  We propose to measure azimuthal correlations of heavy-flavor hadrons to
address the status of thermalization at the partonic stage of light quarks and
gluons in high-energy nuclear collisions. In particular, we show that hadronic
interactions at the late stage cannot significantly disturb the initial
back-to-back azimuthal correlations of DDbar pairs. Thus, a decrease or the
complete absence of these initial correlations does indicate frequent
interactions of heavy-flavor quarks and also light partons in the partonic
stage, which are essential for the early thermalization of light partons.
