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

The BABAR collaboration has demonstrated the first application of the new Tau Polarimetry technique. This polarimetry technique exploits the kinematic coupling of $τ$ decay products to the spin states of the $τ$ and initial state electron, to precisely determine the average beam polarization in an $e^+e^-$ collider. The Tau Polarimetry technique is expected to be used at Belle II following an upgrade to a polarized electron beam, where the precision with which the average beam polarization is known is expected to be the dominant systematic uncertainty in proposed future measurements which require polarized beams. Applying Tau Polarimetry, BABAR has preliminarily measured the PEP-II average beam polarization to be $\langle P\rangle=-0.0010\pm0.0036_{\textrm{stat}}\pm0.0030_{\textrm{sys}}$.