Isolating CP-violating \gamma ZZ coupling in e+e- \to \gamma Z with transverse beam polarizations

TL;DR

This paper investigates how transverse beam polarization at the ILC can be used to isolate and measure CP-violating gamma-Z-Z couplings in the process e+e- to gamma Z, providing limits on the couplings and discussing methods to identify final-state helicities.

Contribution

It introduces a novel approach using final-state spin resolution and transverse polarization to isolate and constrain CP-violating gamma-Z-Z couplings at the ILC.

Findings

Re$\lambda_1$ can be fingerprinted with final-state spin resolution.

Limits on Re$\lambda_1$ are of order 10^{-2} to 10^{-3} at ILC energies.

Quadratic corrections to the cross section are small and manageable.

Abstract

We revisit the process $e^+e^- \to \gamma Z$ at the ILC with transverse beam polarization in the presence of anomalous CP-violating $\gamma Z Z$ coupling $\lambda_1$ and $\gamma \gamma Z$ coupling $\lambda_2$. We point out that if the final-state spins are resolved, then it becomes possible to fingerprint the anomalous coupling {\rm Re}$\lambda_1$.90% confidence level limit on {\rm Re}$\lambda_1$ achievable at ILC with center-of-mass energy of 500 GeV or 800 GeV with realistic initial beam polarization and integrated luminosity is of the order of few times of $10^{-2}$ when the helicity of $Z$ is used and $10^{-3}$ when the helicity of $\gamma$ is used. The resulting corrections at quadratic order to the cross section and its influence on these limits are also evaluated and are shown to be small. The benefits of such polarization programmes at the ILC are compared and contrasted for the process at hand. We also discuss possible methods by which one can isolate events with a definite helicity for one of the final-state particles.