Measurement of Beam Polarization at an $e^+e^-$ $B$-Factory with New Tau Polarimetry Technique

Abstract

Belle II is considering upgrading SuperKEKB with a polarized electron beam. The introduction of beam polarization to the experiment would significantly expand the physics program of Belle II in the electroweak, dark, and lepton flavor universality sectors. For all of these future measurements a robust method of determining the average beam polarization is required to maximize the level of precision. The BABAR experiment has developed a new beam polarimetry technique, Tau Polarimetry, capable of measuring the average beam polarization to better than half a percent accounting for both statistical and systematic uncertainties. Tau Polarimetry strongly motivates the addition of beam polarization to SuperKEKB and could also be used at other $e^+e^−$ colliders with polarized beams.