Study of the capabilities of the CBM detector for open charm elliptic flow measurements

Open charm will be one of the key observables of the future CBM experiment, as it is a penetrating probe of the early dense phase of the collision. In particular, its elliptic flow should be sensitive to the degrees of freedom of the medium at the early stage. In a purely hadronic scenario, open charm has a moderate interaction rate and should develop a low elliptic flow compared to lighter hadrons. The observation of a strong elliptic flow for open charm at FAIR would indicate the creation of a partonic medium [1]. The elliptic flow of open charm particles is also of particular importance for the issue of thermalization. The present study aimed at investigating the capabilities of the CBM detector in measuring the elliptic flow of open charm. The latter will be measured using a Micro-Vertex Detector (MVD) based on MAPS-pixels. Previous studies [2] demonstrated that the experiment will allow to reconstruct a few 10 D-mesons per year. The measurement of their elliptic flow will require the determination of the collision reaction plane. This will be done in CBM using a forward hadron calorimeter, the Projectile Spectator Detector (PSD) [2]. In a first step, we have evaluated the perfomance of the PSD detector for the reaction plane reconstruction. This has been done for Au+Au collisions at 25 AGeV using the UrQMD event generator and the CBM simulation framework (based on GEANT). Simulated events have been transported through a set-up including the MVD, the Silicon Tracking System (STS), dumped in a magnetic dipole field of 1 Tesla, and the PSD. Using the energy deposited in the PSD by spectator particles, the reaction plane of each event has been reconstructed using standard methods [3]. The results show that a very good proton/neutron separation is mandatory, since the protons are deflected by the magnetic field and spoil the azimuthal correlation of the particles with the reaction plane. The expected resolution on the reaction plane (σΦR ), assuming an ideal proton/neutron discrimination, is plotted in Figure 1 as a function of the collision centrality. As one can see, a quite good accuracy can be achieved, in particular for mid-central collisions (3fm ≤ b ≤ 9fm). The elliptic flow signal has been simulated for D mesons, using a thermal model in which the particle azimuthal anisotropy with respect to the reaction plane has been introduced. For each event, the reaction plane resolution was accounted for by smearing the azimutal angle of the particles according to a gaussian distribution with σ = 40 degrees (Figure 1). The v2 parameter has been reconstructed using the Fourier analysis of the simulated azimutal distributions ([3]). After applying a correction factor to Impact parameter [fm] 0 2 4 6 8 10 12 [ d eg re es ]