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

Generators: MC@NLO, Herwig, Pythia Example: distributions on top-anti-top characteristic – Pt of the whole system Pt(tt system) Herwig & MCatNLO agree at low Pt, At large Pt MCatNLO ‘harder’ PYTHIA completely off Many more comparisons: see talks in top meeting Stan Bentvelsen Moriond-QCD 2004

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

Commissioning the detectors 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 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

Commissioning the detectors Most important background for top: W+4 jets Leptonic decay of W, with 4 extra ‘light’ jets Signal plus background at initial phase of LHC 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

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

Can we see the W? (4 jets sample) Select the 2 jets with highest resulting Pt Poor peak No peak in background 150 pb-1 Select 2 jets with invariant mass closest to Mw (80.4 GeV) Large peak in background Enormous bias Not useable! Stan Bentvelsen Moriond-QCD 2004

Stan Bentvelsen Moriond-QCD 2004 Fit to W mass Fit possible as well Not easy to converge Fix width to 6 GeV 150 pb-1 mean σ(stat)  in peak 0.9% 5% Mtop 167.0 0.8 Mw 77.8 0.7 Stan Bentvelsen Moriond-QCD 2004

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 energy scaled Redo all with MC@NLO, Herwig and Pythia Redo analysis with doubled W+4jet background (stat indep) Effect on selection, via selection cuts Effect on W and T mass Quote standard deviation as systematic uncertainty Mtop stat MW mcatnlo 1 158,86 0,73 74,36 0,74 2 163,05 0,76 76,21 3 167,03 77,75 0,66 4 171,40 79,23 0,70 5 175,33 0,77 82,28 0,97 herwig 158,66 0,81 74,67 0,69 162,72 0,87 76,64 0,72 166,85 0,85 77,86 0,65 170,43 0,86 79,43 174,45 81,90 pythia 162,65 75,85 167,62 77,25 171,42 0,79 79,17 0,63 175,23 0,92 81,33 0,64 178,78 0,88 83,34 0,67 Stan Bentvelsen Moriond-QCD 2004

Some results… (still no b-tag) 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   mean std percent Mtop raw 168,12 6,14 3,7% corrected 170,79 3,71 2,2% σ(top) 817,15 94,79 11,6% 806,71 79,74 9,9% Stan Bentvelsen Moriond-QCD 2004

Stan Bentvelsen Moriond-QCD 2004 Lower luminosity? Go down to 30 pb-1 Rebin needed: Both W and T peaks observable 30 pb-1 mean σ(stat)  in peak 0.8% 17% Mtop 170.0 3.2 Mw 78.3 1.0 30 pb-1 Stan Bentvelsen Moriond-QCD 2004

Stan Bentvelsen Moriond-QCD 2004 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

Stan Bentvelsen Moriond-QCD 2004