Search for π0 --> invisible decays with the NA62 experiment
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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.