Measuring the tau polarization at ILC

Abstract

Measurement of the tau lepton polarization in \eett\ is an important electro-weak measurement at ILC and other future electron-positron colliders. In this paper we discuss several methods to extract polarimeter information for \eett\ events at the nominal centre-of-mass energy, and develop a new method, based on charged particle impact parameter measurement, which can accurately reconstruct tau momenta even in events with significant Initial State Radiation. In future work we will extend the study to estimate the precision with which the tau polarization can be measured at ILC-250, both for high-mass tau pairs and for those which radiatively return to the $Z^0$ peak. This will complement our past study which showed that the tau polarization can be measured to better than 1\% at the ILC-500.