Measurement of $A_{LR}$ using radiative return at ILC 250

TL;DR

This study evaluates the potential of the ILC 250 to measure the left-right asymmetry $A_{LR}$ using radiative return, demonstrating a significant improvement in precision over previous measurements, which can enhance constraints on SMEFT parameters.

Contribution

The paper presents a full simulation study showing that the ILC 250 can measure $A_{LR}$ with much higher precision than previous experiments, improving systematic and statistical uncertainties.

Findings

Statistical error on $A_{LR}$ is estimated at 1.8 x 10^{-4}.

Total systematic error on $A_{LR}$ is estimated at 0.00025.

Measurement precision is 8.8 times better than at SLC.

Abstract

For the precision study at the ILC 250, measurement of $A_{LR}$ is important as it can constrain SMEFT parameters. The current best measured $A_{LR}$ value is $A_{LR} = 0.1514 \pm 0.0019\,(stat) \pm 0.0011\,(syst)$ which was measured at the SLC, and a more precise value is required for the global fit for the new physics search in TeV-scale. At the ILC, we can use the $e^+ e^- \to \gamma Z$ process to evaluate the $A_{LR}$. We performed a full simulation study of the $e^+ e^- \to \gamma Z$ process at the center-of-mass energy of 250 GeV and evaluated how much we can improve the precision of this observable. The statistical error on $A_{LR}$ at the ILC 250 turned out to be $1.8 \times 10^{-4}$. Major source of the systematic error was error from the beam polarization. As other sources of the systematic error, the uncorrelated parts of error on the product of luminosity and selection efficiency for each polarization combination contribute. Including those systematic errors, total absolute error on $A_{LR}$ was estimated to be 0.00025, 8.8 times better precision than that from the SLC (0.00219).