A test ofCP-invariance in Z⁰→τ⁺τ⁻ using optimal observables

Abstract

Using 27 490 Z⁰→τ⁺τ⁻ decays, accumulated in 1991, 1992 and 1993 with the OPAL detector at LEP, a direct test ofCP-invariance in the neutral current reactione ⁺ e ⁻→τ⁺τ⁻ is performed by measuringCP-odd observables which are proportional to the weak dipole moment of the τ-lepton. A new method based on optimalCP-odd observables constructed from the τ flight and spin directions is employed. More sensitive measurements of the real and, for the first time, the imaginary part of the weak dipole moment with highest possible signal to noise ratio are obtained. No evidence for a non-zero expectation value of the considered observables and hence forCP-violation is observed. An upper limit on the weak dipole moment of |Re(d ^ω_τ )|<7.8×10⁻¹⁸ e·cm and |Im(d ^ω_τ )|<4.5×10⁻¹⁷ e·cm with 95% confidence level is obtained.