Precision e− beam polarimetry at an e+e− B factory using tau-pair events

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 ±0.002⁢4stat ±0.002⁢9sys. 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.