IHEP 2005 - 20 (In English)
	O.G.Tchikilev, S.A.Akimenko, G.I.Britvich, K.V.Datsko, A.P.Filin, A.V.Inyakin, V.A.Khmelnikov, A.S.Konstantinov, I.Y.Korolkov, V.M.Leontiev, V.P.Novikov, V.F.Obraztsov, V.A.Polyakov, V.I.Romanovsky, V.I.Shelikhov, V.A.Uvarov, O.P.Yushchenko (IHEP, Protvino) V.N.Bolotov, S.V.Laptev, V.A.Duk, A.Yu.Polyarush (INR, Moscow)
	Observation of the Radiative Kaon Decay K- --> \mu-\pio \gamma \nu
	Protvino, 2005. – p. 12, figs. 8, refs.: 14.

Using data collected with the ``ISTRA+'' spectrometer during the 2001 run of the U-70 proton synchrotron in Protvino, we report the first observation of the radiative kaon decay K^- --> \mu^-\pi^o \gamma \nu. We find BR(K_\mu3\gamma, 5 < E^*_\gamma < 30MeV ) / BR(K_\mu3) = 0.270 \pm 0.029(stat) \pm 0.026(syst)% and BR(K_\mu3\gamma, 30 < E^*_\gamma < 60 MeV)/BR(K_{_\mu3}) = 0.0448 \pm 0.0068(stat) \pm 0.0099(syst)%. These ratios are consistent with the theoretical predictions 0.21\% and 0.047\% respectively. The measured angular distribution asymmetry for the region 5< E^*_\gamma < 30MeV, A(\cos \theta^*_{\mu\gamma})= 0.093 \pm 0.141, is two standard deviations away from the theoretical prediction of 0.354. The measured asymmetry in the T-odd variable \xi = \vec p_\gamma \cdot \vec p_\mu \times \vec p_\pi)/m³_K is -0.03 \pm 0.13.

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