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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

= 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.

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APA Citation

Lees, J. P., Poireau, V., Tisserand, V., Grauges, E., Palano, A., Eigen, G., Brown, D. N., Kolomensky, Yu. G., Fritsch, M., Koch, H., Cheaib, R., Hearty, C., Mattison, T. S., McKenna, J. A., So, R. Y., Blinov, V. E., Buzykaev, A. R., Druzhinin, V. P., Kozyrev, E. A., … Wu, S. L. (2023a). Precision .Physical Review D, 108(9).

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