Search for ${\pi }^0$ decays to invisible particles

The NA62 experiment at the CERN SPS reports a study of a sample of $4 \times10^{9}$tagged$\pi^0$mesonsfrom$K^+ \to \pi^+ \pi^0 (\gamma)$, searching for the decay of the ${\pi }^0$ to invisible particles. No signal is observed in excess of the expected background fluctuations. An upper limit of $4.4\times {10}^{-9}$ is set on the branching ratio at 90% confidence level, improving on previous results by a factor of 60. This result can also be interpreted as a model-independent upper limit on the branching ratio for the decay $K^{+}\to {\pi }^{+}X$, where $X$ is a particle escaping detection with mass in the range 0.110-0.155 GeV$\mathrm{/}{c}^2$ and rest lifetime greater than 100 ps. Model-dependent upper limits are obtained assuming $X$ to be an axion-like particle with dominant fermion couplings or a dark scalar mixing with the Standard Model Higgs boson.