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

Abstract

The NA62 experiment at the CERN SPS reports a study of a sample of $4\times 10^9$ tagged ${\pi }^0$ mesons from $K^{+}\to {\pi }^{+}{\pi }^0(\gamma )$, 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$/c^2$ 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.

23 pages, 8 figures, to be submitted to JHEP

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