Improving the phenomenology of K_{l3} form factors with analyticity and unitarity

Understand

The shape of the vector and scalar K_{l3} form factors is investigated by exploiting analyticity and unitarity in a model-independent formalism.

• The method uses as input dispersion relations for certain correlators computed in perturbative QCD in the deep Euclidean region, soft-meson theorems, and experimental information on the phase and modulus of the form factors along the elastic part of the unitarity cut.
• We derive constraints on the coefficients of the parameterizations valid in the semileptonic range and on the truncation error.
• The method also predicts low-energy domains in the complex $t$-plane where zeros of the form factors are excluded.