A measurement of the K+→ π+μ+μ− decay.

A sample of 2.8 × 10⁴K⁺ → π⁺μ⁺μ⁻ 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 ± 0.08) × 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 O (p⁶). [ABSTRACT FROM AUTHOR]