  Despite decades of studies of the photoproduction of hyperons, both their
production mechanisms and their spectra of excited states are still largely
unknown. While the parity-violating weak decay of hyperons offers a means of
measuring their polarization, which could help discern their production
mechanisms and identify their excitation spectra, no such study has been
possible for doubly strange baryons in photoproduction, due to low production
cross sections. However, by making use of the reaction $\gamma p \to K^+ K^+
\Xi^-$, we have measured, for the first time, the induced polarization, $P$,
and the transferred polarization from circularly polarized real photons,
characterized by $C_x$ and $C_z$, to recoiling $\Xi^-$s. The data were obtained
using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab for
photon energies from just over threshold (2.4 GeV) to 5.45 GeV. These
first-time measurements are compared, and are shown to broadly agree, with
model predictions in which cascade photoproduction proceeds through the decay
of intermediate hyperon resonances that are produced via relativistic meson
exchange, offering a new step forward in the understanding of the production
and polarization of doubly-strange baryons.
