CBM-RICH layout optimization

The RICH detector in CBM shall provide electron identification together with the TRD detectors for the measurement of low-mass vector mesons and charmonium in the di-electron decay channel. Both observables are considered key probes for the study of compressed baryonic matter in heavy ion collisions. A RICH design based on first general considerations has been developed and studied in detector simulations and feasibility studies [1, 2]. These studies include full event reconstruction and electron identification in the proposed CBM setup. Current results clearly demonstrate the feasibility of these measurements [2]. Combining RICH, TRD and TOF information a pion suppression of 10 at an electron identification efficiency of 80% is reached for 2 GeV/c < p < 8 GeV/c. Efficiencies and signal-to-background ratios for reconstructed lowmass vector mesons and charmonium are very promising [3, 4]. However, the layout of the RICH detector was only implemented based on a first, educated guess. An optimization of the RICH in particular with respect to minimizing its dimensions is awaiting. This process has been started and first results will be presented in this report. In order to keep the number of detected photoelectrons per electron ring when reducing the overall length of the detector a radiator gas with larger refractive index has to be chosen to compensate the lower yield of Cherenkov photons. We investigated CO2 instead of N2, see table 1. The larger refractive index of CO2 slightly lowers the threshold of Cherenkov light production for pions; otherwise both