Study of Top properties at LHC Stan Bentvelsen

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

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

3 Background: Alpgen W+4 jets n Several samples produced ¡ n Very CPU intense Un-weighting to W lepton (e, , ) decay ¡ Production 1 n Effective : 4390 pb ¡ ¡ ¡ 108401 unweighted events generated (3. 6 10 -6 efficiency) 2. 57% (2784) events pass first top 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 Production 2 n Effective : 2430 pb ¡ ¡ 380740 unweighted events generated (2. 6 10 -5 efficiency) 3. 41% (13002) events pass first selection FAlpgen, Monte Carlo has ‘hard’ matrix element for 4 extra jets (not available in Pythia/Herwig) 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 Stan Bentvelsen Moriond-QCD 2004

4 Commissioning the detectors n Determination cross section and MTop in initial phase of data taking ¡ Use ‘Golden plated’ lepton+jet channel Calibrating detector in comissioning phase Assume pessimistic scenario: -) No b-tagging -) No jet calibration -) But: Good lepton identification n Selection: ¡ ¡ n No background included Isolated lepton with PT>20 Ge. V Exactly 4 jets ( R=0. 4) with PT>40 Ge. V Reconstruction: ¡ Select 3 jets with maximal resulting PT Stan Bentvelsen Moriond-QCD 2004

5 Commissioning the detectors n Most important background for top: W+4 jets ¡ n Leptonic decay of W, with 4 extra ‘light’ jets Signal plus background at initial phase of LHC L = 150 pb-1 (2/3 days low lumi) With extreme simple selection and reconstruction the top -peak should be visible at LHC Stan Bentvelsen Moriond-QCD 2004

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

7 Can we see the W? n (4 jets sample) Select the 2 jets with highest resulting Pt ¡ ¡ Poor peak No peak in background 150 pb-1 n Select 2 jets with invariant mass closest to Mw (80. 4 Ge. V) ¡ ¡ ¡ Large peak in background Enormous bias Not useable! Stan Bentvelsen Moriond-QCD 2004

8 Fit to W mass n Fit possible as well ¡ ¡ Not easy to converge Fix width to 6 Ge. V 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

9 Energy scale / MC dependence n Variation of the jet energy scale to infer systematics ¡ ¡ 1) 2) 3) n n n Bjet scale: Light scale: 0. 92 – 0. 96 – 1. 00 – 1. 04 – 1. 08 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+4 jet background (stat indep) Effect on selection, via selection cuts Effect on W and T mass Quote standard deviation as systematic uncertainty Mtop stat MW stat mcatnlo 1 158, 86 0, 73 74, 36 0, 74 mcatnlo 2 163, 05 0, 76 76, 21 0, 73 mcatnlo 3 167, 03 0, 76 77, 75 0, 66 mcatnlo 4 171, 40 0, 74 79, 23 0, 70 mcatnlo 5 175, 33 0, 77 82, 28 0, 97 herwig 1 158, 66 0, 81 74, 67 0, 69 herwig 2 162, 72 0, 87 76, 64 0, 72 herwig 3 166, 85 0, 85 77, 86 0, 65 herwig 4 170, 43 0, 86 79, 43 0, 66 herwig 5 174, 45 0, 81 81, 90 0, 69 pythia 1 162, 65 0, 81 75, 85 0, 69 pythia 2 167, 62 0, 81 77, 25 0, 72 pythia 3 171, 42 0, 79 79, 17 0, 63 pythia 4 175, 23 0, 92 81, 33 0, 64 pythia 5 Bentvelsen 178, 78 0, 88 83, 34 Stan Moriond-QCD 2004 0, 67

10 Some results… (still no b-tag) n Determine Mtop and σ(top) ¡ ¡ ¡ n ‘Raw’, i. e. no correction for jet scale ‘Corrected’, i. e. apply percentage difference of W-peak to the reconstructed top Input values: n n Mtop ¡ std n ¡ n percent n n raw 168, 12 6, 14 3, 7% corrected 170, 79 3, 71 2, 2% ¡ raw 817, 15 94, 79 11, 6% corrected 806, 71 79, 74 9, 9% since all generators are used, with all energy scales; double counting W+4 jets rate can be measured from data Systematics underestimated: n n σ(top) Systematics overestimated: n Mtop = 175 Ge. V σ(top) = 833 pb mean How to judge these values? No real FSR variation No other backgrounds (e. g. WW, QCD) Trigger Non-uniformities Need further detailed studies! Full simulation Statistic uncertainty already smaller Stan Bentvelsen Moriond-QCD 2004

11 Lower luminosity? n 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 Stan Bentvelsen Moriond-QCD 2004

12 What with b-tagging on? n Now assume full b-tagging ¡ n 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