Probing the spin-parity of the pentaquark $\Theta^+$ baryon

Using the DIANA data on the charge-exchange reaction K^+n --u003e pK^0 on a bound neutron, in which the s-channel formation of the pentaquark baryon Theta^+(1538) has been observed, we analyze the dependence of the background-subtracted Theta^+ --u003e pK^0 signal on the K^0 emission angle in the pK^0 rest frame. The data are consistent with the angular dependence of an asymmetric form dW/dcosTheta_cms ~ (cosTheta_cms + A)^2 with a positive offset A. This form of the cosTheta_cms distribution suggests an interference between the nonresonant $s$-wave and the Theta-mediated higher-wave contributions to the total amplitude of the charge-exchange reaction, and rules out the spin-parity assignment of 1/2$^-$ for the Theta^+ baryon. The selection in cosTheta_cms based on the observed angular dependence of the Theta^+ --u003e pK^0 signal allows to boost the statistical significance of the signal up to 7.1 standard deviations.