An upper limit on the anomalous magnetic moment of the τ lepton

Abstract Using radiative Z 0 → τ + τ − γ events collected with the OPAL detector at LEP at s =M Z during 1990–95, a direct study of the electromagnetic current at the τγ vertex has been performed in terms of the anomalous magnetic form factor F 2 of the τ lepton. The analysis is based on a data sample of 1429 e + e − →τ + τ − γ events which are examined for a deviation from the expectation with F 2 =0. From the non-observation of anomalous τ + τ − γ production a limit of −0.068 2 is obtained. This can also be interpreted as a limit on the electric dipole form factor F 3 as |eF 3 | −16 e cm . The above ranges are valid at the 95% confidence level.