Particle Identification with Calorimeters for the Measurement of the Rare Decay $K^{+} \to \pi^{+}\nu\bar\nu$ at NA62

Flavour physics is one of the most powerful fields in the search of new physics beyond the Standard Model. The kaon sector with the rare decay $K^{+}\rightarrow\pi^{+}\nu\bar{\nu}$ provides one of the cleanest and most promising channels. NA62, a fixed target experiment at the CERN SPS, aims to measure $\pi\nu\nu$ branching ratio with $10\%$ precision to test the Standard Model up to an energy scale of hundreds of TeV. The rejection of the muon background is a crucial task for the measurement of the $K^{+}\rightarrow\pi^{+}\nu\bar{\nu}$ decay. The information from the Cherenkov and the Muon Veto detectors are not sufficient to achieve the required $\mathcal{O}(10^{7})$ muon suppression and an additional factor of $10^2$ has to be reached with a purely calorimetric particle identification. The calibration of the hadron calorimeter (HAC) represents an important challenge, which requires the development of innovative techniques. Since electrons and photons are absorbed by the electromagnetic calorimeter (LKr), the response of the HAC to electromagnetic showers is calibrated by exploiting stopped high energy muons. The obtained HAC energy resolution meets all the demands. The information about energy and shape of the showers detected by the LKr and HAC are combined into a Boosted Decision Trees algorithm to classify the impinging particles as muons, pions or electrons. The particle identification algorithm allows a muon rejection of $10^5$ (including suppression from the muon veto detector) for $90\%$ pion efficiency.