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QCD/W Background estimation using System8 method

Benoit Clément, Daniel Bloch, Denis Gelé,
Sébastien Greder, Isabelle Ripp-Baudot (IReS
Strasbourg)
Outline - General description of the System8
method. - System8 example b-tagging
efficiency - System8 for QCD/W background
estimation - MET distributions - W tranverse
mass distributions
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What is System8 ?
System8 is a method to determine signal and
background fractions, as well as selection
efficiencies.
It uses only data. Some correction factors may be
add from MC.
It allows to extract normalized distribution of
sensitive variables in data.
First developed for b-tagging efficiencies
measurements and used sucessfully. (It as been
adopted as one of the efficiency measurement by
every b-tagging algorithms) DØ Note 4159
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A bit of maths
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A bit more
If the selection criteria are independent, the
efficiencies of applying x criteria ki,i1..x can
be written And then, new equations
applying several criteria

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And finally
The various qj are easily mesured on data.
Selections and samples bring 2t.s
equations, b.(st) unkowns
(fractions and efficiencies). You need as many
equations as unknowns 2t.d b.(dt)
Simplest (???) solutions are (a) s 1 , t 3
, b 2 and (b) s 2 , t 2 , b 2. gt 8
equations, 8 unknowns SYSTEM8.
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First example System8 in b-id
Use the 2 samples solution.
Sample 1 Jets with a soft muon. Sample 2 Jets
with soft muon and back to back tagged
jet Tagging criteria 1 PTrel(muon) gt 0.7
GeV. Tagging criteria 2 JLIP lt 1 (or CSIP, SVT
tag). 1 signal b jets. 1
background clight jets (weak point of the
method).
Output gives the b-tagging efficiency.
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System8 to estimate W/QCD background
Use System8 instead of Matrix Method. QCD and W
fractions in loose samples, as well as W and QCD
efficiencies are output of the method.
Proposed solution 1 sample, 3 criteria.
Sample preselected loose sample in electron
channel (p13) (Emmanuels preselection cuts DØ
note 4274). Selection 1 Electron likelihood LH
gt 0.4 (loose -gt tight). Selection 2 METgt35
GeV. Selection 3 MTW gt 40 GeV. 2 backgrounds
QCD and Wsignal.
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System to solve

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Results

• Problem MET and MTW are not independent.
• Use MC Wjets to estimate correlation factors

For proposed cuts CMET,MTW 1.032 /- 0.01
CMET,LH 0.997 /- 0.01 CLH,MTW 1.002 /-
0.01
System 8 output Efficiencies eLHW (71.6
/- 2.0) eLHQCD (19.2/- 1.8) W
and QCD fractions (loose sample) nW 0.53
/- 0.03 nQCD 0.47 /- 0.02 W and
QCD fractions (tight sample) nW 0.81 /-
0.04 nQCD 0.20/- 0.02
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Jet Multiplicity dependence
QCD and Wsig efficiencies as function of
inclusive jet multiplicity.
71.6
19.2
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Normalized distributions of sensitive variables
from data
Instead of a simple cut, use a bin for one of the
criteria for example, a lt MET lt b. Then the
efficiencies for the new criteria is the fraction
of W and QCD in that interval.
For each bin - Evaluate MC correlation
factors (ALPGEN Wjets p14.02). - Solve the
System8. gt Normalized distributions of the
variable.
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MET correlation factors
MET bins xltMETltx5, x 20,2590 LH gt 0.4,
MTW gt 40 GeV.
MET - MTW
MET - LH
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MET distributions
-W distribution is compared to MC
Wjets. -System8 shows some instability when W
and QCD efficiencies are close (crossing point ).
Crossing point
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MET other crosschecks
Look at the stability of W, QCD fractions and
efficiencies.
Crossing point rejected
Fit 79.3 /- 7.8
Fit 20.7 /- 3.6
Fit 71.0 /- 2.5
Fit 21.2 /- 3.0
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MTW correlation factors
MTW bins xltMTWltx10, x 10,20120 LH gt 0.4,
MET gt 35 GeV.
MET - MTW
MTW - LH
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MTW distributions
-W distribution is compared to MC
Wjets. -System8 shows some instability when W
and QCD efficiencies are close (crossing point ).
Crossing point
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MTW other crosschecks
Look at the stability of W, QCD fractions and
efficiencies.
Fit 21.5 /- 3.5
Fit 77.5 /- 6.9
Fit 73.1 /- 2.7
Fit 21.03 /- 2.9
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Correlation Factors
without correlation factors
without correlation factors
with correlation factors
with correlation factors
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Summary
System8 can be used to determine - QCD and W
background in loose/tight sample. - Loose to
tight (e-LH) efficiencies.
Values are estimated on the analysis sample -
data based method. - no topological effects
(Matix method uses Z-gtee to determine LH
efficiency). - can be used whatever your
preselections are.
Various crosschecks are consistant.
MET and MTW distributions from
data seems ok.
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Plans
To do on System8 - Optimize the cuts
(METgt35 GeV, MTWgt40 GeV ). - Other
variables ? (Topo cuts instead of MET -gt gives
topological selection efficiencies as well.)
Next week - JLIP uptade (p14 TRF) - Fist pass
on data (p13) with JLIP tagging.