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$^o$. 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. 
P. Levi Sandri, G. Mandaglio, V. De Leo, O. Bartalini, V. Bellini,   J.P. Bocquet, M. Capogni, F. Curciarello, A. D'Angelo, ...
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