A new measurement of the $K^\pm\to\pi^\pm\gamma\gamma$ decay at the NA48/2 experiment

Abstract

The NA48/2 experiment at CERN collected two data samples with minimum bias trigger conditions in 2003 and 2004. A measurement of the rate and dynamic properties of the rare decay $K^\pm\to\pi^\pm\gamma\gamma$ from these data sets based on 149 decay candidates with an estimated background of $15.5\pm0.7$ events is reported. The model-independent branching ratio in the kinematic range $z=(m_{\gamma\gamma}/m_K)^2>0.2$ is measured to be ${\cal B}_{\rm MI}(z>0.2) = (0.877 \pm 0.089) \times 10^{-6}$, and the branching ratio in the full kinematic range assuming a particular Chiral Perturbation Theory description to be ${\cal B}(K_{\pi\gamma\gamma}) = (0.910 \pm 0.075) \times 10^{-6}$.