B-tagging meeting overview
Li bo
Shandong University
1
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

2
The trigger study for b-jets

Correlation of b-jet trigger at L2/EF
with offline b-tagging

Various b-jet triggers
rate reduction
3
Correlation with offline taggers


Aims at offline Єb
=60%
The plot shows the
working points of L2
and EF are ~78% and
~68%
4
Rate reduction of different b-jet triggers
The rate reduction is almost constant with different Pt
5
The b-tagging performance
 The b-tagging performance
b-tagging efficiency
rejection of light/c/tau jet

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
6
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
7
2 Impact from primary vertex reconstruction
 Sources



of impacts:
PV reconstruction efficiencies
PV resolutions
Pile up
8
9
2 Impact of Secondary vertex reconstruction on b-tagging
2 Impact from secondary vertex reconstruction
 Source



of impacts:
SV reconstruction efficiencies
SV reconstruction fake rate
SV resolution
10
Impact on b-tagging
11
3 Impact from tracking performance
 Sources


of impacts:
impact parameter resolution
tracking efficiency and fake rate
12
Impact on b-tagging performance
13
Mis-tagging rate
4 4Mis-tagging

Definition:


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.

14

Measurement Method:
1.
2.
3.
4.

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
15
Result of mistag rate
The measured mistag rate is almost close to the actual mistag rate
16
5 Mis-alignment
 Misalignment


sets:
Random 10
Random 5
17
18
19
20
The b-tagging calibration
Purpose:

develop methods to get b-tagging efficiency with data
Method:

using di-jet events

Pt-rel
 System 8

using ttbar events

Tag couting
 Control sample

21
Dijet event
 Pt-rel
method

Muons in μ-jet have
different Pt distributions
w.r.t jet axis originating
from different flavor q/g

use templates for b, c
and light jets to determine
their relative fractions
22
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
23
System 8 method

Two sample with different
heavy flavor content

Two independent taggers

Apply taggers to both
samples
jets passing two taggers
both

Write a system of 8
equations to solve the
tagging efficiency
24
Pt-rel


System8
both methods have good result up to 80Gev(jet Pt )
improvements to system 8 method:


use different release like r13 : improvements to performance of taggers
use different samples and taggers
25
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
26
ttbar results
27
Methods for b-jet sample selection

topological
reconstruct all combinations, apply mass windows

kinematic
apply kinematic fit assuming mbjj and mblv=mtop, and
select combination with best fit x2

likelihood
use jet/lepton energy and angular information
28
29
B-tagging validation
30
31
Summary
The studies for b-jet triggers
The studies for b-tagging performance

the impact of vertex and tracking on b-jet performance

the measurement of mistagging rate

misalignment and error scaling
The studies of b-tagging calibration

measurement of b-tagging efficiency with di-jet and ttbar event

b-jet sample selection with three methods
32