[Paper Review] Probing CP-violating $h\bar{t}t$ coupling in $e^{+}e^{-} ightarrow h \gamma$

Gang Li, Haoran Wang+1 more|arXiv (Cornell University)|Jun 22, 2015

Particle physics theoretical and experimental studies11 citations

TL;DR

This paper proposes probing CP-violating $h\bar{t}t$ couplings via $e^{+}e^{-} \to h\gamma$ at future $e^{+}e^{-}$ colliders, showing up to a 6-fold cross-section enhancement over the SM for $\sqrt{s} = 350~\text{GeV}$ and $\xi = 3\pi/5$. With $\mathcal{L} = 10~\text{ab}^{-1}$, signal significance exceeds 5$\sigma$ at 350 GeV, and forward-backward asymmetry $A_{FB}$ reaches 0.5, offering a clean CP-violation signal despite background challenges.

ABSTRACT

We investigate the possibility of probing the CP-violating $h\bar{t}t$ coupling in the process $e^{+}e^{-} ightarrow h \gamma$ at the future high luminosity $e^{+}e^{-}$ colliders. Our numerical results show that the cross section for this process can be significantly increased for the allowed CP phase $\xi$ and center of mass energy. For example the cross section is about 6 times of that in the standard model (SM) for $\sqrt{s}=350~\mathrm{GeV}$ and $\xi =3\pi/5$ (see text for $\xi$ definition). The simulation for the signal process $e^{+}e^{-} ightarrow h \gamma ightarrow b \bar{b}\gamma$ and its backgrounds shows that the signal significance can reach more than $4.8\sigma$ and $2.3\sigma$ for $\sqrt{s}=350~\mathrm{GeV}, 500~\mathrm{GeV}$ respectively, with the integrated luminosity $\mathcal{L}=3~ ext{ab}^{-1}$ and $\xi\in[\pi/2,3\pi/5]$. For $\mathcal{L}=10~ ext{ab}^{-1}$, the signal significance can be greater than 5 for $\sqrt{s}=350~\mathrm{GeV}$ and 3.9 for $\sqrt{s}=500~\mathrm{GeV}$ with the CP phase $\xi\in[\pi/2,3\pi/5]$. Besides the cross section enhancement, the CP-violating $h\bar{t}t$ coupling will induce the forward-backward asymmetry $A_{FB}$ which is absent in the SM and is a clear signal of new CP violation. Compared with the $A_{FB}$ in the Higgs decay $h ightarrow l^{+}l^{-}\gamma$, the $A_{FB}$ can be greatly enhanced in the production process. For example $A_{FB}$ can reach 0.5 for $\cos\xi\simeq 0.7$ and $\sqrt{s}=500~\mathrm{GeV}$. Due to the large backgrounds, the significance of the expected $A_{FB}$ can only be observed at $1.7\sigma$ with $\mathcal{L}=10~ ext{ab}^{-1}$ and $\sqrt{s}=500~\mathrm{GeV}$. It is essential to trigger the single photon in the final state to separate the bottom jets arising from scalar or vector bosons, in order to isolate the signal from the backgrounds more efficiently.

Motivation & Objective

• To investigate the feasibility of detecting CP-violating $h\bar{t}t$ couplings in the $e^{+}e^{-} \to h\gamma$ process at future high-luminosity $e^{+}e^{-}$ colliders.
• To assess the signal significance of $h \to b\bar{b}\gamma$ final states against dominant backgrounds under realistic luminosity and energy conditions.
• To evaluate the forward-backward asymmetry $A_{FB}$ as a direct probe of new CP violation, distinct from SM Higgs decays.
• To determine optimal kinematic triggers, particularly single-photon tagging, for enhancing signal-background separation.

Proposed method

• Numerical computation of the $e^{+}e^{-} \to h\gamma$ cross section as a function of center-of-mass energy $\sqrt{s}$ and CP phase $\xi$.
• Monte Carlo simulation of the signal process $e^{+}e^{-} \to h\gamma \to b\bar{b}\gamma$ and dominant QED and QCD backgrounds.
• Calculation of the forward-backward asymmetry $A_{FB}$ in the $h\gamma$ production process, defined as $A_{FB} = (\sigma_{\text{forward}} - \sigma_{\text{backward}})/(\sigma_{\text{forward}} + \sigma_{\text{backward}})$.
• Application of kinematic cuts and single-photon triggering to suppress background contributions from $Z\gamma$, $W^+W^-$, and $q\bar{q}\gamma$ processes.
• Estimation of statistical significance using $S/\sqrt{B}$ with integrated luminosities $\mathcal{L} = 3~\text{ab}^{-1}$ and $10~\text{ab}^{-1}$.
• Analysis of $\cos\xi$ dependence to identify regions of maximal CP-violating effects.

Experimental results

Research questions

• RQ1 Can the $e^{+}e^{-} \to h\gamma$ process provide a significant enhancement in cross section due to CP-violating $h\bar{t}t$ couplings?
• RQ2 What is the achievable signal significance for $h \to b\bar{b}\gamma$ at $\sqrt{s} = 350~\text{GeV}$ and $500~\text{GeV}$ with $\mathcal{L} = 3~\text{ab}^{-1}$ and $10~\text{ab}^{-1}$?
• RQ3 To what extent can the forward-backward asymmetry $A_{FB}$ serve as a clean signature of new CP violation in this process?
• RQ4 How effective is single-photon triggering in isolating the signal from dominant QCD and electroweak backgrounds?
• RQ5 What is the sensitivity of the signal to the CP phase $\xi$, particularly in the range $[\pi/2, 3\pi/5]$?

Key findings

• The cross section for $e^{+}e^{-} \to h\gamma$ increases by a factor of approximately 6 compared to the SM for $\sqrt{s} = 350~\text{GeV}$ and $\xi = 3\pi/5$.
• With $\mathcal{L} = 3~\text{ab}^{-1}$, the signal significance exceeds $4.8\sigma$ at $\sqrt{s} = 350~\text{GeV}$ and $2.3\sigma$ at $\sqrt{s} = 500~\text{GeV}$ for $\xi \in [\pi/2, 3\pi/5]$.
• For $\mathcal{L} = 10~\text{ab}^{-1}$, the significance exceeds 5$\sigma$ at $\sqrt{s} = 350~\text{GeV}$ and reaches 3.9$\sigma$ at $\sqrt{s} = 500~\text{GeV}$ under the same CP phase range.
• The forward-backward asymmetry $A_{FB}$ reaches up to 0.5 for $\cos\xi \simeq 0.7$ and $\sqrt{s} = 500~\text{GeV}$, a clear signature absent in the SM.
• Despite large backgrounds, the $A_{FB}$ signal can be observed at $1.7\sigma$ significance with $\mathcal{L} = 10~\text{ab}^{-1}$ at $\sqrt{s} = 500~\text{GeV}$.
• Triggering on the single final-state photon is essential for efficient suppression of background contributions and improved signal isolation.