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Study of Top properties at LHC

Study of Top properties at LHC. Stan Bentvelsen ‘Moriond QCD week’ La Thuile, Mar 28 - Apr 4, 2004. Generators: MC@NLO, Herwig, Pythia. Pt(tt system) Herwig & MCatNLO agree at low Pt, At large Pt MCatNLO ‘harder’ PYTHIA completely off.

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Study of Top properties at LHC

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  1. Study of Top properties at LHC Stan Bentvelsen ‘Moriond QCD week’ La Thuile, Mar 28 - Apr 4, 2004

  2. 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 Many more comparisons: see talks in top meeting Stan Bentvelsen Moriond-QCD 2004

  3. Background: Alpgen W+4jets • Several samples produced • Very CPU intense • Un-weighting to W lepton (e,,) decay • Production 1 • Effective : 4390 pb • 108401 unweighted events generated (3.6 10-6 efficiency) • 2.57% (2784) events pass first top selection • Production 2 • Effective : 2430 pb • 380740 unweighted events generated (2.6 10-5 efficiency) • 3.41% (13002) events pass first selection • W+4 extra light jets • Jet: Pt>10, ||<3.0, R>0.3 • No lepton cuts • Initial grid: 200000*3 • Events: 150·106 • Jobs: 198 • 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 • Alpgen, Monte Carlo has ‘hard’ matrix element for 4 extra jets(not available in Pythia/Herwig) Stan Bentvelsen Moriond-QCD 2004

  4. Determination cross section and MTop in initial phase of data taking Use ‘Golden plated’ lepton+jet channel 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 Commissioning the detectors • Calibrating detector in comissioning phase • Assume pessimistic scenario: • -) No b-tagging • -) No jet calibration • -) But: Good lepton identification No background included Stan Bentvelsen Moriond-QCD 2004

  5. 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 Commissioning the detectors 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 Moriond-QCD 2004

  6. Extraction of top signal • Fit to signal and background • Gaussian signal • 4th order polynomal background Extract cross section and Mtop? Stan Bentvelsen Moriond-QCD 2004

  7. Select the 2 jets with highest resulting Pt Poor peak No peak in background 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 Moriond-QCD 2004

  8. Fit to W mass • Fit possible as well • Not easy to converge • Fix width to 6 GeV Stan Bentvelsen Moriond-QCD 2004

  9. 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 • Redo analysis with jet energyscaled • Redo all with MC@NLO, Herwigand Pythia • Redo analysis with doubled W+4jet background (stat indep) • Effect on selection, via selection cuts • Effect on W and T mass • Quote standard deviationas systematic uncertainty Stan Bentvelsen Moriond-QCD 2004

  10. 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 Input values: Mtop = 175 GeV σ(top) = 833 pb Statistic uncertainty already smaller 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! Full simulation Some results… (still no b-tag) Stan Bentvelsen Moriond-QCD 2004

  11. Lower luminosity? • Go down to 30 pb-1 • Rebin needed: • Both W and T peaks observable 30 pb-1 Stan Bentvelsen Moriond-QCD 2004

  12. 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 Stan Bentvelsen Moriond-QCD 2004

  13. Stan Bentvelsen Moriond-QCD 2004

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