  Possible non-standard tqZ couplings, where q=c or u, originated from general
flavor-changing-neutral-current interactions are studied model-independently
using the effective Lagrangian consisting of several SU(3)xSU(2)xU(1) invariant
dimension-6 operators. After the electroweak symmetry breaking, these operators
are recombined to form four kinds of independent terms whose coefficients are
complex in general. The Lagrangian could therefore include up to eight
independent coupling parameters. Through searches for experimentally allowed
regions of these parameters, it is found that some correlations exist among the
signs and sizes of those couplings.
