We use the reaction $e^{+}$$e^{\mathrm{-}}$→hadrons, in the Mark J detector at the DESY electron-positron collider PETRA, to determine the hadronic cross section up to 46.78 GeV. The production of a top quark with a charge equal to (2/3) is excluded up to 46.6 GeV with 95% C.L. The observed rise in the cross section at higher energies is consistent with the electroweak prediction for a $Z^0$ mass of 93 GeV. We describe some unusual muon inclusive events.

• Received 27 January 1986

DOI:https://doi.org/10.1103/PhysRevD.34.681