  Spin correlations between top-quark and anti-top-quark in pair production are
sensitive to physics beyond the standard model. A complete set of observables,
that can be used to measure all independent parameters of the production spin
density matrix, is necessary to search for anomalous interactions in the
production dynamics. In this letter, we provide a set of asymmetry observables
for measuring all the density matrix elements. These observables are defined
regardless of the production mechanism, and can be specified in any desired
reference frame, particularly the laboratory frame in which the first evidence
of spin correlation was observed experimentally. Our method can be used to find
an optimal Lorentz frame in which the sensitivity to the density matrices is
high for given production process and experimental environment, or to test
possible correlations among the matrix elements.
