ATLAS Higgs Search Strategy and Sources of Systematic Uncertainty Jae Yu For the ATLAS Collaboration 23 June, 2010

Proposed Experiment Portion of the Note Search strategies in ATLAS and CMS – Split into three periods Short term – – Focus on SM higgs with H  γγ, H  WW and H  ZZ – Exclusions and limit settings Mid-term – 14 TeV low lum – SM+MSSM searches for H in early stage – With improved limits Long term – 14 TeV high lum – SM+MSSM+BSM searches Current tools used in experiments – Focus on precision predictions on higgs x-sec and good background modeling – Reduction of systematic uncertainties PDF and scale uncertainties – Differential capability for filtering Sources of systematic uncertainties and necessary improvements for reduction of them

Common Sources for Systematics in ATLAS Detector related – Lepton, photon, jet and MET reconstruction and ID efficiencies – Momentum/Energy resolution – Momentum/Energy correction scale – Luminosity: 10% Theoretical predictions on background x-sec – PDF uncertainties – QCD renormalization and factorization scales, in particular in lower order QCD calculations

H  γγ Low branching ratio and thus low cross section Exclusion limit settings till luminosity becomes significantly higher Controlling systematic uncertainties crucial – Background shapes under signal inv. Mass expected to rely on data using the sidebands Keeps correlations between signal and background systematic uncertainties small Precise knowledge of mass resolution Uncertainty in photon reco efficiency (only for signal)  1% uncertainty affect 0.1% variation in limits Uncertainty on luminosity (only for signal)  10% uncertainty effect 0.4 on the limits What is the things theory can do to reduce these?

H  WW  ll νν One of the most sensitive channels in 130<M H <200 GeV  200pb -1 could give results comparable to Tevatron up to M H <157GeV Studied mass range: 130 to 200 GeV Strategy: Three disjoint analyses looking at exclusive channels, H+0j, 1j, and 2j Signal MC – ggF for H+0j and H+1j  Currently use predictions which includes W  e ν and μν but no τν – VBF for H+2j modeled at the LO using HERWIG Background MC – WW continuum: dominant bck to H+0j qq/qg  WW at NLO using  high loop contribution from gg  WW due to high gluon flux at the LHC

H  WW  ll νν (background MC) WW continuum: dominant bck to H+0j – qq/qg  WW at NLO using – high loop contribution from gg  WW due to high gluon flux at the LHC  generated by gg2WW w/ charged lepton Pt>10GeV filter and | η |<2.7 qq  WWqq relevant to H+2j and modeled at LO by ALPGEN w/ ME up to 3 partons using MLM matching and cross section scaled to NLO using K-factor of 1.21 tt, single t (s and t channels) and Wt: – tt dominant background to H+2j and generated using filtering out hadronic decay of both the W’s The NLO x-sec scaled to NNLO using K-factor of 1.07 – Wt modeled using AcerMC

H  WW  ll νν (background MC) W+jets production: dominant bck to H+0j, 1j and 2j – Still significant even at the fake rate of ~10 -5 due to large x-sec (~10 4 ) – Generated using ALPGEN with ME up to 5 partons – Wbb+jets generated using ALPGEN w/ up to 3 additional partons – X-sec scaled to NLO using K-factor of 1.22 – For H+0j bck PYTHIA inclusive W sample used Z/ ϒ *+jets: bck to ee/ μμ channel from Z/ ϒ *  ll decays – Modeled using ALPGEN with ME up to 5 partons – Z+bb generated using ALPGEN + K-factor 1.22 to inclusive NNLO x-sec – H+0j, Z/ ϒ *  ll samples using PYTHIA used

H  WW  ll νν (background MC) WZ/ZZ production: dominant bck to H+0j, 1j and 2j – Small x-sec but very similar to the signal – Modeled using – For H+2j, qq  WZ(ZZ)qq modeled by ALPGEN with ME up to 3 partons and K-factor 1.21 scaled to NLO x-sec QCD Multi-jet production: split in two parts – bb production using PYTHIA with tight filter requirement – Light quark jets ignored at the moment due to low fake rate γ +jet production: PYTHIA used w/ filter requirement of P T γ >10GeV, | η | 17GeV

H  WW  ll νν (Systematic Uncertainties) Background estimate contributes most significantly Theoretical uncertainty in WW and Top MC Q 2 scale Jet energy scale and Jet Energy Resolution b-tagging level

H  ZZ  llll By itself at 7TeV 1fb-1 not comparable to SM x-sec and Tevatron results  needs to be combined with other channels Precise prediction of irreducible ZZ background crucial Sources of systematic uncertainties – Lepton ID efficiencies – Lepton momentum scale and resolution – Background modeling predicision