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Top at startup of LHC

Top at startup of LHC. Stan Bentvelsen (NIKHEF) October 2 nd , 2004. Top commissioning studies. Top physics at LHC Top one of ‘easiest’ bread and butter Cross section 830 ±100 pb Used as calibration tool Rich in ‘precision and new physics’ Top mass M t , cross section σ t

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Top at startup of LHC

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  1. Top at startup of LHC Stan Bentvelsen (NIKHEF) October 2nd, 2004

  2. Top commissioning studies • Top physics at LHC • Top one of ‘easiest’ bread and butter • Cross section 830±100 pb • Used as calibration tool • Rich in ‘precision and new physics’ • Top mass Mt, cross section σt • Resonances decaying into top • Commissioning for the top group: • Summarize studies already performed • Tasks to do before startup • What do we need as input from others? • What can/should we provide to collaboration? • Goals Semi-leptonic top channel detector tools involved: Lepton reconstruction Missing ET Jets + calibration B-tagging Work done together with Marina Cobal Stan Bentvelsen Commissioning Meeting Freiburg

  3. First evaluation of statistical uncertainty on σtop and Mtop / -st 0.2 % 0.4% 2.5% Initial commissioning studies Gold-plated channel : single lepton • pT (lep) > 20 GeV • pTmiss > 20 GeV • ≥ 4 jets with pT > 40 GeV • ≥ 2 b-tagged jets • | mjjb-<mjjb>|< 20 GeV P. Grenier For initial run at LHC: (L = 1033 cm-2s-1) • Pretty small uncer-tainties after very short time of LHC running! Stan Bentvelsen Commissioning Meeting Freiburg

  4. b-jet scale uncertainty  Mtop 1% 0.7 GeV 5% 3.5 GeV 10% 7 GeV Systematic error on Mtop (TDR performance, 10 fb-1) Initial performance : uncertainty on b-jet scale expected to dominate Cfr: 10% on q-jet scale  3 GeV on Mtop Stan Bentvelsen Commissioning Meeting Freiburg

  5. Various scenarios currently under study • pp collisions • What variations in predictions of t-tbar – which generator to use? • Underlying event parameterization • Background estimation from MC • Try to be as independent from MC as possible. • Detector pessimistic scenarios • Partly or non-working b-tagging at startup • Dead regions in the LArg • Jet energy scale • Get good ‘feel’ for important systematic uncertainties – use data to check data • Software tools • Many studies (not all!) only in fast simulation • It is clear we need to redo most important studies with full simulation • Estimate realistic potential for top physics during the first few months of running Stan Bentvelsen Commissioning Meeting Freiburg

  6. Status of top event generators • ‘Old’ Leading Order MC: • Pythia: full standalone MC • Herwig: full standalone MC • TopRex (include spin correlations – interfaced to Pythia) • ‘New’ NLO QCD calculations implemented in MC • MC@NLO – interfaced to Herwig shower and fragmentation • This is relevant theoretical improvement • Superseeds the old Pythia and Herwig MC’s. • Validation done for this generator • Currently DC2 processes 106 MC@NLO t-tbar events • Crucial for us to analyse these • Waiting for Tier0 exercise to obtain reconstructed objects Stan Bentvelsen Commissioning Meeting Freiburg

  7. Generators: MC@NLO, Herwig, Pythia • PT(tt system) • Herwig & MCatNLO agree at low PT, • At large PT MCatNLO ‘harder’ • PYTHIA completely off Example: distributions on top-anti-top characteristic – PT of the whole system PT of t-tbar system is balanced by ISR & FSR Many more comparisons: see talks in top meeting Stan Bentvelsen Commissioning Meeting Freiburg

  8. Underlying event (UE) / minimum bias • Extremely difficult to predict the magnitude of the UEat LHC • Will have to learn much more from Tevatron before startup • Various models exists • Herwig’s UE and minimum bias shows much less activity compared to Pythia. • This has always been a problem in Herwig. • Jimmy is developed as alternative model for UE at ep collisions • Various ‘tunings’ exist – leading to wildly different results • More work is mandatory here • Wish list to generate fully simulated events with Jimmy during DC2 post-production Stan Bentvelsen Commissioning Meeting Freiburg

  9. Tune of Butterworth Standard Jimmy Standard Pythia Standard Herwig Running at LHC energies Stan Bentvelsen Commissioning Meeting Freiburg

  10. 10 GeV Jimmy UE: Cells & Jets in Atlfast • Herwig vs Jimmy • LO t-tbar • At jet-level effectreduced Cell multiplicity Cluster multiplicity Jet multiplicity Stan Bentvelsen Commissioning Meeting Freiburg

  11. Top peak for various reconstruction methods Difference in mass can be as large as 5 GeV Really need data to check data on UE Study effect better (as said) Reconstruct the top Stan Bentvelsen Commissioning Meeting Freiburg

  12. Background events • Top physics background • Mistags or fake tags • Non-W (QCD) • W+jets, Wbbar, Wccbar • Wc • WW,WZ,ZZ • Z  tt • Single top • AlpGen W+4 jets samples produced • Very CPU intense (NIKHEF grid) • Un-weighting to W lepton (e,,) decay • Production: • Effective : 2430 pb • 380740 unweighted events generated (2.6 10-5 efficiency) • 3.41% (13002) events pass first selection • ~ 150 pb-1 W+4jet background available Largest background is W+4 jet. This background cannot be simulated by Pythia or Herwig shower process. Dedicated generator needed: e.g. AlpGen. Large uncertainties in rate Ultimately, get this rate from data itself. For example, measure Z+4 jets rate in data, and determine ratio (Z+4 jets)/(W+4 jets) from MC W+4 extra light jets Jet: Pt>10, ||<2.5, R>0.4 No lepton cuts Initial grid: 200000*3 Events: 150·106 Jobs: 98 ~1.5 1010 events! Stan Bentvelsen Commissioning Meeting Freiburg

  13. Non-W (QCD-multijet) background • Not possible to realistically generate this background • Crucially depends on Atlas’ capabilities to minimize mis-identification and increase e/ separation • This background has to be obtained from data itself • E.g. method developed by CDF during run-1: • Rely now on e/ separation of 10-5 • Use missing ET vs lepton isolation to define 4 regions: • A. Low lepton quality and small missing ET • Mostly non-W events (i.e. QCD background) • B. High lepton quality and small missing ET • Observation reduction QCD background by lepton quality cuts • C. Low lepton quality and high missing ET • W enriched sample with a fraction of QCD background • D. High lepton quality and high missing ET • W enriched sample, fraction of QCD estimated by (B·C)/(A·D) Stan Bentvelsen Commissioning Meeting Freiburg

  14. Detector scenarios: HV problems • Effects of dead HV regions om Mtop • Argon gap (width ~ 4 mm) is split in two half gaps by the electrode • HV by Dη x Dφ = 0.2 x 0.2 (or 0.1 x 0.2) sectors, separately in each half gap • ~ 33 / 1024 sectors where we may be unable to set the HV on one half gap  multiply energy by 2 to recover particle A.I. Etienvre, J.P. Meyer, J.Schwindling Stan Bentvelsen Commissioning Meeting Freiburg

  15. Analysis EM clusters • 100 000 tt events (~ 1.5 days at LHC at low L) • Simulated using PYTHIA + ATLSIM / G3 (initial detector, h < 3.2) • Reconstructed using ATHENA 7.0.0 • Preselection of events: • At least one recontructed e or μ with PT > 20 GeV and η < 2.5 • ETmiss > 20 GeV • At least 4 jets with PT > 20 GeV and h < 2.5 Jets Stan Bentvelsen Commissioning Meeting Freiburg

  16. Results • If the 33 weak HV sectors die (very pessimistic), the effects on the top mass measurement, after a crude recalibration, are: • Loss of signal: < 8 % • Increase in background: not studied • Displacement of the peak of the mass distribution: -0.2 GeV Mtop(without ) – mtop(with dead regions) • This effect on the Top mass is (much) better known than other systematic uncertainties Stan Bentvelsen Commissioning Meeting Freiburg

  17. Detector scenarios: b-tagging • Precise alignment of ID can be reached only after few months of data taking. • The impact of misalignment can be much larger than having 2 instead of 3 layers • Top events to evaluate b-tagging efficiencies from data • Select a pure t-tbar sample with tight kinematical cuts • Count the number of events with at least 1 tagged jet • Compare 0 vs 1 vs 2 b-tagged jets in top events • Can expect the b-tagging efficiency different in data from MC • In most pessimistic scenario b-tagging is absent at start • Can we observe the top without b-tagging? Stan Bentvelsen Commissioning Meeting Freiburg

  18. Selection: Isolated lepton with PT>20 GeV Exactly 4 jets (R=0.4) with PT>40 GeV Reconstruction: Select 3 jets with maximal resulting PT Non b-tag tops t  bjj V. Kostiouchine M (bjj) Stan Bentvelsen Commissioning Meeting Freiburg

  19. Signal plus background at initial phase of LHC Most important background for top: W+4 jets Leptonic decay of W, with 4 extra ‘light’ jets Non btag: top sample With extreme simple selection and reconstruction the top-peak should be visible at LHC L = 150 pb-1 (2/3 days low lumi) Stan Bentvelsen Commissioning Meeting Freiburg

  20. Extraction of top signal • Fit to signal and background • Gaussian signal • 4th order polynomal Chebechev background • In this fit the width of top is fixed at 12 GeV 150 pb-1 Extract cross section and Mtop? Stan Bentvelsen Commissioning Meeting Freiburg

  21. Select the 2 jets with highest resulting PT W peak visible in signal No peak in background Better ideas well possible! E.g. utilizing 2 body decay in top rest frame. Select 2 jets with invariant mass closest to Mw (80.4 GeV) Large peak in background Enormous bias Not useable! Can we see the W? (4 jets sample) 150 pb-1 Stan Bentvelsen Commissioning Meeting Freiburg

  22. Fit to W mass • Fit signal and background also possible for W-mass • Not easy to converge fit • Fix W width to 6 GeV These numbers for statistical uncertainties are consistent to the earlier study Stan Bentvelsen Commissioning Meeting Freiburg

  23. Jet Energy scale / MC dependence • Variation of the jet energy scale to infer systematics • Bjet scale: 0.92 – 0.96 – 1.00 – 1.04 – 1.08 • Light scale: 0.94 – 0.98 – 1.00 – 1.02 – 1.04 (1) (2) (3) (4) (5) • Analysis with jet energyscaled • All with MC@NLO, Herwigand Pythia; • Redo analysis with doubled W+4jet background (stat indep) • Determine Mtop and σ(top) • ‘Raw’, i.e. no correction for jet scale • ‘Corrected’, i.e. apply percentage difference of W-peak to the reconstructed top • Dependence on top mass reduced by scaling with W: • Rms Raw: 6.2 GeV • Rms Scaled: 1.2 GeV • Large dependence σ(top) on jet energy scale Via event selection. Stan Bentvelsen Commissioning Meeting Freiburg

  24. Using 150 pb-1 of data: Statistic uncertainty already smaller than these systematic variations Note these numbers are very preliminary – Luminosity uncertainty (15-20%) to be added! How to judge these values? Systematics overestimated: since all generators are used, with all energy scales; double counting W+4jets rate can be measured from data Systematics underestimated: No real FSR variation No other backgrounds(e.g. WW, QCD) Trigger Non-uniformities Need further detailed studies! Please don’t thrust any of this without Full simulation Some results… (still no b-tag) Stan Bentvelsen Commissioning Meeting Freiburg

  25. Lower luminosity? • Go down to 30 pb-1 • Both W and T peaks already observable • See something! 30 pb-1 Stan Bentvelsen Commissioning Meeting Freiburg

  26. Fit the lepton+hadronic top • Full kinematic fit to t-tbar system – no b-tagging • Fit the neutrino Px and Py - and get Pz via W constraint • Use W-mass constraints • Require equal top masses for lepton and hadron side • Repeat fit over all possible combinations of jets • Better suited for mass than for cross section • Looks promising but further study needed… Background shows structure Cut on quality of fit (X2) E. Bos Stan Bentvelsen Commissioning Meeting Freiburg

  27. What with b-tagging on? • Now assume full b-tagging • Efficiency 60%, mistag 1% • Background is rapidly decreasing • See for example the W-mass peaks for 1 and 2 b-tags • Same selection: 4 jet events • W reconstructed as dijet mass; |Mjj-80.4| minimal for light-jets j 150 pb-1 Stan Bentvelsen Commissioning Meeting Freiburg

  28. Reconstruct top mass • Sharp top mass peaks with little background • Only use events for which |Mjj-80.4| < 20 GeV • Standard kinematic top reconstruction for 1 and 2 b-tags • Background from W+4jets removed by b-tag requirement • These results are very sensitive to b mis-tag rate 150 pb-1 Stan Bentvelsen Commissioning Meeting Freiburg

  29. Mtt at startup? • Can we determine dσ(tt)/dmtt without b-tagging? • Interesting to SuSy models that modify this cross section • Mtt is invariant mass over all final state to tt products • In principle no assignment of b-jet to top is needed • Suffer a lotfrom background • No reliable measurement of mtt without b-tagging W+4jet background Stan Bentvelsen Commissioning Meeting Freiburg

  30. Resonances decaying to t-tbar • Observe heavy resonances Xt-tbar during commissioning? • Plot invariant mass of 4 jets + lepton + neutrino • No intelligence in determining Pz neutrino here • Insert resonance at 2000 GeV • Cross section * BR(tt) = 350 pb • Heavy resonances with large cross sections visible ‘True’ mtt distributions of resonance and ttbar events Stan Bentvelsen Commissioning Meeting Freiburg

  31. Need checks with full simulation • We are eagerly waiting for reconstructed DC2 events • Repeat with full simulation for Rome next year Stan Bentvelsen Commissioning Meeting Freiburg

  32. Inputs to the top group Estimate of the single electron trigger efficiency Can be done by using the Z triggered as single electron How much time is needed to arrive to a reasonable evaluation of this efficiency? Estimate of the initial lepton identification efficiency Estimate of the integrated luminosity At the beginning the precision on L should be around 10-20%. The ultimate precision should be < 5% Eventually: B-tagging efficiency Jet scales What we have to provide Top candidates enriched samples A “pure” one, obtained with quite tight selection criteria A “loose” one: a more “background enriched” sample, to be used as control sample for background calculations etc… Estimate of a light jet energy scale correction Assume 10% for light and b-quark jets, look at effect on Mtop and stop Assume that at the very beginning only the EM scale is known (means: do not put any weight on the hadronic scale) Output: provide the MW peak to rescale the light jets Estimate of the b-tagging efficiency Top group in Atlas: Stan Bentvelsen Commissioning Meeting Freiburg

  33. Summary / Conclusion Understand the interplay between using the top signal as tool to improve the understanding of the detector (b-tagging, jet E scale, ID, etc..) and top precision measurements • At LHC Top more easily found than W’s in 4 jet channel • Using extreme simple selection, no b-tagging • Need more work on background estimation, both from W’s and QCD, e/ ratio, trigger, lepton ID, etc … • Remove dependence of results on MC as much as possible • Using few days of data taking (150 pb-1) • Current estimate on cross section accuracy of (ball park) 10% plus luminosity uncertainty • Interestingly mass of top via fits to mass peak looks promising(use W-mass as constraint to jet-scale) • Need better ideas to isolate very pure top sample without b-tagging • Its clear we need full simulation • Eagerly awaiting reconstructed DC2 events to repeat/extend these studies Stan Bentvelsen Commissioning Meeting Freiburg

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