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Using the DIANA data on the charge-exchange reaction $K^+n \rightarrow pK^0$on a bound neutron, in which the s-channel formation of the pentaquark baryon$\Theta^+(1538)$ has been observed, we analyze the dependence of thebackground-subtracted $\Theta^+ \rightarrow pK^0$ signal on the $K^0$ emissionangle in the $pK^0$ rest frame. In order to describe the observed$\cos\Theta_K^\mathrm{cms}$ distribution, invoking the interference between thenonresonant s-wave and the $\Theta^+$-mediated higher-wave contributions to theamplitude of the charge-exchange reaction is required at a 2.8$\sigma$ level.The spin--parity assignment of 1/2$^-$ for the $\Theta^+$ baryon is ruled outat a statistical level of 2.9 standard deviations. A physically-meaningfulselection in $\cos\Theta_K^\mathrm{cms}$ based on the observed angulardependence of the $\Theta^+ \rightarrow pK^0$ signal allows to boost thestatistical significance of the signal up to 7.1 standard deviations. This isfar in excess of previously reported signals and renders the $\Theta^+$existence more credible.

Angular distribution in s-channel formation of the pentaquark $\Theta^+$ baryon