Measurements of $R$ and a Search for Heavy-Quark Production in $e^{+}{e}^{-}$ Annihilation at $\sqrt{s}=50\mathrm{} \mathrm{and} 52$ GeV

The ratio $R$ of the cross section for $e^{+}{e}^{-}$ annihilation into hadronic final states to the QED cross section for muon-pair production is measured to be 4.34±0.45±0.30 and 4.23±0.20±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 $-\frac{1}{3}e$ quarks to be 0.19 units of $R$.