Measurement of the spectral functions of vector current hadronic tau decays.

CERN-LEP. A measurement of the spectral functions of non-strange $\tau$ vector current final states is presented, using 124358 $\tau$ pairs recorded by the ALEPH detector at LEP during the years 1991 to 1994. The spectral functions of the dominant two- and four-pion $\tau$ decay channels are compared to published results of $e^+e^-$ annihilation experiments via isospin rotation. A combined fit of the pion form factor from $\tau$ decays and $e^+e^-$ data is performed using different parametrizations. The mass and the width of the $\rho^{\pm}$(770) and the $\rho^0$(770) are separately determined in order to extract possible isospin violating effects. The mass and width differences are measured to be $M_{\rho^\pm(770)} - M_{\rho^0(770)} = (0.0 \pm 1.0)$ MeV/$c^2$ and $\Gamma_{\rho^\pm(770)} - \Gamma_{\rho^0(770)} = (0.1 \pm 1.9)$ MeV/$c^2$. These numbers have been read from the plots in the paper.