Experimental tests of lepton universality in τ decay

Abstract

The branching fractions for τ→eνν(τ), μνν(τ), and hν(τ) are measured using data collected with the CLEO detector at the CESR e(+)e(-) collider: Be=0.1776±0.0006±0.0017, Bμ=0.1737±0.0008±0.0018, and B(h)=0.1152±0.0005±0.0012, where the first error is statistical, the second systematic, and h refers to either a charged π or K. Also measured is the τ mass, mτ=(1778.2±1.4) MeV. Lepton universality is affirmed by the relative branching fractions (B(μ)/B(e)=0.9777±0.0063±0.0087, B(h)/B(e)=0.6484±0.0041±0.0060) and the charged-current gauge coupling-constant ratios (g(μ)/g(e)=1.0026±0.0055, g(τ)/g(μ)=0.9990±0.0098). The τ mass result may be recast as a τ neutrino mass limit, m(ν)(τ)<60 MeV at 95% C.L.