We present a new technique, “tau polarimetry,” for measuring the longitudinal beam polarization present in an e+e− collider through the analysis of e+e−→τ+τ− events. By exploiting the sensitivity of τ decay kinematics to the longitudinal polarization of the beams, we demonstrate that the longitudinal polarization can be measured with a 3 per mil systematic uncertainty at the interaction point using a technique that is independent of spin and beam transport modeling. Using 424.2±1.8 fb−1 of BABAR data at √s=10.58 GeV, the average longitudinal polarization of the PEP-II e+e− collider has been measured to be ⟨P⟩=0.0035±0.0024stat±0.0029sys. The systematic uncertainty studies are described in detail, which can serve as a guide for future applications of tau polarimetry. A proposed e− beam longitudinal polarization upgrade to the SuperKEKB e+e− collider would benefit from this technique.
