QCD/W Background estimation using System8 method
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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

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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

Single Top meeting - B. Clément - 20 nov 2003


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

Single Top meeting - B. Clément - 20 nov 2003


Assuming s independent data samples containing b different signal or backgrounds, one can write for each sample j (j=1..s) an equation:

with n ji: fraction of signal i in sample j.

Moreover supposing t different selection criteria (cuts on various variables), one can write for each sample j (j=1..s) and each criteria k (k=1..t) another equation:

with εki: efficiency of cut k on signal i.

qkj: fraction of remaining events in sample j after cut k.

A bit of maths…

Single Top meeting - B. Clément - 20 nov 2003


A bit more…

If the selection criteria are independent, the efficiencies of applying x criteria ki,i=1..x can be written :

And then, new equations applying several criteria :

Single Top meeting - B. Clément - 20 nov 2003


And finally…

The various qj are easily mesured on data.

Selections and samples bring :

2t.s equations,

b.(s+t) unkowns (fractions and efficiencies).

You need as many equations as unknowns :

2t.d = b.(d+t)

Simplest (???) solutions are :

(a) s = 1 , t = 3 , b = 2 and (b) s = 2 , t = 2 , b = 2.

=> 8 equations, 8 unknowns : SYSTEM8.

Single Top meeting - B. Clément - 20 nov 2003


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) > 0.7 GeV.

Tagging criteria 2 : JLIP < 1% (or CSIP, SVT tag).

1 signal : b jets.

1 background : c+light jets (weak point of the method…).

Output gives the b-tagging efficiency.

Single Top meeting - B. Clément - 20 nov 2003


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)

(Emmanuel’s preselection cuts DØ note 4274).

Selection 1 : Electron likelihood LH > 0.4 (loose -> tight).

Selection 2 : MET>35 GeV.

Selection 3 : MTW > 40 GeV.

2 backgrounds : QCD and W+signal.

Single Top meeting - B. Clément - 20 nov 2003


System to solve

Single Top meeting - B. Clément - 20 nov 2003


Results

  • Problem : MET and MTW are not independent.

  • Use MC W+jets 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 :

εLHW = (71.6 +/- 2.0) % εLHQCD = (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

Single Top meeting - B. Clément - 20 nov 2003


Jet Multiplicity dependence

QCD and W+sig efficiencies as function of inclusive jet multiplicity.

71.6%

19.2%

Single Top meeting - B. Clément - 20 nov 2003


Normalized distributions of sensitive variables from data

Instead of a simple cut, use a bin for one of the criteria : for example, a < MET < 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 W+jets p14.02).

- Solve the System8.

=> Normalized distributions of the variable.

Single Top meeting - B. Clément - 20 nov 2003


MET : correlation factors

MET bins : x<MET<x+5, x = 20,25…90

LH > 0.4, MTW > 40 GeV.

MET - MTW

MET - LH

Single Top meeting - B. Clément - 20 nov 2003


MET distributions

-W distribution is compared to MC W+jets.

-System8 shows some instability when W and QCD efficiencies are close (crossing point ).

Crossing point

Single Top meeting - B. Clément - 20 nov 2003


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 %

Single Top meeting - B. Clément - 20 nov 2003


MTW : correlation factors

MTW bins : x<MTW<x+10, x = 10,20…120

LH > 0.4, MET > 35 GeV.

MET - MTW

MTW - LH

Single Top meeting - B. Clément - 20 nov 2003


MTW distributions

-W distribution is compared to MC W+jets.

-System8 shows some instability when W and QCD efficiencies are close (crossing point ).

Crossing point

Single Top meeting - B. Clément - 20 nov 2003


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 %

Single Top meeting - B. Clément - 20 nov 2003


Correlation Factors

without correlation factors

without correlation factors

with correlation factors

with correlation factors

Single Top meeting - B. Clément - 20 nov 2003


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->ee to determine LH efficiency).

- can be used whatever your preselections are.

Various crosschecks are consistant. MET and MTW distributions from data seems ok.

Single Top meeting - B. Clément - 20 nov 2003


Plans

To do on System8:

- Optimize the cuts (MET>35 GeV, MTW>40 GeV ).

- Other variables ? (Topo cuts instead of MET -> gives topological selection efficiencies as well.)

Next week :

- JLIP uptade (p14 TRF)

- Fist pass on data (p13) with JLIP tagging.

Single Top meeting - B. Clément - 20 nov 2003


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