Abstrakt
A sample of 2.8 x 10(4) K+ -> 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 +/- 0.08) x 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 (p(6)).