b -tagged jet with one muon
DESCRIPTION
Performance of Track and Vertex Reconstruction and b -Tagging Studies with ATLAS Johanna Fleckner , CERN / University of Mainz on behalf of the ATLAS Collaboration. b -tagged jet with one muon. needed to tag this event: tracking momentum resolution transverse impact parameter - PowerPoint PPT PresentationTRANSCRIPT
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Performance of Track and Vertex Reconstruction and b-Tagging Studies with ATLAS
Johanna Fleckner, CERN / University of Mainzon behalf of the ATLAS Collaboration
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ICHEP 22 July 2010
Johanna Fleckner Tracking, Vertexing and b-Tagging with ATLAS
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b-tagged jet with one muonneeded to tag this event: tracking
momentum resolution transverse impact
parameter primary vertexing b-tagging algorithms
primary
vertex
b-decay
muonb-jetsketch of b-decay
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Pixel detector: 80M channels σ(RΦ) = 10 μm
σ(z) = 115 μm Semi Conductor
Tracker (SCT): 6,3M channels σ(RΦ) = 17 μm
σ(z) = 580 μm Transition Radiation
Tracker (TRT): 350.000 channels σ(RΦ) = 130μm (barrel)
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The ATLAS Inner Detector
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using known particle properties
mass distribution sensitive to pT resolution (mass peak
width) pT scale (mass peak
position) K mass spectrum
parameterization:
more information on reconstruction of known particle decays in ATALS by R. di Nardo (poster)
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Estimating Track Momentum Resolution
€
f (m) = t + a ⋅m + b ⋅m2 + N ⋅e−
(xm−m )2
s2
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rescale K pT in Monte Carlo to fit data:
best fit values in the barrel:(consistent with no change)
good agreement between data and nominal Monte Carlo for low momentum studies ongoing for resonances with
higher mass (and pT)ICHEP 22 July
2010Johanna Fleckner Tracking, Vertexing and b-Tagging with ATLAS
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Track Momentum Resolution cont’@ 900 GeV
μ: shift of meanσ: broadening of width
€
with P(δ;μ,σ ) = 12πσ
exp[− (μ −1−δ)2
2σ 2 ]
μ=1.0σ=0.0
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Johanna Fleckner Tracking, Vertexing and b-Tagging with ATLAS
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Primary Vertex Reconstruction iterative finding Algorithm
progressive down-weighting of outlying tracks
resolution determination: split vertex method split tracks in vertex randomly
in two fit both vertices
independently separation gives intrinsic
resolution vertex resolution in data for
ntracks≅ 50: (typical number in dijet events) along x-axis: 40 μm along z-axis: 60 μm
x-Vertex resolution
z-Vertex resolution
without beamspot constraint
without beamspot constraint
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Most b-tagging algorithms rely on the measurable life time and displaced decay vertex of b-hadrons need well measured tracks study the transverse track
impact parameter
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Transverse Impact Parameter
primary
vertex
b-decay
b-jet
sketch of b-decay
track d0trackd0/σ(d0)
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d0 is the d0track
resolution convolved with primary vertex resolution
d0 resolution in bins of pT and θ: fit core of distribution with Gaussian
resolution clearly depends on pT and θ
for fixed theta:
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Transverse Impact Parameter Resolution
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σ(d0) = E 2 + G2
pT+ b2
pT2 sinθ
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d0track can be unfolded in an iterative way
ICHEP 22 July 2010
Johanna Fleckner Tracking, Vertexing and b-Tagging with ATLAS
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Unfolding of track d0 resolution
Low pT: Good agreement
between unfolded and resolution expected from nominal simulation
High pT: discrepancies due to
remaining misalignment
more details on alignment in ATLAS: previous talk by A. Limosani poster by J. Wang
PV uncertainty
measured
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probability for the jet to be compatible with the light jet hypothesis
d0 significance signed wrt jet axis (Sd0) resolution function calculated in data for Sd0:
ICHEP 22 July 2010
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The Jet Probability Tagger (JetProb)
negative
tail
Resolution function R:
resolution
lifetime
symmetrised
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1. track probability:for each track calculate probability to come from primary vertex using resolution Function R
2. jet probability:combine probabilities from all tracks in jet to a jet probability
good agreement between data and Monte CarloICHEP 22 July
2010Johanna Fleckner Tracking, Vertexing and b-Tagging with ATLAS
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Results from the JetProb Tagger
typical cut at 0.058
corresponding cut at 1.2
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explicit reconstruction of b-hadron decay vertex
decay length significance is discriminating variable
ICHEP 22 July 2010
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The Secondary Vertex Tagger SV0
primary
vertex
b-decay
b-jetsketch of b-decay
decay length
typical cut at 7
resolution
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cut on positive decay length significance L/σ(L) clear enhancement of heavy flavour in the mass of
the secondary vertex mvtx
reasonable agreement between data and Monte Carlo
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Results from the SV0 Tagger
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The Soft Muon Tagger exploits semi-leptonic decay of b- and c-hadrons to
muons:
associates good reconstructed muons to jets tag weights from likelihood ratio based on pT distribution wrt
jet axis to be used for calibration of the lifetime tagging algorithms
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€
BR(b→ μνX) + BR(b→ c → μνX) ≈ 20%
tag weights from pT likelihood ratio
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b-tagged jet with one muon
tagger weights: Jet Prob: 10-9
SV0 L/σ(L): 24 soft muon weight: 4.6
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Summary Presented track and vertex performance and first
b-tagging studies in ATLAS reconstruction of known decays K
determine transverse momentum resolution and scale at low pT
primary vertex reconstruction transverse impact parameter resolution
unfolded from primary vertex uncertainty three b-tagging algorithms used for early data
analyses good agreement with Monte Carlo prediction
Inner Detector performs very well in first data solid foundation for ATLAS tracking and b-tagging
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€
S0
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backup slides
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reconstruction requirements: vertex-fit: chi2 < 15 1 pixel hit, 6 SCT hits d0<50 mm, z0<500 mm transverse flight distance to PV > 0.2 (4) mm cosine of angle in transverse plane between K
momentum and K flight direction > 0.99 (0.999) relative pT scale:
and resolution: CONF notes:
ATLAS-CONF-2010-009 ATLAS-CONF-2010-033
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Reconstruction of K
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Estimating Track Momentum Resolution cont’@ 900 GeV
μ=1.01σ=0.0
μ=1.0σ=0.025
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estimating scale factors to compensate incorrect error description
pull for split vertices: ATLAS-CONF-2010-069
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Primary Vertex Reconstruction
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d0 can be unfolded in an iterative way
1st iter.: ith iter.:
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Unfolding of track d0 resolution
d0track resolution in
central barrel at 15 GeV: convolved:24μm unfolded: 18μm MC truth: 15 μm
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sign for the transverse impact parameter wrt jet axis:
track requirements: Pt > 1 GeV nHitsSi >= 7 nHitsPix >= 2 nHitsBlayer >= 1 |d0| < 1mm |z0 * sin(Θ)| < 1.5 mm
tracking note: ATLAS-CONF-2010-
070 b-Tagging notes:
ATLAS-CONF-2010-041
ATLAS-CONF-2010-042
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b-Tagging Cuts and Definitions
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d0 significance signed wrt jet axis (Sd0) sensitive to b-hadron lifetime
orders tracks in the jet according to their Sd0
uses the 2nd highest Sd0 as discriminating variable to identify b-jets
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The Track Counting Tagger
fairly good agreement between data and simulation
2nd highest Sd0 in jet
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1. track probability:
2. jet probability:
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JetProb Probability Calculation
€
Ptrk,i = ℜ(x)dx−∞
− d 0i /σ d0
i
∫
€
Pjet = P0(−lnP0) j
j!j= 0
N−1
∑ , P0 = Ptrk,ii=1
N
∏
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selection cuts: Δ(Rmatch)<0.5 |d0|>4mm pT>4GeV Comb’Muons: chi2/ndf
< 10LowPt: chi2/ndf: cut at 5σ
https://twiki.cern.ch/twiki/bin/view/Atlas/FlavourTaggingPublicResultsCollisionData
likelihood ratio used to identify b-jets taken from simulation
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The Soft Muon Tagger