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Farilla (INFN) On behalf of the ATLAS Collaboration Kruger 2012

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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Farilla (INFN) On behalf of the ATLAS Collaboration Kruger 2012

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  1. 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

  2. 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

  3. 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

  4. Event Display Higgsboson candidate withoneselectedmuon A. Farilla - Kruger 2012

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. Event display (VBF category) H → tlepthad, with t→ mnn A. Farilla - Kruger 2012

  15. 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

  16. 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

  17. 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

  18. ResultsforBoosted: mtt • 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

  19. 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

  20. 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

  21. 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

  22. 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

  23. Backup Slides A. Farilla - Kruger 2012

  24. Backup H→bb: Expected and Observedevents A. Farilla - Kruger 2012

  25. Backup H →bb : Selectioncuts A. Farilla - Kruger 2012

  26. Backup B-tagging A. Farilla - Kruger 2012

  27. Backup H→bb: FlavourFitResults 1-tag controlregion Preb-tag : sum of 0-tag, 1-tag controlregion and 2-tag signalregion A. Farilla - Kruger 2012

  28. Backup H → bb: mbbdistributionfor the sixcategoriesof zero leptonanalysis A. Farilla - Kruger 2012

  29. Backup H→ bb: mbbdistributionfor the fivecategoriesofoneleptonanalysis A. Farilla - Kruger 2012

  30. Backup H→ bb: mbbdistributionfor the fivecategoriesof2leptonsanalysis A. Farilla - Kruger 2012

  31. 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

  32. 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

  33. Backup • H →tt : EventSelectionforlep-hadchannel A. Farilla - Kruger 2012

  34. Backup H →tt : ExamplesofEventYieldfortmthadchannel 7 TeV 8 TeV A. Farilla - Kruger 2012

  35. Backup • H →tt : EventSelectionforhad-hadchannel A. Farilla - Kruger 2012

  36. 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

  37. 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

  38. 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

  39. Backup 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

  40. 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

  41. 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

  42. 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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