Study of K^ - → π ⁰ e^ - ν ₑ γ decay with ISTRA+ setup

Results of study of the K^ - → π ⁰ e^ - ν γ decay at the ISTRA+ setup are presented. We observed 4476 events of this decay. The branching ratio is found to be R = Br(K^ - → π ⁰ e^ - ν ₑ γ )Br(K^ - → π ⁰ e^ - ν ₑ ) = (1.81±0.03(stat.)±0.07(syst.)) × 10−2 for E*γ > 10 MeV and θ*eγ > 10°. For comparison with the previous experiment the branching ratio with cuts E*γ > 10 MeV, 0.6 < cos θ*eγ < 0.9 is calculated: R = Br(K^ - → π ⁰ e^ - ν ₑ γ )Br(K^ - → π ⁰ e^ - ν ₑ ) = (0.47±0.02(stat.) ± 0.03(syst.)) × 10−2. For the cuts E*γ > 30 MeV and θ*eγ > 20°, used in most theoretical papers, Br = (3.06 ± 0.09(stat.) ± 0.14(syst.)) × 10−4. For the asymmetry we get A ξ = −0.015 ± 0.021. At present it is the best estimate of this asymmetry.