First measurement of beam-recoil observables<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>C</mml:mi><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>C</mml:mi><mml:mrow><mml:mi>z</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>in hyperon photoproduction

Spin transfer from circularly polarized real photons to recoiling hyperons has been measured for the reactions $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}}+p\ensuremath{\rightarrow}{K}^{+}+\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\Lambda}}$ and $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}}+p\ensuremath{\rightarrow}{K}^{+}+{\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\Sigma}}}^{0}$. The data were obtained using the CEBAF Large Acceptance Spectrometer (CLAS) detector at the Jefferson Lab for center-of-mass energies $W$ between 1.6 and 2.53 GeV, and for $\ensuremath{-}0.85&lt;\mathrm{cos}{\ensuremath{\theta}}_{{K}^{+}}^{c.m.}&lt;+0.95$. For the \ensuremath{\Lambda}, the polarization transfer coefficient along the photon momentum axis, ${C}_{z}$, was found to be near unity for a wide range of energy and kaon production angles. The associated transverse polarization coefficient ${C}_{x}$ is smaller than ${C}_{z}$ by a roughly constant difference of unity. Most significantly, the total $\ensuremath{\Lambda}$ polarization vector, including the induced polarization $P$, has magnitude consistent with unity at all measured energies and production angles when the beam is fully polarized. For the ${\ensuremath{\Sigma}}^{0}$ this simple phenomenology does not hold. All existing hadrodynamic models are in poor agreement with these results.