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Status Sep 2007 Duc Bao Ta, Markus Cristinziani University of Bonn

Cross section measurement in the dilepton channel. Status Sep 2007 Duc Bao Ta, Markus Cristinziani University of Bonn. Samples v12. 5200 ttbar (dilepton, e,µ, t ) Using TopView 00-12-13, with 1mmfix 5200 ttbar lepton+jets 5144-5146 Z->ll Pythia 5985, 5986, 5987, WW, ZZ, WZ Herwig

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Status Sep 2007 Duc Bao Ta, Markus Cristinziani University of Bonn

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  1. Cross section measurement in the dilepton channel Status Sep 2007 Duc Bao Ta, Markus Cristinziani University of Bonn

  2. Samples v12 • 5200 ttbar (dilepton, e,µ,t) • Using TopView 00-12-13, with 1mmfix • 5200 ttbar lepton+jets • 5144-5146 Z->ll Pythia • 5985, 5986, 5987, WW, ZZ, WZ Herwig • Interesting samples, but statistics very low (after cuts): • W+Jets: 8240-8251 • QCD: 5061-5064

  3. Cut analysis • Preselection cuts: • Two isolated, opposite charged leptons pT>20 GeV • Two Jets, no b-tagging • |h|<2.5 for all visible objects • tight electrons removed between 1.35<||<1.65 (tile gap bug) • tight muons removed ||<0.1 and 1.0<||<1.3 (adopted according to Jorgen’s selection for comparison) • Comparison with Jorgen’s pre-selection ongoing (also in contact with dilepton top mass measurement selection)

  4. Updated numbers for cut analysis: • Cuts for best ratio: • Pre-selection cuts for all object already enough • Muon overlap (DR<0.4) with jets removed if log (|DpT(Jet,µ)|/pT(µ))>1 • Same lepton channels: • Veto on M(ll) 85-95GeV • ee: • ET,Miss>35 GeV • eµ: • ET,Miss>20 GeV • µµ: • ET,Miss>35 GeV

  5. Cut analysis ee eµ Events Events Number of Jets Number of Jets @ 100pb-1 Cut on all other jets: 20 GeV µµ Events Number of Jets

  6. Numbers • Small changes with the additional h cuts • Absolut numbers for background low

  7. Ensemble tests • With preselection cuts only, for all subchannels: • Tested with different S/B and number of sig events • First fit to curve gives, averaged over all S/B • Dsigrel(nsig)=exp(a) nsig-b, with a 0.32±0.02 and b =0.483±0.002 • relative error almost sqrt(n) behaviour

  8. Likelihood method • Using a likelihood -> cross section measurement • pdf variables: • |Df| Lepton0 MET, |Df| Lepton1 MET, |Df| Jet0 MET, |S cos q| Leptons -> |Sh| Leptons, (|Df| Leptons) Example: |Df| Jet0 MET sig bkg Normalized pdf-fit S/B 5:1 mix and random data From pdfs sig bkg

  9. Emsembles test with real cuts, diffucult due to low statistics after cuts: • No cut • µµ channel • µµ channelMET>35GeV • Trying new approach with binned likelihood fit with TFractionFitter • Or use ATLFAST samples when available and validated Example: |Df| Jet0 MET bkg sig

  10. Perspective for 10pb-1, 100pb-1 and 1000pb-1 • For cut analysis: rough estimate of statistical errors with all channels (precuts and MET>30GeV): • Simple approach, take errors from above: • ignoring trigger efficiencies and error on theoretical x-section

  11. Perspective for 10pb-1, 100pb-1 and 1000pb-1 • For likelihood analysis evaluate fit to „relative error vs. number of signal events“ • calculate expected number from cut analysis • No errors on fitted pdfs • For both estimations no systematic errors evaluated so far, result show statistical power of the methods

  12. Both analysis give low statistical error, so most important to evaluate systematics • Cut analysis also limited by error on background numbers

  13. Summary and Outlook • Comparison of pre-selection with other groups ongoing • Cut analysis and likelihood method give already satisfactory statistical errors • More statistics is necessary for both analysis (ATLFAST) • Switch and/or try binned likelihood fit • Evaluate systematic errors: • Vary cuts and jet energies

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