Beam asymmetries in near-threshold $\omega$ photoproduction off the proton

The photoproduction of $\omega$ mesons off protons has been studied at the Bonn ELSA accelerator from threshold to $E_{\gamma }=1700\mathrm{MeV}$. Linearly polarized beams were produced via coherent bremsstrahlung. Large photon asymmetries in excess of 50% were obtained. For the first time the pion asymmetries associated with the $\omega \to {\pi }^0\gamma$ decay were measured and found close to zero. The asymmetries indicate $s$-channel resonance formation on top of $t$-channel exchange processes.

Figure 1

Contributions to $\omega$ photoproduction: natural parity $t$-channel exchange (left), unnatural parity ${\pi }^0$ $t$-channel exchange (middle), $s$-channel intermediate resonance excitation (right).

Figure 2

The ${\pi }^0\gamma$ invariant mass distribution around the $\omega$ signal from the full energy and angular range. The full histogram represents the experimental distribution. Monte Carlo simulations are shown for the $\omega$ signal (circles) and the $2{\pi }^0$ background (triangles). The dashed line is the additional polynomial background (see text), and the crosses show the sum of backgrounds and signal. The mass range accepted in the analysis is indicated by the vertical lines.

Figure 3

Azimuthal distribution of the reconstructed $\omega$ direction with respect to the plane of photon linear polarization ($\Phi$, top abscissa) and to the horizontal $x$ axis of the standard laboratory frame (${\Phi }^{\prime }$, bottom abscissa) within the bin $E_{\gamma }=1200–1300\mathrm{MeV}$ and $\Theta =85°–100°$ ($\omega$ polar angle in the center-of-mass frame of the reaction). The solid curve represents the full fit. The separate fits of the left and right region are denoted dashed line and dotted line (see text).

Figure 4

Photon-beam asymmetries and pion asymmetries with total error bars (statistical and systematic errors added in quadrature). In each figure all different energy bins are included: 1108–1200 MeV (full triangles tip up), 1200–1300 MeV (open squares), 1300–1400 MeV (circles), 1400–1500 MeV (open triangles), and 1500–1700 MeV (full triangles tip down). The full squares represent the full energy range. On the left $\Sigma$ is shown as a function of $|t|$, in the middle $\Sigma$ as a function of $\Theta$, and on the right ${\Sigma }_{\pi }$ as a function of $\Theta$ ($\omega$ polar angle in the center of mass of the reaction).

Figure 5

Comparison of the experimental data for $\Sigma$ (upper row) and ${\Sigma }_{\pi }$ (lower row) to the Bonn-Gatchina PWA. The PWA uses the experimental binning in $E_{\gamma }$ from 1108 to 1700 MeV. Three versions of the PWA are shown: pure ${\pi }^0$ exchange (dashed line), pure pomeron exchange (dotted line), and the full solution (based on a fit to unpolarized data and the GRAAL beam asymmetry; the experimental data presented here were not included).