Elliptic flow and nuclear modification factors of D-mesons at FAIR in a Hybrid-Langevin approach

The Compressed Baryonic Matter (CBM) experiment at the Facility for Anti-proton and Ion Research (FAIR) will provide new possibilities for charm-quark (D-meson) observables in heavy-ion collisions at low collision energies and high baryon densities. To predict the collective flow and nuclear modification factors of charm quarks in this environment, we apply a Langevin approach for the transport of charm quarks in the UrQMD (hydrodynamics + Boltzmann) hybrid model. Due to the inclusion of event-by-event fluctuations and a full (3+1) dimensional hydrodynamical evolution, the UrQMD hybrid approach provides a realistic evolution of the matter produced in heavy-ion collisions. As drag and diffusion coefficients we use a resonance approach for elastic heavy-quark scattering and assume a decoupling temperature of the charm quarks from the hot medium of 130 MeV. Hadronization of the charm quarks to D-mesons by coalescence is included. Since the initial charm-quark distribution at FAIR is unknown, we utilize two different initial charm-quark distributions in our approach to estimate the uncertainty of these predictions. We present calculations of the nuclear modification factor, RAA, as well as for the elliptic flow, v2, in Pb+Pb collisions at Elab = 25 AGeV. The different medium modifications of D-mesons and D̄-mesons at high baryon-chemical potential are explored by modified dragand diffusioncoefficients using the corresponding fugacity factor. Here we find a considerably larger medium modification for Dthan for D-mesons.