estimation of w+jets background in the atlas h ww l l...

Estimation of W+jets backgroundin the ATLAS H → WW → lνlν analysisusing a likelihood-based matrix technique

Phuong Nguyen Dang

University of Freiburg

Graduiertenkolleg − April 30, 2014

IntroductionMatrix method

W+jets background estimationConclusion

Outline

1 Introduction

2 Matrix method

3 W+jets background estimation

4 Conclusion

Phuong Nguyen Dang (Uni. Freiburg) Graduiertenkollegs der Universitat Freiburg − April 30, 2014 2 / 29



Higgs productionHiggs decay channelsH→ WW analysisFake lepton categorization

Introduction

Higgs main production processes:

gluon-gluon Fusion (ggF)

Vector Boson Fusion (VBF)

WH/ZH AssociatedProduction

ttH Associated Production

[GeV] HM80 100 200 300 1000

H+

X)

[pb]

→(p

p σ

-210

-110

1

10

210= 8 TeVs

LH

C H

IGG

S X

S W

G 2

012

H (NNLO+NNLL QCD + NLO EW)

→pp

qqH (NNLO QCD + NLO EW)

→pp

WH (NNLO QCD + NLO EW

)

→pp

ZH (NNLO QCD +NLO EW)

→pp

ttH (NLO QCD)

→pp





Introduction

Higgs decay branching ratios:

tt / bb / cc

gg

ττ / µµ

γγ / Zγ

WW / ZZ

[GeV]HM80 100 120 140 160 180 200

Hig

gs B

R +

Tota

l U

ncert

410

310

210

110

1

LH

C H

IGG

S X

S W

G 2

01

3

bb

ττ

µµ

cc

gg

γγ γZ

WW

ZZ





Introduction

2 leptons selection

Same flavor (SF): ee, µµ

Different flavor (DF): eµ, µe

Missing ET

Jet multiplicities

0 jet, 1 jet, ≥2 jets

Many challenges

Many backgrounds (reducibleand irreducible)

Poor mass resolution (largemissing ET )





Introduction

The W+jets background in the H → WW → lνlν similar to the Higgs signal.

[GeV]Tm

60 80 100 120 140 160 180 200 220 240

Eve

nts

/ 1

0 G

eV

20

40

60

80

100

120 Data stat)⊕ SM (sys

WW γ WZ/ZZ/W

t t Single Top

Z+jets W+jets

H [125 GeV]

ATLAS

1 L dt = 4.7 fb∫ = 7 TeV, s

+ 0 jetsνlνl→(*)

WW→H

Phys. Lett. B 716 (2012) 62-81

d

ν

e+

W+

u

g

u





Fake lepton categorization (1/4)

Sources of fake leptons:

Hadrons reconstructed as leptons (light flavour).

Calorimeter shower from charged hadron fluctuates to look like electronshower.

Charged hadron punches through the calorimeter and is seen in muonsystem.







Semi-leptonic heavy quark decays (heavy flavour).

c

ν

`+

W+

b s

ν

`+

W+

c







Muon from inflight hadron (pion, kaon) decay.







Electron from photon conversion.




Fake factor methodMotivationIdentification variables

General idea of fake factor method

Consider a simple case with two nature type of leptons (real and fake).

Define the loose and tight selections for leptons, which differ from each otherby one (or more) lepton ID variable cuts (e.g. isolation).








Observe number of events in loose and tight selection:

N loose = N loosereal + N loose

fake

Ntight = Ntightreal + Ntight

fake

= εrealNloosereal + εfakeN

loosefake










fake


fake


loosefake

ε is efficiency of loose leptons passing tight cut








fake


fake


loosefake

Matrix form [N loose

Ntight

]=

[1 1εreal εfake

]×

[N loose

realN loose

fake

]






Observe number of events in loose and tight selection


fake


fake


loosefake

Matrix form [N loose

Ntight

]=

[1 1εreal εfake

]×

[N loose

realN loose

fake

]Fake component[

N loosereal

N loosefake

]=

1

εfake − εreal

[εfake −1−εreal 1

]×

[N loose

Ntight

]Ntightfake =

εfake

εreal − εfake(εrealN

loose − Ntight)





Motivation of matrix method

Estimated in Z-rich sample (Z control region).

Ntightfake =

εfake

εreal − εfake( εreal N loose − Ntight)

Estimated in jet-rich sample (di-jet sample, Z+jets sample).





Motivation of matrix method

Estimated in Z-rich sample (Z control region).

Ntightfake =

εfake

εreal − εfake( εreal N loose − Ntight)

Estimated in jet-rich sample (di-jet sample, Z+jets sample).

→ Sample dependence error (∼30−50%)

New matrix method for W+jets background without sample dependence

Using directly di-lepton samples

Based on the response of each lepton category with ID variable cuts.

Matrix built from MC truth

→ systematic uncertainty = MC/data difference.





Identification variables

Isolation variables

Track Isolation =lepPtcone30

lepPt

Calo Isolation =lepEtcone30

lepPt

non-isolated isolated

Track Isolation (lep 2)0 0.5 1 1.5 2

0

0.2

0.4

0.6

0.8

1

ATLAS Work in Progress Gamma

Hadron

Neutral Pion

True Electron

Arb

itra

ryunits

Electron

Track Isolation (lep 2)0 0.5 1 1.5 2

0

0.2

0.4

0.6

0.8

1

ATLAS Work in Progress Light Flavor

Heavy Flavor

True Muon

Arb

itra

ryunits

Muon





Identification variables for electrons

TRT ratio & B-layer hits

TR_ratio10 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35ATLAS Work in Progress Gamma

Hadron

Neutral Pion

True Electron

Arb

itra

ryunits

lepnBLHits10 0.5 1 1.5 2 2.5 3 3.5 4

0

0.2

0.4

0.6

0.8

1ATLAS Work in Progress Gamma

Hadron

Neutral Pion

True Electron

Arb

itra

ryunits





Identification variables for muons

Momentum Imbalance

P1/P1∆

0.2 0 0.2 0.4 0.6 0.8 10

0.1

0.2

0.3

0.4

0.5ATLAS Work in Progress Light Flavor

Heavy Flavor

True Muon

Arb

itra

ryunits

Impact parameter

lepsigd0PV10 5 10 15 20 25 30

0

0.2

0.4

0.6

0.8

1

ATLAS Work in Progress Light Flavor

Heavy Flavor

True Muon

Arb

itra

ryunits




Monte Carlo closure testApplying to dataComparison with fake factor method

Matrix method application

Pre-Selection

Beginningof Cutstage

Building Matrix

MC closure test

Signal Region





Monte Carlo closure test after pre-selection cuts (all jet inclusive)

Matrix is built at the beginning of cutstage.

Assume that lepton identification variables are orthogonal to the kinematicvariables (e.g. mT ) → use one matrix for all bins

[GeV]Tm

60 80 100 120 140 160 180 200 220 2400

50

100

150

200

250

300 ATLAS Work in Progress

Wjets

MC truth

Extracted

Wjets

MC truth = 975.70 ± 263.61

Extraction = 975.54 ± 22.7

Nu

mb

erof

even

ts





Matrix method application

Pre-Selection

Beginningof Cutstage

Building Matrix

Apply

Signal Region





W+jets background results in µe 0jet signal region

[GeV]Tm

60 80 100 120 140 160 180 200 220 2400

20

40

60

80


Wjets

MC truth

Extracted

WjetsN

um

ber

of

even

ts





W+jets background results in µe 1jet signal region

[GeV]Tm

60 80 100 120 140 160 180 200 220 2400

10

20

30

40

50

60


Wjets

MC truth

Extracted

WjetsN

um

ber

of

even

ts





Comparison with fake factor method in µe channel

ATLAS Work in Progress

0jet 1jet 2jet≥

Nu

mb

er

of

W+

jets

eve

nts

0

20

40

60

80

100

120

140

160 Matrix Method

Monte Carlo

Fake Factor




Conclusion

Matrix technique is used for W+jets background in H→WW→ lνlν analysis.

This method is applied directly to dilepton sample, therefore no sample depen-dence error.

The systematic uncertainties of matrix method is about 20% for electron and30% for muon channels.

There is quite agreement between matrix and fake factor methods in signalregions after all selection cutstages.




BACKUP




Event Selection

Preselection

2 tight leptons PT > 22(15)GeV

Mll > 10(12)GeV

|Mll −MZ | > 15GeV

EmissT ,rel > 20(40)GeV

0jet

P llT > 30GeV

Mll < 55GeV∆Φll < 1.8

1jet

b-tag vetoZττ vetoMll < 55GeV∆Φll < 1.8

≥2jet (VBF)

EmissT > 20(45)GeV

b-tag vetoZττ veto∆Yjj > 2.8, Mll > 500GeVMll < 60GeV , ∆Φll < 1.8




General case

In case the fake lepton has more than 1 component:

Nfake = Nfake1 + Nfake2 + ... (2)

We need more than 1 lepton ID variables to separate each lepton component.N loose

Ntight1

Ntight2

...

NtightN

=

1 1 1 ... 1

εtight1real εtight1

fake1 εtight1fake2 ... εtight1

fakeM

εtight2real εtight2

fake1 εtight2fake2 ... εtight2

fakeM... ... ... ... ...

εtightNreal εtightNfake1 εtightNfake2 ... εtightNfakeM

×

N loosereal

N loosefake1

N loosefake2...

N loosefakeM

(3)

The efficiencies are estimated from MC instead of building control region for eachfake component.

Systematic uncertainty mainly come from the incorrect MC description of leptonID variable cuts.




Matrix building

N(C1, x)N(C2, x)

...N(Cn, x)

=

εs1 (C1) εs2 (C1) ... εsm (C1)εs1 (C2) εs2 (C2) ... εsm (C2)... ... ... ...

εs1 (Cn) εs2 (Cn) ... εsm (Cn)

× Ns1 (x)

Ns2 (x)...

Nsm (x)

(4)

Ci = WJET (2 tight lepton selection), STD (1tight+1loose lepton selection), ISO(passing isolation cut), INV ISO (passing inverted isolation cut), TRT, BL (Blayer), IMB (imbalance momentum),...

In this study, the variable x is one of the kinematic quantities: MT , MET , Mll , ∆φll ,. . .

The matrix elements are obtained from the MC truth.→ The systematic uncertainty is due to the MC/data difference.

The matrix does not rely on the knowledge of cross section of non-Wjetsbackground.

Not only the total number of events from each source but also the shape ofkinematic variables can be extracted.




Monte Carlo closure test

[rad]ll

φ∆

0 0.5 1 1.5 2 2.5 30

50

100

150

200

250

300


Wjets

MC truth

Extracted

Wjets

Figure: ∆Φll distributions extracted after EmissT cut in µe channel




Comparison with fake factor method

Cross-check with MC W+jets and fake factor method.

Only statistic uncertainties are presented for both data-driven methods.


Channel Number of jets Matrix method MC W+jets Fake factorµµ 0jet 9.45 ± 3.82 0.01 ± 0.01 11.34 ± 1.31

1jet 4.95 ± 1.25 0.35 ± 0.35 2.77 ± 0.91≥2jet 0.49 ± 0.65 0.00 ± 0.00 0.05 ± 0.39

eµ 0jet 101.28 ± 30.81 207.66 ± 68.40 32.38 ± 1.881jet 27.13 ± 5.73 30.35 ± 12.13 19.25 ± 1.58≥2jet 1.52 ± 1.76 0.31 ± 0.31 0.93 ± 0.35

µe 0jet 46.89 ± 3.34 89.40 ± 34.62 55.12 ± 1.611jet 32.94 ± 3.68 83.92 ± 36.42 20.65 ± 1.28≥2jet 0.29 ± 0.45 0.00 ± 0.00 0.89 ± 0.36

ee 0jet 10.78 ± 0.89 32.87 ± 18.20 12.20 ± 0.551jet 3.63 ± 0.94 14.30 ± 7.90 2.11 ± 0.24≥2jet 0.16 ± 0.23 0.00 ± 0.00 0.08 ± 0.02


estimation of w+jets background in the atlas h ww l l...

Documents