Closing in on the radiative weak chiral couplings

We point out that, given the current experimental status of radiative kaon decays, a subclass of the O(p⁴) counterterms of the weak chiral lagrangian can be determined in closed form. This involves in a decisive way the decay K^± → π^±π⁰l⁺l^-, currently being measured at CERN by the NA48/2 and NA62 collaborations. We show that consistency with other radiative kaon decay measurements leads to a rather clean prediction for the O(p⁴) weak couplings entering this decay mode. This results in a characteristic pattern for the interference Dalitz plot, susceptible to be tested already with the limited statistics available at NA48/2. We also provide the first analysis of K_S → π⁺π^-γ*, which will be measured by LHCb and will help reduce (together with the related K_L decay) the experimental uncertainty on the radiative weak chiral couplings. A precise experimental determination of the O(p⁴) weak couplings is important in order to assess the validity of the existing theoretical models in a conclusive way. We briefly comment on the current theoretical situation and discuss the merits of the different theoretical approaches. (orig.)