesaswell,andsotheelectron

|

η

|

<

1

.

37 requirement significantly reduces the impact of this background. Thechargemis-idrateduetotrackcurvaturemismeasurementfor electronsandmuonsisnegligible.

The charge mis-id probability is determined by a maximum-likelihoodfitusing Z

→

ee eventsreconstructedassame-signand as opposite-sign pairs, as a function of electron

η

and pT. This

probability functionisthenappliedtoasample ofeventspassing the2



0

τ

hadselectionexceptthattheleptonpairisrequiredtobe

oppositesign.Thechargemis-idprobabilityfromtherelativelylow momentum Z daughtersisextrapolatedtohigherpT usingscaling

functions extractedfrom Monte Carlosimulations. The dominant uncertaintyisduetothestatisticalprecision ofthechargemis-id probabilitydetermination,andis

≈

40% inthesignalregions.

7.4. Non-promptlightleptons

A significant backgroundarises from leptons not produced in decays of electroweak bosons (non-prompt leptons), which can promote(forexample)asingle-leptont¯t eventintoa2



0

τ

had

cat-egoryoradileptont¯t eventtothe3



or2



1

τ

hadcategories.These

backgrounds in the signal regions are expected to be dominated byt¯t orsingletopquarkproductionwithleptonsproducedin de-cays ofheavy-flavourhadrons.Productionoft¯t withanadditional photon whichconvertsinthe detectormaterialisa subdominant contribution.Withthetightobjectselectionrequirementsapplied in this analysis, almost all reconstructed electron and muon ob-jects correspond to real electrons and muons; the fraction aris-ing from incorrect particle identification is negligible. Estimates ofthesebackgrounds areobtainedfromdata.Eachchannel hasa slightlydifferentprocedure,motivatedbythespecificevent topol-ogyandthestatisticalpoweravailableinthecontrolregions. The methods are discussed below,andthe expectednon-prompt lep-ton contributions to thevarious categoriesare shownin

Table 3

. In the following, a tight lepton isa lepton that passesthe nom-inal selection, a sideband lepton isdefined asa lepton candidate which satisfies different criteria than the tight lepton selection (identification selection, isolation, or pT), and (sideband) control

regions either require one or more sideband leptons to replace a tight lepton in the signal region selection, or have the same lepton selection as the signal region but different jet require-ments.

7.4.1. 2



0

τ

hadcategories

The non-prompt lepton yields in the signal regions are es-timated by extrapolating from sideband control regions in data whichare enrichedint¯t non-promptcontributions.Forelectrons, sidebandobjects areselected byinverting theelectron identifica-tion and isolation requirements; formuons the sideband objects havelow transverse momentum,6

<

pT

<

10 GeV,butotherwise

areselectedthesamewayasnominalmuons.Transferfactorsare used toextrapolatefromeventswithonetight andonesideband

lepton,but which otherwise passthe signal region selections, to thesignalregionswithtwotightleptons.Thesetransferfactorsare determinedfromadditionaldatacontrolregions(tight+sideband andtwotightleptons)withlowerjetmultiplicity (1

≤

njet

≤

3 for

electrons,2

≤

njet

≤

3 formuons).Inallregionstheexpected

con-tribution from processes producing prompt leptons is subtracted beforeextractingtransferfactorsorusingtheyieldsfor extrapola-tion.Forchannelswithelectrons,thechargemis-idbackgroundis alsosubtracted,andadileptonmassvetoisappliedinthecontrol regionstosuppresscontributionsfrom Z

→

e+e− decays.A cross-checkonthemuonestimate,usingan extrapolationinmuon iso-lationinsteadofmuon pT,agreeswellwiththenominalprocedure

andprovidesadditionalconfidenceintheestimate.

Thesystematicuncertaintiesonthisprocedureareestimatedby checkinga)itsabilitytosuccessfullypredictthenon-prompt back-groundint¯t simulationandb)thestabilityofthepredictionusing datawhenthe selection ofthecontrol regions isaltered. Forthe former,differentpartonshower andb-hadron decaymodels were checked,aswas the result ofremoving the b-tagged jet require-ment.Inaddition,forelectrons,theeffectsofrelaxingthe pseudo-rapidityrequirement to

|

η

|

<

2

.

5 and ofraising the pT threshold

werestudied.Thesechecksshowstabilityatthe25–30%level, lim-itedbythestatisticalprecisionofthesimulations.Thestabilityin datais checked by alteringthe pT required for theb-tagged jet,

applyingarequirementon missingtransversemomentum3 Emiss_T , extractingthetransferfactorsonlyfromeventswiththreejets,or (formuons)using10–15 GeV muonsasthesidebandobjects.This checkshowsstabilityofthepredictionsto14%formuonsand19% forelectrons.Additionalsystematicuncertaintiesintheprediction arisefromthestatisticaluncertaintiesontheyieldsinthecontrol regionsandthesubtractionofpromptandchargemis-id contribu-tions.Theoveralluncertaintiesonthenon-promptyieldprediction inanygivencategoryrangefrom32%to52%,andcorrelations be-tween the categories due to uncertainties in the transfer factors areincludedinthefit(seeSection9).

7.4.2. 3



category

Sidebandleptonsaredefinedbyreversingtheisolation require-mentforelectronsandmuonsand,forelectrons,requiringthatthe candidatefailthetightelectronidentificationdiscriminant require-mentoftheanalysisbutpassa looserselection.The non-prompt leptoncontributioninthesignalregionisestimatedby extrapolat-ingfromdataregionswithtwotightandonesidebandlepton, us-ingtransferfactorsestimatedfromMonteCarlosimulation.These eventstypicallycontaintwopromptopposite-signleptonsandone non-promptlepton,whichnecessarilymustbeofthesamesignas one ofthe prompt leptons. Thereforethe non-prompt lepton es-timationprocedure isapplied onlyto thetwo same-sign leptons. The simulation-derived transfer factor is validatedin a region of lower jet multiplicity (2

≤

njet

≤

3 and exactly one b-tagged jet).

Goodagreementisobservedinthisvalidationregionbetweenthe prediction (11

.

8

±

2

.

3) and the observed yield (9

.

8

±

4

.

9 events afterpromptbackgroundsubtraction). Systematicuncertainties in theprocedurearederivedbystudyingtheagreementbetweendata andsimulationinthevariablesusedfortheextrapolation,whichis

≈

20%forbothelectronsandmuons.Additionaluncertaintiesarise fromthe statisticaluncertainties on the yields in the control re-gionsandinthet¯t simulation.

3 _{Thisiscalculatedusingcalorimeterenergydeposits,calibratedaccordingto}

as-sociatedreconstructedphysicsobjects,andalsoincludingthetransversemomenta ofreconstructedmuons.

7.4.3. 2



1

τ

hadcategory

Reconstructing twosame-signlight leptonsfromt¯t production

or similar sources requires that one of the light leptons is non-promptorhasitschargemisidentified.Inthe2



1

τ

hadcategory,the

charge mis-id contributionis negligible andtheprimary concern is non-prompt light leptons. Around half of the

τ

had candidates

in these events come from W

→

τ ν

decays, while the remain-der arise from misidentified light-quarkor gluon jets. Regardless ofwhetherthe

τ

hadcandidateisafake,thereisalsoanon-prompt

light lepton.Due tothis fact,sidebands inthe light-lepton selec-tioncriteriaareused,analogouslytothe2



0

τ

hadand3



categories.

Sincetheratioofrealandfake

τ

hadcandidatesissimilarinthe

sig-nalandallcontrolregions,fake

τ

had candidatesarenotaccounted

forseparately; thesmallvariations in theratiointhe control re-gions are found to have negligible impact on the total estimate inthe signalregion. Inorderto maintain similarorigin composi-tion of the non-prompt leptons, the ET isolation requirement is

inverted, the pT isolation requirement is relaxed, and for

elec-tronstheidentificationcriteriaarealsorelaxedtoalooserworking point. The low jet multiplicity region 2

≤

njet

≤

3 isused to

de-termineatransferfactorfromsidebandto tightleptonselections. The expectednon-promptlepton yieldinthe signalregion is ob-tained by usingthis transferfactor to extrapolate froma control region withthe same jet selection as thesignal region butwith one tight and one sideband light lepton. The procedure is vali-dated by checking that it correctly reproduces the signal region yield expected in t¯t simulations. The assigned systematic uncer-tainty (27%) isdominated by thestatistical precision ofthistest. Theoveralluncertaintyonthenon-promptbackgroundprediction is dominatedby the limitedstatistics ofthe highjet multiplicity controlregion.

7.4.4. 4



category

The non-prompt lepton contribution in this category is ex-pectedtobenegligibleandisestimatedtobe



10−3 eventsinthe

Z -enriched sample and



10−4 _{events inthe Z -depletedsample.}

Inbothcasesthisrepresents



2% ofthetotalbackground expecta-tion.Theseestimatesareobtainedusingthetransfer factorsfrom the3



channelandappropriatecontrolregionswithtwoloose lep-tonsandrelaxedjetmultiplicityrequirements.

7.5.

τ

hadmisidentificationinthe1



2

τ

hadcategory

The nominal estimate for the fake

τ

had yield is derived from

t¯t simulation. To obtaina sufficientlylarge sample size,fast sim-ulation using parameterized calorimeter showers is used. At all preselection stages the simulationis found to give an acceptable description of thett background,

¯

both inkinematic distributions andtotalyield.Thisestimateiscross-checkedwiththedata-driven methoddescribedbelow.

Ofthetwo

τ

hadcandidates,oneisopposite insigntothelight

lepton (OS) andthe other hasthe same sign(SS). The SS candi-dateisalmostalwaysafake

τ

had,whilethelightleptonisprompt

andthe OS

τ

had candidateis oftenreal (

≈

30%). A sideband

τ

had

isdefinedasacandidatepassingalooseidentificationBDT selec-tion butnot thenominaltight one. Assuming the

τ

had candidate

fakeprobabilitiesare notcorrelatedbetweenjetsidentifiedasOS and SScandidates, control regions can be used to predict yields inthesignalregion.Therearethreecontrolregions,dependingon whetheronlytheOS,onlytheSS,orboththeOSandSS

τ

had

can-didates are sidebandobjects. The two regions with sideband OS

τ

had candidates are used to obtain the transfer factor for the SS

τ

had candidate,which is then applied to theregion witha tight

OSandsidebandSScandidatetoobtainthepredictionforthe sig-nal region where both are tight. The transfer factoris measured

Fig. 2. Thespectrumofthenumberofjetsexpectedandobservedineachsignalregion.Fordisplaypurposesthesix20τhadcategories(ee/eμ/μμand =4/ ≥5 jets)are

combinedintooneplot,asarethetwo4categories( Z -enrichedandZ -depleted).Thehatchedbandsshowthetotaluncertaintyonthebackgroundpredictionineachbin. Thenon-promptandchargemis-idbackgroundspectraaretakenfromsimulationoftt,¯ singletop,Z→ +_−_{+jets,andothersmallbackgrounds,withnormalizationas}

describedinthetext(inparticularthe=4/ ≥5 jetregionsofthe20τhadplothavetheratiogivenbythedata-drivenprediction).Theoverlaidredlineshowsthett H signal¯

fromtheStandardModel.Forvisibility,thett H signal¯ ismultipliedbyafactorof2.4inthe20τhad,3,and12τhad plots.(Forinterpretationofthereferencestocolorin

thisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

asa function of the pT,

η

,number oftracks, andb-tag

discrim-inant value of the SS

τ

had candidate. The data-driven method is

cross-checked in t¯t simulation and found to successfully predict theyieldsinthesignalregion.Themainlimitationofthismethod isthestatisticalpowerofthecontrolregions.

Thesimulation-drivenmethodistakenastheprimaryestimate, asthevalidationofthemethodatpreselectionstagesismore pre-cise than the data-driven method due to larger event yield for the former. The comparison of the simulation- and data-driven techniques gives a 36% uncertainty in the prediction in the

sig-nal region, which is taken as the systematic uncertainty on the estimate.

8. Othersystematicuncertainties

Systematicuncertaintiesnotalreadydiscussedaresummarized below.

The uncertaintyonthe integratedluminosity is2.8%.This un-certainty isderived froma calibrationoftheluminosityscale de-rived from beam-separation scans performedin November 2012, followingthesamemethodologyasthatdetailedinRef.[94].

Leptonreconstruction and identification uncertainties are ob-tained from Z

→ 

, Z

→ 

γ

,

ϒ

→ 

, and J

/ψ

→ 

events

[80–82].Uncertaintiesonthedetectorresponseareassessed simi-larlytootherATLASanalyses.Themodellingoftheefficiencyofthe tightisolationrequirementsinsimulationisexplicitlycheckedasa functionofthenumberofjetsintheevent. Thesecorrectionsare foundto bevery small, withuncertainties limitedby data statis-tics.

The largest jet-related systematic uncertainty arises from the jet energyscale,in particular contributionsfrom thein-situ cali-brationindata,thedifferentresponsetoquarkandgluonjets,and thepileupsubtraction.The impactoftheb-taggingefficiency un-certaintyonthesignalstrength

μ

=

σ

t¯t H,obs

/

σ

t¯t H,SMatthebest-fit

valueof

μ

is



μ

=

₋+0₀._.08₀₆.Becauseonlyone(oftypicallytwo)b-jets

presentinsignalort¯t V eventsisrequiredtobetagged,the uncer-tainty ontheb-tagging efficiency(while included) doesnothave aslargean effectinthisanalysisasitdoesinother t¯t H searches

suchasthosetargetingthe H

→

bb decay.

¯

Theuncertaintiesontheinclusivet¯t H productioncrosssection are discussed in Section 3. Additionally, the effects of PDF un-certainty,QCD scale choice, andpartonshower algorithmon the signalacceptanceineachanalysiscategoryareconsidered.The re-sulting relative uncertainties on the acceptance are 0.3–1.4% for PDF,0.1–2.7% forscalechoice,and1.5–13% forpartonshower al-gorithm.

FormostbackgroundstheuncertaintiesfromMonteCarlo sim-ulationsample sizes are negligible. Forthe dibosonbackgrounds, however,thesecanreach50%ofthetotaldibosonyield uncertain-tiesshownin

Table 3

.

9. Results

Theobservedyields,andacomparisonwiththeexpected back-grounds andt¯t H signal, are shown in Table 3. The distributions of the number of jetsin the events passing signal region selec-tionsareshownin

Fig. 2

.Thebest-fitvalueofthesignalstrength

μ

=

σ

t¯t H,obs

/

σ

t¯t H,SM is determined using a maximum likelihood

fit to the data yields of the categories listed in Table 3, which aretreatedasindependentPoissontermsinthelikelihood.Thefit isbased onthe profile-likelihood approachwhere the systematic uncertaintiesaretreatedasnuisanceparameterswithprior uncer-taintiesthatcanbefurtherconstrainedbythefit

[95]

.The

μ

=

1 hypothesis assumesStandard Model Higgs boson productionand decaywithmH

=

125 GeV;forallothervaluesof

μ

onlythet¯t H productioncrosssectionisscaled(theHiggsbosonbranching frac-tionsarefixedtotheirSMvalues).

Systematicuncertaintiesare allowed to floatinthe fitas nui-sanceparametersandtake ontheir best-fitvalues.Theonly con-straintson nuisanceparameter uncertainties found by thefit are for non-prompt lepton transfer factors and normalization region yieldsinthe2



0

τ

hadcategoriesandthefake

τ

hadbackgroundyield

inthe 1



2

τ

hadcategory. Theformerallhavelargestatistical

com-ponentsandsotheadditionalinformationfromthesignal regions isexpected to constrain them. The latter hasa very large initial uncertaintywhichthe fitis ableto constrainas

μ

is requiredto bethesameinallcategories. Thelargestdifferencebetween pre-andpost-fitnuisanceparametervaluesisinthe1



2

τ

hadfake

esti-mate,whichshiftsby

−

1

.

0

σ

duetothedeficitofobservedrelative toexpectedevents.Thenextlargesteffectisa

+

0

.

4

σ

shiftinthe 2



0

τ

hadnon-prompt

μ

transferfactor.

The results of the fit are shownin Fig. 3. The impact of the mostimportantsystematicuncertaintiesonthemeasuredvalueof

μ

in thecombinedfit isshown inTable 4.Ineach category, the uncertaintieson

μ

are mainlystatistical,exceptforthecombined 2



0

τ

had result where the statistical and systematic uncertainties

Fig. 3. Best-fitvaluesofthesignalstrengthparameterμ=σtt H¯ ,obs/σt¯t H,SM.Forthe

4Z -depletedcategory,μ<−0.17 resultsinanegativeexpectedtotalyieldandso theloweruncertaintyistruncatedatthispoint.

Table 4

Leadingsourcesofsystematicuncertaintyandtheirimpactonthemeasuredvalue ofμ.

Source μ

20τhadnon-prompt muon transfer factor +0.38 −0.35

t¯t W acceptance +0.26 −0.21

t¯t H inclusive cross section +0.28 −0.15

Jet energy scale +0.24 −0.18

20τhadnon-prompt electron transfer factor +0.26 −0.16

t¯t H acceptance +0.22 −0.15

t¯t Z inclusive cross section +0.19 −0.17

t¯t W inclusive cross section +0.18 −0.15

Muon isolation efficiency +0.19 −0.14

Luminosity +0.18 −0.14

are of comparable size. In the 4



Z -depleted category, a (non-physical)signalstrength

μ

<

−

0

.

17 resultsinanegativeexpected totalyieldandadiscontinuityintheprofiledlikelihood;theerror baristhereforetruncatedatthispoint.Theresultsarecompatible withthe StandardModel expectationandwithprevious searches fort¯t H productioninmultileptonfinalstates

[18]

.Combinedover all categories, thevalue of

μ

isfound to be 2

.

1+₋1₁._.4₂.In the pres-enceof a signal of SM strength, the combinedfit is expectedto return

μ

=

1

.

0₋+₁1._.2₁. The

μ

=

0 hypothesis has an observed (ex-pected) p-valueof0.037(0.18),correspondingto1

.

8

σ

(0

.

9

σ

).The

μ

=

1 hypothesis (the SM)has an observed p-value of0.18, cor-respondingto0

.

9

σ

.Thelikelihoodfunctioncanbeusedtoobtain 95%confidencelevel(CL)upperlimitson

μ

usingtheCLs method

[95,96],leadingtotheresultsin

Table 5

.Theobserved(expected) upperlimit,combiningallchannels,is

μ

<

4

.

7 (2.4).

This analysisis a search fort¯t H production; assuch, produc-tion oft Hqb andt H W isconsidered asa backgroundandsetto StandardModelexpectation.Includingthiscontributionasa back-groundinduces a shift of



μ

= −

0

.

04 compared tosetting it to zero.AfullextractionoflimitsonthetopquarkYukawacoupling includingtherelevantmodificationsofsingletopplusHiggsboson productionisreportedinRef.[97].

Theresultsaresensitivetotheassumedcrosssectionsfort¯t W

andt¯t Z production,andusetheoretical predictionsforthese val-uesasexperimentalmeasurementsdonotyethavesufficient pre-cision. The best-fit

μ

value as a function of thesecross sections is

Table 5

Observedandexpected95%CLupperlimits,derivedusingtheCLsmethod,onthestrengthparameterμ=σt¯t H,obs/σtt H¯ ,SMforaHiggsbosonofmassmH=125 GeV.The lastcolumnshowsthemedianexpectedlimitinthepresenceofat¯t H signalofStandardModelstrength.

Channel Observed limit Expected limit

−2σ −1σ Median +1σ +2σ Median (μ=1) 20τhad 6.7 2.1 2.8 3.9 5.7 8.4 5.0 3 6.8 2.0 2.7 3.8 5.7 8.5 5.1 21τhad 7.5 4.5 6.1 8.4 13 21 10 4 18 8.0 11 15 23 39 17 12τhad 13 10 13 18 26 40 19 Combined 4.7 1.3 1.8 2.4 3.6 5.3 3.7

μ

(

tt H

¯

)

=

2

.

1

−

1

.

4



σ

(

t

¯

t W

)

232 fb

−

1



−

1

.

3



σ

(

t

¯

t Z

)

206 fb

−

1



.

10. Conclusions

A search for t¯t H production in multilepton final states has been performed using 20

.

3 fb−1 of proton–proton collision data at

√

s

=

8 TeV recordedbytheATLAS experimentattheLHC.The best-fitvalueoftheratio

μ

oftheobservedproductionratetothat predictedby theStandard Modelis 2

.

1+₋₁1_..4₂. Thisresultis consis-tentwiththeStandardModelexpectation. A95% confidencelevel limit of

μ

<

4

.

7 isset.The expected limitin the absenceoft¯t H

signalis

μ

<

2

.

4.Theobserved(expected)p-valueoftheno-signal hypothesiscorrespondsto1

.

8

σ

(0

.

9

σ

).

Acknowledgements

We thankCERN for the very successfuloperation of theLHC, aswell asthe support stafffrom ourinstitutions without whom ATLAScouldnotbeoperatedefficiently.

WeacknowledgethesupportofANPCyT,Argentina;YerPhI, Ar-menia;ARC,Australia;BMWFW andFWF,Austria;ANAS, Azerbai-jan;SSTC,Belarus; CNPqandFAPESP,Brazil;NSERC, NRCandCFI, Canada;CERN;CONICYT,Chile;CAS,MOSTandNSFC,China; COL-CIENCIAS,Colombia;MSMTCR,MPOCRandVSCCR,Czech Repub-lic; DNRF, DNSRC and Lundbeck Foundation, Denmark; EPLANET, ERC and NSRF, European Union; IN2P3-CNRS, CEA-DSM/IRFU, France;GNSF,Georgia;BMBF,DFG,HGF,MPGandAvHFoundation, Germany; GSRT and NSRF, Greece; RGC, Hong Kong SAR, China; ISF,MINERVA,GIF,I-COREandBenoziyoCenter,Israel;INFN,Italy; MEXTand JSPS,Japan; CNRST, Morocco; FOMandNWO, Nether-lands; BRF and RCN, Norway; MNiSW and NCN, Poland; GRICES andFCT,Portugal;MNE/IFA, Romania;MES ofRussiaandNRCKI, RussianFederation;JINR;MSTD,Serbia;MSSR,Slovakia;ARRSand MIZŠ, Slovenia; DST/NRF, South Africa; MINECO, Spain; SRC and Wallenberg Foundation, Sweden;SER, SNSF andCantons of Bern and Geneva, Switzerland; NSC, Taiwan; TAEK, Turkey; STFC, the Royal Society and Leverhulme Trust, United Kingdom; DOE and NSF,UnitedStatesofAmerica.

The crucial computingsupport fromall WLCG partners is ac-knowledgedgratefully,inparticularfromCERNandtheATLAS Tier-1 facilities at TRIUMF (Canada), NDGF (Denmark, Norway, Swe-den),CC-IN2P3(France),KIT/GridKA(Germany),INFN-CNAF(Italy), NL-T1(Netherlands),PIC(Spain),ASGC(Taiwan),RAL(UK)andBNL (USA)andintheTier-2facilitiesworldwide.

References

[1]S.L. Glashow, Partial symmetries of weak interactions, Nucl. Phys. 22 (1961)

579.

[2]S. Weinberg, A model of leptons, Phys. Rev. Lett. 19 (1967) 1264.

[3]A. Salam, Weak and electromagnetic interactions, in: N. Svartholm (Ed.),

El-ementary Particle Theory: Relativistic Groups and Analyticity. Proceedings of the Eighth Nobel Symposium, Almqvist & Wiksell, Stockholm, 1968, p. 367.

[4]F. Englert, R. Brout, Broken symmetry and the mass of gauge vector mesons,

Phys. Rev. Lett. 13 (1964) 321.

[5]P.W. Higgs, Broken symmetries, massless particles and gauge fields, Phys. Rev.

Lett. 12 (1964) 132.

[6]P.W. Higgs, Broken symmetries and the masses of gauge bosons, Phys. Rev. Lett.

13 (1964) 508.

[7]G. Guralnik, C.R. Hagen, T.W.B. Kibble, Global conservation laws and massless

particles, Phys. Rev. Lett. 13 (1964) 585.

[8]ATLAS Collaboration, Observation of a new particle in the search for the

Stan-dard Model Higgs boson with the ATLAS detector at the LHC, Phys. Lett. B 716 (2012) 1, arXiv:1207.7214 [hep-ex].

[9]CMS Collaboration, Observation of a new boson at a mass of 125 GeV with

the CMS experiment at the LHC, Phys. Lett. B 716 (2012) 30, arXiv:1207.7235 [hep-ex].

[10]ATLAS Collaboration, Measurement of Higgs boson production in the diphoton

decay channel in pp collisions

at center-of-mass energies of 7 and 8 TeV with

the ATLAS detector, Phys. Rev. D 90 (2014) 112015, arXiv:1408.7084 [hep-ex].

[11]CMS Collaboration, Observation of the diphoton decay of the Higgs boson and

measurement of its properties, Eur. Phys. J. C 74 (2014) 3076, arXiv:1407.0558 [hep-ex].

[12]ATLAS Collaboration, Measurements of Higgs boson production and couplings

in the four-lepton channel in pp collisions

at center-of-mass energies of 7 and

8 TeV with the ATLAS detector, Phys. Rev. D 91 (2015) 012006, arXiv:1408.5191 [hep-ex].

[13]CMS Collaboration, Measurement of the properties of a Higgs boson in the

four-lepton final state, Phys. Rev. D 89 (2014) 092007, arXiv:1312.5353 [hep-ex].

[14]ATLAS Collaboration, Observation and measurement of Higgs boson decays to

WW∗with the ATLAS detector, Phys. Rev. D 92 (2015) 012006, arXiv:1412.2641 [hep-ex].

[15]CMS Collaboration, Measurement of Higgs boson production and properties in

the WW decay

channel with leptonic final states, J. High Energy Phys. 1401

(2014) 096, arXiv:1312.1129 [hep-ex].

[16]ATLAS Collaboration, Evidence for the Higgs-boson Yukawa coupling to tau

leptons with the ATLAS detector, J. High Energy Phys. 1504 (2015) 117, arXiv:1501.04943 [hep-ex].

[17]CMS Collaboration, Evidence for the 125 GeV Higgs boson decaying to a pair

of τleptons, J. High Energy Phys. 1405 (2014) 104, arXiv:1401.5041 [hep-ex].

[18]CMS Collaboration, Search for the associated production of the Higgs boson

with a top-quark pair, J. High Energy Phys. 1409 (2014) 087, arXiv:1408.1682 [hep-ex], Erratum: J. High Energy Phys. 1410 (2014) 106.

[19] ATLASCollaboration,SearchforthestandardmodelHiggsbosonproducedin associationwithtopquarksanddecaying intobb in¯ pp collisionsat√s=

8 TeV withtheATLASdetector,Eur.Phys.J.C75(2015)349,http://dx.doi.org/ 10.1140/epjc/s10052-015-3543-1,arXiv:1503.05066[hep-ex].

[20]ATLAS Collaboration, Search for H→γ γ produced in association with top quarks and constraints on the Yukawa coupling between the top quark and the Higgs boson using data taken at 7 TeV and 8 TeV with the ATLAS detector, Phys. Lett. B 740 (2015) 222, arXiv:1409.3122 [hep-ex].

[21]ATLAS Collaboration, ATLAS experiment at the CERN Large Hadron Collider, J.

Instrum. 3 (2008) S08003.

[22]S. Dawson, C. Jackson, L. Orr, L. Reina, D. Wackeroth, Associated Higgs

produc-tion with top quarks at the Large Hadron Collider: NLO QCD correcproduc-tions, Phys. Rev. D 68 (2003) 034022, arXiv:hep-ph/0305087.

[23]L. Reina, S. Dawson, Next-to-leading order results for t¯th production

at

the Tevatron, Phys. Rev. Lett. 87 (2001) 201804, arXiv:hep-ph/0107101.

[24]W. Beenakker, et al., NLO QCD corrections to t¯t H production

in hadron

colli-sions, Nucl. Phys. B 653 (2003) 151, arXiv:hep-ph/0211352.

[25]W. Beenakker, et al., Higgs radiation off top quarks at the Tevatron and the

LHC, Phys. Rev. Lett. 87 (2001) 201805, arXiv:hep-ph/0107081.

[26]LHC Higgs Cross Section Working Group Collaboration, S. Dittmaier, et

al., Handbook of LHC Higgs cross sections: 1. Inclusive observables, arXiv: 1101.0593 [hep-ph].