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B tagging performances for ttbar semileptonic events

B tagging performances for ttbar semileptonic events. Philippe Doublet, Roman Pöschl , François Richard – LAL ILD Detector and Optimisation Meeting September 8th , 2010. Introduction.

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B tagging performances for ttbar semileptonic events

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  1. B tagging performances for ttbarsemileptonicevents Philippe Doublet, Roman Pöschl, François Richard – LAL ILD Detector and Optimisation Meeting September 8th, 2010

  2. Introduction • tt  (bW)(bW)  (bqq)(bqq) = 6 jets(bqq)(blv) = 4 jets + 1 lepton (blv)(blv) = 2 jets + 2 leptons • Wewant to understand the b tagging performances in thiscrowdedenvironment • We use ILCSOFT v01-06 withLCFIVertex v00-03 and the ILD LOI DSTs • Data shownhere : tt  bbqqµv (ep+1,em-1) similarresultswith e- and (ep-1,em+1)

  3. Pre-treatment • Semileptonicevents are analysed by removing the highestenergeticlepton • 80% within ±5 GeV • Then« SatoruJetFinder » isapplied and forced to 4 jets • Then the wholechain of b taggingisappliedwith default pre-trained Neural Nets. • In the end : 1 lepton + 4 b tagged jets (withtrueflavour)

  4. Why43% of the B jets have a b tag lessthan0.8 ? First overview of b tagging Light jets (udsc) B jets Counts for all jets (4 x Nb events)

  5. Second overview of B tagging • For tt events, wemust find 2 B jets • The 2ndB jet is more difficult to tag Jet with the highest B tag among the 4 Jet with the second highest B tag Real B jet Light jet (large contamination) Real B jet Light jet (negligible) Bad taggingefficiency

  6. Variables used for the input of the NNet • For each jet, 9 variables are of interest • Number of vertices • Impact parameter joint probabilities : P(r-φ), P(r-z) • Number of tracks • Decaylength L, itssignificance = L / σL • Rawmomentum • « pTcorrected » mass = invariant mass corrected for neutral B and D decays • Probability of secondaryvertices (deducedfrom the chi2 of the tracks’ fit)  does not seem to be discriminant

  7. Comparison : real B jets and light jets More vertices Main differencies Largersignificance Mb ~ 4 GeV Not useful

  8. Comparison : real B jets withbtag>0.8 Small significance Small decaylength

  9. Possible reasons • Manybad(btag<0.8) cases seem to occurbecausethe B decayis in the beampipe as indicated in the decaylength. • Significanceissmallerthanexpected: badtrackmeasurementcombinedwithsmalldecaylength ? • pTcorrected mass and rawmomentum are smaller : someparticles (neutrals ?) lost • Can weperformbetter ?

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