Update on searches for ZZ signal

in the 3 electron channel Azeddine Kasmi

Bob Kehoe

Many Thanks to:

David , Pavel, Julia

LO

NLO

OUTLINE

A search for the non-resonant ZZ signal in the ZZ3e channel was carried out.

Pt, ETmiss, Isolation and Z mass cuts were used to

reduce the WZ, ttbar, and Zbb backgrounds. The signal efficiencies and background

rejections will be shown Conclusion and Future Plan

PT of ElectronsZbb (left) ZZ (right)

PT of ElectronsWZ (left) ttbar (right)

Signal and Background after Pt cuts

Signal Cross Section (fb)

= 3 rec ele/truth

Events with 3 ele

Pt>7 & one Pt >20

* ( fb )

ZZ 4e (NLO)

19 fb 33 % 6

Background Cross Section (fb)

Ratio

Bkg : Signal

Ratio

Bkg : Signal

WZ 3e + X 190 10:1 25 % 47 8:1

tt 3e + X 8000 421:1 3 % 240 40:1

Zbb 3e + X 15000 790:1 10 % 1500 250:1

Signal + Background (scaled) Zbb3l Omitted

ttbar

WZ

ZZ

ETmiss in signal and Background

WZ

ttbar

Zbb

ZZ

M3e

ETmiss

ETmiss in signal and Background vs M3e

Zbb

WZ

ttbar

ZZ

ETmiss

M3eM3e

ETmiss

Pt cut & ETmiss < 20 GeV

Zbb Omitted

ZZ

ttbar

WZ

Signal Efficiency and Background

Rejection of ETmiss

ETmiss

ZZ (on shell)

Signal

Efficiency

WZ

Efficiency

ttbar

Efficiency

Zbb

Efficiency

20 GeV 87% 13 % 6 % 68%

So far

ET miss is a good cut to reduce the WZ and ttbar background

Further background reduction needs to be done

Zbb 3l remains dominant

Signal and Background after Pt cuts & ET

miss

Signal Cross Section (fb)

= 3 e/truth

Events with 3 ele

Pt>7 & one Pt >20

* ( fb )

ZZ 4e (NLO)

19 fb 33 % 6 Pt cut Pt cut &

ETmiss cut

Background Cross Section (fb)

Ratio

Bkg : Signal

Ratio

Bkg : SignalRatio

Bkg : Signal

WZ 3e + X 190 10:1 25 % 47 8:1 1:1

tt 3e + X 8000 421:1 3 % 240 40:1 3:1

Zbb 3e + X 15000 790:1 10 % 1500 250:1 196:1

Isolation

The electrons from ZZ should be isolated unlike the electrons from Zbb

Etcone 30 cut may be used to discriminate the Zbb

IsolationEtcone/Et in case of Zbb (left) ZZ (right)

b electron

Cut on 0.2

IsolationEtcone/Et in case of ttbar (left) WZ (right)

Pt cut & ETmiss < 20 GeV & etcone/et <0.2

ZbbZZ

WZ

ttbar

Signal and Background after Pt cuts & ET

miss & Isolation

Signal Cross Section (fb)

3 electrons

Pt>7 & one Pt >20

* ( fb )

ETmiss

& Isolation

ZZ 4e (NLO)

19 fb 33 % 6 Pt cuts Signal Efficiency

77 %

Background Cross Section (fb)

Ratio

Bkg : Signal

Ratio

Bkg : Signal

Ratio

Bkg : Signal

WZ 3e + X 190 10:1 25 % 47 8:1 1:1

90% Rejection

tt 3e + X 8000 421:1 3 % 240 40:1 1:2

99% Rejection

Zbb 3e + X 15000 790:1 10 % 1500 250:1 52:1

(84% Rejection)

Use Z mass as a cut

Find which combination of 2 electrons makes a Z for both samples ZZ and Zbb

Define a variable Z_Best which takes the closest invariant mass to PDG Z of 1&3 or 2&3

This Z_Best will be used to distinguish between ZZ and Zbb

Z

Z e1e3

e2

e4

Z

e1

e2

b

b

Mz12 (left) && Z_Best (right)

Zbb_Mz12 (not scaled)

ZZ_Mz12Zbb_Z_Best

(not scaled)

ZZ_Z_Best

Mz12 vs Mz_Best

ZZZbb

Pt cut & ETmiss < 20 GeV & etcone/et <0.2

|MzPDG – Z_Best| < 20 GeV

Sig_eff = 51 %

Bkg_rej = 94%

Bkg:sig = 30:1

Z_Best vs Isolation

ZZ

ZbbThis shows the M_ZBest as a function of the isolation of the 3rd leading electron in both cases ZZ and Zbb.

Pt cut & ETmiss < 20 GeV &

Isolation on 3rd leading electron < 0.1 |MzPDG – Z_Best| < 10 GeV

Sig_eff = 40 %

Bkg_rej = 98%

Bkg:sig = 15:1

Conclusion && Future Plan

The ETmiss is good in case of WZ and ttbar background.

Zbb was reduced considerably but still not enough. Looking for another cut to reduce Zbb is crucial.

Cut on jets: the Zbb event has two leading jets from the b-quarks give some discrimination power

Try to find the 4th candidate electron to use it to find the second Z peak

Optimize the cuts Try the analysis for L= 1 fb-1 - 10 fb-1 Higher statistics for ttbar