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

TL;DR

This paper introduces a new tau polarimetry technique for measuring beam polarization at $e^+e^-$ colliders, demonstrating its application at BABAR and its potential for future experiments like Belle II.

Contribution

The paper presents the first application of a novel tau polarimetry method to measure beam polarization with high precision at an $e^+e^-$ collider.

Findings

Preliminary measurement of PEP-II beam polarization: -0.0010 ± 0.0036 (stat) ± 0.0030 (sys)

Tau Polarimetry technique exploits decay product kinematics to determine spin states

Potential for high-precision polarization measurements at Belle II with upgrades.

Abstract

The BABAR collaboration has demonstrated the first application of the new Tau Polarimetry technique. This polarimetry technique exploits the kinematic coupling of $\tau$ decay products to the spin states of the $\tau$ 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 $\langle P\rangle=-0.0010\pm0.0036_{\textrm{stat}}\pm0.0030_{\textrm{sys}}$.