scott mcgarvie determining the cp properties of a light higgs boson

Scott McGarvie Atlas UK, 15/12/03

Scott McGarvie

Determining the CP Properties of a Light Higgs Boson

Royal Holloway, University of London


•Introduction

•Heavy Higgs CP

•Light Higgs CP

•Production/Decay Channel

•Selection procedure

•CP sensitive variables

•Conclusions and Future Plans

Contents


IntroductionThe 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


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

Carena, Ellis, Mrenna, Pilaftsis, Wagner. Nucl.Phys. B659 (2003)

145-178 - hep-ph/0211467

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


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


Heavy Higgs CP

•CP even parallel decay planes,

•CP odd perpendicular

Buszello, Fleck, Marquard, Van der Bij, SN-ATLAS-2003-025

CP Information is contained in the decay planes of H → ZZ* → 4l


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


Higgs Decays


Higgs production


Interaction Lagrangian

Light Higgs CP

2 2 2 2M ( )QM c d

For a light Higgs the H → ZZ* branching fraction is too small

Use H → bb, , + decay channels

c is the CP-even coupling and d is the CP-odd coupling

SM has c = 1 and d = 0

Using ideas from Gunion, He PRL 76, 24, 4468 (1996)

Only top quark will have significant sensitivity to CP


CP sensitive variables

CP information contained in the momenta of the tops


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


Comment

•Very small event rate

•In the context of the MSSM, the HVV coupling is absent

The ttH → will probably not be the best channel in which to measure CP


Comment II

the ttH → bb, +- channels have advantages and disadvantages.

,

Couplings

Branching fractions increase

production cross-section decreases

Much higher signal rate, much higher backgrounds


Selection Procedure

•2 or more light jets

•2 bjets

•2 photons (pT 25 GeV)

•1 electron or 1 muon ( pT 25 GeV)


Selection efficiencies

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

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 6.2 5.3 3.8 2.5


Hadronic W mass from all jj pairs


Hadronic top mass from all bjj


Top Reconstruction

4-vectors of the two tops are found by selecting the combination which minimizes the 2

The selected tops are then used to Determine to values of the CP sensitive variables

Reconstruct a top for all bjj and bl combinations.

mt = 175 GeV, mw = 80 GeV, from TDR


Reconstructed hadronic W


Reconstructed hadronic top


Reconstructed semi-leptonic top


CP sensitive variables: a1, a2

CP odd

CP evenCP even

CP odd


CP sensitive variables: b1, b2

CP odd

CP even

CP even

CP odd


CP sensitive variables: b3, b4

CP odd

CP evenCP even

CP odd


Future PlansSeveral 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

•MC@NLO


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

scott mcgarvie determining the cp properties of a light higgs boson

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