First measurement of beam-recoil observables Cx and Cz in hyperon photoproduction

Spin transfer from circularly polarized real photons to recoiling hyperons has been measured for the reactions gamma+p -> K++Lambda and gamma+p -> K++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 -0.85 < cos theta(+c.m.)(K)<+0.95. For the 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 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 Sigma(0) this simple phenomenology does not hold. All existing hadrodynamic models are in poor agreement with these results.