First measurement of beam-recoil observables C(x) and C(z) in hyperon photoproduction

Spin transfer from circularly polarized real photons to recoiling hyperons has been measured for the reactions γ ⃗+ p → K ⁺ + Λ ⃗ and γ ⃗+ p → K ⁺ + Σ ⃗ 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 -0.85θ _K ^{+ c . m .} 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 Λ 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 Σ ⁰ this simple phenomenology does not hold. All existing hadrodynamic models are in poor agreement with these results.