  As a new observable for heavy flavor correlations the momentum imbalance
$A_D$ of D mesons is proposed. It is defined analogously to the jet momentum
imbalance $A_J$ of fully reconstructed jets. However, since D mesons are flavor
tagged particles, no jet reconstruction is necessary. $A_D$ quantifies the
influence of the medium created in heavy-ion collisions on correlated charm
pairs. We present results with the partonic transport model Boltzmann Approach
to MultiParton Scatterings (BAMPS), which describes well the nuclear
modification factor and elliptic flow of all heavy flavor particles at RHIC and
LHC. The $A_D$ distribution in heavy-ion collisions at LHC is shifted to larger
values of $A_D$ compared to proton-proton collisions. We argue that this shift
is due to medium effects and can be explained partially by a path length
imbalance of charm pairs and partially by momentum fluctuations in the initial
charm pair distribution.
