Higgs Physics in ATLAS

Higgs Physics in ATLASBruce MelladoUniversity of Wisconsin

US ATLAS Software MeetingBNL, August 27 2003

Bruce Mellado, US ATLAS Software Meeting , BNL 08/27/03

OutlineIntroductionWhy is the Higgs so important?

SM HiggsStatus of feasibility studiesProduction modes at LHCImpact of Vector Boson Fusion (VBF)Experimental issues

MSSM HiggsStatus of feasibility studies


SM Higgs (1)Standard Model:Huge success in describing the world of elementary particlesMany properties of quarks, leptons and forces carriers well establishedAgent of spontaneous electro-weak symmetry breaking, the Higgs, not observed yet!


SM Higgs (2)Direct searches at LEPSome excess of events consistent with MH=115 GeVPlaced 95% CL exclusion limit at MH=114 GeVGlobal fit to dataElectro-weak parameters sensitive to Log(MH)MH=91+58-37 GeVMH

SM Higgs (3)Higgs mass sensitive to top mass:New D0 and preliminary CDF RunII measurementsMT=179.4 GeVNew EW fit:MH~130 GeVMH

LHCLHC, a p-p collider

Current schedule:Early 2007 (pilot run)10 fb-1 by end of 2007Starting 2008:30 fb-1 first yearThen 100 fb-1 per year

Center of mass E14 TeVDesign Luminosity1034 cm-2 s-1Luminosity Lifetime10 hBunch spacing25 ns


Cross-sections at LHCSearch for Higgs and new physics hindered by huge background ratesKnown SM particles produced much more copiouslyThis makes low mass Higgs specially challenging. Need to rely onNarrow resonancesComplex signaturesHiggs in association with tops,W,Z and jets.


SM Higgs at the LHC (1)Production:Directgg HDominantLarge background for low massesVector Boson Fusion (VBF) via qq qqHSecond largest cross-sectionDistinct final stateAssociatedttH, WH, ZHInteresting final stateSmall cross-section


SM Higgs at the LHC (2)VBFggH


SM Higgs at the LHC (3)Main discovery modesMH2MZ:H ZZ 4lqqH qqZZ qqll qqH qqZZ qqlljjqqH qqWW qqljjStandard analyses:Higgs with forward jets (VBF) looked at only for heavy Higgs


SM Higgs at the LHC (4)Sensitivity All region excluded @ 95% CL after 1 month Fast discovery for MH>150 GeVDiscovery with 10-15 fb-1 for MH

SM Higgs via ttHComplex channel (Hbb):One lepton (trigger)4 b-jets + 2 jetsb-tagging essentialNeed to know well background (ttjj)Good for discovery and Yukawa coupling determination80 < MH < 120 GeVATLAS+CMS30 fb-1CMS study:Use likelihood for t decays and kinematics


SM Higgs, H Need good calorimetryAchieve ~1% resolution in Higgs mass reconstructionNeed good rejection against jetsBackgroundNon-resonant productionEasily determined from side bandsGood for discovery of low mass Higgs100 < MH

SM Higgs, H->ZZ->4lVery clean signatureNarrow resonanceSmall background contributionMain experimental issuesLepton isolationZbb and ttbb rejectionGood for discovery in wide Higgs mass range120 < MH < 600 GeV


Low Mass Higgs via VBF (1)Wisconsin Pheno (D.Rainwater, D.Zeppenfeld et al.):Two high PT jets with large Dh separationStrong discovery potential for low Higgs mass Can measure Higgs couplingsInvisible decaysHadron level analyses very promising:CMS & ATLAS seriously looking into VBF modes


Low Mass Higgs via VBF (2)H->WW*->ll,lqq. Strong for MH>120 GeVMain background:tt EW WWjjW + 4 jetsH->->ll,lh (+ptmiss). Good around LEP limitMain backgroundQCD and EW Zjj

H->bb. Useful for Yukawa coupling measurementH->Main background real and fake non-resonant


VBF H->WW* (1)Background: tt, EW WWjj Understanding of tt production is crucial

Background suppression:Well separated forward jets + central jet vetob-jet vetoLepton angular correlationsMH=160 GeVe


VBF H->WW* (2)Evidence of Spin-0 resonance in H->WW->ll modesLook into difference in between leptons MT>175 GeVMT

VBF H->H analyses (->ll,lh):M reconstruction using collinear approximationMass resolution ~10%

Main backgroundsEW & QCD Zjjtt and W productionH-> ->ll30 fb-1


Multivariate Analysis (1)Classical cut analysis uses rectangular signal-like phase spaceContour if signal-like phase space may be of any shapeDisadvantage of cut analysis gets larger with increasing number of discriminating variablesUse Neural Networks as a multivariate tool


Multivariate Analysis (2)Complex final state (VBF)Two forward jetsTwo leptonsMissing momentumClassical cut analysis misses correlationsIdeal for multivariate analysis


Multivariate Analysis (3)Signal significance improvement with neural network based analysis:Neural network output used as a discriminating variableResults improve by 45-50%


One experiment may reach a 5 signal significance with only 10 fb-1 of dataPreliminaryATLAS 10 fb-1


Luminosity plotPreliminaryATLAS


VBF for Intermediate MassesPreliminaryPreliminaryVBF H->WW->llVBF H->ZZ->llqq


MCs for VBFVBF analysis is an exclusive searchTwo hard and well separated jets (tagging jets)Veto on third jet in central region of the detector.Need to distinguish between QCD and EW processesSevere requirements on MC:Simulate X+3jA lot of MC development needed before turn on!PRD54 6680


SM Higgs MassDetermined best with H->ZZ->4lH->Resolution ~1%Electro-magnetic energy scale uncertainty ~0.1%Calibration with Z->eeHiggs width grows fast with massPrecision decreases for heavy Higgs


Coupling Measurements


Major Experimental Issues (1)


Major Experimental Issues (2)


VBF Experimental IssuesTagging forward jets:Efficiencies criticalDouble tag efficiency ~50%Jet calibrationCentral jet veto:Pile up effects introduce fake central jetsEffect small at low luminositySerious concern at high luminosityNeed to improve analysis based on more realistic full simulation


MSSM Higgs (1)Minimal super-symmetric extension of Higgs sectorFive Higgs: h (light), H, A, H (heavy)Parameter space reduced to two: MA,tanbTheoretical limit on light MSSM Higgs: h

MSSM Higgs (2)Large multiplicity of discovery modes:SUSY particles heavy:SM-like: h,bb,,WW; H4lMSSM-specific: A/H,,tt; Hhh, AZh; HSUSY accessible:H/A 02 02, 02 h 01Small impact on Higgs branching ratio to SM particlesConsider different MSSM scenarios Different upper limits to light MSSM Higgs (h)


MSSM Higgs Discovery PotentialMost of plane explored with 10 fb-1>1 MSSM Higgs observable:Disentangle SM and MSSMVBF not used yet!5 s Contours


MSSM Higgs and VBFThe addition of VBF modes in MSSM searches enhances sensitivity to low mass HiggsOne experiment with 10 fb-1 covers almost entirely MSSM planeOnly light Higgs (h)


Heavy MSSM HiggsbbH/Ahad had :Full simulation studiesTrigger efficiency-jets efficiencyMass reconstruction15% resolutionReconstruction efficiencygbtH, H:Only transverse massAlmost background free100% polarization enhances signal


Sensitivity to Heavy Higgs (1)Sensitivity to heavy MSSM Higgs dominated by in final state Little sensitivity to intermediate tan with SM decaysNeed new scenariosSUSYHiggs decaysFrom cascade5 s Contours


Sensitivity to Heavy Higgs (2)H/A02 024l+EtmissLook for isolated e, and EtmissStrong background suppression:SM (ZZ, tt)MSSM (ss)MSSM plane:230

Software DevelopmentHiggs will be observed with more than one channelHiggs searches spans over a number of final statesLeptons, photons, Ptmiss, jets, tausNeed to understand a lot of experimental issuesLepton/ identification, isolation, calibration, rejection against jetsTau reconstruction, rejection against jetsMissing momentum, b-taggingForward jets and central jet veto: calibration, tagging efficiency, rejection against pile-up, underlying eventMost of the studies based on full simulation date back to TDR. A lot of work needs to be done


SummarySM Higgs may be observed with 10 fb-1Known channels (H bb,,4l) well assessedVector Boson Fusion significantly enhances sensitivity for Higgs of low mass:Forward jet tagging efficiency crucialNeural Nets enhance signal significance by ~50%A lot of MC development needed before turn on!Most of MSSM plane explored with 10 fb-1VBF enhances sensitivity to light MSSM HiggsSensitivity to heavy Higgs dominated by channels with in final stateLots of software development to be done before turn on!


The LHC is coming!


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Higgs Physics in ATLAS