The main goal of the NA62 experiment is to measure the branching ratio of the K+ → π+ ν ν̄ decay with ∼10% precision. The NA62 RICH is crucial for the identification of charged particles from kaon and pion decays. In particular, the detector should perform well enough to provide a muon suppression factor of at least 100 for the pion sample in the momentum range between 15 and 35 GeV/c while keeping a reasonably high efficiency for the pion selection.
The RICH performance is traditionally described in terms of parameters like ring radius resolution, single hit resolution and the average number of hits per event which are evaluated for electron tracks in order to avoid the momentum dependence. One of the most important factors for the performance improvement is the precise mirror alignment.
In the first part of the talk the mirror alignment procedure is described in detail. The procedure was accomplished for the first time in 2016 and allowed to improve the relative mirror misalignment from ∼500 to ∼30 μrad in terms of the mirror orientation. Given the focal length of mirrors equal to 17 m, the latter value corresponds to ∼1 mm residual misalignment in terms of the Cherenkov ring centre position in the mirror focal plane.
The second part of the talk is dedicated to the measurement of the basic performance using electron tracks: ring radius resolution, ring centre resolution, single hit resolution and mean number of hits. The main constituents of the single hit resolution are discussed. The contribution of the residual mirror misalignment to the performance is evaluated.