Observation of the $p\eta'$ Cusp in the New Precise Beam Asymmetry $\Sigma$ Data for $\gamma p\to p \eta$

TL;DR

This paper reports the first observation of a cusp effect in the beam asymmetry data for $ o p \, \eta$ photoproduction, revealing a branch-point singularity at the $p\eta'$ threshold and supporting the existence of the $N(1895)\frac{1}{2}^-$ resonance.

Contribution

The study provides the first measurement of a cusp effect in polarization observables and demonstrates the significance of including singularities in partial wave analyses for resonance identification.

Findings

Observation of a strong cusp effect at the $p\eta'$ threshold.

Indication of the $N(1895)\frac{1}{2}^-$ nucleon resonance.

Enhanced partial wave analysis accuracy with new data.

Abstract

Data on the beam asymmetry $\Sigma$ in the photoproduction of $\eta$ mesons off protons are reported for tagged photon energies from 1130 to 1790 MeV (mass range from $W=$ 1748 MeV to $W=$ 2045 MeV). The data cover the full solid angle that allows for a precise moment analysis. For the first time, a strong cusp effect in a polarization observable has been observed that is an effect of a branch-point singularity at the $p\eta'$ threshold [$E_\gamma=$ 1447 MeV ($W=$ 1896 MeV)]. The latest BnGa partial wave analysis includes the new beam asymmetry data and yields a strong indication for the $N(1895)\frac{1}{2}^-$ nucleon resonance, demonstrating the importance of including all singularities for a correct determination of partial waves and resonance parameters.