Top paper status W Verkerke NIKHEF Wouter Verkerke

Top paper status W. Verkerke (NIKHEF) Wouter Verkerke, NIKHEF

Introduction • Goal of top paper – observation of ttbar production and measurement of its cross section • Physics of SM top quark pair production, s=165 pb Dominant at Tevatron Dominant at LHC • Virtually all top quarks decay to t Wb • Thus final states classified by W decay modes: W qq (2/3) or W ln (1/3) – Dilepton channel (2 x. W ln), 1/9 of cross section – Lepton+jets channel (1 x. W ln), 4/9 of cross section – All Hadronic channel(0 x. W ln), 4/9 of cross section Wouter Verkerke, NIKHEF

Analysis strategy • Lepton+jets Look for events with 4 jets, of which 2 are b-jets, hard lepton, missing ET – Can reconstruct also invariant mass of jets from t W(qq)b, but simple procedures suitable for early data taking have low efficiency (~20 -30%) Not primary tool for early data analysis – Strategy: Evidence for top based on excess of events w. r. t known backgrounds • Dilepton Look for events with 2 jets, (of which 2 bjets) two hard leptons, missing ET – No easy clean signature in events that is characteristic for top – Strategy: Evidence for top based on excess of events w. r. t known backgrounds • Understanding of backgrounds is key – Use data driven approach wherever possible

General status of analysis • Lepton+jets – We think analysis procedures now all sorted out. – Ironing out last details, some discussion still ongoing on a few systematic uncertainties – Still need to update to final b-tagging efficiency uncertainties – Once procedures are finalized still need few days to (re)propagate latest changes in background estimations etc to cross section results • Dileptons – Analysis essentially complete – Still need some time to calculate final cross sections and combined cross section • All plots and numbers shown in this presentation are still subject to change Wouter Verkerke, NIKHEF

Data sample • All data taken in DQ periods A-F that pass ‘top’ GRL – Total luminosity = 2. 9 pb-1 • Triggers used (some time evolution) – L 1_ MU 10 or EF_mu 10_MSonly – L 1_EM 10, EF_g 17_etcut or EF_e 10_medium • Object definitions for events selection – Electron = ‘medium robust electron’ with p. T>20, E/p and b-layer cuts |ηcluster|<2. 47 (excluding 1. 37<h<1. 52), isolation: ET(R=0. 2)< 4 + 0. 023*ET(el) Ge. V – Muon = ‘MUID tight’, p. T>20 Ge. V, |h|<2. 5 ET(R=0. 3)<4 Ge. V, PT(R=0. 3)<4 Ge. V DR>0. 4 w. r. t nearest jet with p. T>20 Ge. V – Jet = Anti. Kt 4 Topo. Cluster jets EM+JES. DR(jet-el)>0. 2. – B-jet = Jet + SV 0 cut at 50% MC efficiency point – Missing ET = Simplified METRef. Final Details in https: //twiki. cern. ch/twiki/bin/view/Atlas. Protected/Top. Common. Objects Wouter Verkerke, NIKHEF

Event selection for lepton+jets channel • Event samples named 1, 2, 3, 4 -jet pretag defined as – The appropriate single electron or muon trigger has fired – Exactly one lepton (el or mu) with p. T>20, matching the corresponding trigger object – ETmiss>20 Ge. V and ETmiss+m. T(W)>60 Ge. V – Exactly N jets with p. T>25 Ge. V (N=1, 2, 3) or >=4 jets (N=4) Wouter Verkerke, NIKHEF

Event selection for lepton+jets channel • Corresponding samples after b-tagging: 1, 2, 3, 4 -jet tagged signal region – Added requirement = 1 b-tagged jet with p. T>25 20 evts 17 evts ttbar events Wouter Verkerke, NIKHEF

Background in the lepton+jets analysis • QCD multijet events – Contributions with non-prompt leptons (semileptonic decays resulting in hard non-prompt leptons) – Contributions with fake leptons (in e+jets channel) – Large (and uncertain) cross section, small (and uncertain) acceptance rate simulation is not reliable – Developed fully data driven estimate of this background • W+jets events – Sizeable contributions from each of W+light jets, W+bb+jets, W+cc+jets, W+c+jets – Partly irreducible (W+bb+jets has same final state as signal) – W+4 jets cross section has large uncertainty. HF composition of this sample has additional substantial theoretical uncertainty. – Developed nearly fully data driven estimate • single top, Z+jets, dibosons – Small background, can use MC estimates, large cross section error (Z+jets) has small impact Wouter Verkerke, NIKHEF

Data driven estimate of QCD background • Use matrix method to estimate # QCD events with fake leptons in m+jets channel – Measure event count with loose and tight(=standard) lepton ID cuts – Can related Nloose, Ntight to Ntight(fake) if you know relative ereal, efake – Can measure ereal from Z(mm) events = 0. 990 ± 0. 003 – Can measure efake from QCD dominated control region • QCD control regions – A: ETmiss < 10 Ge. V efake = 0. 382 ± 0. 007 (stat only) – B: d 0 -sig(m)>5 && ETmiss>20 efake 0. 295 ± 0. 025 (stat only) – Key: Is control region (kinematically) representative of signal region? – Check 1 – Test on QCD MC sample 30% systematic – Check 2 – Compare control regions (no MC dependence) use difference as metric of systematic uncertainty – Total systematic uncertainty is combination of both Wouter Verkerke, NIKHEF

Data driven estimate of QCD background • Can test-drive method on 1 -jet samples 1 -jet pretag sample without MT(w) cut 1 -jet tagged sample without MT(w) cut Data W+jets simulation QCD (data driven – matrix method) Wouter Verkerke, NIKHEF

Data driven estimate of QCD background • Estimate QCD with MM in pretag samples – Statistics too low after flavor tagging – Instead estimate contribution in tagged samples by multiplying pretag numbers with measured tag rates • Use fitting method to estimate #QCD events with fake leptons in e+jets events – Matrix Method less reliable for e+jets due to sizeable contribution from both HF and non-HF sources which introduces additional issues in representativeness of control samples – Not described here due to time constraints. • Results in 4 -jet tagged samples – e+jets: N(QCD) = 4. 9 ± 2. 9 – m+jets: N(QCD) = 1. 7 ± 1. 1 – Numbers include systematic uncertainties Wouter Verkerke, NIKHEF

Estimation of W+jets background • Start with estimation in 4 -jet pretag bin – Exploit approximate constant ratio of W+N+1 jets/W+Njets cross sections to extrapolate W+1, 2 jets to W+4 jets – ‘Berends-Giele’ scaling. Systematic uncertainties related to this idea relatively well explored in existing publications (24%) – Measure W+1, 2 jets by subtracting data driven QCD estimate from 1, 2 -jet pretag bins (+ MC estimates for other processes) • Estimate in 4 -jet tagged samples with – Measure 2 -jet tag fraction from data (subtracting QCD and other bkg) – Account for evolution of HF composition between 2 -jet and 4 -jet bin with MC factor Wouter Verkerke, NIKHEF

Estimation of W+jets background • Results e+jets: 11. 4± 2. 2(stat. )± 4. 8(syst. ) mu+jets: 19. 8± 4. 2(stat. )± 5. 4(syst. ) ( 6. 3± 1. 8(stat. )± 0. 2(syst. ) )% 2. 4 ± 0. 8 (syst. ) • Note on MC input uncertainties on f(2 4) – HF composition of 2 -jet sample: 300% relative uncertainty on f(bb), f(cc), 100% on f(c) – Per-component tag fractions: uncertainties from tagging group – 2 4 jet extrapolation ratios: 2 x magnitude of effect of Alpgen param. variation studies (40 -60% per component) W(eν)tagged-≥ 4 jet = 1. 6± 1. 0 (63%) W(μν)tagged-≥ 4 jet = 3. 0± 1. 7 (58%) Wouter Verkerke, NIKHEF

Lepton+jets event yield • Calculate ttbar event yield – subtract all background estimates from data yield in 4 -jet tagged sample • e+jets: Nobs = 17 N(ttbar) = 9. 9 ± 3. 1 • m+jets: Nobs = 20 N(ttbar) = 15. 6 ± 1. 8 Wouter Verkerke, NIKHEF

Lepton + jets cross section • Calculate cross section using MC acceptance – e+jets: e(MC@NLO) = 0. 0282 ± 24% – m+jets: e(MC@NLO) = 0. 0285 ± 24% 29% 28% stat, syst ~40% each σ(e+j) = 122 ± 51(stat) +59 -47(syst) pb σ(m+j) = 189 ± 54(stat) +69 -47(syst) pb stat, syst ~30% each [ s(theory)=165 pb ] – Combined cross section calculation in progress Need careful propagation of correlated systematic errors. – Have tool to do this – was developed from dilepton cross section combination in 2009 Summer PUB notes. Expect numbers by end of this week NUMBERS NOT FINAL – CHANGES STILL EXPECTED Wouter Verkerke, NIKHEF

Lepton+jets systematic uncertainties • Systematic uncertainties on cross section by component – b/c. Tag uncertainty based on preliminary 24%/48% flavor tagging uncertainty This number can still change – Jet Energy Scale ~10% at 25 Ge. V

Alternative calculation of e/m+jets cross section • A Counting method based on 1, 2, 4 jet bins • B Can also extract yield from 3, 4 jet bins – But more complex as 3 -jet bin has substantial ttbar contribution • C Can also exploit information of 4 -jet untagged sample – With supporting information from 3 -jet tagged, untagged bins as well • Solutions B, C implemented as template fits to m(jjj) distribution – Shape information gives some discrimination power between signal and background – Introduce known correlations between signal and background in various bins Wouter Verkerke, NIKHEF

Alternative calculation lepton+jets cross section • Results from B (3+4 jet tagged samples) s(e+jets) = 102. 1 pb s(m+jets)= 186. 2 pb +42. 2/ +13. 6/ -59. 1 (stat) -30. 9(syst) -9. 6 (lumi) +78. 4/ +73. 7/ +23. 6/ -69. 6 (stat) -33. 0(syst) -16. (lumi) +67. 2 • Results from C (4 jet tagged/untagged) Verkerke, NIKHEF NUMBERS NOT FINAL – CHANGES STILLWouter EXPECTED

Summary on lepton+jets • Observe clear excess of events in 3, 4 -jet tagged samples consistent with ttbar • Have fully data driven estimate of QCD background • Have almost fully data driven estimate of W+jets background • Can extract ttbar yield/ cross section using different combination of control regions – Counting (1, 2, 4), Fitting (3, 4) Fitting (4, 4 -untag) • Cross section results consistent with SM prediction • Signal significance in l+jets likely just short of 5 sigma Wouter Verkerke, NIKHEF

Event selection for the dilepton channel • Three channels: ee, mm, em • Event selection – Two opposite sign leptons (one must match trigger object) – Cosmic veto – |d 0|>0. 5 mm , df>3. 1 – ETmiss>40 Ge. V (ee), ETmiss>30 Ge. V (mm), HT>150 Ge. V (em) – Z mass veto (± 5 Ge. V ee, ± 10 Ge. V mm) – At least 2 jets with p. T>20 Ge. V (signal region) ee mm em Wouter Verkerke, NIKHEF

Event selection for the dilepton channel ee mm em Wouter Verkerke, NIKHEF

Backgrounds in the dilepton analysis • W+jets – W+ 2 jets events with additional fake lepton pass event selection – Relatively large cross section, MC fake rate not reliable – Data driven estimation (procedure includes other smaller backgrounds with two fake leptons) • Z+jets (Drell-Yan) – Most Z+jets events rejected with Z mass veto on lepton invariant mass, but some leakage – Insufficient data available for fully data driven estimate, have partially data driven method • Single top, Dibosons, Z tt – Comparatively small contributions Use MC estimates Wouter Verkerke, NIKHEF

Data driven estimate of fake lepton bkg (W+jets) • Fake lepton background measured with Matrix Method. – Similar to lepton+jets, but now have 4 x 4 matrix due to 4 permutations of Loose, Tight for 2 leptons – Value of ereal measured again from Z ll events – Value of efake measured in control sample with single loose lepton • Additional ‘weighting’ and ‘fitting’ method as cross check Wouter Verkerke, NIKHEF

Data assisted estimate of Z+jets background • Schematic illustration Rescale A+C from MC with data/MC ratio in B+E N(ee) = 0. 25 ± 0. 18, N(mm) = 0. 67 ± 0. 38, N(em)=0 (full systematics included) Wouter Verkerke, NIKHEF

Dilepton event yield • Calculate ttbar event yield – subtract all background estimates from data yield • ee: Nobs = 2 N(ttbar) = 1. 31 ± 0. 37 • mm: Nobs = 3 N(ttbar) = 2. 10 ± 0. 41 • em: Nobs = 4 N(ttbar) = 2. 97 ± 0. 57 Wouter Verkerke, NIKHEF

Dilepton cross section • Calculate cross section using MC acceptance – ee: e(MC@NLO) = 0. 15 ± 0. 02 – mm: e(MC@NLO) = 0. 23 ± 0. 02 – em: e(MC@NLO) = 0. 25 ± 0. 02 σ(ee) = 175 ± 195(stat)± 69(syst) pb σ(mm) = 179 ± 150(syst) ± 53(syst) pb σ(em) = 118 ± 82(stat) ± 19(syst) pb – Combined cross section calculation in progress Need careful propagated of correlated systematic errors. – Have tool to do this – was developed from dilepton cross section combination in 2009 Summer PUB notes. Expect numbers by end of this week NUMBERS NOT FINAL – CHANGES STILL Wouter EXPECTED Verkerke, NIKHEF

Dilepton acceptance systematics • Systematic uncertainties on acceptance by component Wouter Verkerke, NIKHEF

Next steps • Await calculation of final numbers (days). Once that is done: • Schedule top-wg analysis approval meeting • Send paper draft to Ed Board Wouter Verkerke, NIKHEF

Documentation status • Paper draft (~22 pages) is reasonably good shape, mostly needs final numbers • Series of 11 internal notes in preparation since 4 months documenting all analysis aspects in detail – 1: Lepton selection and performance 2: Jet selection and performance 3: ETmiss performance 4: B-tagging performance 5: Fake lepton background 6: W+jets background 7: Drell-Yan background 8: Top. Inputs and D 2 PD software 9: MC samples 10: Lepton+jets analysis strategy 11: Dilepton analysis strategy – Expect updated versions of notes 1 -9 by the end of this week – Working on making updated notes 10, 11 available next week. Will include summary of essential results of Notes 1 -9 here to make these the primary supporting documents for collaboration circulation. (But all notes will go into CDS as COM notes) Wouter Verkerke, NIKHEF