Commissioning the detectors

1. 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 Commissioning

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

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

4. 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 Fully consistent to earlier studies as shown by Marina Stan Bentvelsen Commissioning

5. Scale variations: 5 scales (see previous slide) for each of the three generators (MC@NLO Pythia Herwig) And for MC@NLO with 2 times background added Correction • 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 • Not granted Mjj gives correct MW, i.e. for hard FSR events… • Dependence on top mass reduced by scaling with W: • Rms of top masses: • Raw: 6.2 GeV • Scaled: 1.2 GeV • Note: Here simple rescaling of Top mass – not of the jet-energies themselves! • Large dependence σ(top) on jet energy scale • Via event selection. Stan Bentvelsen Commissioning

6. Using 150 pb-1 of data: Statistic uncertainty already smaller than these systematic variations Note these numbers are very preliminary – No luminosity uncertainty included here! 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

7. Is this an idea: ‘Purify’ W-sample • Select events for which reconstructed top mass: 150<Mtop<200 GeV • Look at W-mass: • Better S/B ratio • Need pursued further Stan Bentvelsen Commissioning

8. 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 Stan Bentvelsen Commissioning

9. 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 m with PT > 20 GeV and h < 2.5 • ETmiss > 20 GeV • At least 4 jets with PT > 20 GeV and h < 2.5 Jets Stan Bentvelsen Commissioning

10. 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) • The effect on the mass distribution should be known (much) better than the effects from the other systematic errors Stan Bentvelsen Commissioning

11. etag = probability to tag at least one jet in a top event etag = eb-tag + enon-b – (eb-tag .enon-b) enon-b = ec-tag + enonhf eb-tag is the sum of these possibilities: Probability to tag 1 b-jet in the event, when 1 is found in the detector Probability to tag 1 b-jet when 1 is found in the detector Probability to tag 2 b-jets when 2 are found in the detector First simple evaluation (counting method): Select a very pure ttbar sample with tight kinematical cuts Count the number of events with at least 1 tagged b-jet Divide this number by the number of pre-tag candidate events B-tagging efficiency Stan Bentvelsen Commissioning

12. B-tagging algorithm Estimate of the non-perfect ID alignment E.g. for the pixels (from S. Rozanov): - after 2 months: 100 mm after 4 months: 20 mm (0.6 effect on b-tag efficiency) after 6 months: 10 mm after 8 months: 5 mm (design performance!!!) asymptotic ?\ Staging pixel detector 2 layers -> 0.6 effect on b-tag efficiency “Inputs” to the top group Stan Bentvelsen Commissioning

13. e’s are measured in MC (where flavor content of jets in top events can be precisely determined). The tagging algorithm however will perform differently on data and on MC. This difference can be accounted for by introducing a scale factor: SF F1b = fraction of events with 1 taggable jets F2b = fraction of events with 2 taggable jets B-tagging efficiency Probability to tag one B-jet when two are found Probability to tag one B-jet when one is found Probability to tag two B-jets when two are found Stan Bentvelsen Commissioning

14. Cross Section • The ttbar cross section formula may be written as: • We define the ttbar event detection efficiency as: • ekin= fraction of ttbar events which pass the requirements • etrig = trigger efficiency for identifying high PT leptons • etag event = efficiency to tag at least one tight jet in a ttbar event • Need to insert trigger efficiencies and kinematic acceptances. Stan Bentvelsen Commissioning