Measurements of charged kaon semileptonic decay branching fractions K(+/-) --> pi(0)mu(+/-)v and K(+/-) --> pi(0)e(+/-)v and their ratio

Measured ratios of decay rates for R_{K e 3 / K2pi}, R_{K mu3 / K2pi} and R_{K mu3 / Ke3} are presented. These measurements are based on K± decays collected in a dedicated run in 2003 by the NA48/2 experiment at CERN. The results obtained are R_{K e 3 / K2pi} = (0.2496 +- 0.0009 stat +- 0.0004 syst) and R_{K mu3 / K2pi} = (0.1637 +- 0.0006 stat +- 0.0003 syst). Using the PDG average for the K± --> π±π0 normalisation mode, both values are found to be larger than the current values given by the particle data book and lead to a larger magnitude of the |Vus| CKM element than previously accepted. When combined with the latest particle data book value of |Vud|, the result is in agreement with unitarity of the CKM matrix. In addition, a new measured value of R_{K mu3 / Ke3} = (0.656 +- 0.003 stat +- 0.001 syst) is compared to the semi-empirical predictions based on the latest form factor measurements.