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Experimental search for radiative decays of the pentaquark baryon Θ^+(1540)

The data on the reactions K^+Xe --> K^0 γX and K^+Xe --> K^+ γX, obtained with the bubble chamber DIANA, have been analyzed for possible radiative decays of the Θ^+(1540) baryon: Θ^+ --> K^0 p γand Θ^+ --> K^+ n γ. No signals have been observed, and we derive the upper limits Γ(Θ^+ --> K^0 p γ) / Γ(Θ^+ --> K^0 p) < 0.032 and Γ(Θ^+ --> K^+ n γ) / Γ(Θ^+ --> K^+ n) < 0.041 which, using our previous measurement of Γ(Θ^+ --> KN) = (0.39+-0.10) MeV, translate to Γ(Θ^+ --> K^0 p γ) < 8 keV and Γ(Θ^+ --> K^+ n γ) < 11 keV at 90% confidence level. We have also measured the cross sections of K^+ -induced reactions involving emission of a neutral pion: σ(K^+n --> K^0 p π^0) = (68+-18) \mub and σ(K^+N --> K^+ N π^0) = (30+-8) \mub for incident K^+ momentum of 640 MeV.