  We present a new observable, $\Delta \phi$, an azimuthal angle difference
between $t$ and $\bar{t}$ quarks in $t\bar{t}$ pair production, at hadron
colliders as an interesting probe of the radiative quantum chromodynamics
process as well as a high-order correction in the high-mass regime. This
variable also enables good discrimination on some new physics models that may
explain the forward-backward charge asymmetry of $t\bar{t}$ production measured
at the Tevatron. With a reliable estimation of the dataset obtained up to 2011
at the Tevatron and Large Hadron Collider, we present an opportunity for
testing the standard model as well as searching new physics models with the
$\Delta \phi$ observable.
