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

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