Abstract

The ratio $R$ of the cross section for ${e}^{+}{e}^{\ensuremath{-}}$ annihilation into hadronic final states to the QED cross section for muon-pair production is measured to be 4.34\ifmmode\pm\else\textpm\fi{}0.45\ifmmode\pm\else\textpm\fi{}0.30 and 4.23\ifmmode\pm\else\textpm\fi{}0.20\ifmmode\pm\else\textpm\fi{}0.21 at c.m. energies of 50 and 52 GeV, respectively. From these values of $R$ and an analysis of the event shapes we deduce a 95%-confidence-level upper limit for the production rate of new heavy charge $+\frac{2}{3}e$ or $\ensuremath{-}\frac{1}{3}e$ quarks to be 0.19 units of $R$.