Transcript On b-tagging in ATLAS - University of Glasgow

A.Cheplakov, S.Thompson
On b-tagging in ATLAS
• List of taggers
• Our CSC contribution
• Ongoing analyses
Glasgow bi-weekly, 23 Oct 2007
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b - taggers (12.0.x)
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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
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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…)
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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
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lhSig = IP1D + IP2D + SecVtxBU
weight = SV1+IP3D – recommended tagger for CSC analyses
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For details and references: https://twiki.cern.ch/twiki/bin/view/Atlas/FlavourTagging
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MORE taggers are welcome!
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Space and vertex b-tagging (from A.Rozanov @ Marseille’May 2007)
Jet Jet
weight
weight
N(2 track SVX)
SVX energy fraction
SVX mass
SVX mass
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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.
b – tagging basic performance (BT0)
Impact of jet reconstruction algorithms
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 (?)