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Fully-leptonic ttbar + jets events at ATLAS

Fully-leptonic ttbar + jets events at ATLAS. Introduction to top quarks at ATLAS MC based di-leptonic cross-section measurement Study of effect of jet algorithm choice Study of effect of ISR model variation on simulated cross-section. Kenneth Wraight.

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Fully-leptonic ttbar + jets events at ATLAS

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  1. Fully-leptonic ttbar + jets events at ATLAS • Introduction to top quarks at ATLAS • MC based di-leptonic cross-section measurement • Study of effect of jet algorithm choice • Study of effect of ISR model variation on simulated cross-section Kenneth Wraight Thanks to Craig Buttar and Sarah Allwood-Spiers

  2. Part I Introduction to top quarks at ATLAS

  3. The ubiquitous top quark • The strong force is measured in top pair production. • The weak force is measured in top decay and single top production. • Fundamental top parameters like mass, spin and charge still require precise measurement . • Tops decay before hadronisation, passing spin information to daughters. This provides unique environment for Beyond Standard Model searches. • Ultimate test of multi-scale QCD calculations used to predict top and new physics properties.

  4. The top quark as background LHC will be the world’s first proper top quark factory: ~1 ttbar event per second for σ=833pb at L=1033cm-2s-1 Background to... associated Higgs production multi-jet SUSY decays new & exotic physics

  5. LHC Muon detectors Calorimeters Proton Proton Inner detector Magnets Diameter = 25m, Length = 46m, Weight = 7000 tonnes ATLAS (the detector) ATLAS ATLAS is a general purpose detector designed to measure collision products in order to test the Higgs theory and look for signatures of ‘New Physics’.

  6. Part II MC based di-leptonic cross-section measurement* *based on work done for Atlas Public Note: ATL-PHYS-PUB-2009-086

  7. At 14 TeV total σ(LHC) = 125 ± 25 mb σ(ttbar) = 833 ± 12% pb  try to extract ~1 signal event from 20 million total. Signal Vs. Background • Dileptonic (e/μ) events are only ~5% of the ttbar cross-section. • BUT • Clean channel • Distinctive trigger • No need to reconstruct top mass Z+jets  W+jets 

  8. Event Characteristics fully leptonic 2 opposite charge leptons (e/μ) 2 neutrinos large missing energy 2 b quarks + ISR/FSR 2 or more jets

  9. Experimental Object Definitions (@ 10TeV) • Jets & MET: • Cone jets • pt > 20 GeV • |η| < 2.5 • MET>20/35GeV • Overlap*: • select electrons • remove jets within ΔR < 0.2 of electron • remove muons within ΔR < 0.3 of jet • Electrons: • shower shape requirement • isolation: etcone20 < 6 GeV • pt > 20 GeV • 0 < |η| < 1.37 or 1.52 < |η| < 2.47 • Muons: • match inner det. & spectrometer • isolation: etcone20 < 6 GeV • pt > 20 GeV • |η| < 2.5 *electrons & jets share same container

  10. Samples & Event Selection (@ 10TeV) • single flavor channel (ee, μμ): • trigger: 2 high-ptleptons (EF_e15_medium , EF_mu15, resp.) • two opposite charged leptons, pt > 20 GeV & |η|<2.5 • Etmiss > 35 GeV • ≥2 jets of pt > 20 GeV • Z-pole veto 86 < mll < 96 GeV • mixed flavor channel (eμ): • trigger: 2 high-ptleptons (EF e15 medium or EF mu15, resp.) • two opposite charged leptons, pt > 20 GeV & |η|<2.5 • Etmiss > 20 GeV • ≥2 jets of pt > 20 GeV

  11. exclusive*: cumulative*: *quoted errors are from sample statistics only di-electron channel selection cutflow (for L=200pb-1 @10TeV) ↑ATLAS work in Progress↓

  12. non-fully leptonic ttbar,Zee,Zμμ,Zττ,Weν,Wμν,Wτν,Wbb,WW,WZ,ZZ,t-chan,W-chan Results Ns=228 Nb=45 • The selection procedure above results in the following S/√(S+B) ratios for 200pb-1: ee = 13.1, μμ = 16.3 and eμ= 24.5, cf. ee = 13.2, μμ = 16.4 and eμ= 24.6 from the Pub Note ATLAS work in progress • with the following cross-sections: ee = 216.96 ± 23.9 pb, μμ = 217.03 ± 19.1 pb, eμ= 216.67 ± 13.4 pb  σlepcomb.= 217.41± 10.0 pb, cf. 217.06 pb* *MC@NLO sample cross-section*K-factor --from Pub. Note: ATL-PHYS-PUB-2009-086

  13. Part III Study: systematic effect of jet algorithm choice

  14. effect of jet algorithms • A few jet algorithms about these days... • Atlas Cone (old favourite) – cone based, unsafe • Kt – cluster based, IR&Collinear safe • anti-Kt – cluster based, IR&Collinear safe • SIS cone – cone based, IR&Collinear safe • effects come from resolution & JES differences between algorithms • observed distributions can change • signal & background acceptances can change • S/(S+B) & significance determine importance of acceptance changes • R= 0.4 for kt cut-off and SIS cone size. • Pseudo-event samples and selection same as in Top PubNote. • Main backgrounds (based on Pub Note selections) ... • - ee : Zee + jets && Wenu + jets • - mumu : Zmumu + jets • - mix : Z tautau + jets

  15. Signal & background... for L=200pb-1 @ 10TeV ↑ATLAS work in Progress • Varying jet algorithm choice does not substantially effect signal or (main) background distributions. • Acceptance variance is within level of a few percent which is comparable with other systematics • Relative (i.e.S/(S+B)) selection is constant across algorithms. • S/√(S+B) varies slightly across algorithms. All compare well with Pub. Note: • ee: 14.2, μμ: 17.3, eμ: 26.2

  16. Part IV Study: systematic effect of ISR model variation on simulated cross-section This research project has been supported by a Marie Curie Early Stage Research Training Studentship of the European Community’s Sixth Framework Programme under contract number (MRTN-CT-2006-035606-MCnet) Special thanks to Peter Skands

  17. ttbar system tops leptons W bosons 320 = Perugia 0 Pythia tune: benchmark 321 = Perugia Pythia tune: larger ISR phase-space & harder hadron’n 322 = Perugia Pythia tune: smaller ISR phase-space & softer hadron’n ISR effects on ttbar pt spectra (hadron level) ATLAS work in progress ATLAS work in progress ATLAS work in progress ATLAS work in progress Initial differences in ttbar system washed-out by decay-chain

  18. 320 = Perugia 0 Pythia tune: benchmark 321 = Perugia Pythia tune: more pert. activity, less non-pert. particles 322 = Perugia Pythia tune: less pert. activity, more non-pert. particles ISR effects on ttbar hadrons & jets ATLAS work in progress ATLAS work in progress hadron level FS hadrons atlfast # hadrons hadron pt ATLAS work in progress ATLAS work in progress selected Cone jets # cone jets cone jet pt No. of jets determined by hadron ptnot multiplicity

  19. 320 = Perugia 0 Pythia tune: benchmark 321 = Perugia Pythia tune: more pert. activity, less non-pert. particles 322 = Perugia Pythia tune: less pert. activity, more non-pert. particles ISR effects on sample selection • 100k events for each sample • used (close to) Pub. Note selection (see above) Effects of ISR more important in background estimation

  20. Summary • Completed a MC based cross-section measurement • including all relevant backgrounds (except QCD di-jets) • results in accordance with published results • Investigated the effects of jet algorithm choice • on ttbar signal and major channel backgrounds • Investigated the effects of ISR variations • on ttbar signal and selected background • Currently on MCnet studentship at CERN • working on min. bias & UE focussed analyses

  21. Backup

  22. non-fully leptonic ttbar,Zee,Zμμ,Zττ,Weν,Wμν,Wτν,Wbb,WW,WZ,ZZ,t-chan,W-chan selection stack plots (di-electron channel) – Jets & Etmiss Ns=228 Nb=45 ATLAS work in progress ATLAS work in progress ATLAS work in progress ATLAS work in progress

  23. non-fully leptonic ttbar,Zee,Zμμ,Zττ,Weν,Wμν,Wτν,Wbb,WW,WZ,ZZ,t-chan,W-chan selection stack plots (di-electron channel) – Electrons Ns=228 Nb=45 ATLAS work in progress ATLAS work in progress ATLAS work in progress

  24. Systematics (details) --from Pub. Note: ATL-PHYS-PUB-2009-086

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