Evidence for top quark production in<i>p</i>¯<i>p</i>collisions at √<i>s</i>=1.8 TeV

We present the results of a search for the top quark in 19.3 ${\mathrm{pb}}^{\mathrm{\ensuremath{-}}1}$ of p\ifmmode\bar\else\textasciimacron\fi{}p collisions at \ensuremath{\surd}s =1.8 TeV. The data were collected at the Fermilab Tevatron collider using the Collider Detector at Fermilab (CDF). The search includes standard model tt\ifmmode\bar\else\textasciimacron\fi{} decays to final states ee\ensuremath{\nu}\ensuremath{\nu}\ifmmode\bar\else\textasciimacron\fi{}, e\ensuremath{\mu}\ensuremath{\nu}\ensuremath{\nu}\ifmmode\bar\else\textasciimacron\fi{}, and \ensuremath{\mu}\ensuremath{\mu}\ensuremath{\nu}\ensuremath{\nu}\ifmmode\bar\else\textasciimacron\fi{} as well as e+\ensuremath{\nu}+jets or \ensuremath{\mu}+\ensuremath{\nu}+jets. In the (e,\ensuremath{\mu})+\ensuremath{\nu}+jets channel we search for b quarks from t decays via secondary vertex identification and via semileptonic decays of the b and cascade c quarks. In the dilepton final states we find two events with a background of 0.${56}_{\mathrm{\ensuremath{-}}0.13}^{+0.25}$ events. In the e,\ensuremath{\mu}+\ensuremath{\nu}+jets channel with a b identified via a secondary vertex, we find six events with a background of 2.3\ifmmode\pm\else\textpm\fi{}0.3. With a b identified via a semileptonic decay, we find seven events with a background of 3.1\ifmmode\pm\else\textpm\fi{}0.3. The secondary vertex and semileptonic-decay samples have three events in common. The probability that the observed yield is consistent with the background is estimated to be 0.26%. The statistics are too limited to firmly establish the existence of the top quark; however, a natural interpretation of the excess is that it is due to tt\ifmmode\bar\else\textasciimacron\fi{} production. We present several cross-checks. Some support this hypothesis; others do not. Under the assumption that the excess yield over background is due to tt\ifmmode\bar\else\textasciimacron\fi{}, constrained fitting on a subset of the events yields a mass of 174\ifmmode\pm\else\textpm\fi{}${10}_{\mathrm{\ensuremath{-}}12}^{+13}$ GeV/${\mathit{c}}^{2}$ for the top quark. The tt\ifmmode\bar\else\textasciimacron\fi{} cross section, using this top quark mass to compute the acceptance, is measured to be 13.${9}_{\mathrm{\ensuremath{-}}4.8}^{+6.1}$ pb.