Flavor-changing neutral currents in the Standard Model and its extensions involving singlet quarks

Phenomenology of singlet quarks (SQs), which are SU(2) singlets with the hypercharges Y-h = -1/3 and masses m(h) > m(t), is reviewed. At accelerator energies, the existence of SQs is predominantly manifested through the flavor-changing neutral currents (FCNCs). The Standard Model (SM), in which similar processes occur only at the loop level, treats them as a background for identifying new-physics signals. It is shown that experimental data on rare processes and corresponding SM predictions indicate that extensions of the SM are applicable to an analysis of phenomena induced by the FCNCs. The generalized matrix of mixing standard quarks with each other and with SQs is constructed. Bounds on the mixing angles and lower limit m(h) greater than or similar to 0.5 TeV on the SQ mass are obtained. The possibilities of increasing the confidence level of the SQ-existence hypothesis so that these quarks would be directly observed are discussed. The cross sections for the nondiagonal production of an SQ in a pair with a standard quark in e(+)e(-), ep, and p (p) over bar collisions are analyzed and a unique signature of these events is described. A model involving an up singlet quark with the hypercharge Y-U = 2/3 and application to t-quark physics is discussed.