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H IGGS SEARCH IN THE H bb CHANNEL IN ATLAS Ricardo Gonalo Royal Holloway University of London on behalf of the ATLAS Collaboration International Europhysics Conference on High Energy Physics, Grenoble, July 21-27 2011

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Outline ATLAS and the LHC Higgs physics at the LHC ZH/WH H bb in ATLAS Event selection Backgrounds Systematic errors Results Boosted VH Summary and Outlook Ricardo GonaloEPS-HEP 2011 - Grenoble - July 20112

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ATLAS and the LHC Ricardo GonaloEPS-HEP 2011 - Grenoble - July 20113

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Ricardo GonaloEPS-HEP 2011 - Grenoble - July 20114 JINST (2008) 3 S08003 Inner Tracker: | |

ZH llbb Selection Trigger: e (p T e >20GeV) or (p T >18GeV) 2e/2 trigger (p T >12GeV) Exactly 2 leptons p T > 20GeV Opposite charge for Z mass cut: 76 < m ll < 106 GeV E T miss < 50GeV Two leading jets b tagged Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201114

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ZH llbb Backgrounds Z + b(bb ) dominates: Shape from MC Normalization from sideband (m bb < 80GeV); Cross check in single-b control region m bj Top: From MC Cross check in m ll control regions: [60GeV, 76GeV] or [106GeV, 150GeV] Multijet: e channel: multijet sample from reverting electron quality cuts channel: negligible (from MC) Diboson ZZ, WZ: from MC Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201115

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WH lbb Selection Trigger: e (p T e >20GeV) or (p T >18GeV) Exactly 1 lepton p T > 25GeV M T = 2p T l p T (1 - cos l ) > 40 GeV E T miss > 25GeV Exactly 2 jets (anti-k T 0.4; E T > 25GeV) to reduce top background Both jets b tagged Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201116

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WH lbb Backgrounds W+jets and top dominant: W+jets important at low m bb : Use m jj from data as template Top dominant at high m bb : Describe shape with MC Cross check m bb in 3-jet bin Normalisation from m bb sidebands [40GeV, 80 GeV] and [140GeV, 250GeV] Z+jets: use same normalisation of MC as measured in ZH Multijet Data templates from QCD enhanced samples (anti- isolation) Diboson WW, WZ from Monte Carlo Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201117

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Dominant systematic errors from b-tagging efficiency in both analyses Followed by jet energy scale Points at the next things things to improve! Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201118 Systematic Uncertainties ZH llbb WH lbb

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H bb Analysis Results Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201119

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ZH llbb Good description of the background No excess observed Single-channel exclusion of 20x Standard Model Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201120

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ZH llbb Good description of the background No excess observed Single-channel exclusion of 20x Standard Model Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201121

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ZH llbb Good description of the background No excess observed Single-channel exclusion of 20x Standard Model Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201122

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ZH llbb Good description of the background No excess observed Single-channel exclusion of 20x Standard Model Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201123

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WH lbb Good description of the background No excess observed Single-channel exclusion of 20-30x Standard Model Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201124

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Boosted VH with H bb Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201125

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Select W l events and search for a H bb jet 1.Search for high-p T jet (Cambridge-Aachen algorithm, R=1.2) 2.Search jet clustering history in reverse and look for large mass drop 3.Re-cluster with small R parameter to find sub jets Peak consistent with W jj in tt events Proof of principle for future analysis Alternative to inclusive channels: search for high-pT Higgs to bb: J. M. Butterworth, A. R. Davison, M. Rubin, and G. P. Salam, Phys. Rev. Lett. 100 (2008) 242001, arXiv:0802.2470 [hep-ph] p T H > 200GeV 5% of inclusive cross section but improved significance Ricardo Gonalo26 See talk by D.Miller in QCD yesterday

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Conclusions & Outlook Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201127

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First direct search for H bb at the LHC in WH and ZH channels Combined sensitivity to 10- 20x Standard Model Room for improvement! Reduce systematics New channels to explore Multivariate analyses Lots of data expected! Proof of principle for boosted Higgs analysis Watch this space! Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201128 Ref.: ATLAS-CONF-2011-103

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B ACKUP Ricardo Gonalo EPS-HEP 2011 - Grenoble - July 2011 29

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ATLAS & LHC: Efficiency and Pileup Very good ATLAS performance Data-taking efficiency above 95% Luminosity-weighted distribution of the mean number of interactions per crossing for 2011 Calculated from the instantaneous luminosity as: =L x inel / (n bunch x f r ) L: instantaneous luminosity inel : inelastic cross section (71.5 mb) n bunch : number of colliding bunches f r : LHC revolution frequency Entries at 0 from pilot bunches See: arXiv:1101.2185 Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201130

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Monte Carlo Samples Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201131

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Common event selection: Lepton Identification: Jet reconstruction: Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201132

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Detector related uncertainties Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201133

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Normalization Systematic Uncertainties Ricardo GonaloEPS-HEP 2011 - Grenoble - July 201134