Measurement of the K⁺ → π⁺γγ decay

A sample of 3984 candidates of the 𝐾⁺→𝜋⁺𝛾𝛾 decay, with an estimated background of 291±14 events, was collected by the NA62 experiment at CERN during 2017–2018. In order to describe the observed di-photon mass spectrum, the next-to-leading order contribution in chiral perturbation theory was found to be necessary. The decay branching ratio in the full kinematic range is measured to be (9.61 ± 0.17) × 10⁻⁷. The first search for production and prompt decay of an axion-like particle with gluon coupling in the process 𝐾⁺→𝜋⁺𝑎, 𝑎→𝛾𝛾 is also reported.