Higgs → t + t - in Vector Boson Fusion

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

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

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

4. ET Event Topology Distinctive experimental signature t t • Experimental signature: • 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

5. 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:

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

7. 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

8. 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 Cross Sections: Signal/tt Background

9. tt Background Studies

10. 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 tt Background Studies

11. 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 Reduction of tt background Veto on wrong sign events rejects 20% of ttbar background in lepton hadron mode, 25% in dilepton mode

12. 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) Reduction of tt background 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

13. 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) Reduction of tt and Z+jets background Includes: Dh between jets, mass(jets), central jet veto h requirement

14. 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) Jet Studies (UCL)

15. 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

16. 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

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

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

19. t /Z t • 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 VBF Production of Z Bosons (Oxford)

20. 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 Available MC’s

21. 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

22. Di-lepton mode: tt Background Studies