Momentum imbalance of D mesons in ultra-relativistic heavy-ion collisions at LHC
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.