Search for π ⁰ decays to invisible particles

The NA62 experiment at the CERN SPS reports a study of a sample of 4 × 10 ⁹ tagged π ⁰ mesons from K ⁺ → π ⁺π ⁰(γ), searching for the decay of the π ⁰ to invisible particles. No signal is observed in excess of the expected background fluctuations. An upper limit of 4.4 × 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 ⁺ → π ⁺X, where X is a particle escaping detection with mass in the range 0.110–0.155 GeV/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.