CURRENT ALGEBRA, KL3+ FORM FACTORS, AND RADIATIVE KL3+ DECAY

The complete gauge-invariant matrix element for the decays K+→π0l+νγ is derived up to and including terms linear in the photon energy k. The contributions of order k-1 and k0 are evaluated through Low's theorem, while the contributions linear in k, which represent the most important structure-dependent terms, are related to the vector and axial-vector couplings in K+→l+νγ via the hypothesis of partially conserved axial-vector current (PCAC). It is shown that the spectra and rates from the radiative decay provide an independent means of measuring the Kl3 parameters f+(0), ξ, and λ±. Results are given for the rates and photon spectra in both K+→π0e+νγ and K+→π0μ+νγ decays for representative values of the Kl3 parameters. With improved experimental data, it is then possible to check the predictions of various theoretical models both for the t dependence of the Kl3 form factors and for the size of the structure-dependent matrix elements. © 1969 The American Physical Society.