Measurement of charged kaon semileptonic decay branching fractions and their ratio at the NA-48/2 experiment at CERN

Measurements of the ratios of charged kaon decay rates for Ke3/K2 π, K μ 3/K2 π and K μ 3/Ke3 are presented. These measurements are based on charged kaon decays collected in a dedicated run in 2003 by the NA48/2 experiment at CERN. The results obtained are Ke3/K2 π = 0.2470 ± 0.0009 ( stat ) ± 0.0004 ( syst ) and K μ 3/K2 π = 0.1637 ± 0.0006 ( stat ) ± 0.0003 ( syst ). Using the PDG average for the K2pi normalization 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 V us parameter in the Cabibbo-Kobayashi-Maskawa (CKM) matrix than previously accepted. When combined with the latest Particle Data Book value of | V ud |, | V us | is in agreement with unitarity of the CKM matrix. A new measured value of the ratio of the semileptonic decay rates, K μ 3/Ke3 = 0.663 ± 0.003(stat) ± 0.001(syst) is compared to semi-empirical predictions based on the latest form factor measurements.