The NA48/2 experiment at CERN reports the first observation of the K±→π0π0μ±ν decay based on a sample of 2437
candidates with 15% background contamination collected in 2003--2004. The decay
branching ratio in the kinematic region of the squared dilepton mass above
0.03~GeV2/c4 is measured to be (0.65±0.03)×10−6. The
extrapolation to the full kinematic space, using a specific model, is found to
be (3.45±0.16)×10−6, in agreement with chiral perturbation
theory predictions.Comment: 16 pages, 9 figure