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On b-tagging in ATLAS

List of taggers Our CSC contribution Ongoing analyses. On b-tagging in ATLAS. A.Cheplakov, S.Thompson. 1. Glasgow bi-weekly, 23 Oct 2007. b - taggers (12.0.x). JetProb – simply combines probabilities of track to originate from the primary vertex (from ALEPH time)

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On b-tagging in ATLAS

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  1. List of taggers Our CSC contribution Ongoing analyses On b-tagging in ATLAS A.Cheplakov, S.Thompson 1 Glasgow bi-weekly, 23 Oct 2007

  2. b - taggers (12.0.x) • JetProb – simply combines probabilities of track to originate from the primary vertex (from ALEPH time) • IP1D, IP2D, IP3D – calculated from the signed transverse (a0) and longitudinal (z0) impact parameters using significance S of a0 and z0 and likelihood functions P_b and P_u: track weight W_t = P_b(S)/P_u(S) and jet weight is sum of ln(W_t) for all good tracks in the jet • SoftMuonTag, SoftElectronTag – calculates discriminating functions for soft (<20GeV/c) lepton identification inside b-jet for a number of variables (#tracks in TRT, a0, relative pT…) • SecVtxBU, SV1, SV2 – all two track vertices are combined to find a secondary vertex inside the jet at a certain distance from the primary vertex, discriminating variable is obtained as a logarithm of ratio of two PDFs calculated from 3D (SV2) or 2D+1D (SV1) calibration histograms for inv.mass of all particles from reconstructed secondary vertex, fraction of total jet energy and #2-track vertices • lhSig = IP1D + IP2D + SecVtxBU • weight= SV1+IP3D – recommended tagger for CSC analyses • For details and references: https://twiki.cern.ch/twiki/bin/view/Atlas/FlavourTagging • MORE taggers are welcome! 2

  3. Space and vertex b-tagging (from A.Rozanov @ Marseille’May 2007) Jet weight Jet weight SVX energy fraction SVX mass N(2 track SVX) SVX mass 3

  4. Analysis Details • t-tbar events: T1_McAtNlo_Jimmy, run5200, statistics ~500K - trig1_misal1_mc12 using Geometry CSC-01-02-00 (12000604) • ESDs used to generate fresh AODs containing addition jet collections: - ConeTower/Topo, Rcone = 0.2 –:– 0.8 - KtTower/Topo, RkT = 0.2–:– 0.8 - MidPointTower Rcone= 0.4, 0.7 & an optimised parameter set • B-tagging calibration performed on all the jet containers - e.g. +20% shift in rejection observed when using default calibration • Events analysed using tag information from the new reference files. • EventView with GroupArea 12.0.6.7 used for the Analysis.

  5. b – tagging basic performance (BT0)

  6. Impact of jet reconstruction algorithms

  7. Ongoing analyses • First complete draft of CSC Notes by 22 October - impact of misalignment ( as discussed at the Flavour tagging performance phone meeting 02 Oct 2007 http://indico.cern.ch/conferenceDisplay.py?confId=21733 ) - a slight increase in the rate of fake tracks - significant loss of b-tagging performance (60-85%) • Jet shape as another tagger (?)

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