First measurement of the $K^{+} rightarrow pi^{+}nuoverline{nu}$ decay with the NA62 experiment at CERN
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Abstract
The decay $K^{+} rightarrow pi^{+}nuoverline{nu}$ with a very
precisely predicted branching ratio of $8.4times 10^{-10}$ is one of
the best candidates to reveal indirect effects of new physics at the
highest mass scales. The NA62 experiment at CERN SPS is designed to
measure the branching ratio of the
$K^{+} rightarrow pi^{+}nuoverline{nu}$ decay with a
decay-in-flight technique, novel for this channel. The search for the
$K^{+} rightarrow pi^{+}nuoverline{nu}$ decay is presented in
this thesis, based on a sample of about $10^{11}$ $K^+$ decays
collected in 2016 by the NA62 experiment. The Single Event Sensitivity
is found to be $3times 10^{-10}$ corresponding to about 0.26
$K^{+} rightarrow pi^{+}nuoverline{nu}$̄ and 0.15 background
events expected. The analysis has revealed one candidate event
compatible with the expectations. Interpreting the candidate as
background leads to a 95$%$ CL upper limit of $1.4times10^{-9}$ on
the $K^{+} rightarrow pi^{+}nuoverline{nu}$ branching ratio.
Prospects for future developments are also reviewed.