Experimental tests of lepton universality in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>τ</mml:mi></mml:math>decay

Abstract

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