Kenneth Wraight Fullyleptonic ttbar jets events at ATLAS
Kenneth Wraight 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 Thanks to Craig Buttar and Sarah Allwood-Spiers
Part I Introduction to top quarks at ATLAS 2
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. 3
The top quark as background LHC will be the world’s first proper top quark factory: ~1 ttbar event per second for σ=833 pb at L=1033 cm-2 s-1 Background to. . . associated Higgs production multi-jet SUSY decays new & exotic physics 4
Diameter = 25 m, Length = 46 m, Weight = 7000 tonnes LHC ATLAS (the detector) Muon detectors Calorimeters Proton ATLAS Magnets Inner detector 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’. 5
Part II MC based di-leptonic cross-section measurement* *based on work done for Atlas Public Note: ATL-PHYS-PUB-2009 -086 6
At 14 Te. V 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 crosssection. BUT • Clean channel • Distinctive trigger • No need to reconstruct top mass Z+jets W+jets 7
Event Characteristics fully leptonic 2 opposite charge leptons (e/μ) 2 neutrinos large missing energy 2 b quarks + ISR/FSR 2 or more jets 8
Experimental Object Definitions (@ 10 Te. V) • Electrons: – – shower shape requirement isolation: etcone 20 < 6 Ge. V pt > 20 Ge. V 0 < |η| < 1. 37 or 1. 52 < |η| < 2. 47 • Muons: – – • Jets & MET: match inner det. & spectrometer isolation: etcone 20 < 6 Ge. V pt > 20 Ge. V |η| < 2. 5 – – Cone jets pt > 20 Ge. V |η| < 2. 5 MET>20/35 Ge. V • Overlap*: – selectrons – remove jets within ΔR < 0. 2 of electron – remove muons within ΔR < 0. 3 of jet *electrons & jets share same container 9
Samples & Event Selection (@ 10 Te. V) sample σ * k-factor ttbar (non hadronic) 202. 86 * 1. 07 W eν +jets 13253. 84 * 1. 22 W μν +jets 13242. 22 * 1. 22 W τν +jets 13237. 72 * 1. 22 Z ee +jets 1183. 66 * 1. 22 Z μμ +jets 1182. 41 * 1. 22 Z ττ +jets 1178. 57 * 1. 22 Wbb 14. 64 * 1. 22 WW 15. 62 * 1. 69 WZ 1. 37 * 1. 42 ZZ 4. 87 * 1. 81 single top (t-channel) 41. 12 * 1. 05 single top (W-channel) 14. 41 * 0. 99 • single flavor channel (ee, μμ): – trigger: 2 high-pt leptons (EF_e 15_medium , EF_mu 15, resp. ) – two opposite charged leptons, pt > 20 Ge. V & |η|<2. 5 – Etmiss > 35 Ge. V – ≥ 2 jets of pt > 20 Ge. V – Z-pole veto 86 < mll < 96 Ge. V • mixed flavor channel (eμ): – trigger: 2 high-pt leptons (EF e 15 medium or EF mu 15, resp. ) – two opposite charged leptons, pt > 20 Ge. V & |η|<2. 5 – Etmiss > 20 Ge. V 10 – ≥ 2 jets of pt > 20 Ge. V
*quoted errors are from sample statistics only cumulative*: exclusive*: di-electron channel selection cutflow (for L=200 pb-1 @10 Te. V) ↑ATLAS work in Progress↓ 11
non-fully leptonic ttbar, Z ee, Z μμ, Z ττ, W eν, W μν, W τν, Wbb, WW, WZ, ZZ, t-chan, W-chan Ns=228 Nb=45 Results ATLAS work in progress • The selection procedure above results in the following S/√(S+B) ratios for 200 pb-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 • with the following cross-sections: ee = 216. 96 ± 23. 9 pb, μμ = 217. 03 ± 19. 1 pb, eμ = 216. 67 ± 13. 4 pb σlep comb. = 217. 41 ± 10. 0 pb, cf. 217. 06 pb* *MC@NLO sample cross-section*K-factor --from Pub. Note: ATL-PHYS-PUB-2009 -086 12
Part III Study: systematic effect of jet algorithm choice 13
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 Pub. Note. • Main backgrounds (based on Pub Note selections). . . - ee : Z ee + jets && W enu + jets - mumu : Z mumu + jets - mix : Z tautau + jets 14
Signal & background. . . for L=200 pb-1 @ 10 Te. V alg. cone Kt anti-Kt SIS cone chan. ee μμ eμ S 259 350 778 249 338 761 250 337 761 252 336 765 B 30. 7 51. 8 28. 4 24. 1 49. 9 25. 8 27. 2 50. 3 26. 9 31. 4 48. 3 27. 4 S/ 0. 89 0. 87 0. 97 0. 88 0. 87 0. 90 0. 87 0. 97 0. 89 0. 88 0. 97 S+B S/ 15. 2 17. 5 27. 4 14. 8 17. 2 27. 1 15. 0 17. 1 27. 2 √(S+B) ↑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: 15 ee: 14. 2, μμ: 17. 3, eμ: 26. 2
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-2006035606 -MCnet) Special thanks to Peter Skands 16
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 ttbar system ATLAS work in progress tops ATLAS work in progress leptons ATLAS work in progress W bosons Initial differences in ttbar system washed-out by decay-chain 17
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 # cone jets atlfast # hadrons ATLAS work in progress hadron level FS hadrons hadron pt ATLAS work in progress selected Cone jets cone jet pt No. of jets determined by hadron pt not multiplicity 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 sample selection • 100 k events for each sample • used (close to) Pub. Note selection (see above) sample ee μμ eμ comb. ttbar 9390 +5. 7% -3. 5% 10975 +3. 2% -4. 2% 23631 +3. 2% -2. 7% 43996 +3. 7% -3. 2% W +jets 629 9. 7% -25. 6% -- 947 +3. 5% -28. 9% 1576 +6. 0% -27. 6% Z +jets 463 453 +10. 6% -29. 2% +6. 4% -23. 8% 3 +100% -33. 3% 919 +8. 8% -26. 6% WW +jets 323 451 +27. 2% -17. 0% +7. 1% -22. 8% 913 1687 +21. 5% -15. 3% +18. 7% -17. 6% Effects of ISR more important in background estimation 19
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 20
Backup 21
non-fully leptonic ttbar, Z ee, Z μμ, Z ττ, W eν, W μν, W τν, Wbb, WW, WZ, ZZ, t-chan, W-chan Ns=228 Nb=45 selection stack plots (di-electron channel) – Jets & Etmiss ATLAS work in progress 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 Ns=228 Nb=45 selection stack plots (di-electron channel) – Electrons ATLAS work in progress 23
Systematics (details) --from Pub. Note: ATL-PHYS-PUB-2009 -086 24
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