The Callan-Treiman low-energy theorem offers an opportunity to test electroweak couplings of light quarks to the gauge boson W. To that aim, we introduce a model-independent and accurate dispersive parametrization of the two K_{ l3} form factors. We then discuss three applications to the analysis of K_{ e3} and K_{ mu3} measurements: the prediction of the ratios Gamma(K_{ mu3})/Gamma(K_{ e3}), the extraction of |f_+(0)V_{ us}| and finally the possible measurement of m_u - m_d induced isospin breaking asymmetry.
Categories: hep-ph hep-ex