Measurements of $C_x$, $C_z$ and $d_\sigma/d\Omega$ for $K^+\Lambda$ and $K^+\Sigma^0$ Photoproduction

The photoproduction reactions $\gamma + p \rightarrow K^+ +\Lambda$ and $\gamma + p \rightarrow K^+ +\Sigma^0$ have been measured from threshold to 2.9 GeV photon energy using the CLAS spectrometer at Jefferson Lab. Using a circularly polarized real photon beam, we have obtained first-ever data for the double polarization observables $C_x$ and $C_z$. These correspond to the transfer of polarization from the photon to the produced hyperon. Results for both hyperons span a wide range of kaon production angles. $C_z$ for the $K^+ \Lambda$ reaction shows a striking trend toward complete polarization transfer to the hyperon over a wide range of kaon production angles, while $C_x$ tends towards zero. Several effective Lagrangian reaction models utterly fail to predict these newly-measured observables. We have extended our previous measurements \footnote{J. W. C. McNabb (CLAS Collaboration), {\it et al.}, Phys. Rev. C {\bf 69} 042201(R) (2004).} of the differential cross sections, $d\sigma/d\Omega$, to cover the nearly complete angular range $-0.85\le \cos\theta(K^+_{c.m.})\le+0.95$ and the extended photon energy range from 2.4 to 2.9 GeV. The results support our previous conclusions that multiple resonance-like contributions are present in $K^+ \Lambda$ production between $W$ of 1.8 and 2.0 GeV. $t$-channel scaling is found to be significant in $K^+\Lambda$ production, but is much less significant in $K^+ \Sigma^0$ production.