The results of a search for π ⁰ decays to a photon and an invisible massive dark photon at the NA62 experiment at the CERN SPS are reported. From a total of 4.12 × 10⁸ tagged π ⁰ mesons, no signal is observed. Assuming a kinetic-mixing interaction, limits are set on the dark photon coupling to the ordinary photon as a function of the dark photon mass, improving on previous searches in the mass range 60–110 MeV/c ². The present results are interpreted in terms of an upper limit of the branching ratio of the electro-weak decay $$ {\pi}^0\to \gamma \nu \overline{\nu} $$ π 0 → γ ν ν ¯ , improving the current limit by more than three orders of magnitude.

The NA62 experiment reports an investigation of the $$ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $$K + → π + ν ν ¯ mode from a sample of K⁺ decays collected in 2017 at the CERN SPS. The experiment has achieved a single event sensitivity of (0.389 ± 0.024) × 10⁻¹⁰, corresponding to 2.2 events assuming the Standard Model branching ratio of (8.4 ± 1.0) × 10⁻¹¹. Two signal candidates are observed with an expected background of 1.5 events. Combined with the result of a similar analysis conducted by NA62 on a smaller data set recorded in 2016, the collaboration now reports an upper limit of 1.78 × 10⁻¹⁰ for the $$ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $$K + → π + ν ν ¯ branching ratio at 90% CL. This, together with the corresponding 68% CL measurement of ( $$ {0.48}_{-0.48}^{+0.72} $$0.48 − 0.48 + 0.72 ) × 10⁻¹⁰, are currently the most precise results worldwide, and are able to constrain some New Physics models that predict large enhancements still allowed by previous measurements.