Tau polarization measurements at LEP using τ → ϱν and τ → a1ν

Abstract

Measurements of the tau polarization by the LEP collaborations using the decay modes τ → ϱν and τ → a 1 ν are reviewed. The kinematics of these final states involving vector hadrons is discussed as is the use of different polarization-sensitive variables. These tau decay modes involve reconstruction of photons and π 0 's which presents special challenges at LEP energies. The τ → ϱν decay mode is the most powerful of the five used at LEP ( eυυ¯ , πνν¯, πν, ρν , and a 1 ν and carries about 40% of the weight in the LEP average. The a 1 ν mode carries the least weight, about 6%. The results from the four experiments are presented with discussion of the systematic errors. Estimates are given for the final uncertainty on the tau polarization achievable using these two channels when LEP running near the Z resonance is finished.