1. Update on Invisible Higgs analysis in the tth channel
Review of previous results
Cut optimisation
Conclusion
Ricardo Goncalo
Ricardo Gonçalo
Atlas Higgs UK RHUL

2. Atlas Higgs UK meeting @ RHUL
Analysis overview
Cut-based analysis
Only signal and tt backg so far
Difficulties:
Two components of missing momentum: can’t reconstruct tbl
ttbar is the most significant background and is very similar to signal
Signal/Background ~10-3
Very much “ongoing work”!…. And talk cobbled together in a hurry (sorry!)
Process
xBR
tth
330 (*) fb tt fb
bbW, W  l fb
bbZ, Z  l+l- fb
ttW, W  l
420 fb
ttZ, Z  
190 fb
(*) =520 fb in reference analysis, with SM couplings
Ricardo Gonçalo
Atlas Higgs UK RHUL

3. Atlas Higgs UK meeting @ RHUL
Cuts 
Reference analysis cuts:
1 electron (pT>25GeV; ||<2.5)
or 1 muon (pT>20GeV; ||<2.5)
Veto on additional electron (pT>10GeV)
or additional muon (pT>6GeV)
2 b-tagged jets
2 or more un-tagged jets
tbjj reconstruction:
|mjj-mW|<15GeV; ||<2.0 for jets in Wjj
|mbjj-mt|<25GeV
mT > 120 GeV
Missing ET > 150GeV
Scalar sum of pT of reconstructed l j j b b (+ missing ET): ET>250GeV
In reconstructed W  jj: Rjj =(2jj+2jj) < 2.2 (to reject lep-tau decays)
Ricardo Gonçalo
Atlas Higgs UK RHUL

4. Atlas Higgs UK meeting @ RHUL
Simulation
Channel: Higgs (mh = 120 GeV) decaying to neutralinos in MSSM
(tan= 5, mA= 1 TeV; M1= 44 GeV, M2= 220 GeV, M3= 1 TeV)
PYTHIA for signal and background
Generated 142 M tt (L~25fb-1)+ 1 M tth (L~2pb-1)
Atlfast simulation, ATLAS release 7.0.2
Low luminosity setting
Cone jets (Rcone = 0.4)
Jet tagging: b jets 60%; c mistag 10%; u,d,s, mistag 1%
CTEQ5L PDFs
mtop = 175 GeV (..historical)
Interfaced code to Atlfast within Athena to produce dedicated ntuple
Ricardo Gonçalo
Atlas Higgs UK RHUL

5. Atlas Higgs UK meeting @ RHUL
Results so far…
Most background comes from lep-tau and lep-lep decays of tt, as concluded in reference paper
 decays increase the missing ET
Wjj reconstructed from ISR/FSR jets in lep-tau and lep-lep events
lep-had
lep-lep
lep-tau
tau-tau
had-had
Ricardo Gonçalo
Atlas Higgs UK RHUL

6. Atlas Higgs UK meeting @ RHUL
Results so far
Accepted events for 30 fb-1
tth x-section scaled to SM value as used in reference analysis
tth: x-section = 520 fb-1
Signal = 44.3 events
tt: x-section = fb-1
Background = 812 events
S/B = 1.55
Ricardo Gonçalo
Atlas Higgs UK RHUL

7. Atlas Higgs UK meeting @ RHUL
Kinematic fit
Tried something different:
Assume pTmiss comes from  and h only
Build grid of points in pT and (l,) and calculate pZ
pZ can be found from pT and plep assuming W on mass-shell
From pb , p and plep, calculate mt for each point
Propagate errors in mt from grid spacing to obtain mt and calculate 2 mW W b t
e/  mt
Ricardo Gonçalo
Atlas Higgs UK RHUL

8. Atlas Higgs UK meeting @ RHUL
Kinematic fit results
Kinematic fit ~works for signal: (pTh)~85GeV
Will try to use fit results for discrimination against tt background
The hope is that this allows other cuts to be relaxed
Correlations to mT and ETmiss may be important
Ricardo Gonçalo
Atlas Higgs UK RHUL

9. Variables used in event selection
Applying cuts as in standard analysis:
Missing ET: Signal …and tt background
Ricardo Gonçalo
Atlas Higgs UK RHUL

10. Atlas Higgs UK meeting @ RHUL
Transverse W mass
Signal… … and tt background
Ricardo Gonçalo
Atlas Higgs UK RHUL

11. Rjj (distance in eta-phi between jets from W decay)
Signal… … and tt background
Ricardo Gonçalo
Atlas Higgs UK RHUL

12. Atlas Higgs UK meeting @ RHUL
Scalar sum of event ET
Problem found: cutting on ET of lepton, 2 jets, 2 b jets AND missing ET…means that truth histo will be wrong (no ETmiss), but preselection ok
Signal …tt background
Ricardo Gonçalo
Atlas Higgs UK RHUL

13. Lepton and jet variables
Signal… … and tt background
Ricardo Gonçalo
Atlas Higgs UK RHUL

14. New variables to use in selection?
Higgs pT from the kinematic fit
Signal tt background
Ricardo Gonçalo
Atlas Higgs UK RHUL

15. Atlas Higgs UK meeting @ RHUL
Cut on l,Etmiss
Naively expect lepton to be mainly away from ETmiss direction in signal and close to ETmiss direction in background (with lepton and  from W decay)
In fact, most tt background which passes standard cuts comes from lep-tau with ETmiss from both  and W decay and one detected lepton
Ricardo Gonçalo
Atlas Higgs UK RHUL

16. Atlas Higgs UK meeting @ RHUL
Cut optimisation
Varied several cuts over a few threshold values to optimise significance = S/(S+B)
All cut combinations considered
Cuts varied:
mT (standard cut: > 120 GeV)
ETmiss (standard cut: > 150GeV)
Scalar sum of pT of reconstructed l j j b b: ET (standard cut: >250GeV
Added pTH obtained from kinematic fit
Added  between reconstructed lepton and ETmiss
Also tried varying electron and muon pT cuts; still trying things out
Ricardo Gonçalo
Atlas Higgs UK RHUL

17. Atlas Higgs UK meeting @ RHUL
Cut optimisation
Significance for 30 fb-1 increases from ~ 1.6 to ~ 3
Neglecting the cut on pTH,fit doesn’t seem to make much difference
other thresholds must vary to achieve equal significance
Ricardo Gonçalo
Atlas Higgs UK RHUL

18. Atlas Higgs UK meeting @ RHUL
Cut optimisation
Some cuts have more impact than others
Results point to optimal thresholds:
mTW cut ~ 115GeV
ETmiss cut ~ 220 GeV
l,Etmiss cut < 2.8 rad
No clear solution for ET or pTH,fit
Large correlations between variables should be expected
Ricardo Gonçalo
Atlas Higgs UK RHUL

19. Atlas Higgs UK meeting @ RHUL
Optimised cuts
Signal… … and tt background
Ricardo Gonçalo
Atlas Higgs UK RHUL

20. Atlas Higgs UK meeting @ RHUL
Sum ET
Signal… … and tt background
Ricardo Gonçalo
Atlas Higgs UK RHUL

21. Atlas Higgs UK meeting @ RHUL
Transverse W mass
Signal… … and tt background
Ricardo Gonçalo
Atlas Higgs UK RHUL

22. Lepton and jet variables
Signal… … and tt background
Ricardo Gonçalo
Atlas Higgs UK RHUL

23. Not using Rjj as selection variable
Signal… … and tt background
Ricardo Gonçalo
Atlas Higgs UK RHUL

24. Atlas Higgs UK meeting @ RHUL
Higgs pT from fit
Signal… … and tt background
Ricardo Gonçalo
Atlas Higgs UK RHUL

25. Atlas Higgs UK meeting @ RHUL
l,Etmiss
Signal… … and tt background
Ricardo Gonçalo
Atlas Higgs UK RHUL

26. Atlas Higgs UK meeting @ RHUL
Conclusions
Studying major background to tth  inv (other backgrounds much less significant)
Improvement in significance may be achieved through cut on l,Etmiss
Kinematic fit to semileptonic top decay: still assessing usefulness as discriminating variable
Simple optimisation of cut thresholds done (ongoing work)
Ricardo Gonçalo
Atlas Higgs UK RHUL