We study first the box-diagram contribution to the $gamma p,rightarrow ,K^0Sigma^+$ process to understand the anomaly of the kaon photoproduction cross section from CBELSA/TAPS experiment at Electron Stretcher Accelerator (ELSA), where the imaginary part of the scattering amplitude from the box-diagrams is calculated by using Cutkoskys rules in the on-shell approximation while the real part of the amplitude is derived by dispersion relation calculations. Together with the results of the K-MAID model, the contribution of the box-diagrams fails to provide the sudden drop of the differential cross-section between the $K^*,Lambda$ and $K^*,Sigma$ thresholds. In addition, we include the $Delta(1940)$ resonance in the process to complete the description of the differential cross-section. Combining the contributions from the K-MAID model, the box-diagrams and the $Delta(1940)$ resonance, we have obtained the theoretical differential cross-section the $gamma p,rightarrow ,K^0Sigma^+$ process, which is compatible with the CBELSA/TAPS experimental data.
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