Search for CP violation in the decay Z → τ+τ−

Data collected by ALEPH in the years 1990, 1991 and 1992 have been used to update a previous search for CP violation in the decay of the Z into τ+τ-. The measurement of the weak dipole form factor of the τ lepton has been performed by studying correlations between the τ leptons. No signal of CP violation was found. The weak dipole form factor is found to be d~τ = (+0.15 +/- 0.58stat +/- 0.38sys) 10-17 e . cm, obtained with 19628 identified τ+τ- events. This gives an upper limit on the weak dipole form factor of d~τ < 1.5 . 10-17 e . cm at the 95% confidence level.