Measurement of neutral pion photoproduction off the proton with the large acceptance electromagnetic calorimeter BGOegg

Differential cross sections and photon beam asymmetries for the reaction $\gamma p\to {\pi }^{0}p$ have been measured in the photon beam energy range of $1.3–2.4\mathrm{GeV}$ and the ${\pi }^0$ polar angle range of $-1<cos{\theta }_{{\pi }^0}^{\mathrm{c}.\mathrm{m}.}<0.7$. The measurement has been done by using a large acceptance electromagnetic calorimeter in the SPring-8 LEPS2/BGOegg experiment. The results of the photon beam asymmetry in a wide ${\pi }^0$ polar angle range are new for the photon beam energies exceeding $1.9\mathrm{GeV}$. None of the existing partial wave analysis models describe the observed asymmetries in the high energy region. The contribution of multipole amplitudes with an orbital angular momentum of 5 in the $\pi N$ system has been found to be important at these energies.

Figure 1

Schematic view of the tagging detector. Only the part near the storage ring is shown.

Figure 2

Setup of the BGOegg experiment. Only the apparatus closely related to the signal selection is shown in a side view.

Figure 3

Distribution of the $\gamma \gamma$ invariant mass in the final signal sample.

Figure 4

Acceptance factors obtained by geant4-based MC simulation with the realistic detector setup. Two $50\mathrm{MeV}$-bins, corresponding to the photon beam energies of 1625 and $2225\mathrm{MeV}$, are shown as examples.

Figure 5

Differential cross sections for $\gamma p\to {\pi }^{0}p$ at the photon beam energies ($E_{\gamma }$) of $1300–1900\mathrm{MeV}$. The present results from the BGOegg experiment are indicated by closed circles with statistical errors. The histograms show the systematic errors of the BGOegg measurement. The other data points come from the existing experimental results: CLAS (open square) [15], CB-ELSA (open circle) [10], CBELSA/TAPS (open triangle) [14], GRAAL (open diamond) [9], and LEPS Collaborations (open cross) [17].

Figure 6

Differential cross sections for $\gamma p\to {\pi }^{0}p$ at the photon beam energies ($E_{\gamma }$) of $1900–2400\mathrm{MeV}$. The present results from the BGOegg experiment are indicated by closed circles with statistical errors. The histograms show the systematic errors of the BGOegg measurement. The symbols for the other data points have the same meanings as in Fig. 5.

Figure 7

Modulation of the measured $\gamma p\to {\pi }^{0}p$ signal yield as a function of the azimuthal angle of the reaction plane relative to the photon beam polarization vector. Two cases of the ${\pi }^0$ polar angle ranges are shown for $2200<E_{\gamma }<2300\mathrm{MeV}$.

Figure 8

Photon beam asymmetries for $\gamma p\to {\pi }^{0}p$ at the photon beam energies ($E_{\gamma }$) of $1300–2400\mathrm{MeV}$ as a function of $cos{\theta }_{{\pi }^0}^{\mathrm{c}.\mathrm{m}.}$. The present results from the BGOegg experiment are indicated by closed circles with statistical errors. The histograms show the systematic errors of the BGOegg measurement. The other data points come from the existing experimental results: CLAS (open square) [16], CBELSA/TAPS (open triangle) [13], GRAAL (open diamond) [9], LEPS (open cross) [17], Daresbury (closed and open stars) [5, 6], CEA (asterisk) [3], and Yerevan Collaborations (open circle) [4].

Figure 9

Comparison of the existing PWA results and the present results (closed circle) for the differential cross sections at $1300<E_{\gamma }<1900\mathrm{MeV}$. The solid, dashed, and dotted curves show the PWA model calculations by the Bonn-Gatchina [31], SAID [32], and ANL-Osaka Collaborations [33], respectively. The histograms show the systematic errors of the BGOegg measurement.

Figure 10

Comparison of the existing PWA results and the present results (closed circle) for the differential cross sections at $1900<E_{\gamma }<2400\mathrm{MeV}$. The meanings of the individual lines and the histograms are the same as in Fig. 9.

Figure 11

Comparison of the existing PWA results and the present results (closed circle) for the photon beam asymmetries at $1300<E_{\gamma }<2400\mathrm{MeV}$. The solid, dashed, and dotted curves show the PWA model calculations by Bonn-Gatchina [31], SAID [32], and ANL-Osaka Collaborations [33], respectively. The histograms show the systematic errors of the BGOegg measurement.

Figure 12

The existing SAID calculation results of (a) differential cross section and (b) photon beam asymmetry with the orbital angular momenta of a $\pi N$ system up to 5 (solid line) and 4 (dashed line) in high energy regions: $2200–2250\mathrm{MeV}$ for the differential cross section and $2200–2300\mathrm{MeV}$ for the photon beam asymmetry. The symbols in (a) and (b) represent the experimental results shown in Figs. 6 and 8.

Figure 13

The existing Bonn-Gatchina calculation results of (a) differential cross section and (b) photon beam asymmetry with the orbital angular momenta of a $\pi N$ system up to 9 (solid line), 7 (dashed line), 5 (dotted line), and 4 (dash-dotted line) in the high energy regions, same as in Fig. 12. The experimental results indicated by symbols are also the same as those in Fig. 12.

Figure 14

The existing PWA calculation results of differential cross sections [(a) and (c)] and photon beam asymmetries [(b) and (d)] in a low energy region ($1600–1650\mathrm{MeV}$). (a) and (b) show the SAID results with the orbital angular momenta of a $\pi N$ system $L$ up to 5 (solid line), 4 (dashed line), and 3 (dotted line). (c) and (d) display the Bonn-Gatchina curves with $L\le 9$ (solid line), $L\le 7$ (dashed line), $L\le 5$ (dotted line), and $L\le 3$ (dash-dotted line). The experimental results are simultaneously plotted with the same symbols as in Figs. 5 and 8.