1. ttH (Hγγ) search and CP measurement
Scott McGarvie
Pedro Teixeira-Dias
ATLAS-UK Higgs mtg, Durham, 18/9/06

2. Motivation
Higgs may be CP-even (e.g. SM), CP-odd (e.g. A in MSSM), or a CP-mixed state (CP-violating 2HDM, complex MSSM)
ttH channel offers possibility to measure CP of light Higgs scalar (i.e., mH<2mZ)

3. CP measurement in ttH: method
Suggested by Gunion & He, PRL76, 24 (1996) 4468;
CP-sensitive variables depend only on momenta of top quarks, not Higgs … …though in practice, accuracy of top reconstruction will depend on Higgs decay mode /A

4. Feasibility study at parton level
Perfect top quark momentum reconstruction
Establish power of method to measure/distinguish CP-even, CP-odd, mixed-CP signal
No backgrounds

5. CP-sensitive variables (I) – tth/A
a2-even
a2-odd
a1-even
a1-odd
a3-even
a3-odd
b1-even
b1-odd

6. CP-sensitive variables (II) - tth/A
b3-even
b3-odd
b2-even
b2-odd
b4-even
b4-odd
b5-even
b5-odd

7. Maximum Likelihood
Use the method of maximum likelihood to determine the mixing parameter .
Where f(x) are the PDFs for the test statistic, which in our case are just the individual variables (e.g. x=b4 …)

8. Measurement of α αtrue = 50% < αreco > = 51.8%
Distribution of αreco for 500 toy MC experiments, with s+b=100 events each (≈250 /fb)

9. Feasibility study result (parton level)
αrecon = αtrue
 used b4 variable only
 method provides unbiased CP determination
 “15-20% uncertainty” (L ≈ 250 /fb)

10. Detector-level study (ATLFAST)
tth/A  bjj blυ γγ (BR (Hγγ) ≈ 10-3)
Selection Procedure: 
 2 light jets
2 bjets
2 photons (pT  25 GeV)
1 electron or muon (pT  25 GeV) 

11. CP sensitive variables: a1 , a2
a1-even
a2-even
ATLFAST LEVEL
a1-odd
a2-odd
a1-even
a2-even
PARTON LEVEL
a1-odd
a2-odd

12. CP sensitive variables: b4 , b5
b4-even
b5-even
ATLFAST LEVEL
b4-odd
b5-odd
b5-even
b4-even
PARTON LEVEL
b4-odd
b5-odd

13. Detector-level study (ATLFAST)
CP-sensitive variables retain discriminant power
…must include backgrounds
…and improve event selection for higher background rejection
TDR analysis carried out long ago and using older technology (PYTHIA 5.7 for signal and bgd; relied on PHOTOS MC to generate ISR/FSR γs… spectrum too soft)
Now: use MADGRAPH ME calculation for all bgds; HERWIG 6.5 is used for CP-even/odd signals
 TDR bgd estimate was found to be ~5x lower than current estimates…!!

14. Backgrounds (MADGRAPH)
Process
σ (fb)
Equiv. Lumi generated (fb)
ttγ
3326
155
ttγγ 16
49,700
ttγj
3704
220
ttjj
268
960
bbγγ
3944 26
bbγj
348
570
bbjj
3921 25
Wγγ
123
50,700
Wγγj
134
1,540
mH=120 GeV (HERWIG)
1.5
205,000
generated event samples: ~ 6M background events; ~ 2M signal events (mH=115 – 150 GeV)

15. Photon pT cuts optimization

16. Photon pT cuts optimization
pT(γ1) pT(γ2)
signal
signal
ttγγ bgd
ttγγ bgd

17. Mass window optimization, Δm
Count events in mass window of size Δm:
mh- Δm < mγγ < mh+ Δm
Δm (MeV)
 Maximum significance for 2.3 GeV (cf 1.7 GeV in TDR)

18. Event selection optimization (mh=120 GeV; 100 /fb)
Use the current signal and background event samples and try to improve the event selection signal-to-noise… there is room for improvement:
TDR-inspired:
pT(e/μ)> 25 GeV; pT(γ1), pT(γ2)> 25 GeV
mh-Δm < m(γγ) < mh + Δm (Δm =1.7 GeV)
Total b = 32.1
s = 19.7
Optimized event selection:
pT(e/μ)> 25 GeV; pT(γ1)> 50 GeV, pT(γ2)> 30 GeV
mh-Δm < m(γγ) < mh + Δm (Δm =2.3 GeV)
Total b = 19.2
s = 17.8

19. Event selection (cont’d)
Additional cuts? (...can’t afford large additional cuts in signal rate, otherwise compromise CP-even/odd discriminant power…)
Requiring 1 b-tagged jet (reduces Wxx bgds)  only a slight improvement in significance
Top-quark reconstruction: required for CP measurement; will use bjj blv combination that minimizes

20. Summary & Future Plans
Established power of CP-method in tth channel at LHC (parton level study)
Detector level tth (hγγ) (ATLFAST):
CP-sensitive variables seem to retain power
much-improved bgd calculation (wrt TDR) done
event selection improved
still need to establish finally the precision of CP determination, for eg. L=100 /fb and 300 /fb
finish writing up and get PhD.
Future plans: investigate possible increase in CP-sensitivity by 1) using more than just one CP-sensitive variable 2) using a higher BR Higgs decay mode (HWW, bb) 3) combining CP-measurement from Hγγ and HWW, bb

21. Backup slides

22. Detector-level event reconstruction
For events which meet these requirements:
Reconstruct a hadronic W from all jet-jet pairs
Assume missing momentum is due to the neutrino, use W mass constraint to fully reconstruct the neutrino momentum.
Reconstruct a leptonic W in events which have 1 or more neutrino solutions
Reconstruct the top quarks by combining the b-jets and Ws 4-vectors

23. top-quark reconstruction
Reconstruct a top for all bjj and bl combinations.
Four-vectors of the two tops are found by selecting the combination which minimizes the 2
mt = 175 GeV, mw = 80 GeV,  from TDR
The reconstructed tops are then used to determine the values of the CP-sensitive variables