Experimental tests of lepton universality in tau decay from CLEO

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: B e = 0.1776±0.0006±0.0017, B μ =0.1737±0.0008±0.0018, and B h =0.1152±0.0005±0.0012, where the first errors are statistical, the second systematic, and h refers either to a charged π or K . Also measured is the tau mass, m τ =(1778.2±1.4) MeV. Lepton universality is affirmed by the resulting 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 tau mass result may be recast as a tau neutrino mass limit, m ν τ , <60 MeV at 95% CL.