$M_{p\eta}$ invariant mass distribution for $E_\gamma$ = 1400-1500 MeV.

$M_{p\eta}$ invariant mass distribution for $E_\gamma$ = 1500-1600 MeV.

$M_{p\eta}$ invariant mass distribution under the conditions: $E_\gamma$ = 1400-1500 MeV; 1120 MeV $\leq M_{p\pi^0} \leq$ 1220 MeV; $\theta_p^{lab} \leq$ 25°; 25° $\leq \theta_\gamma^{lab} \leq$ 155°.

$M_{p\eta}$ invariant mass distribution under the conditions: $E_\gamma$ = 1400-1500 MeV; 1120 MeV $\leq M_{p\pi^0} \leq$ 1220 MeV; $\theta_p^{lab} \leq$ 25°; 1° $\leq \theta_\gamma^{lab} \leq$ 155°.

$M_{p\eta}$ invariant mass distribution for $E_\gamma$ = 1400-1500 MeV and $M_{p\pi^0} \leq$ 1190 MeV, allowing for all proton and photon laboratory angles.

$M_{p\eta}$ invariant mass distribution for the incident photon energy range $E_\gamma$ = 1420-1540 MeV and $M_{p\pi^0} \leq$ 1170 MeV.

$M_{p\eta}$ invariant mass distribution for the incident photon energy range $E_\gamma$ = 1420-1540 MeV and $M_{p\pi^0} \leq$ 1180 MeV.

$M_{p\eta}$ invariant mass distribution for the incident photon energy range $E_\gamma$ = 1420-1540 MeV and $M_{p\pi^0} \leq$ 1190 MeV.

$M_{p\eta}$ invariant mass distribution for the incident photon energy range $E_\gamma$ = 1420-1540 MeV and $M_{p\pi^0} \leq$ 1200 MeV.

$M_{p\eta}$ invariant mass distribution for the incident photon energy range $E_\gamma$ = 1420-1540 MeV and $M_{p\pi^0} \leq$ 1210 MeV.

$M_{p\eta}$ invariant mass distribution for the incident photon energy range $E_\gamma$ = 1420-1540 MeV and $M_{p\pi^0} \leq$ 1220 MeV.

$M_{p\eta}$ invariant mass distribution for the incident photon energy range $E_\gamma$ = 1400-1420 MeV and $M_{p\pi^0} \leq$ 1190 MeV.

$M_{p\eta}$ invariant mass distribution for the incident photon energy range $E_\gamma$ = 1420-1460 MeV and $M_{p\pi^0} \leq$ 1190 MeV.

$M_{p\eta}$ invariant mass distribution for the incident photon energy range $E_\gamma$ = 1460-1500 MeV and $M_{p\pi^0} \leq$ 1190 MeV.

$M_{p\eta}$ invariant mass distribution for the incident photon energy range $E_\gamma$ = 1500-1540 MeV and $M_{p\pi^0} \leq$ 1190 MeV.

$M_{p\eta}$ invariant mass distribution for the incident photon energy range $E_\gamma$ = 1540-1600 MeV and $M_{p\pi^0} \leq$ 1190 MeV.

$M_{p\eta}$ invariant mass distribution for the incident photon energy range $E_\gamma$ = 1420-1540 MeV and $M_{p\pi^0} \leq$ 1190 MeV.

Excess yield relative to the PWA distributions in the invariant mass distributions $M^2_{p\eta}$.

Excess yield relative to the PWA distributions in the invariant mass distributions $M^2_{\pi^0\eta}$.

Excess yield relative to the PWA distributions in the invariant mass distributions $M^2_{p\pi^0}$, with a cut on the structure in the $M^2_{p\eta}$ invariant mass distribution.

Excess yield relative to the PWA distributions in the $p-\eta$ opening angle distribution in the $\gamma p$ centre-of-mass system.

Excess yield relative to the PWA distributions in the $p-\eta$ opening angle distribution in the $\gamma p$ centre-of-mass system.

Excess yield relative to the PWA distributions in the $\pi^0-\eta$ opening angle distribution in the $\gamma p$ centre-of-mass system.

Excess yield relative to the PWA distributions in the $\pi^0-\eta$ opening angle distribution in the $\gamma p$ centre-of-mass system.

Differential cross section $d\sigma/dM_{\pi^0\eta}$ for $E_\gamma$ = 1540-1600 MeV.

$M_{p\eta}$ invariant mass distribution, taken as difference to the PWA calculations (s. Fig. 11).

$M_{\pi^0\eta}$ invariant mass distribution, taken as difference to the PWA calculations (s. Fig. 11).

$\theta_{p\eta}$ opening angle distribution, taken as difference to the PWA calculations (s. Fig. 12).

$\theta_{\pi^0\eta}$ opening angle distribution, taken as difference to the PWA calculations (s. Fig. 12).

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THE DATA WERE GROUPED IN ENERGY BINS 50 MEV WIDE. ONE HALF OF THE BIN WAS TAKEN BY US AS THE ERROR ON THE ENERGY.. THE DATA HAVE BEEN READ FROM A LARGE-SCALE VERSION OF FIG. 9 WHICH IS CONSIDERED BY AUTHORS TO BE SUFFICIENTLY ACCURATE AS A SOURCE (PRIVATE COMMUNICATION OF M. J. LONGO, FEBRUARY 10, 1972).