Transcript Overviwe_of_b-tagging_meetings

B-tagging meeting overview
Li bo
Shandong University
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outline
Purpose

information on b-tagging studying
The b-tagging meetings mainly deal with
Trigger studies for b jets
 The performance of b-tagging
 B-tagging calibration
 The validation of software release
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The trigger study for b-jets
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Correlation of b-jet trigger at L2/EF
with offline b-tagging
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Various b-jet triggers
rate reduction
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Correlation with offline taggers
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Aims at offline Єb
=60%
The plot shows the
working points of L2
and EF are ~78% and
~68%
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Rate reduction of different b-jet triggers
The rate reduction is almost constant with different Pt
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The b-tagging performance
 The b-tagging performance
b-tagging efficiency
rejection of light/c/tau jet
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Study on b-tagging performance
1 Studies for the various jet algorithms
2 Impact from vertex reconstruction
3 Impact from tracking performance
4 Measurement of mistag-rate
5 Misalignment
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1 Studies for the various jet algorithms
Requirement that jet has more than one track makes b
tagging procedure more robust against jet reconstruction
algorithms
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2 Impact from primary vertex reconstruction
 Sources
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of impacts:
PV reconstruction efficiencies
PV resolutions
Pile up
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2 Impact of Secondary vertex reconstruction on b-tagging
2 Impact from secondary vertex reconstruction
 Source
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of impacts:
SV reconstruction efficiencies
SV reconstruction fake rate
SV resolution
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Impact on b-tagging
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3 Impact from tracking performance
 Sources
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of impacts:
impact parameter resolution
tracking efficiency and fake rate
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Impact on b-tagging performance
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Mis-tagging rate
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Definition:
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the ratio between the number of light jets
tagged as b-jet and the number of light jets
Sources leading to mis-tagging:
finite resolution of the reconstructed
track/vertex parameters
 Tracks/vertices from the long-live particles
that decay in jets.
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Measurement Method:
1.
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3.
4.
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taking negative weight of all jets
inverting to positive to reperesent positive
weight distribution of light jets
measure the mis-tagging rate
with MC, correction for the long-live meson
presence in the light jet and heavy flavor jet
contribution.
Other method: using templates
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Result of mistag rate
The measured mistag rate is almost close to the actual mistag rate
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5 Mis-alignment
 Misalignment
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sets:
Random 10
Random 5
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The b-tagging calibration
Purpose:
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develop methods to get b-tagging efficiency with data
Method:
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using di-jet events
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Pt-rel
 System 8
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using ttbar events
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Tag couting
 Control sample
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Dijet event
 Pt-rel
method
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Muons in μ-jet have
different Pt distributions
w.r.t jet axis originating
from different flavor q/g
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use templates for b, c
and light jets to determine
their relative fractions
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Pt-rel method
Fraction of b-jets can be extracted
from fitting to templates, before
tagging (Fb ->u) and atfer tagging
(Ftag b->u)
F tag b u
efficiency calculated as
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System 8 method
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Two sample with different
heavy flavor content
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Two independent taggers
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Apply taggers to both
samples
jets passing two taggers
both
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Write a system of 8
equations to solve the
tagging efficiency
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Pt-rel
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System8
both methods have good result up to 80Gev(jet Pt )
improvements to system 8 method:
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use different release like r13 : improvements to performance of taggers
use different samples and taggers
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Using ttbar event
count number of events with
1,2,3 b-tags(lepton+jet) or 1,2
b-tags (delepton)
likelihood fit gives b-tag
efficiency
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ttbar results
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Methods for b-jet sample selection
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topological
reconstruct all combinations, apply mass windows
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kinematic
apply kinematic fit assuming mbjj and mblv=mtop, and
select combination with best fit x2
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likelihood
use jet/lepton energy and angular information
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B-tagging validation
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Summary
The studies for b-jet triggers
The studies for b-tagging performance
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the impact of vertex and tracking on b-jet performance
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the measurement of mistagging rate
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misalignment and error scaling
The studies of b-tagging calibration
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measurement of b-tagging efficiency with di-jet and ttbar event
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b-jet sample selection with three methods
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