Commissioning and performance of the NA62 RICH detector

NA62 is the last generation kaon experiment at CERN SPS aiming to study the decay K$^+$ $\rightarrow\pi^+ \nu \bar{\nu }$. The goal of the experiment is to measure the decay branching ratio, $O$ (10$^{-10}$) , with 10% precision collecting about 100 K$^+$ $\rightarrow\pi^+ \nu \bar{\nu }$ events in three years of data taking and assuming a 10% signal acceptance. The weak signal and the huge background make the experiment very challenging. The NA62 detector must be able to reject background events from decay channels with branching ratios up to 10 orders of magnitude higher than the signal one and with similar experimental signature. In order to achieve the designed goal, good Particle Identification (PID), kinematic rejection and precise timing are required. The key element of the PID in NA62 is the Ring Imaging Cherenkov detector (RICH) to identify pions and muons and to measure the particle arrival time. After a pilot run in 2014, installation and commissioning of the detector were completed in 2015, when also the first physics run took place. Results on the detector performance will be presented together with preliminary results from the 2015 and 2016 physics runs.