Measurement of polarisation observables in $K^0_sSigma^+$ photoproduction off the proton

The reaction $gamma , p rightarrow K^0_S,Sigma^+$ is studied in the photon energy range from threshold. Linearly polarised photon beams from coherent bremsstrahlung enabled the first measurement of photon beam asymmetries in this reaction up to $E_gamma = 2250$ MeV. In addition, the recoil hyperon polarisation was determined through the asymmetry in the weak decay $Sigma^+ rightarrow p pi^0$ up to $E_gamma = 1650$ MeV. The data are compared to partial wave analyses, and the possible impact on the interpretation of a recently observed cusp-like structure near the $K^*$ thresholds is discussed.

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

The $gamma + p rightarrow K^0 + Sigma^+$ 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 $K^*$ 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 $K^*Lambda$ and $K^*Sigma$ thresholds. It is speculated whether this signals the turnover of the reaction mechanism from t-channel exchange below the $K^*$ production threshold to an s-channel mechanism associated with the formation of a dynamically generated $K^*$-hyperon intermediate state.