Study of Electron-Positron Collisions at Center-of-Mass Energies of 27.4 and 27.7 GeV at PETRA

This paper reports on the first results of the study of $e^{+}{e}^{-}$ collisions at $\sqrt{s}=27.4\mathrm{}$ GeV and $\sqrt{s}=27.7\mathrm{}$ GeV at PETRA, using the $4\pi$-sr electromagnetic and calorimetric detector MARK-J. We obtain an average $R=\frac{\sigma (e^{+}{e}^{-}\to \mathrm{hadrons})}{\sigma (e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-})}=3.8\mathrm{}\pm 0.3 \left(\mathrm{statistical}\right)\pm 0.6 \left(\mathrm{systematic}\right)$ and a relative $R=1.0\mathrm{}\pm 0.2$ between the two energies. The $R$ values, the measured thrust distribution, and average spherocity show no evidence for the production of new quark flavors.