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Photoproduction $γp \to K^+Λ(1690)$ in an effective Lagrangian approach

A gauge-invariant model is constructed for the $γp \to K^+Λ(1690)$ reaction within a tree-level effective Lagrangian approach with the purpose to understand the underlying production mechanisms and to study the resonance contributions in this reaction. In addition to the $t$-channel $K$ and $K^\ast$ exchanges, the $s$-channel nucleon exchange, and the interaction current, the $s$-channel nucleon resonance exchanges are also included in constructing the reaction amplitudes to describe the data. It is found that the contributions from the $s$-channel $N(2570)5/2^-$ exchange are required to describe the most recently measured total cross-section data for $γp \to K^+Λ(1690)$ from the CLAS Collaboration. Further analysis shows that the interaction current dominates the $γp \to K^+Λ(1690)$ reaction near the threshold as a result of gauge invariance. The $t$-channel $K$ exchange contributes significantly, while the contributions from the $t$-channel $K^\ast$ exchange as well as the $s$-channel nucleon exchange turn out to be negligible. The contributions from the $s$-channel $N(2570)5/2^-$ exchange are found to be responsible for the bump structure shown in the CLAS total cross-section data above the center-of-mass energy $W \approx 2.7$ GeV. The predictions of the differential cross sections for $γp \to K^+Λ(1690)$ are shown and discussed, which can provide theoretical guidances for the future experiments.