Experimental tests of lepton universality in $\tau$ decay

The branching fractions for $\tau \to e\nu {\nu }_{\tau },$ $\mu \nu {\nu }_{\tau },$ and $h{\nu }_{\tau }$ are measured using data collected with the CLEO detector at the CESR $e^{+}{e}^{-}$ collider: ${\mathcal{B}}_e$=0.1776$\pm$0.0006$\pm$0.0017, ${\mathcal{B}}_{\mu }$=0.1737$\pm$0.0008$\pm$0.0018, and ${\mathcal{B}}_h$=0.1152$\pm$0.0005$\pm$0.0012, where the first error is statistical, the second systematic, and $h$ refers to either a charged $\pi$ or $K$. Also measured is the $\tau$ mass, $m_{\tau }$=(1778.2$\pm$1.4) MeV. Lepton universality is affirmed by the relative branching fractions $({\mathcal{B}}_{\mu }$/${\mathcal{B}}_e$=0.9777$\pm$0.0063$\pm$0.0087, ${\mathcal{B}}_h$/${\mathcal{B}}_e$=0.6484$\pm$0.0041$\pm$0.0060) and the charged-current gauge coupling-constant ratios ${(g}_{\mu }{/g}_e$=1.0026$\pm$0.0055, $g_{\tau }{/g}_{\mu }$=0.9990$\pm$0.0098). The $\tau$ mass result may be recast as a $\tau$ neutrino mass limit, $m_{{\nu }_{\tau }}<$60 MeV at 95% C.L.