Experimental study of direct photon emission in K/it− → π−π0γ decay using ISTRA+ detector

The branching ratio in the charged-pion-kinetic-energy region of 55 to 90 MeV for the direct photon emission in the K− → π−π0γ decay has been measured using in-flight decays detected with the ISTRA+ setup operating in the 25-GeV/c negative secondary beam of the U-70 PS. The value Br(DE)=[0.37±0.39(stat.)±0.10(syst.)]×10−5 obtained from the analysis of 930 completely reconstructed events is consistent with the average value of two stopped-kaon experiments, but it differs by 2.5 standard deviations from the average value of three in-flight-kaon experiments. The result is also compared with recent theoretical predictions.