Isospin breaking in Kℓ4 decay of the neutral kaon Original Research Article

In the presence of photons, the neutral Kℓ4 decay, K0→π0π−ℓ+νℓ, can be parameterized in terms of three vectorial, one anomalous, and one tensorial form factors. We present here analytic expressions of two vectorial form factors, f and g, calculated at one-loop level in the framework of chiral perturbation theory based on the Effective Lagrangian including mesons, photons, and leptons. These expressions may then be used to disentangle the isospin breaking part from the measured form factors and hence improve the accuracy in the determination of ππ scattering parameters from Kℓ4 experiments.