Measurement of the Absolute Branching Ratio for the K+ -> mu+ nu(gamma) Decay with the KLOE Detector

We have measured the fully inclusive K+ -> mu+nu(gamma) absolute branching ratio with the KLOE experiment at DAFNE, the Frascati Phi-factory. From some 865,283 K+ -> mu+nu(gamma) decays obtained from a sample of about 5.2x10^8 Phi-meson decays, we find BR(K+ -> mu+ nu (gamma)) = 0.6366 +-0.0009(stat.) +-0.0015(syst.), corresponding to an overall fractional error of 0.27%. Using recent lattice results on the decay constants of pseudoscalar mesons one can obtain an estimate for the CKM mixing matrix element |Vus|=0.2223+-0.0026.