The Imaging X-Ray Polarimetry Explorer¹ (IXPE) is a NASA small explorer mission selected in early 2017. IXPE will be a pathfinder mission opening a new window on the x-ray sky by enabling polarimetry measurements on essentially all classes of the brightest cosmic x-ray sources. Launched on 2021 December 9, with a 2-year baseline mission, IXPE will perform a study of dozens of sources in its first year, with follow-on more detailed observations of selected targets in year two.

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.