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

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.