Hyperon photoproduction in the nucleon resonance region

High-statistics cross sections and recoil polarizations for the reactions $\gamma +p\to K^{+}+\Lambda$ and $\gamma +p\to K^{+}+{\Sigma }^0$ have been measured at CLAS for center-of-mass energies between 1.6 and $2.3\text{GeV}$. In the $K^{+}\Lambda$ channel we confirm a resonance-like structure near $W=1.9\text{GeV}$ at backward kaon angles. Our data show more complex $s$- and $u$- channel behavior than previously seen, since structure is also present at forward angles, but not at central angles. The position and width change with angle, indicating that more than one resonance is playing a role. Large positive $\Lambda$ polarization at backward angles, which is also energy dependent, is consistent with sizable $s$- or $u$-channel contributions. Presently available model calculations cannot explain these aspects of the data.

Figure 1

(Color online) Angular distributions for $\Lambda$ (left) and ${\Sigma }^0$ (right) hyperon photoproduction measured at CLAS (solid circles) at $W=2.01\text{GeV}$. The error bars combine statistical and estimated point-to-point systematic errors. Data from SAPHIR [8] (open triangles) are also shown. The curves are for effective Lagrangian calculations computed by Kaon-MAID [4] (solid) and Janssen et al. [12] (dashed), and a Regge-model calculation of Guidal et al. [9, 10] (dot-dashed).

Figure 2

(Color online) Energy dependence of the $\Lambda$ cross section at the most forward angle measured (top), and at intermediate forward and backward angles (middle), (bottom). The error bars combine statistical and estimated point-to-point systematic errors. The curves and other data are the same as in Fig. 1.

Figure 3

(Color online) Recoil polarization of $\Lambda$ hyperons as a function of $W$ for the center-of-mass kaon angle of $\cos \left({\theta }_K^{\mathrm{\text{c.m.}}}\right)=-0.3$ (top) and $\cos \left({\theta }_K^{\mathrm{\text{c.m.}}}\right)=+0.3$ (bottom). Vertical bars on CLAS data (solid points) combine statistical and systematic errors and horizontal error bars span regions of weighted averaging. The curves and other data are the same as in Figs. 1 and 2.