Electron pair reconstruction in CBM

The reconstruction of low-mass vector mesons (ρ, ω, φ) emitted out of the hot and dense phase of relativistic heavy ion collisions is one of the experimental goals of the CBM experiment. In this contribution the status on feasibility studies for their measurement will be summarized. Dominant background sources are random combinations of e− and e from π-Dalitz decay and γ conversion, the latter mostly in the target and to a smaller extent in the tracking stations, the beam pipe and the magnet yoke. In order to minimize electrons from conversions a single 25 μm gold target was assumed. A characteristic feature of conversion and π-Dalitz decays is the moderate decay momentum of the electron pair. This generally leads to small opening angles and comparatively small laboratory momenta. A special challenge of the current concept is the fact that electron identification is not provided in front of the magnetic field. By adding correction coils it is possible to suppress the field between the target and first Silicon Tracking Station. The excellent two hit resolution in the MAPS detector (< 100μm) gives then a chance to reject close pairs. To increase the acceptance for low momentum particles the magnetic field was reduced to Bmax = 0.7 T and the size of STS 1 to 6 was increased. The strategy of background rejection described in [1] was applied. The invariant mass