1. Determining the CP Properties of a Light Higgs Boson
Scott McGarvie
Determining the CP Properties of a Light Higgs Boson
Royal Holloway, University of London

2. Contents Introduction Heavy Higgs CP Light Higgs CP
Production/Decay Channel
Selection procedure
CP sensitive variables
Conclusions and Future Plans

3. Introduction
The search for the Higgs boson is one of the most important tasks for the LHC
Demonstrated for the TDR that ATLAS can cover the whole SM parameter space
Once the Higgs is discovered, we will need to study its properties

5. SM Higgs is spin 0 and CP-even
This may not be the case in models with extended Higgs sectors:
general 2HDM
MSSM with complex parameters
MSSM predicts 3 neutral Higgs particles. h, H (CP-even) A (CP-odd)
Presence of complex phases causes mixing and the mass eigenstates (h1, h2, h3) then have mixed CP
Carena, Ellis, Mrenna, Pilaftsis, Wagner. Nucl.Phys. B659 (2003) hep-ph/

6. Experimental evidence that demonstrates the CP quantum numbers, would be very useful in determining the correct model
Making a precision measurement such as this will be very difficult in the messy environment of the LHC

7. Heavy Higgs CP
CP Information is contained in the decay planes of H → ZZ* → 4l
CP even parallel decay planes,
CP odd perpendicular
Buszello, Fleck, Marquard, Van der Bij, SN-ATLAS

8. It was shown that they can distinguish between a CP-even and a CP-odd with 100fb-1, for a Higgs mass  200 GeV
The linear collider uses a similar technique to determine the CP
Running in the  mode, the polarisation of the photons can be changed from parallel to perpendicular to preferentially produce either a CP-even or CP-odd state

9. Higgs Decays

10. Higgs production

11. Light Higgs CP
For a light Higgs the H → ZZ* branching fraction is too small
Use H → bb, , + decay channels
Using ideas from Gunion, He PRL 76, 24, 4468 (1996)
Interaction Lagrangian
c is the CP-even coupling and d is the CP-odd coupling
SM has c = 1 and d = 0
Only top quark will have significant sensitivity to CP

12. CP sensitive variables
CP information contained in the momenta of the tops

13. Aims Use the associated ttH →  channel
Select events with required topology
Reconstruct the physics objects
Select events using a 2 technique, to reduce the combinatorial background
Demonstrate that the CP information is contained with the proposed variables

14. Comment
The ttH →  will probably not be the best channel in which to measure CP
Very small event rate
In the context of the MSSM, the HVV coupling is absent

15. Branching fractions increase production cross-section decreases
Comment II
the ttH → bb, +- channels have advantages and disadvantages.
Much higher signal rate, much higher backgrounds
Couplings ,
Branching fractions increase
production cross-section decreases

16. Selection Procedure g g 2 or more light jets 2 bjets
2 photons (pT  25 GeV)
1 electron or 1 muon ( pT  25 GeV) g g

17. Selection efficiencies
Cut
Nbjet =2
Nljet  2
1e or 1 
n 2
pT  25 , e, 
 1  soln.
CP even (%)
19.4
19.1
6.2
4.9
2.9
2.0
CP odd (%)
19.3
19.2
5.3
3.8
2.5
For the selected events
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 quark by combining the b-jet and W 4-vectors

18. Hadronic W mass from all jj pairs

19. Hadronic top mass from all bjj

20. Top Reconstruction Reconstruct a top for all bjj and bl combinations.
4-vectors of the two tops are found by selecting the combination which minimizes the 2
mt = 175 GeV, mw = 80 GeV,  from TDR
The selected tops are then used to Determine to values of the CP sensitive variables

21. Reconstructed hadronic W

22. Reconstructed hadronic top

23. Reconstructed semi-leptonic top

24. CP sensitive variables: a1, a2
CP even
CP even
CP odd
CP odd

25. CP sensitive variables: b1, b2
CP even
CP even
CP odd
CP odd

26. CP sensitive variables: b3, b4
CP even
CP even
CP odd
CP odd

27. Future Plans Several things still to do
Finish the semi-leptonic ttH →  channel and obtain significances for distinguishing between CP-even and CP-odd
Backgrounds
Better operators with greater sensitivity to CP
Sensitivity to a Higgs with mixed CP
Other Decay channels
ttH → , both tops hadronic
ttH →bb
ttH →+-
Combined analysis of different channels

28. Conclusions
ATLAS does have some sensitivity to the CP properties of a light higgs.
A precision measurement like this needs high statistics, and may require a combined ATLAS/CMS analysis