On some aspects of isospin breaking in the decay $$K^\pm \rightarrow \pi ^0 \pi ^0 e^\pm \mathop {\nu _e}\limits ^{_{(-)}}$$

Two aspects of isospin breaking in the decay \(K^\pm \rightarrow \pi ^0 \pi ^0 e^\pm \mathop {\nu _e}\limits ^{_{(-)}}\) are studied and discussed. The first addresses the possible influence of the phenomenological description of the unitarity cusp on the extraction of the normalization of the form factor from data. Using the scalar form factor of the pion as a theoretical laboratory, we find that this determination is robust under variations of the phenomenological parameterizations of the form factor. The second aspect concerns the issue of radiative corrections. We compute the radiative corrections to the total decay rate for \(K^\pm \rightarrow \pi ^0 \pi ^0 e^\pm \mathop {\nu _e}\limits ^{_{(-)}}\) in a setting that allows comparison with the way radiative corrections were handled in the channel \(K^\pm \rightarrow \pi ^+ \pi ^- e^\pm \mathop {\nu _e}\limits ^{_{(-)}}\). We find that, once radiative corrections are included, the normalizations of the form factor as determined experimentally from data in the two decay channels come to a better agreement. The remaining discrepancy can easily be accounted for by other isospin-breaking corrections, mainly those due to the difference between the masses of the up and down quarks.