We present the result from a precision measurement of the mass of the $\tau$ lepton, $M_\tau$, based on $423 fb^{ -1}$ of data recorded at the $\Upsilon(4S)$ resonance with the BaBar detector. Using a pseudomass endpoint method, we determine the mass to be $1776.68 \pm 0.12 (stat) \pm 0.41 (syst) MeV$. We also measure the mass difference between the $\tau^+$ and $\tau^-$, and obtain $(M_{ \tau^{ +}}-M_{ \tau^{ -}})/M^\tau_{ AVG} = (-3.4 \pm 1.3 (stat) \pm 0.3 (syst)) \times 10^{ -4}$, where $M^\tau_{ AVG}$ is the average value of $M_{ \tau^+}$ and $M_{ \tau^-}$.
Categories: hep-ex