It is convenient to adopt the Gottfried Jackson frame. In particular, we boost into the reference frame of the produced resonance and orient the coordinate system such that the photon is in the +z direction and the x-axis is co-planar to the recoiling proton, thus defining xz as the production plane. To project the photon polarization state onto the production plane, we relate the linear polarization bases in the laboratory and production coordinate systems: | It is convenient to adopt the Gottfried Jackson frame. In particular, we boost into the reference frame of the produced resonance and orient the coordinate system such that the photon is in the +z direction and the x-axis is co-planar to the recoiling proton, thus defining xz as the production plane. To project the photon polarization state onto the production plane, we relate the linear polarization bases in the laboratory and production coordinate systems: |