Search for the standard model higgs boson in the h bb and h tt decay modes with the atlas detector
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Search for the Standard Model Higgs Boson in the H → bb and H → tt decay modes with the ATLAS detector. Farilla (INFN) On behalf of the ATLAS Collaboration Kruger 2012. H → bb and H → tt in ATLAS. Phys . Lett . B 716 (2012) . The Higgs particle has been observed for

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Search for the standard model higgs boson in the h bb and h tt decay modes with the atlas detector
Search for the Standard Model Higgs Boson in theH →bb and H →tt decay modes with the ATLAS detector

Farilla (INFN)

On behalfof the ATLAS Collaboration

Kruger 2012

A. Farilla - Kruger 2012


H bb and h tt in atlas
H → bb and H → ttin ATLAS

Phys. Lett. B 716 (2012)

  • The Higgsparticlehasbeenobserved for

    the first time at LHC (by the ATLAS and CMS

    experiments) throughitsdecaysinto

    bosonpairs (ZZ, gg, WW)

  • Butwhatabout the Higgsdecays

    intofermions?

  • H → bb and H → ttdecays : important to

    establish the properties of the HiggsBoson

    in the fermionicsector!

  • This talk:

    ATLAS results for 4.7 fb-1√s = 7 TeV (2011) & 13 fb-1√s = 8 TeV (2012)

  • ATLAS publishedresultsalreadyavailable for 4.7 fb-1√s = 7 TeV (2011) :

  • http://arxiv.org/abs/1207.0210 (H →bb)

  • http://link.springer.com/article/10.1007%2FJHEP09%282012%29070 (H →tt)

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Higgsdecayto b-quark pair

  • High branching ratio

  • FormH = 125 GeV BR(H → bb) ~ 58%

  • The searchfor H → bbisperformed in associated

  • production with W/Z bosontoclean the sample from

  • the hugemultijet background and have a clean

  • signature for triggering

  • The multijet background isreduced at the price of a

  • lowereventyield: the cross section of VH associated

  • production is ~10 timessmallerthan the mainHiggs

  • production mode (gluon-gluon fusion)

A. Farilla - Kruger 2012


Event display
Event Display

Higgsboson candidate withoneselectedmuon

A. Farilla - Kruger 2012


Search analysis s trategy
Search/AnalysisStrategy

  • SearchforHiggstob-pair in

    associated production with a Z or

    W±boson:

  • Better background rejection, goodsignaturefortriggering

  • The analysisisdividedintothreechannels (based on leptonmultiplicity):

  • Two (llbb), one (lnbb) or zero (nnbb) chargedlepton (l=e or m)

  • Cuts common toallchannels:

  • Two or threejets : first jet pT> 45 GeV, otherjetspT > 20 GeV

  • Twob-tags: 70% efficiency per tag (mistag ~1%)

  • ZH →llbb

  • Twoleptons

  • ETmiss < 60 GeV

  • 83 < mZ < 99 GeV

  • Single & di-lepton trigger

  • WH →lnbb

  • Onelepton

  • ETmiss > 25 GeV

  • 40 < mTW < 120 GeV

  • Single lepton trigger

  • ZH →nnbb

  • No leptons

  • ETmiss >120 GeV

  • ETmisstrigger

A. Farilla - Kruger 2012


Analysis overview
AnalysisOverview

  • Signalover Background ratioincreaseswithpTbb

  • Toenhance the sensitivity, the analysisiscategorisedas a functionofpTV

  • 16 signalcategories:

  • 0-lepton: ETmiss [120-160] [160-200] [>200] GeV (2 jets or 3 jets)

  • 1 & 2 leptons: pTW/Z [0-50], [50-100], [100-150], [150-200], [>200] GeV

  • Recentimportantimprovements:

  • Cuts are optimizedforeachcategory (~30% increase in sensitivity)

  • Muonenergy (pT>4 GeV) addedforb-jets (~10% increase in resolution)

  • Additionalttbarbasedb-taggingcalibration (~50% reduction in systematics)

A. Farilla - Kruger 2012


Background and mc
Background and MC

  • Background shapesfrom

  • simulation and normalised

  • usingflavour and data fit

  • Multijetbkgdeterminedby

  • data-driventecniques

  • WZ/ZZ (with Z→bb) bkg

  • normalisation and shapefrom

  • simulation

  • Signal WH/ZH Pythia8

  • Multijet Data Driven

  • Diboson WW/WZ/ZZ Herwig

  • ttbarMC@NLO

  • Single top Acer/MC@NLO

  • W+bPowheg

  • W+c/light jetsAlpgen

  • Z+b/c/light jetsAlpgen/Sherpa

1 lepton

0 lepton

2 leptons

Z+jets

Top/W+jets

Z+jets/W+jets/Top

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Background and flavoursfit

Eventswith 0,1 and 2 b-jetshavedifferentflavourcontribution: thisallows

tofit the normalisationof the severalcomponents

  • Flavourmaximumlikelihoodfittosignal and controlregions:

    • DetermineV+jets (b/c/light) and top scale factors

    • One scale factorappliedforeachbkg

    • Scale factors are consistentfor the 7 TeV and 8 TeV data samples

      (Z+cfactorchanges due todifferent MC treatment)

  • Almostall SF ~1 : the MC simulationisperformingverywell !

  • Improvedunderstandingof background in recentanalysis:

    • Analyzing the 8 TeV data wediscoveredthat the V pTspectrumfalls

      more rapidly in data thanexpectedfrom MC

    • W + jets and Z + jets: 5-10% correctionrequired

    • Top bkg: 15% correctionrequired

A. Farilla - Kruger 2012


Examples of m bb distributions
Examples of mbbdistributions

2 leptons( 50 < pTZ< 100 GeV)

  • In this slide: threeexamples of finalmbb in specificbins of

  • pTZ , pTW , Etmiss

  • (more plots in backup slides)

  • Full mbbdistributionisused in the limitsettingprocedure

0 lepton ( 120 < ETmiss< 160 GeV )

1 lepton( pTW > 200 GeV)

A. Farilla - Kruger 2012


Diboson production
Diboson Production

  • WZ & ZZ production with Z →bb:

    • Signaturesimilarto VH with H →bbbut cross section 5 timeslarger

  • Perform a separate fittosearchdibosonevents and to validate the analysis:

    • Profilelikelihoodfitperformed (with full systematics)

    • Allbackgrounds (exceptdiboson) subtracted

    • Allleptonchannelscombined, no separation in pTVbins

  • Dibosonpeakclearlyvisible

    in the data

  • Result in agreement with SM:

    • s/sSM = mD= 1.09 ± 0.20 (stat) ± 0.22 (sys)

    • Significance = 4.0 s

A. Farilla - Kruger 2012


Systematics uncertainties
SystematicsUncertainties

Background systematics

  • Mainexperimentaluncertainties:

    • Jets: components (7 JES, 1pTReco, resol.)

    • ETmiss-scale and resolutionof soft

      components. Data/MC forETmiss trigger

    • bTagging-light, c & 6 pTbinsfor b-jet efficiency

    • Lepton – energy, resolution, efficiency

    • Multijet / Diboson / Luminosity /MC stat

  • Maintheoreticaluncertainties:

    • W/Z+jetmbb (20%) and V pT (5-10%)

    • Single top/topnormalisation

    • W+c/W+jets (30%), Z+c/z+jets (30%)

    • Diboson (11%)

    • BR(H →bb) @mH=125 GeV

Signalsystematics

A. Farilla - Kruger 2012


Results
Results

2011 & 2012

2011

2012

  • Combinedresults (2011&2012):

    • Use standard CLsprofilingmethod

    • Fit full mbbdistribution

    • Observed (expected) limit at mH = 125 GeV:

    • 1.8 (1.9) x SM prediction

    • Observed (expected) p0 value: 0.64 (0.15)

    • s/sSM = m = -0.4 ±0.7 (stat) ± 0.8 (syst)

    • No significantexcessobserved

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Higgsdecaytot-leptonpair

  • H →ttisoneof the leadingdecaymodes

  • formH = 125 GeV

  • Veryimportanttoestablish the couplingsto the

  • fermions

  • Three Higgs production processes are considered

  • in thissearch: ggF, VBF, VH

  • VBF has the highestsensitivitythankstoits

  • signature (2 high-pTjetswith high Dhjjand high mjj)

Weselectonly

hadronic

decaysof V

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Event display (VBF category)

H → tlepthad, with t→ mnn

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

AnalysisCategories (8 TeV)

  • Analysisdividedintochannelsbased on tdecays

  • In eachchannel the searchisoptimizedseparately in a set of(mutually

  • exclusive) analysiscategories, whichdifferbynumberofjets (0,1 or 2 jets) and

  • kinematicfeatures

  • Categoriesfor 8 TeV data:

    • H →t lept lep(BR ~12.4%)2-jets VBF , Boosted, 2-jet VH, 1-jet

    • H →t lept had(BR ~ 45.6%) 2-jets VBF, Boosted, 1-jet, 0-jet

    • H →t hadt had(BR ~ 42%) 2-jets VBF, Boosted

  • Quitesimilarcategoriesfor 7 TeV data

  • (more details in the backup slides)

  • In eachchannel and category the finaldiscriminant (usedforlimitsetting ) is the invariant mass of the t-pairmtt

  • mttreconstructedwith the Missing Mass Calculatorusingmeasuredmomenta, ETmiss and the simulateddistributionof the opening angle betweenvisible and missingmomenta

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backgroundestimation

  • The main background forallanalysiscategoriesis due to Z → ttprocess

  • Data drivenestimationof Z → tt :

    • Use Z → mm data and replace the m’s

    • witht’s from MC simulation (t-embedded)

    • Allotherobjects (jets, underlyingevent, etc)

    • are obtainedfrom data

    • Good agreement between data and MC

  • For VBF lep-hadcategory the estimation

  • ofZ → ttisperformedwith high statistic MC

  • sampleswithjet-selectioncuts at generator

  • level (VBF-filter), plus correction and

  • normalizationfrom data

  • Other background sources:

    • Z (→ee/mm) + jets, Top, di-boson

    • (estimatedfrom MC + correctionsfrom data)

    • Multijet and W+jets (data drivenestimation)

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Resultsfor VBF: mtt

  • VBF categoryhas the highestsensitivity

  • Limitedstatisticsbutgood S/B ratio

  • In this slide: resultsfor 8 TeV data (more plots in the backup slides)

lep-lep

lep-had

had-had

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

  • Boostedcategoryhas the best sensitivityamongnon-VBFcategories

  • In this slide: resultsfor 8 TeV data (more plots in the backup slide)

lep-lep

lep-had

had-had

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Systematicsuncertainties

  • Mainsystematicuncertaintiesfor Z→tt Background and the Signal

  • Dominantsystematic : Embedding, t Energy Scale, Jet Energy Scale

Uncertaintieswith (S) are alsoappliedbin-by-bin (affect the shapeof the finaldistribution)

A. Farilla - Kruger 2012


Results1
Results

2011

2012

2011 & 2012

  • Combinedresults (2011&2012):

    • Use standard CLsprofilingmethod

    • Fit full mttdistribution

    • Observed (expected) limit at mH = 125 GeV:

    • 1.9 (1.2) x SM prediction

    • s/sSM = m = 0.7 ± 0.7

    • No significantexcessobserved

A. Farilla - Kruger 2012


Results2
Results

  • Contour plot obtainedintroducing, in the likelihood

  • function, signalstrenghtparametersmi = si / s i,SM

  • foreach production mode ggF, VBF, VH

  • (mVBF and mVH are combinedsincetheybothdepend on the VVH coupling)

  • Notenoughsensitivityto probe SM Higgsagainst

  • the background onlyprediction, butprovidesgood

  • constraint in VBF and ggFplane

Observed (expected) p0 value:

1.1s (1.7s) formH = 125 GeV

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Conclusions

  • UpdatedATLAS resultson H →bb and H →tt are nowavailable

  • for 4.7 fb-1at√s = 7 TeV(in 2011) and 13 fb-1at√s = 8 TeV(in 2012)

  • For H →bb @ mH = 125 GeV:

  • m= -0.4 ±0.7 (stat) ± 0.8 (syst)

  • Observed (expected) p0 value: 0.64 (0.15)

  • For H →tt@ mH = 125 GeV:

  • m = 0.7 ± 0.7

  • Observed (expected) p0 value: 0.135 (0.047)

  • No excess observed yet for H →bb and H →tt

  • Including the new data sample of ~10 fb-1at √s = 8 TeV

  • collectedby ATLAS in the last weeks couldprovide new insightsinto

  • thesetwoextremelyinterestingsHiggsdecays

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup Slides

A. Farilla - Kruger 2012


Backup
Backup

H→bb: Expected and Observedevents

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup

H →bb : Selectioncuts

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup

B-tagging

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup

H→bb: FlavourFitResults

1-tag controlregion

Preb-tag : sum of 0-tag, 1-tag

controlregion and 2-tag signalregion

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup

H → bb: mbbdistributionfor the sixcategoriesof zero leptonanalysis

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup

H→ bb: mbbdistributionfor the fivecategoriesofoneleptonanalysis

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup

H→ bb: mbbdistributionfor the fivecategoriesof2leptonsanalysis

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup

thadidentification

  • Hadronicdecaysoft leptons: one or threechargedhadrons plus a neutrino

  • and possiblyneutralhadronsgiving a collimated jet withfewassociatedtracks

  • The visibledecayproducts are combined in thadcandidates

  • Jetsforthadcandidates are re-calibratedto account for the differentcalorimeter

  • responsetohadronict-decaysw.r.t.hadronicjets

  • Multivariate analysisbased on BoostedDecisionTree (BDT) isperformedtoreject

  • mis-identifiedthadjets (tracking and calorimeter information isusedby BDT)

  • Loose, medium and tight thadcandidates are foundwithefficienciesof

  • 60%, 50% and 30% respectively and jet mis-identificationprobability < 1%

  • Selectionofthadcandidates: | h | < 2.5, pT > 20 GeV, one or threeassociatedtracks

  • (withpT > 1 GeV) withinDR < 0.2 around the thad direction and a total chargeof ±1

  • computedfrom the associatedtracks.

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup

H→tt : mttreconstructionwithMissing Mass Calculator (MMC)

  • MMC is a more sophisticated versionof the collinearapproximation

  • Mainimprovement : requirethat the relative orientationsof the n’s and the other

  • decayproducts are consistentwith mass and kinematicsof a tdecay

  • Thisisachievedbymaximasing a probabilitydefined in the kinematically

  • allowedphasespaceregion

  • Thisresults in very high reconstructionefficiency (>99%) and 13-20% resolution

  • in mtt, depending on eventtopology and final state

  • Collinearapproximation (usedforlep-lep 7 TeV data with at least 1 jet) :

  • for high pTt’s we can assume that the n direction isalong the visible

  • decayproducts

  • Forlep-lep data at 7 TeVwith 0-jets the effective mass mtteff (the

  • invariant mass of the di-lepton and ETmiss system) isusedsince the performance

  • of the collinearapproximationisnotoptimalwhen the t-decayproducts are

  • back-to-back in the transverseplane

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup

  • H →tt : EventSelectionforlep-hadchannel

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

H →tt : ExamplesofEventYieldfortmthadchannel

7 TeV

8 TeV

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup

  • H →tt : EventSelectionforhad-hadchannel

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup

  • Backgroudestimationfor H →tt (lep-lepchannel):

  • Top contributionestimatedfrom MC and correctedusing data in

  • top-enrichedControlRegion (CR)

  • Fakeleptoncontributionobtainedfrom data with a templatemethodusing a CR in

  • whichleptonisolationrequirementisreversed (templatefrompTdistribution

  • of the sub-leadinglepton)

  • Dibosoncontributionestimatedfrom MC and validated in a 3-lepton CR enrichedwith WZ events

  • Backgroudestimationfor H →tt (had-hadchannel):

  • Data drivenestimationof the dominantbck (Z →tt and multijet) both

  • fornormalisation and shape

  • Two-dimensionaltemplatefittotrackmultiplicitydistributions

  • Templatemodelledby the simulationfor Z →tt and fromsame-sign (SS) data formultijet

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup

  • Backgroudestimationfor H →tt (lep-hadchannel):

  • Bckgestimation (except VBF category) based on assumptionthatformultijet the shapeofmtt in signalregionis the sameforopposite-sign (OS) and same-sign (SS) eventsafterallkinematiccuts (exceptchargerequirement)

  • The numberof SS events and the shapeofmttderivedfrom CR obtainedbyinverting the OS requirement

  • SS sample dominatedbymultijetbutcontaincontributionfromotherbckg: forthesebck OS-SS terms are requiredto cover anyexcessof OS componentsover SS components

  • W+jetsbckgnormalisedto data usingbckg-enriched CR

  • For VBF categoryW+jet and multijetbckgestimatedusing a FakeFactor F = Nid / N anti-id

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup

H→tt: Resultsfor VBF

  • VBF categoryhas the highestsensitivity

  • Limitedstatisticsbutgood S/B ratio

  • In this slide: resultsfor 7 TeV data

lep-lep

lep-had

had-had

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

H →tt: ResultsforBoosted

  • Boostedcategoryhas the best sensitivityamongnon-VBFcategories

  • In this slide: resultsfor 7 TeV data

lep-lep

lep-had

had-had

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup

Jet energy resolution for samples with different values of <μ>, the mean number of pile-up interactions per bunch crossing.

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

Backup

The improvement in the turn-on curve forsimulatedpp → ZH → nn̄bb (usingPythia and mH = 120 GeV) usingeither the lowestunprescaled trigger chain in 2011 (L1 XE50 → L2 xe55 noM → xe60 verytightnoMu) or in 2012 (L1 XE40 BGRP7 → L2 xe45T → xe80T tclcwloose) are shown.

A. Farilla - Kruger 2012


Farilla infn on behalf of the atlas collaboration kruger 2012

The ATLAS detector components

The Magnet system:

Four Superconducting magnets:

1 Central Solenoid

3 Air core Toroids

The Inner Detector

EM and hadronic

Calorimeters

The Muon Spectrometer

Mauro Iodice - PRAGA 09 - date …, 2009


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