NA62 Collab.
Search for π0 decays to invisible particles
The NA62 experiment at the CERN SPS reports a study of a sample of 4×109 tagged π0 mesons from K+→π+π0(γ), searching for the decay of the π0 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/c2 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.