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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−1ofproton–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 orμ)
withoutahadronically decayingτ
lepton; threelightleptons;twosame-charge lightleptons withahadronically decayingτ
lepton; fourlightleptons;and one lightleptonandtwo hadronically decayingτ
leptons. No significant excess of events is observed above the background expectation.Thebestfitforthet
¯t H production crosssection,assumingaHiggsbosonmassof125 GeV, is2.1+−1.41.2timestheSMexpectation,andtheobserved(expected)upperlimitatthe95%confidencelevel is4.7(2.4)timestheSMrate.Thep-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 reportedforH
→
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
) withnoadditionalhadronicallydecayingτ
lepton;threelight lep-tons;twosame-sign lightleptons withonehadronically decayingτ
lepton;fourlightleptons;andonelightleptonwithtwo hadron-ically decayingτ
candidates. Thesechannels are sensitive to the Higgs decays H→
W W∗,τ τ
, and Z Z∗ produced in associationE-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 denotedt¯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.Chargedparticlesinthepseudorapidity1range
|
η
| <
2.
5 arereconstructedwiththeinnertrackingdetector,which is immersedin a2 T magnetic field parallel to the detectoraxis1 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 ofthepolarangleθ
asη= −ln tanθ/2.ThetransversemomentumisdefinedaspT=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, yieldingσ
(
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 isσ
(
t H W)
=
4.
7+−00..43 (scale)+0.8
−0.6(PDF)fb.Theinterferenceoft H W productionwithtt H ,
¯
whichappearsat 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 thevalueis206
±
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) andrenormaliza-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-shellγ
∗/
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 Powhegdescriptionsof 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 appliedbased 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 v2.14[65] leading order(LO) generator.Samples are merged withmatrixelement-partonshoweroverlapsremoved usingMLMTable 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 Tauola1.20[74]for
τ
decays.The Herwig++ samplesmodel pho-ton radiation with Photos but use the internalτ
decay model. Samplesusing Pythia 8.1and Sherpa usethosegenerators’internalτ
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.
52be-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(EconeT ) isbasedonthe sumof transverse energies ofcalorimeter cells within a cone of radius
R
≡
(φ)
2+ (
η
)
2=
0.
2 around the electroncandi-datedirection. Thisenergysumexcludescellsassociatedwiththe
electron and is corrected for leakage from the electromagnetic shower and ambient energy in the event. The second (pconeT ) is definedbasedontrackswith pT
>
1 GeV withinacone ofradiusR
=
0.
2 aroundthe electron candidate. Both isolation energies are separately required to be lessthan 0.
05×
ET. Thelongitudi-nal impact parameter of the electron track with respect to the selectedeventprimaryvertex, multipliedby thesineofthepolar angle,
|
z0sinθ
|
,isrequired tobe lessthan1 mm. The transverseimpactparameterdividedbytheestimateduncertaintyonits mea-surement,
|
d0|/
σ
(
d0)
,mustbe lessthan4.Iftwoelectrons closerthan
R
=
0.
1 are selected, only the one with the higher pT isconsidered. 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 GeVare kept. Additionally, muonsare required to be separated by at least
R
>
0.
04+ (
10 GeV)/
pT,μ fromanyselectedjets(seebe-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|
z0sinθ
|
isrequiredtobelessthan1mm,while
|
d0|/
σ
(
d0)
mustbelessthan3.Hadronicallydecaying
τ
candidates(τ
had)arereconstructedus-ingclustersintheelectromagneticandhadroniccalorimeters.The
τ
candidates are required to have pT greater than 25 GeV and|
η
|
<
2.
47. The number of charged tracks associated with theτ
candidatesisrequiredtobeoneorthreeandthecharge ofthe
τ
candidates, determined from the associated tracks, must be
±
1. Theτ
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 hadronicallydecayingτ
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 andη
-dependent calibration factors, derived from simulations, to the meanenergyofstableparticles insidethejets. Additionalcorrec-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, anyjetswithinR
=
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 insimu-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∗ andτ τ
decay modes.Consideringthe decayofthett system¯
aswell, theset¯t HeventscontaineitherW 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 leastonelightleptonisrequiredtomatchaleptonselectedbythe 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 leptonswithnoτ
had(
20
τ
had)
,threelightleptons(3),two
same-signlight leptons andone
τ
had (21
τ
had), four light leptons (4),
andone lightlepton andtwo
τ
had (12
τ
had). The categorieswithτ
had candidates targetthe H→
τ τ
decay;the othersareprimar-ily sensitive to H
→
W W∗ witha very small contribution fromH
→
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 Modelt¯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
hadcategoriesSelected events are required to includeexactly two light lep-tons, which must have the same charge. Events with
τ
hadcan-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 EconeT/
pT<
0.
05 and pTcone/
pT<
0.
05.The angularacceptanceof 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 Wbackgrounds,eventsmustincludeatleastfourreconstructedjets. In order to suppress diboson and single-boson backgrounds, at leastone ofthesejetsmust beb-tagged.The selectedeventsare separatedby lepton flavour (e±e±,e±
μ
±,andμ
±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 isappliedtothetwoleptonswiththesamecharge.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
hadcategorySelected events are required to include exactly two light leptons, with the same charge and leading (subleading) pT
>
25
(
15)
GeV, and exactly one hadronicτ
candidate. The recon-structedchargeoftheτ
had candidatehastobeoppositetothatofthelightleptons.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 massFig. 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 numberof
τ
hadcandidates.Inordertosuppressthett¯
+
jets andt¯t Vback-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→
4and 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
hadcategorySelectedeventsarerequiredtoincludeexactlyonelightlepton withpT
>
25 GeV andexactlytwohadronicτ
candidates.Theτ
hadcandidates 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 theτ
hadτ
hadsystem(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 12
τ
had category the primaryconcernisfake
τ
hadcandidates,whicharemodelledusingsimula-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 themajortt V contribution
¯
inthe 20
τ
had categoriesandcomparableto t¯t Z in the 2
1
τ
had category. Thet¯t Z process hassimilarmul-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
hadcategories.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 20
τ
hadselectionexceptwithtwoormoreb-taggedjetsandeithertwoor threejets,wherethet¯t W purityis
≈
30%,andarefoundtobe con-sistent within uncertainties. Thespectra ofthenumberof jetsin thesevalidationregionsare showninFig. 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+
band 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 analogousprocessesμ
±→
μ
±e+e− andμ
±→
μ
±μ
+μ
−havenegligible ratesforthe selected events.The t¯t and Z/
γ
∗→
+−
+
jets events thatun-dergo this process contribute to 2
0
τ
had in the ee and eμ
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 electronη
and pT. Thisprobability functionisthenappliedtoasample ofeventspassing the2
0
τ
hadselectionexceptthattheleptonpairisrequiredtobeoppositesign.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
hadcat-egoryoradileptont¯t eventtothe3
or2
1
τ
hadcategories.Thesebackgrounds 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) controlregions 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
hadcategoriesThe 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,butotherwiseareselectedthesamewayasnominalmuons.Transferfactorsare used toextrapolatefromeventswithonetight andonesideband
lepton,but which otherwise passthe signal region selections, to thesignalregionswithtwotightleptons.Thesetransferfactorsare determinedfromadditionaldatacontrolregions(tight+sideband andtwotightleptons)withlowerjetmultiplicity (1
≤
njet≤
3 forelectrons,2
≤
njet≤
3 formuons).Inallregionstheexpectedcon-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,agreeswellwiththenominalprocedureandprovidesadditionalconfidenceintheestimate.
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 thresholdwerestudied.Thesechecksshowstabilityatthe25–30%level, lim-itedbythestatisticalprecisionofthesimulations.Thestabilityin datais checked by alteringthe pT required for theb-tagged jet,
applyingarequirementon missingtransversemomentum3 EmissT , 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
hadcategoryReconstructing twosame-signlight leptonsfromt¯t production
or similar sources requires that one of the light leptons is non-promptorhasitschargemisidentified.Inthe2
1
τ
hadcategory,thecharge mis-id contributionis negligible andtheprimary concern is non-prompt light leptons. Around half of the
τ
had candidatesin these events come from W
→
τ ν
decays, while the remain-der arise from misidentified light-quarkor gluon jets. Regardless ofwhethertheτ
hadcandidateisafake,thereisalsoanon-promptlight lepton.Due tothis fact,sidebands inthe light-lepton selec-tioncriteriaareused,analogouslytothe2
0
τ
hadand3categories.
Sincetheratioofrealandfake
τ
hadcandidatesissimilarinthesig-nalandallcontrolregions,fake
τ
had candidatesarenotaccountedforseparately; 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 tode-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 eventsintheZ -enriched sample and
10−4 events inthe Z -depletedsample.Inbothcasesthisrepresents
2% ofthetotalbackground expecta-tion.Theseestimatesareobtainedusingthetransfer factorsfrom the3channelandappropriatecontrolregionswithtwoloose lep-tonsandrelaxedjetmultiplicityrequirements.
7.5.
τ
hadmisidentificationinthe12
τ
hadcategoryThe nominal estimate for the fake
τ
had yield is derived fromt¯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 insigntothelightlepton (OS) andthe other hasthe same sign(SS). The SS candi-dateisalmostalwaysafake
τ
had,whilethelightleptonispromptandthe OS
τ
had candidateis oftenreal (≈
30%). A sidebandτ
hadisdefinedasacandidatepassingalooseidentificationBDT selec-tion butnot thenominaltight one. Assuming the
τ
had candidatefakeprobabilitiesare notcorrelatedbetweenjetsidentifiedasOS and SScandidates, control regions can be used to predict yields inthesignalregion.Therearethreecontrolregions,dependingon whetheronlytheOS,onlytheSS,orboththeOSandSS
τ
hadcan-didates are sidebandobjects. The two regions with sideband OS
τ
had candidates are used to obtain the transfer factor for the SSτ
had candidate,which is then applied to theregion witha tightOSandsidebandSScandidatetoobtainthepredictionforthe 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-tagdiscrim-inant value of the SS
τ
had candidate. The data-driven method iscross-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-fitvalueof
μ
isμ
=
−+00..0806.Becauseonlyone(oftypicallytwo)b-jetspresentinsignalort¯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-fitvalueofthesignalstrengthμ
=
σ
t¯t H,obs/
σ
t¯t H,SM is determined using a maximum likelihoodfit 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]
.Theμ
=
1 hypothesis assumesStandard Model Higgs boson productionand decaywithmH=
125 GeV;forallothervaluesofμ
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
hadcategoriesandthefakeτ
hadbackgroundyieldinthe 1
2
τ
hadcategory. Theformerallhavelargestatisticalcom-ponentsandsotheadditionalinformationfromthesignal regions isexpected to constrain them. The latter hasa very large initial uncertaintywhichthe fitis ableto constrainas
μ
is requiredto bethesameinallcategories. Thelargestdifferencebetween pre-andpost-fitnuisanceparametervaluesisinthe12
τ
hadfakeesti-mate,whichshiftsby
−
1.
0σ
duetothedeficitofobservedrelative toexpectedevents.Thenextlargesteffectisa+
0.
4σ
shiftinthe 20
τ
hadnon-promptμ
transferfactor.The results of the fit are shownin Fig. 3. The impact of the mostimportantsystematicuncertaintiesonthemeasuredvalueof
μ
in thecombinedfit isshown inTable 4.Ineach category, the uncertaintiesonμ
are mainlystatistical,exceptforthecombined 20
τ
had result where the statistical and systematic uncertaintiesFig. 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 ofμ
isfound to be 2.
1+−11..42.In the pres-enceof a signal of SM strength, the combinedfit is expectedto returnμ
=
1.
0−+11..21. Theμ
=
0 hypothesis has an observed (ex-pected) p-valueof0.037(0.18),correspondingto1.
8σ
(0.
9σ
).Theμ
=
1 hypothesis (the SM)has an observed p-value of0.18, cor-respondingto0.
9σ
.Thelikelihoodfunctioncanbeusedtoobtain 95%confidencelevel(CL)upperlimitsonμ
usingtheCLs method[95,96],leadingtotheresultsin
Table 5
.Theobserved(expected) upperlimit,combiningallchannels,isμ
<
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 isTable 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.
4σ
(
t¯
t W)
232 fb−
1−
1.
3σ
(
t¯
t Z)
206 fb−
1.
10. ConclusionsA 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-fitvalueoftheratioμ
oftheobservedproductionratetothat predictedby theStandard Modelis 2.
1+−11..42. Thisresultis consis-tentwiththeStandardModelexpectation. A95% confidencelevel limit ofμ
<
4.
7 isset.The expected limitin the absenceoft¯t Hsignalis
μ
<
2.
4.Theobserved(expected)p-valueoftheno-signal hypothesiscorrespondsto1.
8σ
(0.
9σ
).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.
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