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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 $\vecγ+ p \to K^+ + \vecΛ$ and $\vecγ+ p \to K^+ + \vecΣ^0$. The data were obtained using the CLAS detector at Jefferson Lab for center-of-mass energies $W$ between 1.6 and 2.53 GeV, and for $-0.85<\cosθ_{K^+}^{c.m.}< +0.95$. For the $Λ$, 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 {\it 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.