Measurement of the absolute branching ratio for the K+->mu (+) v(gamma) decay with the KLOE detector

We have measured the fully inclusive K+ -> mu(+)v(y) absolute branching ratio with the KLOE experiment at DM phi NE, the Frascati phi-factory. From some 865283 K+ -> mu(+)v(gamma) decays obtained from a sample of similar to 5.2 x 10(8), phi-meson decays, we find BR(K+ -> mu+v(mu),(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 vertical bar V(us)vertical bar = 0.2223 +/- 0.0026. (c) 2005 Elsevier B.V. All rights reserved.