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Measurements at 8 TeV of TTbar events with additional particles in the final state

Measurements at 8 TeV of TTbar events with additional particles in the final state. Boris Mangano (ETH Zürich) on behalf of the CMS collaboration. Outline. 7 TeV. 8 TeV. TTbar plus what? T T bar+jets TTbar+BBbar TTbar+V : TTbar+gamma TTbar+W TTbar+Z. TOP-12-018. PAS TOP-12-041.

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Measurements at 8 TeV of TTbar events with additional particles in the final state

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  1. Measurements at 8 TeV of TTbarevents with additional particles in the final state Boris Mangano (ETH Zürich) on behalf of the CMS collaboration

  2. Outline 7 TeV 8 TeV • TTbar plus what? • TTbar+jets • TTbar+BBbar • TTbar+V: • TTbar+gamma • TTbar+W • TTbar+Z TOP-12-018 PAS TOP-12-041 8TeV results released during the last 12 months PAS TOP-12-024 PAS TOP-13-010 PAS TOP-13-011 Were these processes seen also at 8TeV ? TOP-12-014 new results LHCP 2014

  3. Motivation About ½ of TTbar events contain extra jets with |h| < 2.4, pT>30 GeV. Possibility to test higher order QCD calculations. • TTbar plus what? • TTbar+jets • TTbar+BBbar • TTbar+V: • TTbar+gamma • TTbar+W • TTbar+Z Irreducible non-resonant background for ttH (H->bb) analysis. Potential to verify if couplings to the top quark are compatible with SM or altered by BSM effects. LHCP 2014

  4. tt+jets @ 8TeV (19.6 /fb) • Event selection: • Dilepton OS events (el and mu) • For same-flavour pairs, Z-veto and require MET • N-jets >=2 • N-bjets >=1 PAS TOP-12-041 • Extra jets definition: • Kinematic reconstruction method to identify b-jets from ttbar decay. • Other extra jets are then analysed • Unfolding: • cross-section in N-jets bins is presented at particle level Main source of uncertainty: Up to 20% uncertainty from jet-energy-scale and signal model uncertainties. LHCP 2014

  5. tt+jets @ 8TeV (19.6 /fb) The whole is greater than the sum of its parts (Aristotle) MC@NLO + Herwig does not generate sufficiently large jet multiplicities. LHCP 2014

  6. tt+jets @ 8TeV (19.6 /fb) The whole is greater than the sum of its parts (Aristotle) Second Leading extra jet pT (GeV) Leading extra jet pT (GeV) These distributions are at reconstruction level LHCP 2014

  7. tt+bb@ 8TeV (19.6 /fb) PAS TOP-13-010 • Event selection: • Dilepton OS events (el and mu) • For same-flavour pairs, Z-veto andrequire MET • N-jets >=4 • N-bjets (tight) >=2 • Normalization of simulated samples: • Performed by means of a fit to the measured b-tagging algorithm discriminator. • Templates for different tt+X samples (including tt+bb signal) are taken from simulation. Main source of uncertainty: 20-25% total uncertainty. Mostly from uncertainties in b-tagging discriminator templates. LHCP 2014

  8. tt+bb@ 8TeV (19.6 /fb) First additional jet Second additional jet The whole is greater than the sum of its parts (Aristotle) Simultaneous template fit of two data distributions LHCP 2014

  9. tt+bb@ 8TeV (19.6 /fb) Factor 0.38 to account for difference in selection efficiency between ttbb and ttjj (from simulation) The whole is greater than the sum of its parts (Aristotle) Cross-section ratio at particle level: using again selection efficiencies from simulation LHCP 2014

  10. tt+gamma @ 8TeV (19.7 /fb) PAS TOP-13-011 • Event selection: • one isolated central mwith pT > 26 GeV • Veto on extra leptons • N-jets >=4, with pT > 55, 45, 35, 20 GeV • N-bjets >=1 • One central photon ET > 25 GeV • DR (photon,muon) > 0.7 • Signal extraction: • Template fit of charged-hadron-isolation discriminant distribution in data. • Templates for signal and misidentified photons from simulations. Main source of uncertainty: 20-25% total uncertainty from modelling of the background template. LHCP 2014

  11. tt+gamma @ 8TeV (19.7 /fb) N-1 plots for some of the observables used to select the signal photons. LHCP 2014

  12. tt+gamma @ 8TeV (19.7 /fb) Fit of distribution of the charged-hadron-isolation discriminant The whole is greater than the sum of its parts (Aristotle) Inclusive ttbar cross section (TOP-12-017) LHCP 2014

  13. ttW and ttZ @8TeV (19.5 /fb) TOP-12-036 pre-print to be released in the next days • Both ttW and ttZ processes are very rare and the measurement of their cross sections can be used to test the internal consistency of SM. • ttZ production allows the probe of tZ coupling. LHCP 2014

  14. ttW in same-sign dilepton Analysis type: counting experiment • Signal region definition: • - Tight prompt-lepton selection • same-sign dilepton (el or mu) • third lepton vetoed only if it makes a Z-candidate • N-jets >= 3 • N-bjets >=1 • lepton pT>40 GeV and HT > 155 GeV Against misidentified prompt leptons and ttbar background Against WZ and other diboson processes. Remove overlap with ttZ analysis. Final optimization against ttbar background LHCP 2014

  15. ttW in same-sign dilepton • Dominant background: • ttbar with misidentified leptons is estimated from data. Uncertaintyis about 50%. • Other backgrounds: • ttZ, WZ and other rarer processes are estimated from simulations. • Z+jets with mis-measured lepton charge estimated from data Six signal regions to exploit larger S/B in plus-plus channels. LHCP 2014

  16. ttZ in trilepton Analysis type: counting experiment • Signal region definition: • Three tight leptons (el or mu) • Two same-flavour opposite-sign making a Z-candidate • N-jets >= 4 • N-bjets (loose) >=2 & N-bjet (medium) >=1 Against misidentified leptons and ttbar background Against WZ and other diboson processes. Remove overlap with ttW analysis. LHCP 2014

  17. ttZ in trilepton • Dominant background: • non-top-quark background processes with b-jets from ISR. Uncertainty is about 50%. • Other backgrounds: • TTbarwith misidentified leptons is estimated from data. • Rarer, and irreducible, SM processes estimated from simulation Only one signal region is used for the statistical interpretation of the data LHCP 2014

  18. ttZ in four-lepton Analysis type: counting experiment • Signal regions definition: • Four tight leptons (el or mu) • Two same-flavour opposite-sign making a Z-candidate • Veto on second Z-candidate • N-bjets =1 • N-bjets >1 Against WZ and lepton misidentification Against ZZ background Two non-overlapping signal regions LHCP 2014

  19. ttZ in four-lepton • Backgrounds: • ZZ(*) in four leptons is estimated from MC • Z+X events with misidentified leptons • Limits and benefits of this channel: • statistically limited • Large S/B ratio LHCP 2014

  20. ttV (V = W or Z) Results in agreement with SM predictions: cross section ttW = 206 ± 23 fb cross section ttZ = 197 ± 25 fb Numbers from MG5_aMC@NLO. They arecompatible with first NLO calculations from MCFM and PowHel. LHCP 2014

  21. Summary • TTbar plus what? • TTbar+jets • TTbar+BBbar • TTbar+V: • TTbar+gamma • TTbar+W • TTbar+Z PAS TOP-12-041 8TeV results released during the last 12 months PAS TOP-13-010 PAS TOP-13-011 TOP-12-036 pre-print to be released in the next days New Results LHCP 2014

  22. Backup slides Latsis Symposium 2013

  23. ttV (V = W or Z) LHCP 2014

  24. tt+jets The whole is greater than the sum of its parts (Aristotle) LHCP 2014

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