First Measurement of the $\Sigma$ Beam Asymmetry in $\eta^{\prime}$ Photoproduction off the Proton near Threshold

TL;DR

This paper reports the first measurement of the $oldsymbol{oldsymbol{ extSigma}}$ beam asymmetry in $oldsymbol{oldsymbol{ exteta^{ extprime}}}$ photoproduction off protons near threshold energies, revealing structures not explained by current models.

Contribution

It provides the first experimental data on $oldsymbol{ extSigma}$ asymmetry in $oldsymbol{ exteta^{ extprime}}$ photoproduction near threshold, challenging existing theoretical calculations.

Findings

Asymmetry shows a clear structure with sign change around 90°

Data suggest P-wave D-wave or S-wave F-wave interference

Current models do not reproduce the observed asymmetry patterns

Abstract

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.