Precision e^- Beam Polarimetry at an e⁺e^- B Factory using Tau-Pair Events

2023/08/01 by The BABAR Collaboration
#FOS: Physical sciences #High Energy Physics - Experiment (hep-ex)

paper · doi:10.48550/arxiv.2308.00774

Abstract

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 ± 0.0024_\textrmstat± 0.0029_\textrmsys. 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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