New measurement of the K+ -> pi(+)mu(+)mu(-) decay

The flavor-changing neutral current decay K+ -> pi(+)mu(+)mu(-) (K-pi mu mu) is well suited to explore the Standard Model structure. A new measurement of this decay branching fraction and form factor has been performed by the NA62 experiment at CERN based on data collected in 2017-2018. From the selected sample of 28011 K-pi mu mu event candidates with a negligible background, the decay form factor parameters a(+) = -0.592 +/- 0.013(stat) +/- 0.007(syst) 0.001(ext) and b(+) = -0.699 +/- 0.046(stat) +/- 0.035(syst) 0.003 ex t, as well as the branching fraction Br = (9.27 +/- 0.07(stat) 0.08(syst) +/- 0.04(ext)) x 10(-8) have been measured.