[Paper Review] Probing $WW\gamma$ coupling through $e^- \gamma ightarrow u_e W^-$ at ILC

Satendra Kumar, P. Poulose|arXiv (Cornell University)|Jan 7, 2015

Particle physics theoretical and experimental studies30 references4 citations

TL;DR

This paper proposes probing anomalous $WW\gamma$ couplings via $e^- \gamma \to \mu_e W^-$ at the ILC, leveraging a clean single-lepton final state. With 100 fb$^{-1}$ at $\sqrt{s} = 500$ GeV and unpolarized beams, it achieves a sensitivity of $\pm 0.004$ to $\delta\kappa_\gamma$, while angular and energy distributions enhance sensitivity to $\lambda_\gamma < 0$, improving limits to a few per-mil and enabling discrimination between $\lambda_\gamma < 0$ and $\lambda_\gamma \geq 0$ cases.

ABSTRACT

The anomalous $WW\gamma$ coupling is probed through $e\gamma ightarrow u W$ at the ILC. With a spectacular single lepton final state, this process is well suited to study the above coupling. Cross section measurements can probe $\delta \kappa_\gamma$ to about $\pm 0.004$ for a luminosity of 100 fb$^{-1}$ at $500$ GeV center-of-mass energy with unpolarized electron beam. The limits derivable on $\lambda_\gamma$ from the total cross section are comparatively more relaxed. Exploiting the energy-angle double distribution of the secondary muons, kinematic regions sensitive to these couplings are identified. The derivable limit on $\lambda_\gamma < 0$ could be improved to a few per-mil, focusing on such regions. More importantly, the angular distributions at fixed energy values, and energy distribution at fixed angles present very interesting possibility of distinguishing the case of $\lambda_\gamma <0$ and $\lambda_\gamma \ge 0$.

Motivation & Objective

• To investigate the sensitivity of the ILC to anomalous $WW\gamma$ couplings using the $e^- \gamma \to \mu_e W^-$ process.
• To evaluate the potential of this process for probing $\delta\kappa_\gamma$ and $\lambda_\gamma$ couplings with high-precision measurements.
• To identify kinematic regions in energy-angle distributions that enhance sensitivity to $\lambda_\gamma < 0$.
• To explore the discriminative power of angular and energy distributions in distinguishing $\lambda_\gamma < 0$ from $\lambda_\gamma \geq 0$.

Proposed method

• Utilizes the $e^- \gamma \to \mu_e W^-$ process at the ILC with a center-of-mass energy of 500 GeV and unpolarized electron beams.
• Analyzes the total cross section to derive limits on $\delta\kappa_\gamma$ and $\lambda_\gamma$.
• Examines the energy-angle double differential distribution of the final-state muon to identify kinematic regions sensitive to anomalous couplings.
• Focuses on regions with high sensitivity to $\lambda_\gamma < 0$ by selecting fixed energy or fixed angle configurations.
• Compares angular distributions at fixed energy and energy distributions at fixed angles to distinguish $\lambda_\gamma < 0$ from $\lambda_\gamma \geq 0$.
• Applies kinematic reconstruction and statistical analysis to extract coupling constraints from simulated data with 100 fb$^{-1}$ luminosity.

Experimental results

Research questions

• RQ1 What is the sensitivity of the $e^- \gamma \to \mu_e W^-$ process at the ILC to anomalous $WW\gamma$ couplings?
• RQ2 How do energy-angle distributions of the final-state muon enhance sensitivity to $\lambda_\gamma < 0$?
• RQ3 Can angular distributions at fixed energy values distinguish between $\lambda_\gamma < 0$ and $\lambda_\gamma \geq 0$?
• RQ4 What are the achievable limits on $\delta\kappa_\gamma$ and $\lambda_\gamma$ using total cross section measurements?
• RQ5 In which kinematic regions is the sensitivity to $\lambda_\gamma$ maximized?

Key findings

• The total cross section measurement achieves a sensitivity of $\pm 0.004$ to $\delta\kappa_\gamma$ with 100 fb$^{-1}$ luminosity at $\sqrt{s} = 500$ GeV and unpolarized beams.
• Limits on $\lambda_\gamma$ from the total cross section are comparatively relaxed, indicating the need for differential analysis.
• Focusing on specific kinematic regions identified via energy-angle distributions, the limit on $\lambda_\gamma < 0$ can be improved to a few per-mil.
• Angular distributions at fixed energy values show strong potential to distinguish $\lambda_\gamma < 0$ from $\lambda_\gamma \geq 0$.
• Energy distributions at fixed angles also present a promising method for discriminating the sign of $\lambda_\gamma$.
• The combination of energy-angle distributions enables enhanced sensitivity and discrimination power beyond total cross section measurements.