First measurement of the $\Sigma$ beam asymmetry in $\eta^{\prime}$ photoproduction off the proton near threshold

The \( \Sigma\) beam asymmetry in \( \eta^{\prime}\) photoproduction off the proton was measured at the GrAAL polarised photon beam with incoming photon energies of 1.461 and 1.480 GeV. For both energies the asymmetry as a function of the meson production angle shows a clear structure, more pronounced at the lowest one, with a change of sign around \( 90^{\circ}\). The observed behaviour is compatible with P-wave D-wave (or S-wave F-wave) interference, the closer to threshold the stronger. The results are compared to the existing state-of-the-art calculations that fail to account for the data.