Measurement of the K_L \to \pi\mu\nu form factor parameters with the KLOE detector

Using 328 pb^{-1}of data collected at DAFNE corresponding to $\sim$ 1.8 million $K_L\to \pi\mu\nu$ decays, we have measured the $K_{\mu 3}$ form factor parameters. The structure of the $K-\pi$ vector-current provides information about the dynamics of the strong interaction; its knowledge is necessary for evaluation of the phase-space integral required for measuring the CKM matrix element $V_{us}$ and for testing lepton universality in kaon decays. Using a new parametrization for the vector and scalar form factors, we find $\lambda_+$=\pt(25.7\pm 0.6),-3, and $\lambda_0$=\pt(14.0\pm 2.1),-3,. Our result for $\lambda_0$, together with recent lattice calculations of $f_\pi$, $f_K$ and $f(0)$, satisfies the Callan-Trieman relation