Measurement of the radiative K-e3 branching ratio

Abstract.We present a measurement of the relative branching ratio of the decay K 0 → π ± e ∓ νγ (K e3γ ) withrespect to K 0 → π ± e ∓ ν (γ) (K e3 + K e3γ ) decay. The result is based on observation of 19 000K e3γ and 5.6 · 10 6 K e3 decays. The value of the branching ratio is Br(K e03γ ,E γ∗ > 30 MeV,θ ∗γ >20 o )/Br(K 0e3 ) = (0.964 ± 0.008 +0.011−0.009 )%. This result agrees with theoretical predictions but is atvariance with a recently published result. 1 Introduction The study of radiative K L decays can give valuable information on the kaon structure. It allows agood test of theories describing hadron interactions and decays, like Chiral Perturbation Theory(ChPT). Here we present a study of the radiative K e3 decay.There are two distinct photon components in the radiative K 0e3 decays - inner bremsstrahlung (IB)and direct emission. K 0e3 decays are mainly sensitive to the IB component because of the smallelectron mass m e . A big contribution to the rate, dominated by the IB amplitude, comes from theregion of small photon energies E