Anomaly in the K^0_S Sigma^+ photoproduction cross section off the proton at the K* threshold

The $\gamma +p\to K^0+{\mathrm{\Sigma }}^{+}$ photoproduction reaction is investigated in the energy region from threshold to $E_{\gamma }=2250$,MeV. The differential cross section exhibits increasing forward-peaking with energy, but only up to the $K^{\ast }$ threshold. Beyond, it suddenly returns to a flat distribution with the forward cross section dropping by a factor of four. In the total cross section a pronounced structure is observed between the $K^{\ast }\mathrm{\Lambda }$ and $K^{\ast }\mathrm{\Sigma }$ thresholds. It is speculated whether this signals the turnover of the reaction mechanism from t-channel exchange below the $K^{\ast }$ production threshold to an s-channel mechanism associated with the formation of a dynamically generated $K^{\ast }$-hyperon intermediate state.