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

Abstract

The branching ratio in the charged-pion kinetic energy region of 55 to 90 MeV for the direct photon emission in the K − → π − π 0 γ 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 valueBr(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.

Measurement of the direct emission and interference terms and search for CP violation in the decay K ±→π ± π 0 γ

“…The systematic uncertainty assigned to this measurement corresponds to half of the difference observed in the result varying the T * π upper cut from 80 MeV to 65 MeV. Using the theoretical prediction for the branching ratio of the inner bremsstrahlung component as given in [3] and the current value for the branching ratio of the direct emission component as quoted in the PDG [20] It should be noted that NA48/2 has been able to keep the rate of wrong solutions for the odd photon to < 0.1% while in other measurements [13,14,15,16,17,18,19] the wrong solutions fraction was always larger than 10%. The relative background contamination is < 10 −4 while in other experiments it is at the level of 10 −2 or larger.…”

“…It should be noted that NA48/2 has been able to keep the rate of wrong solutions for the odd photon to <0.1% while in other measurements [13][14][15][16][17][18][19] the wrong solutions fraction was always larger than 10%. The relative background contamination is < 10 −4 while in other experiments it is at the level of 10 −2 or larger.…”

“…In section 7.2 the possibility of e being non zero will be investigated. Most of the previous experiments [13,14,15,16,17,18,19] have measured DE and INT terms in the kinematical region of 55 MeV < T * π < 90 MeV, obtaining a value for the INT contribution compatible with zero. Therefore, the values quoted in these works for the DE branching ratio have been obtained from fits where the INT term has been set to zero.…”