1. Z' search with a full reconstruction of the ttbar final state
S Calvet, R Febbraro, D Pallin
LPC group meeting
Nov 22, 2010

2. Strategy Strategy Selection
Full reconstruction of the (ttbar) final state
to gain on Z’ mass resolution,
to reduce tails at high masses from jets combination
Background removed largely using b-tag
Simple method for the choice of the best combination (as baseline)
Mtop is known
Loop over all combinations
Best comb. according to lowest Combination quality factor (2) given by
QF= [(Mjj -MW)/W] 2 + [(Mjjj -Mth)/th] 2 + [(Mjlv -Mtl)/tl] 2
Difference with ATL-PHYS-PUB
Don’t perform separated recons scheme in function of highest evt jet mass
don’t add an additional jet to account for Z’-> ttbar X-> (jjb) (lb) X
Selection
Standard Top X-sec selection with Topinputs
~50% of SM evts are reconstructible (all parton from ttbar are in acceptance)
Among the reconstructible SM evts the right combination is found in ~65% 2

3. Tops reconstruction
Selection efficiency 3

4. Signal Samples characteristics
Signal Z’-> (ttbar) X-> (jjb) (lb) X
(ttbar) masses : of-peak tail due to missing X jet
Additional X (jets) tends to be close to one
of the produced tops Z’
ttbar
Top had
Top Lept 4

5. Z’ reconstruction (0.5 TeV)
Reconstructed Z’ mass Signal Z’-> (ttbar) X-> (jjb) (lb) X
M= M(TT) signal+ comb bkg /Signal
M= M(TT)-MTh-MTL + 2*Mtop signal+ comb bkg /Signal
Signal
+comb BKG
Signal
Mass resolution is better
Signal
+comb BKG
Signal 5

6. Z’ reconstruction (0.7 TeV)
Reconstructed Z’ mass Signal Z’-> (ttbar) X-> (jjb) (lb) X
M= M(TT) signal+ comb bkg /Signal
M= M(TT)-MTh-MTL + 2*Mtop signal+ comb bkg /Signal
Signal
+comb BKG
Signal
Mass resolution is better
Signal
+comb BKG
Signal 6

7. Z’ reconstruction (1 TeV)
Reconstructed Z’ mass Signal Z’-> (ttbar) X-> (jjb) (lb) X
M= M(TT) signal+ comb bkg /Signal
M= M(TT)-MTh-MTL + 2*Mtop signal+ comb bkg /Signal
Signal
+comb BKG
Signal
Signal
+comb BKG
Signal 7

8. Z’ reconstruction (0.5 ; 0.7 ; 1.0; 1.5 TeV)
Reconstructed Z’ mass (Good comb)
M= M(TT)
M= M(TT)-MTh-MTL + 2*Mtop
Linearity
Mass resolution
Z’ true mass 8

9. DATA/MC comparison (electron stream)
C1: Exactly 1 lepton with pT>20 GeV
C11: Last cut e-mu overlap removal 9

10. DATA/MC comparison (electron stream)
C1: Exactly 1 lepton with pT>20 GeV
C11: Last cut e-mu overlap removal 10

11. Results Results equivalent as in PUB note Resolution
Linearity true%recons mass
To be done
To understand data/MC difference
263 data evts vs 236 for CEA
MC in agreement except for QCD (15 evts vs 34 for CEA)
Detail look at all variables with full 2010 dataset (ongoing)
Use of a kinematic fit
Mainly to improve resolution on Z’ mass
Marginaly to improve the efficiency to choose the right combination
Comb BKG removal
Use QF and b-jet SV0 prob ?
Use additionnal cuts on Pt (ttbar), Pt (light jets), Mjj before selction of best comb …
Systematics , Limit extraction 11

12. BACKUP 12

13. Ttbar reconstruction M= M(TT) M= M(TT)-MTh-MTL + 2*Mtop
Reconstructed TTbar mass (Good comb)
M= M(TT)
M= M(TT)-MTh-MTL + 2*Mtop 13

14. Ttbar reconstruction M= M(TT) M= M(TT)-MTh-MTL + 2*Mtop
TTbar mass resolution
M= M(TT)
M= M(TT)-MTh-MTL + 2*Mtop 14

15. Ttbar reconstruction
Reconstructed TTbar mass (Good comb)
M= M(TT) 15

16. Ttbar reconstruction M= M(TT)-MTh-MTL + 2*Mtop
Reconstructed TTbar mass (Good comb)
M= M(TT)-MTh-MTL + 2*Mtop 16