Study of the upstream background for the measurement of the branching ratio of the $K^+ \to \pi^+ \nu\bar\nu$ decay with the NA62 experiment at CERN

The $K^+ \to \pi^+ \nu\bar\nu$ decay is one of the most sensitive channels suitable for finding indirect signal of new physics at high energy scales. The NA62 experiment at the CERN SPS plans on measuring the branching ratio of this decay with an unprecedented precision using a decay-in-flight technique. The kinematic selection of the signal is based on the reconstruction of the squared missing mass, obtained from the momentum of the parent $K^+$ and the $\pi^+$ in the final state. The main background sources include more abundant kaon decays such as $K^+ \to \pi^+ \pi^0$ , $K^+ \to \mu^+ \nu_{\mu}$ and $K^+ \to \pi^+ \pi^+ \pi^- $, that under certain circumstances can mimic the experimental signature of the signal; in order to reduce the background due to these kaon decays, NA62 relies on the kinematic selection, an excellent timing between the sub-detectors, a good particle identification and muon rejection, and a hermetic photon veto. The main residual source of background is due to the so-called upstream background, which is any kaon decay or interaction happening before the fiducial decay region, matched to a pile-up particle in the upstream part of the detector. In this thesis I performed a $K^+ \to \pi^+ \nu\bar\nu$ selection that allowed to study and understand the features of the upstream background and measure it in the 2017 data set. This task is particularly difficult because of the non-trivial topology that these kind of events has in order to contribute to the background.