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Semileptonic ttH(H bb ) studies at UCL

This study at UCL involves cut-based analysis for semileptonic ttH(H→bb) events, focusing on jet reconstruction, top quark and Higgs boson reconstruction, and background shape studies.

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Semileptonic ttH(H bb ) studies at UCL

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  1. Semileptonic ttH(Hbb) studies at UCL Lily Asquith, Catrin Bernius, Sebastian Boeser, Simon Dean, Nikos Konstantinidis Atlas UK Meeting, Higgs Session 11/01/2008

  2. Cut-Based Analysis • Preselection • 1 isolated lepton, pTe>25GeV, pTμ>20GeV, |η|<2.5 • At least 6 jets, pT>15GeV, |η|<5 • At least 4 tagged as b-jets • Wl • constrain invariant mass of lto W mass, solve for pzv • Wjj • Pairs of non b-jets with mjj=mW±25GeV • Reconstruction of 2 top quarks • Minimise 2 with top masses • Reconstructed top masses must be within ±20GeV of mt • Reconstruction of Higgs • Use the two highest-pT remaining b-jets Atlas UK Meeting, Higgs Session

  3. Studies Shown Here • Jets in the crack region • Background invariant mass shape • Fast simulation jets • Generator samples Atlas UK Meeting, Higgs Session

  4. Jets in the Crack Region (Lily Asquith) • Uninstrumented angular region = "crack" • Considered 1.4 < || < 1.6 EM accordion calorimeters Hadronic tile calorimeters Hadronic LAr endcap calorimeters Forward LAr calorimeters Atlas UK Meeting, Higgs Session

  5. Jets in the Crack Region • Abundance of jets in ttH final state • Poor jet reconstruction may seriously affect overall event reconstruction • Study uses cone 0.4 jets in release 12.0.6 ttH and ttbb samples (5870, 5871 & 5855) • Matched to truth-jets using dR < 0.2 • In ||-bins of 0.2, Gaussian fits made to ET(reco) / ET (true) • Linearity = mean • Resolution = width / mean Atlas UK Meeting, Higgs Session

  6. Light Jet Performance • Low and high energy jet samples • ~4% linearity dip in crack region • Resolution worse at lower pT • Deviations at high eta due to low fit stats Atlas UK Meeting, Higgs Session

  7. B-Jet Performance • No b-tagging outside Inner Detector (|| < 2.5) • ~4% dip again observed in linearity • Resolution also worse at lower pT • More work to show impact on hadronic W, tops and Higgs Atlas UK Meeting, Higgs Session

  8. J. Drohan signal Background Shape (Catrin Bernius) Sinead M. Farrington, "B Physics Lessons from the Tevatron", YETI'08 • Broad bb mass peak • Kinematic selection produces similar signal and background shape • Would like better discrimination • Started by considering ttH vs. ttbb (QCD) Atlas UK Meeting, Higgs Session

  9. Background Shape • Formulated "tight selection" on Higgs b-jets at low pt more jets in ttbb than ttH Tight selection: pT(b1Higgs)> 40 GeV, pT(b2Higgs)> 30 GeV Atlas UK Meeting, Higgs Session

  10. Tight Selection ET(b2higgs) vs ET(b1higgs) in mass window 90GeV<mbb<150GeV to check cut correlation Cut out Tight selection: pT(b1Higgs)> 40 GeV, pT(b2Higgs)> 30 GeV Atlas UK Meeting, Higgs Session

  11. Shape Comparison • Normalised to 30 fb-1 ("combined sample") Atlas UK Meeting, Higgs Session

  12. Correct/Incorrect Combinations in ttH Signal • Correct combinations: b-jets (from Higgs) correctly matched • Fit Gaussian to correct combinations, Landau to incorrect combinations Atlas UK Meeting, Higgs Session

  13. Fitting Background & Signal shapes • Fit gaussian + landau to ttH signal sample • Landau also describes ttbb background very well Atlas UK Meeting, Higgs Session

  14. Fit background + signal shape to the combined sample, keeping parameters fixed but with scaling factors applied Combined Fit Standard Selection Tight Selection Mass window: 90-150 GeV: • signal (gauss+landau) = 437.9 • ttbb(QCD) (landau) = 3089.1 --------------------------------------------- • signal/(signal+ttbb) = 0.12 • signal(gauss)/(all signal) = 0.28 Mass window: 90-150 GeV: • signal (gauss+landau) = 377.9 • ttbb(QCD) (landau) = 1923.2 -------------------------------------------- • signal/(signal+ttbb) = 0.15 • signal(gauss)/(all signal) = 0.31 Atlas UK Meeting, Higgs Session

  15. Next Steps For Shape Study • Include ttbb(EW) and ttjj backgrounds • Explore behaviour of combined fit when varying analysis cuts • eg... b-weight cut at different values Atlas UK Meeting, Higgs Session

  16. Fast Simulation Jets • Fast simulation uses different cone jet finder than accessed by JetRec • Atlfast currently only supplies the results of cone 0.4 jet finder in AOD output • Can also run using kT algorithm • Done via KtEvent interface, same as JetRec • In principle, re-running Atlfast using kT and comparing with full-chain kT jets should give closer agreement Atlas UK Meeting, Higgs Session

  17. 12.0.6 cone jets Full Fast • 7000 signal events considered • Cone 0.4 • Included for both full and fast in 12.0.6 AOD files • Noticeably higher number of jets from Atlfast • Overestimation of jet efficiency at low pT, as seen in other analyses • Consistent with Bill Murray (RAL) Atlas UK Meeting, Higgs Session

  18. 12.0.6 kT jets Full Fast • Full chain kT jets included in 12.0.6 AODs, fast not • Atlfast run directly from evgen files used in standard production • R-parameter = 0.4 • Shift to higher pT in fast • Cone jets should be more consistent from r14 as FastKtJet interface will be adopted by Atlfast (deliverable) Atlas UK Meeting, Higgs Session

  19. Generator Studies (Sebastian Boeser) • ttHTruthCollector tool used to inspect signal and background truth samples • identify the hard scatter vertex, i.e. the one with either an outgoing higgs or two outgoing tops • identify any additional b-partons at the hard scatter vertex • follow the top-branches to identify the b-partons and W's • follow the W-branches to identify the jets or lepton-neutrino pairs Atlas UK Meeting, Higgs Session

  20. tWs decays • 38950 ttH events • Missing b-quarks from top decays compensated by finding s-quarks • The relative number of tWs/tWb is in good agreement with |Vts|2 as given by the PDG • tWs decays supported in PYTHIA sample, neglected in other generators Atlas UK Meeting, Higgs Session

  21. W decays • 5000 ttjj events • 7.4% missing qq pairs from W decays • Equally high number of -pairs is found in their place. • W being accepted as "hadronic" decay in the semi-leptonic sample. • (In some cases there are extra b-quarks at the hard-scatter vertex. Happens if the hard scatter is connected to the underlying event with a b-quark) Atlas UK Meeting, Higgs Session

  22. Next Steps in Generators Study • Understand effect on analysis of additional W decays • Understand correct way of dealing with ttbb component in ttjj sample, to avoid double-counting events (not covered here) Atlas UK Meeting, Higgs Session

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