Improved tau polarisation measurement

Using 22 pb(-1) of data collected at LEP in 1992 on the peak of the Z resonance, the ALEPH collaboration has measured the polarisation of the tau leptons decaying into e (nu)over bar nu , mu (nu)over bar nu,pi nu and a(1) nu from their individual decay product distributions. The measurement of the tau polarisation as a function of the production polar angle yields the two parameters A(tau) and A(e), where, in terms of the axial and vector couplings g(A_l) and g(V_l), A(l) = 2 g(V_ l) g(A_l)/(g(V_ l)^2 + g(A_l)^2). This analysis follows to a large extent the methods devised for the 1990 and 1991 data but with improvements which bring a better understanding of the systematic uncertainties. Combining the 1992 measurements with our previously published results yields A(tau) = 0.136 +/- 0.012 +/- 0.009 and A(e) = 0.129 +/- 0.016 +/- 0.005. Assuming e-tau universality, the measurements imply an effective weak mixing angle of sin(2) theta(W)(eff) = 0.2332 +/- 0.0014.