Kinematic evidence for top quark pair production inW+multijet events inpp¯collisions at √s=1.8 TeV

We present a study of W+multijet events that compares the kinematics of the observed events with expectations from direct QCD W+jet production and from production and decay of top quark pairs. The data were collected in the 1992–93 run with the Collider Detector at Fermilab (CDF) from 19.3 ${\mathrm{pb}}^{\mathrm{-}1}$ of proton-antiproton collisions at √s =1.8 TeV. A W+≥2 jet sample and a W+≥3 jet sample are selected with the requirement that at least the two or three jets have energy transverse with respect to the beam axis in excess of 20 GeV. The jet energy distributions for the W+≥2 jet sample agree well with the predictions of direct QCD W production. From the W+≥3 jet events, a ‘‘signal sample’’ with an improved ratio of tt¯ to QCD produced W events is selected by requiring each jet to be emitted centrally in the event center of mass frame. This sample contains 14 events with unusually hard jet ${\mathit{E}}_{\mathit{T}}$ distributions not well described by expectations for jets from direct QCD W production and other background processes. Using expected jet ${\mathit{E}}_{\mathit{T}}$ distributions, a relative likelihood is defined and used to determine if an event is more consistent with the decay of tt¯ pairs, with ${\mathit{M}}_{\mathrm{top}}$=170 GeV/${\mathit{c}}^2$, than with direct QCD W production. Eight of the 14 signal sample events are found to be more consistent with top-quark than direct QCD W production, while only 1.7 such top-quark–like events are expected in the absence of tt¯.