A measurement of the $K^{+}\to {\pi }^{+}{\mu }^{+}{\mu }^{-}$ decay

A sample of $2.8\times {10}^4$ $K^{+}\to {\pi }^{+}{\mu }^{+}{\mu }^{-}$ candidates with negligible background was collected by the NA62 experiment at the CERN SPS in 2017–2018. The model-independent branching fraction is measured to be $(9.15\pm 0.08)\times {10}^{-8}$, a factor three more precise than previous measurements. The decay form factor is presented as a function of the squared dimuon mass. A measurement of the form factor parameters and their uncertainties is performed using a description based on Chiral Perturbation Theory at $\mathcal{O}(p^6)$.