Differential cross sections, photon beam asymmetries, and spin density matrix elements of $\omega$ photoproduction off the proton at $E_{\gamma }=1.3-2.4$ GeV

For the reaction $\gamma p\to \omega p$, differential cross sections, photon beam asymmetries, and spin density matrix elements have been measured in the BGOegg experiment at the SPring-8 LEPS2 beamline by using a large acceptance electromagnetic calorimeter. A 1.3–2.4 GeV photon beam with high linear polarization has enabled a precision measurement of photon beam asymmetries $\mathrm{\Sigma }$ and polarized spin density matrix elements ${\rho }_{1-1}^1$ over a wide angular range, especially above the total energy of about 2 GeV for the first time. The measured ${\rho }_{1-1}^1$ indicates the dominance of $s$-channel resonances even at higher energies. The $\mathrm{\Sigma }$ values at backward angles are not reproduced by the existing partial-wave analysis in the highest-energy region, providing new constraints to the contribution of nucleon resonances with high spins.

Figure 1

Distributions of ${\pi }^0\gamma$ invariant mass. Panel (a) shows the distribution for the whole selected sample, while panels (b) and (c) are examples of fits with signal and background functions in two kinematical bins, whose conditions are displayed in each panel.

Figure 2

Differential cross sections of the reaction $\gamma p\to \omega p$. The BGOegg results are shown by solid blue circles with statistical uncertainties. The histograms indicate systematic uncertainties. The open squares, open crosses, and open triangles come from other experimental results by the CLAS [12], LEPS [13], and LEPS-NTPC [14] Collaborations, respectively. The open circles and open stars are two independent results from Refs. [15, 16], respectively, by the CBELSA/TAPS Collaboration. The overlaid lines represent the existing results of a partial-wave analysis by the Bonn-Gatchina group [17, 18].

Figure 3

Photon beam asymmetries of the reaction $\gamma p\to \omega p$. The BGOegg results are shown by solid blue circles with statistical uncertainties. The open diamonds, open triangles, open squares, and solid stars come from other experimental results by the GRAAL [19], LEPS-NTPC [14], CLAS [20], and CLAS-FROST [18] Collaborations, respectively. The overlaid lines represent the existing results of a partial-wave analysis by the Bonn-Gatchina group [17, 18].

Figure 4

Spin density matrix elements ${\rho }_{1-1}^1$ (upper row), ${\rho }_{1-1}^0$ (middle row), and ${\rho }_{00}^0$ (lower row) of the reaction $\gamma p\to \omega p\to {\pi }^0\gamma p$. The BGOegg results are shown by solid blue circles with statistical uncertainties. The open squares and open circles come from other experimental results by the CLAS [12] and CBELSA/TAPS [16] Collaborations, respectively.

Figure 5

Total energy dependence of the measured differential cross sections for three $cos{\theta }_{\omega }^{\mathrm{c}.\mathrm{m}.}$ bins. The BGOegg results are shown with statistical uncertainties only.

Figure 6

Total energy dependence of the measured photon beam asymmetries for two $cos{\theta }_{\omega }^{\mathrm{c}.\mathrm{m}.}$ bins. The BGOegg results are shown with statistical uncertainties only.