Measurements of the τ polarisation in Z decays.

A sample of Z→ττ events observed in the DELPHI detector at LEP in 1991 and 1992 is analysed to measure the τ polarisation in the exclusive decay channels $$ev\bar v$$ , $$\mu v\bar v$$ , πν, ρν and aν. The τ polarisation is also measured with an inclusive hadronic analysis which benefits from a higher efficiency and a better systematic precision than the use of the exclusive decay modes. The results have been combined with those published on the 1990 data. A measurement of the τ polarisation as a function of production angle yields the values for the mean τ polarisation 〈 P〉=−0.148±0.022 and for the Z polarisation P=−0.136±0.027. These results are used to determine the ratio of vector to axial-vector effective couplings for taus $$\bar v_\tau /\bar a_\tau = 0.074 \pm 0.011$$ , and for electrons $$\bar v_e /\bar a_e = 0.068 \pm 0.014$$ , compatible with e-τ universality. With the assumption of lepton universality, the ratio of vector to axial-vector effective couplings for leptons $$\bar v_l /\bar a_l = 0.072 \pm 0.008$$ is obtained, implying a value of the effective weak mixing angle sinθ=0.2320±0.0021. [ABSTRACT FROM AUTHOR]