Higgs → t+t- in Vector Boson Fusion S. Burdin, M. Campanelli, S. Farrington, C. Hays, A. Larner, K. Leney, J. Monk, C. Taylor 11th January 2008

Higgs Physics Where to look? Indirect evidence points to low mass (<160GeV) Higgs boson Which production mechanism? Which decay? Decay rate Vector Boson Fusion Production rate

Signal Significance at 30fb-1 In low mass region, Vector Boson Fusion H  tt is one of the most promising search modes Signal Significance

Event Topology Distinctive experimental signature t  t  Necessitates understanding of: Electron/muon ID Tau ID and tau-tau mass resolution Trigger efficiencies Missing ET Jets (including jet energy scale, jet algorithms, forward calorimeter performance) Backgrounds Two tau leptons Forward jets Missing Et

Event Selection In UK we focus on lepton-hadron and di-lepton modes at the moment. Trigger: 20GeV Muon / 25 GeV Electron Cuts: jets not back-to-back missing Et >40GeV Dh between tag jets >4.4 Tag jets invariant mass >700GeV Central jet veto: no third jet in |h| <3.2 with pt >20GeV tau candidates should be between tag jets Cut flow:

UK Efforts tt background studies: Liverpool/Oxford Jet studies: UCL VBF Production of Z bosons: Oxford

tt Background Studies (Liverpool/Oxford) W decays mimic t decays W leptonic decay to e and m resemble leptonic t decay W hadronic decay to jets resemble hadronic t decay b jets resemble VBF tagging jets

Cross Sections: Signal/tt Background Apply baseline H→ tt cuts to signal and tt background samples Signal: Herwig VBF120tautaulh 5334, VBF120tautaull 5333 tt: MC@NLO ttbar 5200 + Liverpool samples. No all hadronic decays. Not enough events to calculate total efficiency for ttbar separate the cuts into three independent sets tau correlated, jet correlated, and a third set correlated to both tau and jet properties Multiply the three sets of efficiencies together

tt Background Studies Efficiencies Lepton-hadron Lepton-Lepton Tau cuts 1.7x10-4 6.4x10-3 Jet cuts 1.4x10-2 Angular cut - 0.46 Centrality 0.81 0.79 Central jet veto 0.14 0.21 Total Efficiency 3x10-7 7x10-6 Effective stt 0.11fb 3.2fb Effective sH→tt 0.35fb 0.43fb

tt Background Studies Using truth information we can see which W decay modes contribute the most background after cuts Lepton-hadron mode: After requiring two leptons in the event still have significant contribution from lepton + jets W decays Suggests lepton from b decay may be reconstructed as leptonic t Using truth information we see that in cases where a jet “fakes” a muon, ~70% of the time it is matched to a b jet and is a real muon problem is ameliorated by isolation cut on electron looking into applying muon isolation cut

Reduction of tt background Charge Correlation helps to reject background Lepton hadron mode: a jet faking a tau may come from either b Lepton lepton mode: either b hadron can decay to a lepton The proportion of wrong sign events is known (BRs + b mixing) so number of wrong sign events in the data is a way to calibrate the size of this background Veto on wrong sign events rejects 20% of ttbar background in lepton hadron mode, 25% in dilepton mode

Reduction of tt background b jets in ttbar decays can mimic the forward jets in the VBF Higgs signal b jet veto helps in rejecting background Apply veto to any jet in the event, not just those tagged as VBF anti b-tag veto has been tuned: require weight <1 (default b-tag selection cut is at 6.75) S/√(S+B) b tag weight b tag weight Veto on b jets rejects a further 60% of ttbar background in lepton hadron and dilepton modes After charge correlation and b jet veto the signal loss is 4% in lepton hadron mode, 10% in lepton lepton mode

Reduction of tt and Z+jets background Studies of Log Likelihood variable combining jet cuts indicate great improvements in background rejection (note: ttbar and Z+≥ 2 jets background) Preliminary studies indicate powerful background suppression (factor 10) but this is performed with low statistics samples (few events are left after the other cuts have been applied) Includes: Dh between jets, mass(jets), central jet veto h requirement

Jet Studies (UCL) Identifying VBF signature requires understanding forward jets Which algorithms work well in forward region? Do jets behave as we expect? Several jet algorithms studied (using Spartyjet to compare them)

Truth/Reconstructed Jet Matching Compare W+jets MC with VBF Higgs MC: VBF jets are in the more forward regions of the detector Calorimeter geometry is not projective QCD jets truth/reco match slightly better than VBF jets KT 04 and SIS Cone 04 algorithms show best matching h h

Truth/Reconstructed Jet Matching Matching efficiency as function of h t jets in central region are included in this plot W+jets h h VBF Liquid Argon calorimeter endcap region

Truth/Reconstructed Jet Matching Jet resolution as function of pt and h 5% pt correction required only in endcap pt h

Jet Studies Work in progress studies of central jet veto Pythia vs Herwig comparisons inclusion of pile-up and multiple interactions

VBF Production of Z Bosons (Oxford) Seek Vector Boson Fusion signature for Z ee,mm,(tt) Never observed Expect ~4x the Higgs rate Will look almost exactly like the Higgs signature Useful way to study backgrounds, as well as being an interesting first observation in itself /Z t

Available MC’s VBF Z production not implemented in Herwig/Pythia MC’s on the market which we can use: Madgraph, VBFNLO, SHERPA Comparison of MC’s taking place see large differences in predicted cross sections working with MC authors to understand these Moving towards generating full simulation samples

Outlook All of this work is included in the CSC note Now looking beyond CSC note Optimisation of cuts under realistic conditions, including all background contributions Jet triggers are under study There is now significant UK contribution to a fascinating Higgs mode

tt Background Studies Di-lepton mode: