Measurement of the $K^{+}\to {\pi }^{+}\gamma \gamma$ decay

Abstract

A sample of 3984 candidates of the $K^{+}\to {\pi }^{+}\gamma \gamma$ decay, with an estimated background of $291\pm 14$ 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 $(9.61\pm 0.17)\times 10^{-7}$. The first search for production and prompt decay of an axion-like particle with gluon coupling in the process $K^{+}\to {\pi }^{+}a$, $a\to \gamma \gamma$ is also reported.

Comments: Revised version, accepted for publication in Physics Letters B