Kaon identification and the search for heavy neutrinos at NA62

A search for heavy neutrinos in the decay K⁺→μ⁺_h has been performed using the data collected by the NA62-R_K experiment in 2007. The signal event selection is presented, along with an estimate of the expected background in the signal region, obtained by Monte Carlo simulation as well as data driven techniques. This estimate is supported by studies of the detector performance in muon identification efficiency and momentum resolution. Using a sample corresponding to 6x10^7 kaon decays in the fiducial volume, limits of order 10^-^5 have been set on the branching ratio, Br(K^+→μ^+ν_h ), for neutrino masses in the range 275 MeV/c^2 – 375 MeV/c^2, under the assumption that the heavy neutrino does not decay into visible particles inside the detector. In the range 330 MeV/c^2 – 375 MeV/c^2, there are no previous limits from peak searches. Over the rest of the range, the new limits do not improve upon existing results. Improved limits might come from the NA62 experiment designed to measure the branching ratio of the ultra rare kaon decay K+→π^+νν, which began taking data in 2015. Kaon identification is performed by a differential Cherenkov detector. The timing resolution of the photo-detectors was measured in a pilot run in 2014 and used to improve the simulation of the detector. An algorithm was developed to align the detector during live data taking, and this was used at the start of the 2015 run.