Differential cross sections and photon beam asymmetries of $\eta$ photoproduction on the proton at $E_{\gamma }=1.3\text{–}2.4$ GeV

We report exclusive measurements for the $\eta$ photoproduction on a proton target. The differential cross sections and photon beam asymmetries of the $\gamma p\to \eta p$ reaction are measured in a center-of-mass energy ($W$) range of 1.82–2.32 GeV and a polar angle range of $-1.0<cos{\theta }_{\mathrm{c}.\mathrm{m}.}^{\eta }<0.6$ by using a large acceptance calorimeter (BGOegg) and forward-angle charged-particle detectors at the SPring-8 LEPS2 beamline. The reaction is identified with a kinematic fitting method to select a recoil proton and two photons produced in an $\eta$ meson decay. A bump structure at $W$ = 2.0–2.3 GeV in the differential cross section is confirmed at extremely backward $\eta$ polar angles, where the existing data are inconsistent with each other. This bump structure is likely associated with high-spin resonances that couple with $s\overline{s}$ quarks. The photon beam asymmetries in a wide $\eta$ polar angle range for the photon beam energies above 2.1 GeV are reported for the first time, providing an additional constraint to nucleon resonance studies at high energies.

Figure 1

A schematic drawing of experimental setup at the LEPS2 beamline (top view).

Figure 2

A three-dimensional figure of ToF wall that is formed by 32 RPCs.

Figure 3

The correlation between the missing energy of a $\gamma \gamma$ pair detected at the BGOegg calorimeter and the velocity of a charged particle measured using the RPC. The black dots in panel (a) show the data before the kinematic fit but after requiring the existence of two neutral clusters at the BGOegg calorimeter with a loose cut on the missing mass. The vertical bands come from electron events, which originate from the different electron bunches in the SPring-8 storage ring. Panel (b) shows the data after passing the 99% confidence level cut in the kinematic fit.

Figure 4

The invariant mass distributions for $\gamma \gamma$ pairs detected at the BGOegg calorimeter. The black solid histogram shows the events surviving after applying a loose condition, where the missing mass of a $\gamma \gamma$ pair is less than 1200 $\mathrm{MeV}/c^2$. The red dashed histogram shows the events that survive after applying the 99% confidence level cut.

Figure 5

An example of the template fitting at the kinematic bin of $W$ = 1.87 GeV and $cos{\theta }_{\mathrm{c}.\mathrm{m}.}^{\eta }=0$. The invariant mass spectrum for $\gamma \gamma$ pairs detected at the BGOegg calorimeter in the real data is plotted with statistical uncertainties in panel (a). The red solid, green dashed, blue dotted, and magenta dash-dotted histograms show the template mass spectra obtained from MC simulations of the $\eta$, ${\pi }^0{\pi }^0$, $\eta {\pi }^0$, and $\omega$ photoproduction processes, respectively. The thick black solid line shows a sum of all the template spectra. The plot in panel (b) shows the missing mass spectrum of a $\gamma \gamma$ pair in the real data. The line colors and styles of fit results are defined in the same way as those in (a).

Figure 6

Differential cross sections $d\sigma /d\mathrm{\Omega }$ as a function of $W$ for the reaction $\gamma p\to \eta p$. The individual panels correspond to different bins of the $\eta$ emission angle in the center-of-mass system. The present results are shown by red solid circles with statistical uncertainties. The gray histograms are the associated systematic uncertainties. The green triangles, blue squares, black inverted triangles, and black circles come from other experimental results by the CBELSA/TAPS [6], CLAS [5], LEPS [4], and MAMI A2 [7] Collaborations, respectively.

Figure 7

Comparison of the differential cross sections with and without the use of RPC for the extremely backward $\eta$ angles $-1.0<cos{\theta }_{\mathrm{c}.\mathrm{m}.}^{\eta }<-0.95$. The red solid circles and green squares are the results by using the DC only and both the RPC and DC, respectively.

Figure 8

Differential cross sections $d\sigma /d\mathrm{\Omega }$ as a function of $cos{\theta }_{\mathrm{c}.\mathrm{m}.}^{\eta }$ for the reaction $\gamma p\to \eta p$. The present results are shown by red solid circles with statistical uncertainties. Estimated systematic uncertainties are indicated by the gray histograms. The blue solid, magenta dotted, green dashed, and black dash-dotted curves show the PWA model calculations by EtaMAID2018 [19], SAID2009 [21], Bonn-Gatchina2019 [22], and ANL-Osaka2016 [24], respectively.

Figure 9

Photon beam asymmetries $\mathrm{\Sigma }$ as a function of $cos{\theta }_{\mathrm{c}.\mathrm{m}.}^{\eta }$ for the reaction $\gamma p\to \eta p$. The present results are shown by the red solid circles with statistical uncertainties and the associated systematic uncertainties are shown by the gray histograms. The black triangles, blue squares, and green inverted triangles come from other experimental results by GRAAL [26], CLAS [8], and CBELSA/TAPS [9] Collaborations, respectively. The blue solid, green dashed, magenta dotted, black dash-dotted, and blue long-dashed curves represent the PWA results by the EtaMAID2018 [19], Bonn-Gatchina2019 [22], SAID2009 [21], ANL-Osaka [24], and Jülich-Bonn [27] models, respectively.

Figure 10

The differential cross sections of the $\eta$, ${\pi }^0$, and $\omega$ photoproduction processes as a function of the total energy $W$ at the angles $cos{\theta }_{\mathrm{c}.\mathrm{m}.}=-0.95$ (red circles), $-0.85$ (green squares), $-0.75$ (blue triangles), $-0.65$ (magenta inverted triangles).

Figure 11

The existing PWA results calculated by EtaMAID2018 [(a) and (b)] and Bonn-Gatchina2019 [(c) and (d)] with the orbital angular momenta $L$ up to 1 (red dashed lines), 2 (green dotted lines), 3 (blue dotted-dashed lines), 4 (magenta long dashed-dotted lines), and 5 (cyan dotted lines). The full PWA calculation with all the orbital angular momenta is shown by black solid lines. The BGOegg results are plotted as red circles.