[Paper Review] Determination of the Higgs CP property in Hadron Colliders

Akihiro Matsuzaki, Hidekazu Tanaka|arXiv (Cornell University)|Jan 11, 2011

Particle physics theoretical and experimental studies4 references4 citations

TL;DR

This paper proposes three model-independent methods to determine the CP quantum number of the Higgs boson at hadron colliders: (1) analyzing production via vector boson fusion, (2) applying CP selection rules to b-quark decay modes, and (3) measuring the angular distribution of leptons from top quark decay in t\bar{t} final states. The methods are effective across a wide Higgs mass range, with Monte Carlo simulations showing clear discrimination between CP-even and CP-odd Higgs bosons using as few as 20 events.

ABSTRACT

We propose three ways to determine the CP eigenvalue of the Higgs boson at the hadron collider as follows: 1. We determine the Higgs CP eigenvalue from the production cross section which is affected by the CP eigenvalue of the Higgs boson. 2. We adopt the CP selection rules to determine the Higgs CP eigenvalue. 3. We determine the CP property by the momentum distribution of the decay products of the Higgs boson. Our methods can be applied for a wide range of the Higgs mass.

Motivation & Objective

• To determine the CP eigenvalue of the Higgs boson in a model-independent way at hadron colliders.
• To address the challenge of identifying CP properties when Higgs decays into Z, W, or top quarks are kinematically forbidden due to low mass.
• To provide experimental strategies applicable across a broad Higgs mass spectrum, especially for light Higgs bosons.
• To distinguish CP-even from CP-odd Higgs bosons using production mechanisms, decay selection rules, and momentum distributions of decay products.

Proposed method

• Using vector boson fusion production: CP-even Higgs bosons can be produced via VV fusion, while CP-odd Higgs bosons are forbidden by selection rules.
• Applying CP selection rules to Higgs decays into b-quark pairs: CP-even Higgs bosons favor decay modes with no photon emission from B* mesons, unlike CP-odd Higgs bosons.
• Measuring the opening angle distribution of leptons from top quark decays in t\bar{t} final states to infer the parity and thus CP eigenvalue of the Higgs boson.
• Using Monte Carlo simulations to model the D2/D1 ratio of angular distributions in the Higgs rest frame, where D2/D1 = 1 for CP-even and D2/D1 = -1 for CP-odd.
• Analyzing the invariant mass reconstruction of the Higgs boson in t\bar{t} and VV fusion processes to distinguish CP properties.
• Evaluating the sensitivity of the methods under realistic luminosity conditions, such as 10^34 cm^{-2}s^{-1} for one year of LHC operation.

Experimental results

Research questions

• RQ1 Can the CP quantum number of the Higgs boson be determined independently of specific new physics models?
• RQ2 How can the CP property be extracted when the Higgs boson decays into Z, W, or top quarks are kinematically forbidden?
• RQ3 Can production mechanisms like vector boson fusion be used to distinguish CP-even from CP-odd Higgs bosons?
• RQ4 How do selection rules in b-quark decay modes differ between CP-even and CP-odd Higgs bosons?
• RQ5 Can the angular distribution of leptons from top quark decays in t\bar{t} final states be used to determine the CP eigenvalue of the Higgs boson?

Key findings

• For a Higgs boson with mass 400 GeV, the D2/D1 ratio from Monte Carlo simulation was measured as 1.4 ± 0.7 in a sample of 20 events, consistent with CP-even Higgs.
• For a Higgs boson with mass 1000 GeV, the D2/D1 ratio was 1.8 ± 1.1 in a sample of 20 events, again indicating CP-even Higgs.
• The method based on vector boson fusion production can identify CP-odd Higgs bosons if the S-V-V coupling is non-zero; however, it fails when the coupling is suppressed, such as in the case β = α or α + π/2 in 2HDM or MSSM.
• The CP selection rule method shows that CP-even Higgs bosons are less likely to emit photons from B* mesons than CP-odd Higgs bosons, providing a clear experimental signature.
• In the MSSM with tanβ = 1.5 and mA = 400 GeV, about 10 signal events of pp → A + t\bar{t} are expected per year at 10^34 cm^{-2}s^{-1} luminosity, with background reduced to 10% after cuts.
• The three methods together allow determination of the CP eigenvalue across a wide Higgs mass range, and any deviation from CP-even or CP-odd behavior suggests CP violation.