[Paper Review] Determining the CP properties of the Higgs boson

P. S. Bhupal Dev, A. Djouadi+3 more|arXiv (Cornell University)|Jul 19, 2007

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper proposes that the CP properties of a Standard Model-like Higgs boson can be unambiguously determined in $e^{+}e^{-}$ collisions by measuring the total cross section and top quark polarization in associated $t\bar{t}H$ production. The method distinguishes scalar ($0^{++}$) and pseudoscalar ($0^{-+}$) Higgs states via distinct threshold behaviors and polarization asymmetries, with sensitivity to CP mixing parameter $b$ reaching $\Delta b \sim 0.01$ for a pseudoscalar-like state at 500 fb$^{-1}$ luminosity.

ABSTRACT

The search and the probe of the fundamental properties of Higgs boson(s) and, in particular, the determination of their charge conjugation and parity (CP) quantum numbers, is one of the main tasks of future high-energy colliders. We demonstrate that the CP properties of a Standard Model-like Higgs particle can be unambiguously assessed by measuring just the total cross section and the top polarization in associated Higgs production with top quark pairs in e+e- collisions.

Motivation & Objective

• To unambiguously determine the CP quantum numbers of a Standard Model-like Higgs boson in high-energy $e^{+}e^{-}$ collisions.
• To overcome the limitations of existing CP-sensitive observables at the LHC, which suffer from large QCD backgrounds and limited sensitivity.
• To provide a simple, direct, and statistically robust method independent of model-dependent normalization or higher-order corrections.
• To probe CP violation by measuring the mixing parameter $b$ between CP-even and CP-odd Higgs components in the $t\bar{t}H$ coupling.

Proposed method

• Measure the total cross section $\sigma(e^{+}e^{-}\to t\bar{t}H)$ as a function of center-of-mass energy $\sqrt{s}$, which exhibits distinct threshold behaviors for scalar and pseudoscalar Higgs bosons.
• Measure the top quark polarization $P_t$, which is strongly sensitive to the CP nature of the Higgs boson due to interference between $\gamma$ and $Z$ exchange diagrams.
• Use the ratio of $P_t$ values for scalar and pseudoscalar Higgs bosons at $\sqrt{s}=800$ GeV to extract the relative strength of the $Zt\bar{t}$ axial-vector coupling, yielding a factor of three difference.
• Employ the parameter $b$ to quantify CP mixing, where $b=0$ corresponds to pure CP-even and $b=1$ to pure CP-odd Higgs states.
• Utilize polarized $e^{+}e^{-}$ beams to enhance the top quark polarization signal and improve statistical sensitivity to $b$.
• Analyze the azimuthal angle asymmetry of the antitop quark relative to the top-electron plane, which is linear in $b$ and directly probes CP violation.

Experimental results

Research questions

• RQ1 Can the CP quantum numbers of a SM-like Higgs boson be determined unambiguously in $e^{+}e^{-}$ collisions without relying on complex angular correlations?
• RQ2 How do the total cross section and top quark polarization differ between a CP-even and CP-odd Higgs boson in $t\bar{t}H$ production?
• RQ3 What is the sensitivity of the cross section and polarization to the CP mixing parameter $b$ at realistic luminosity and energy?
• RQ4 Can a P-odd observable such as the up-down asymmetry in the antitop quark distribution directly probe CP violation in the Higgs sector?

Key findings

• The total cross section for $e^{+}e^{-}\to t\bar{t}H$ shows a steep dependence on the CP mixing parameter $b$ near the threshold, enabling high sensitivity to CP properties.
• At $\sqrt{s} = 800$ GeV and $\mathcal{L} = 500$ fb$^{-1}$, the sensitivity to $b$ reaches $\Delta b \sim 0.01$ for a pseudoscalar-like Higgs boson ($b=1$), significantly better than the $\sim10\%$ precision from $\Phi \to \tau\tau$ decays.
• Top quark polarization $P_t$ provides a clean, CP-odd-sensitive observable: the ratio of $P_t$ values for scalar and pseudoscalar Higgs bosons is approximately three at $\sqrt{s}=800$ GeV, due to the axial-vector nature of the $Zt\bar{t}$ coupling.
• The up-down asymmetry of the antitop quark with respect to the top-electron plane is linear in $b$ and reaches up to 5% for $M_\Phi = 120$ GeV at $\sqrt{s} = 800$ GeV, offering a direct probe of CP violation.
• The method is robust against normalization uncertainties and higher-order corrections because it relies on ratios of cross sections and polarization asymmetries, not absolute rates.
• Polarized initial beams enhance the top polarization signal by a factor of three and improve the statistical precision of $b$ determination, especially for $b \sim 1$.