The KEDR collaboration has precisely measured the τ lepton mass. This continues a series of high-precision measurements of masses of particles and resonances at the VEPP-4M collider at the Budker Institute of Nuclear Physics in Novosibirsk (see CERN Courier November 2005 p9). In 2004 and 2005 masses of the J/Ψ and Ψ' mesons were measured with a relative accuracy of 4 × 10-6 and 7 × 10-6, respectively.

The τ lepton mass is a fundamental parameter of the Standard Model. Its value can be used with the τ lifetime and the decay probability to eνbareντ to test the principle of lepton universality, one of the postulates of modern electroweak theory. Up to now the accuracy of the measurement of the Beijing Spectrometer (BES) has dominated the accuracy of the τ mass. Like BES, the KEDR experiment determines the τ mass by measuring the energy dependence of the τ+τ- cross-section near threshold, and the key factor in such experiments is the accuracy of the beam-energy determination.

While previous experiments relied on the extrapolation based on the J/Ψ and Ψ' meson masses (measured in Novosibirsk) as reference points, the new KEDR-VEPP experiment uses two independent high-precision methods for the beam-energy measurement. During data-taking the beam energy was monitored through Compton backscattering of infrared laser light with a precision of 5 × 10-5. The beam energy was absolutely calibrated daily with a precision of 1 × 10-5 using the resonant depolarization method.

The preliminary result, presented at ICHEP'06 (see CERN Courier December 2006 p21), based on 6.7 pb-1 of data, is mτ = 1776.80-0.23+0.25 ±0.15 MeV. This value agrees well with the current world average mτ = 1776.99-0.26+0.29 MeV and has comparable accuracy. It is also in good agreement with the recent preliminary result from the Belle experiment at KEK of mτ = 1776.71±0.13±0.32 MeV, which was also reported in Moscow. Detector-related uncertainties currently dominate in the systematic error presented for KEDR, but they will be reduced with more detailed data analysis. The experiment started a new run of data-taking at the threshold for t production in October, with the aim of reducing the statistical error.