Measurements of the τ mass and the mass difference of the τ+ and τ− at BABAR

Abstract

We present the result from a precision measurement of the mass of the τ lepton, M τ , based on 423 fb − 1 of data recorded at the Υ ( 4 S ) resonance with the BABAR detector. Using a pseudomass endpoint method, we determine the mass to be 1776.68 ± 0.12 ( stat ) ± 0.41 ( syst ) MeV . We also measure the mass difference between the τ + and τ − , and obtain ( M τ + − M τ − ) / M τ AVG = ( − 3.4 ± 1.3 ( stat ) ± 0.3 ( syst ) ) × 10 − 4 , where M τ AVG is the average value of M τ + and M τ − .