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

A sample of 3120 K±→π±μ+μ−K±→π±μ+μ− decay candidates with (3.3±0.7)%(3.3±0.7)% background contamination has been collected by the NA48/2 experiment at the CERN SPS, allowing a detailed study of the decay properties. The branching ratio was measured to be BR=(9.62±0.25)×10−8BR=(9.62±0.25)×10−8. The form factor W(z)W(z), where z=2(Mμμ/MK)z=(Mμμ/MK)2, was parameterized according to several models. In particular, the slope of the linear form factor W(z)=W0(1+δz)W(z)=W0(1+δz) was measured to be δ=3.11±0.57δ=3.11±0.57. Upper limits of 2.9×10−22.9×10−2 and 2.3×10−22.3×10−2 on possible charge asymmetry and forward–backward asymmetry were established at 90% CL. An upper limit BR(K±→π∓μ±μ±)<1.1×10−9BR(K±→π∓μ±μ±)<1.1×10−9 was established at 90% CL for the rate of the lepton number violating decay.