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 ZZ  3e channel was carried out. Pt, E T miss, 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 Electrons Zbb (left) ZZ (right)

PT of Electrons WZ (left) ttbar (right)

Signal and Background after Pt cuts SignalCross Section (fb)  = 3 rec ele/truth Events with 3 ele Pt>7 & one Pt >20  *  ( fb ) ZZ  4e (NLO) 19 fb33 %6 BackgroundCross Section (fb) Ratio Bkg : Signal Ratio Bkg : Signal WZ  3e + X19010:125 %478:1 tt  3e + X :13 %24040:1 Zbb  3e + X :110 % :1

Signal + Background (scaled) Zbb  3l Omitted ttbar WZ ZZ

E T miss in signal and Background WZ ttbar Zbb ZZ M 3e E T miss

E T miss in signal and Background vs M 3e Zbb WZ ttbar ZZ E T miss M 3e E T miss

Pt cut & E T miss < 20 GeV Zbb Omitted ZZ ttbar WZ

Signal Efficiency and Background Rejection of E T miss E T miss ZZ (on shell) Signal Efficiency WZ Efficiency ttbar Efficiency Zbb Efficiency 20 GeV87%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 & E T miss SignalCross Section (fb)  = 3 e/truth Events with 3 ele Pt>7 & one Pt >20  *  ( fb ) ZZ  4e (NLO) 19 fb33 %6Pt cut Pt cut & E T miss cut BackgroundCross Section (fb) Ratio Bkg : Signal Ratio Bkg : Signal Ratio Bkg : Signal WZ  3e + X19010:125 %478:1 1:1 tt  3e + X :13 %24040:1 3:1 Zbb  3e + X :110 % :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

Isolation Etcone/Et in case of Zbb (left) ZZ (right) b electron Cut on 0.2

Isolation Etcone/Et in case of ttbar (left) WZ (right)

Pt cut & E T miss < 20 GeV & etcone/et <0.2 Zbb ZZ WZ ttbar

Signal and Background after Pt cuts & E T miss & Isolation SignalCross Section (fb) 3 electrons Pt>7 & one Pt >20  *  ( fb ) E T miss & Isolation ZZ  4e (NLO) 19 fb33 %6Pt cuts Signal Efficiency 77 % BackgroundCross Section (fb) Ratio Bkg : Signal Ratio Bkg : Signal Ratio Bkg : Signal WZ  3e + X19010:125 %478:1 1:1 90% Rejection tt  3e + X :13 %24040:1 1:2 99% Rejection Zbb  3e + X :110 % :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 e1 e3 e2 e4 Z e1 e2 b b

Mz12 (left) && Z_Best (right) Zbb_Mz12 (not scaled) ZZ_Mz12 Zbb_Z_Best (not scaled) ZZ_Z_Best

Mz12 vs Mz_Best ZZ Zbb

Pt cut & E T miss < 20 GeV & etcone/et <0.2 |Mz PDG – Z_Best| < 20 GeV Sig_eff = 51 % Bkg_rej = 94% Bkg:sig = 30:1

Z_Best vs Isolation ZZ Zbb This shows the M_ZBest as a function of the isolation of the 3 rd leading electron in both cases ZZ and Zbb.

Pt cut & E T miss < 20 GeV & Isolation on 3 rd leading electron < 0.1 |Mz PDG – Z_Best| < 10 GeV Sig_eff = 40 % Bkg_rej = 98% Bkg:sig = 15:1

Conclusion && Future Plan The E T miss 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 4 th candidate electron to use it to find the second Z peak Optimize the cuts Try the analysis for L= 1 fb fb -1 Higher statistics for ttbar