Authors:	Peruzzo, Letizia
Title:	Search for π0 --> invisible decays with the NA62 experiment
Online publication date:	19-May-2021
Year of first publication:	2021
Language:	english
Abstract:	The search for new physics beyond the Standard Model is one of the most active fields in particle physics. Complementary to direct searches for new processes at high-energy scale, $\mathcal{O}(\text{TeV})$, rare or forbidden Standard Model decays are investigated to look for deviations from the predictions. The NA62 experiment at the CERN SPS, designed for the measurement of the ultra-rare decay $K^+ \rightarrow \pi^+ \nu \bar{\nu}$, investigates new-physics contributions in the kaon and pion sector. The highly efficient, hermetic photon-veto system makes NA62 a perfect apparatus for a high-sensitivity search for $\pi^o$ decays into invisible particles. In a fraction of data collected by NA62 in 2017, about $8.4 \times 10^9$ $\pi^o$ mesons have been tagged by the reconstruction of the second most-abundant $K^+$ decay, $K^+ \rightarrow \pi^+ \pi^o (\gamma)$. The background rejection inefficiency for visible $\pi^o$ decays, ranging from $\mathcal{O}(10^{-8})$ to $\mathcal{O}(10^{-9})$, is estimated by the combination of single-photon detection efficiencies measured in data and Monte Carlo simulations of $K^+ \rightarrow \pi^+ \pi^o (\gamma)$ decays, a novel experimental technique with respect to that used in the most sensitive previous experimental result. The analysis is performed with the blind technique for a cut-based signal region. The $\pi^+$ momentum range is optimized to have the best sensitivity for the estimated upper limit on the $\pi^o \rightarrow invisible$ branching ratio in absence of a signal. In the signal region with $\pi^+$ momentum between 25 and 40~GeV/$c$, a total of about $4 \times 10^9$ tagged $\pi^o$ mesons are collected out of which 12 events survive the signal selection criteria. Given an estimated rejection inefficiency of $( 2.8^{+5.9}_{-2.1} ) \times 10^{-9}$, no signal is observed beyond the expected background fluctuations. The resulting upper limit on the branching ratio is \begin{displaymath} \mathrm{BR}(\pi^o \rightarrow invisible) \leq 4.4 \times 10^{-9} \end{displaymath} at 90\% confidence level, which improves on the previous best limit by a factor of 60. As a by-product of the analysis, the decay $K^+ \rightarrow \pi^+ X$ is investigated, where $X$ is a neutral particle escaping detection with a mass in the range 109--155~MeV/$c^2$. Model-dependent scenarios are investigated with the assumption that $X$ is an axion-like particle with dominant fermion couplings or a dark scalar with mixing with the Standard Model Higgs.
DDC:	530 Physik 530 Physics
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 08 Physik, Mathematik u. Informatik
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-5911
URN:	urn:nbn:de:hebis:77-openscience-7082af54-1c89-4e69-97e2-0ead33d7517d3
Version:	Original work
Publication type:	Dissertation
License:	In Copyright
Information on rights of use:	http://rightsstatements.org/vocab/InC/1.0/
Extent:	xiv, 174, xvii Seiten, Illustrationen, Diagramme
Appears in collections:	JGU-Publikationen