The NA48/2 experiment at CERN created great opportunities to analyze rare charged kaon decays. A large part of the 2003 year data was acquired with a newly introduced kaon spectrometer. Along with magnet spectrometer, calorimeters and muon detector systems a precise reconstruction of kaon decays became possible. Particularly, the decay K+ → π+π-μ+ν can be studied with calcuable parameters of all particles, including the neutrino.
As a result, about 2500 events were selected. This sample is about 360 times larger than any previous result - and so, it brings a significant improvement in branching ratio evaluation. Relative small amount of background will possibly allow calculation of form-factors and of π+π- scattering length.