New measurement of the $K^+ \to \pi^+\mu^+\mu^-$ decay

The flavor-changing neutral current decay K+ → π+μ+μ− (Kπμμ) 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 Kπμμ event candidates with a negligible background, the decay form factor parameters a+ = 10 − 0.592± 0.013stat ± 0.007syst ± 0.001ext and b+ = −0.699 ± 0.046stat ± 0.035syst ± 0.003ext, as well as the branching fraction Br = (9.27 ± 0.07stat ± 0.08syst ± 0.04ext) × 10−8 have been measured.