A measurement of the τ mass and the first test of CPT-invariance with τ leptons

The mass of the τ lepton is derived from the complete OPAL data set taken on the Z0 resonance during the LEP1 period. For the measurement two different pseudomass techniques are applied to more than 95% of the semileptonic decays of the produced τ leptons. The result is mτ 1775.1 ± 1.6 ± 1.0 MeV/c2 where the first error is statistic and the second one systematic. To test CPT invariance we compare the masses of the positively and negatively charged τ leptons. The relative mass difference is found to be smaller than 3.0 x 10-3 at the 90% confidence level. This report summarises the main aspects of the analysis described in more detail in [1]