Beam-Polarization Asymmetries for the $p(\overrightarrow{\gamma },K^{+})\Lambda$ and $p(\overrightarrow{\gamma },K^{+}){\Sigma }^0$ Reactions for $E_{\gamma }=1.5–2.4\mathrm{G}\mathrm{e}\mathrm{V}$

Beam polarization asymmetries for the $p(\overrightarrow{\gamma },K^{+})\Lambda$ and $p(\overrightarrow{\gamma },K^{+}){\Sigma }^0$ reactions are measured for the first time for $E_{\gamma }=1.5–2.4\mathrm{G}\mathrm{e}\mathrm{V}$ and $0.6<\cos ({\theta }_{K^{+}}^{\mathrm{c}.\mathrm{m}.})<1.0$ by using linearly polarized photons at the Laser-Electron-Photon facility at SPring-8 (LEPS). The observed asymmetries are positive and gradually increase with rising photon energy. The data are not consistent with theoretical predictions based on tree-level effective-Lagrangian approaches. Including the new results in the development of the models is, therefore, crucial for understanding the reaction mechanism and to test the presence of baryon resonances which are predicted in quark models but are thus far undiscovered.

Figure 1

Missing-mass spectrum for the $p(\overrightarrow{\gamma },K^{+})X$ reaction taken at the LEPS/SPring-8 facility.

Figure 2

Asymmetry spectra for the $p(\overrightarrow{\gamma },K^{+})\Lambda$ (a) and $p(\overrightarrow{\gamma },K^{+}){\Sigma }^0$ (b) reactions for all events. A fit to the data with $C\cos 2\phi$ is superimposed. Vertical (horizontal) errors indicate statistical uncertainties (angle intervals).

Figure 3

Beam polarization asymmetries for the $p(\overrightarrow{\gamma },K^{+})\Lambda$ (left) and $p(\overrightarrow{\gamma },K^{+}){\Sigma }^0$ (right) reactions as a function of $\cos ({\theta }_{K^{+}}^{\mathrm{c}.\mathrm{m}.})$ for different photon-energy bins. The error bars are mostly smaller than the markers. Theoretical predictions using the KAON-MAID program [15] (dashed lines) and by Janssen et al. [7, 8] (solid lines) are compared with the experimental data.