First search for $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ using the decay-in-flight technique

Abstract

The NA62 experiment at the CERN SPS reports the first search for $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ using the decay-in-flight technique, based on a sample of $1.21\times 10^{11}$ $K^{+}$ decays collected in 2016. The single event sensitivity is $3.15\times 10^{-10}$, corresponding to 0.267 Standard Model events. One signal candidate is observed while the expected background is 0.152 events. This leads to an upper limit of $14\times 10^{-10}$ on the $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ branching ratio at 95\% CL.

18 pages, 13 figures