The Top Quark

member of the weak-isospin doublet containing the bottom quark (see our review on the \Standard Model of Electroweak Interactions" for more information). This note summarizes its currently measured properties, and provides a discussion of the experimental and theoretical issues involved in the determination of its parameters (mass, production cross section, decay branching ratios, etc.). B. Top quark production at the Tevatron: All direct measurements of top quark production and decay have been made by the CDF and DD experiments at the Fermilab Tevatron collider in pp collisions at p s = 1.8 TeV. Here top quarks are produced dominantly in pairs from the QCD processes qq ! tt and gg ! tt. At this energy, the production cross section in these channels is expected to be approximately 5 pb for m t = 175 GeV/c 2 , with a 90% contribution from qq annihilation. Smaller contributions are expected from electroweak single-top production mechanisms, namely qq 0 ! W ! tb and qg ! q 0 tb, the latter mediated by virtual-W exchange (\W-gluon fusion"). The combined rate from these processes is approximately 2.5 pb at m t = 175 GeV/c 2 (see Ref. 1 and references therein). The expected contribution of these channels is further reduced relative to the dominant pair-production mechanisms because of larger backgrounds and poor detection eeciency. With a mass above the Wb threshold, the decay width of the top quark is expected to be dominated by the two-body channel t ! Wb. Neglecting terms of order m 2 b =m 2 t , 2 s and those of order (s ==)m 2 W =m 2 t , this is predicted in the Standard Model to be 2]: