Alignment and Cherenkov angle resolution of the DELPHI RICH

The particle separation power of a RICH depends crucially on the obtained resolution in Cherenkov angle. In order to obtain the highest possible resolution, a precise knowledge of the positions of detector elements, the refractive indices and the photon detection points is required. The photon detectors are equipped with calibration systems for the determination of drift velocities and Lorentz angles. An off-line alignment of the detector elements is performed to improve the Cherenkov angle resolution. Events of the type Z) → μ+ μ− are used, and special care is taken to minimize the influence of the background content on the Cherenkov angle data. Results from the 1994 physics run are presented. The obtained single photon resolution is compared with simulations. While the gas radiator resolution is in very good agreement, some additional smearing of the simulated photon positions is needed to reproduce the liquid data.