Measurement of the K+/sub mu3/ decay spectrum and form factors

The Dalitz-plot density of a sample of 3973 K+/sub mu3/ decays in flight at 1.8 GeV/c (K+ → π0μ+ν/sub μ/) was studied to determine lambda/sub +/ and lambda0, the linear coefficients of the t dependence of the vector and scalar form factors, where t is the four-momentum transfer to the lepton pair. The μ+ was detected by scintillator hodoscopes, identified by thick-plate optical spark chambers and momentum analyzed in a magnetic spectrometer. The π0 was detected by its decay into two γ's which converted in a lead--optical-spark-chamber--scintillator sandwich detector. The measured coefficients were lambda/sub +/=0.050 +- 0.013 and lambda0=0.029 +- 0.011. The chi2 of the fit was 63 for 57 degrees of freedom