The role of the NA62 RICH in the $BR(K^+ \to \pi^+ \nu \bar\nu)$ measurement

The NA62 experiment aims to measure the $BR$($K^+$ $\rightarrow$ $\pi^+$ $v$ $\overline{v}$) with a 10% precision. One of the main backgrounds comes from the decay $K^+$ $\rightarrow$ $\mu^+$ $v$ , therefore a highly powerful pion/muon separation is needed. The NA62 RICH, together with the calorimeter system, provides an accurate particle identification which has been essential to obtain the first results of the NA62 $K^+$ $\rightarrow$ $\pi^+$ $v$ $\overline{v}$ analysis, based on 2016 data. Two different algorithms have been exploited which, used in combination, allowed to get a pion reconstruction and identification efficiency of 75%, with a muon suppression factor of about 500, in the momentum range 15–35 GeV/c.