Measurement of the very rare K+→π+ννˉK^+ \to \pi^+ \nu \bar\nu decay

The decay K+→π+νν¯ , with a very precisely predicted branching ratio of less than 10−10 , is among the best processes to reveal indirect effects of new physics. The NA62 experiment at CERN SPS is designed to study the K+→π+νν¯ decay and to measure its branching ratio using a decay-in-flight technique. NA62 took data in 2016, 2017 and 2018, reaching the sensitivity of the Standard Model for the K+→π+νν¯ decay by the analysis of the 2016 and 2017 data, and providing the most precise measurement of the branching ratio to date by the analysis of the 2018 data. This measurement is also used to set limits on BR(K+→π+X ), where X is a scalar or pseudo-scalar particle. The final result of the BR(K+→π+νν¯ ) measurement and its interpretation in terms of the K+→π+X decay from the analysis of the full 2016-2018 data set is presented, and future plans and prospects are reviewed

First measurement of the K+→π+νν‾K^{+} \rightarrow \pi^{+}\nu\overline{\nu} decay with the NA62 experiment at CERN

The decay $K^{+} \rightarrow \pi^{+}\nu\overline{\nu}$ with a very precisely predicted branching ratio of $8.4\times 10^{-10}$ 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 $K^{+} \rightarrow \pi^{+}\nu\overline{\nu}$ decay with a decay-in-flight technique, novel for this channel. The search for the $K^{+} \rightarrow \pi^{+}\nu\overline{\nu}$ 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 $3\times 10^{-10}$ corresponding to about 0.26 $K^{+} \rightarrow \pi^{+}\nu\overline{\nu}$̄ 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 $1.4\times10^{-9}$ on the $K^{+} \rightarrow \pi^{+}\nu\overline{\nu}$ branching ratio. Prospects for future developments are also reviewed

First measurement of the K+rightarrowpi+nuoverlinenuK^{+} rightarrow pi^{+}nuoverline{nu} decay with the NA62 experiment at CERN

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$$ CL upper limit of $1.4times10^{-9}$ on the $K^{+} rightarrow pi^{+}nuoverline{nu}$ branching ratio. Prospects for future developments are also reviewed.Der Zerfall $K^{+} rightarrow pi^{+}nuoverline{nu}$ mit einem sehr genau vorhergesagten Verzweigungsverh"{a}ltnis von $8.4times 10^{-10}$ ist einer der besten Kandidaten, um indirekte Effekte neuer Physik auf h"{o}chsten Massenskalen zu finden. Das NA62-Experiment am CERN SPS wurde entwickelt, um das Verzweigungsverh"{a}ltnis des $K^{+} rightarrow pi^{+}nuoverline{nu}$-Zerfalls mit einer f"{u}r diesen Kanal neuartigen textit{Decay-in-flight}-Technik zu messen. Die Suche nach dem $K^{+} rightarrow pi^{+}nuoverline{nu}$-Zerfall wird in dieser Arbeit mit einem Datensatz von etwa $10^{11}$ $K^+$-Zerf"{a}llen im Jahr 2016 mit dem NA62-Experiment aufgezeichnet wurden, durchgef"{u}hrt. Die Single-Event-Sensitivit"{a}t ist $3times 10^{-10}$ und entspricht etwa 0.26 $K^{+} rightarrow pi^{+}nuoverline{nu}$- und 0.15 Hintergrundereignissen. Die Analyse findet einen Kandidaten und ist mit dem Erwartungen vereinbar. Wenn man den gefundenen Kandidaten als Untergrund interpretiert, f"{u}hrt dies zu einer Obergrenze vom $1.4times 10^{-9}$ bei 95$$ CL auf das $K^{+} rightarrow pi^{+}nuoverline{nu}$-Verzweigungsverh"{a}ltnis. Aussichten f"{u}r zuk"{u}nftige Entwicklungen werden ebenfalls dargestellt