Abstrakt
The NA62 experiment at the CERN SPS reports a study of a sample of 4 x 10(9) tagged pi (0) mesons from K+ -> 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 x 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+ -> pi 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.