Measurements of the form-factors slopes of KL → π μν decay with the KLOE detector

We present a measurement of the K-π form-factor parameters for the decay KL → πμν. We use 328 pb-1 of data collected in 2001 and 2002, corresponding to ∼ 1.8 million Kμ3 events. Measurements of semileptonic form factors provide information about the dynamics of the strong interaction and are necessary for evaluation of the phase- space integral Iμ K needed to measure the CKM matrix element |Vus| for KL → πμν decays and to test lepton universality in kaon decays. Using a new parameterization for the vector and scalar form factors we find λ+=(25.6 ± 0.4stat. ± 0.3syst.) × 10-3 and λ0=(14.3 ± 1.7stat. ± 1.1stat.) × 10-3. In the more usual quadratic expansion of the form factor the above result is corresponds to λ' + = λ+, λ''+ = 2λ2 +, λ' 0 = λ0 and λ'' 0 = (λ20 +0.000416)/2. Our results, together with recent lattice calculations of fπ, fK and f(0), satisfy the Callan-Trieman relation.