Baryon Resonance Studies Via Meson Photoproduction in the LEPS2/BGOegg Experiment

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Introduction
The constituent quark models [1] describe the ground-state baryon resonances successfully.However, most of the excited states predicted in the constituent quark models have not been observed experimentally or have different masses than expected.Especially, in the energy region above W = 1.8 GeV, a significant number of predicted states have not been established by the experiments.Most of the initially observed states were found with πN scattering.During experiments involving πN scattering, the flavors of constituent quarks in the pion have an impact on the behavior of the excited nucleons.In contrast, the photon induced reaction can couple to a q q pair including the strangeness in a few GeV energy.Here the η meson photoproduction of is a good tool to study the resonances containing the strangeness.The η meson photoproduction off proton has an advantage to search for isospin 1/2 resonances.This is because an η meson is isospin 0 and cannot produce intermediate states that are isospin 3/2.Moreover, the η meson contains hidden strangeness (s s), and so it is expected to be coupled to a baryon resonance with a large s s component.
We carried out experiments at the LEPS2 beamline in SPring-8 using the BGOegg calorimeter [4].To identify the reaction γ p → ηp → γ γ p, we detected all the final-state particles and applied a kinematic fit.We measured the differential cross sections and photon beam asymmetries for the γ p → ηp reaction in the center-of-mass energy of 1.82 < W < 2.32 [2].We observed the bump structure for the differential cross sections at cos θ η c.m. < −0.6 in W >2.0 GeV.In this article, we report the results and discussed the observed structure.

Experimental Setup and Data Analysis
We carried out the experiment at the SPring-8/LEPS2 beamline [3] in 2014 using photon beam.A linearly polarized photon beam was generated via the backward Compton scattering using ultraviolet laser photons off 8 GeV electrons in the SPring-8 storage ring.The typical photon beam intensity was 2 × 10 6 photons/s.Fig. 1 The differential cross sections dσ/d are plotted as a function of W for the reaction γ p → ηp.Each panel corresponds to a different bin of the η emission angle in the center-of-mass system.The results obtained by the BGOegg collaboration are represented by red circles, with statistical uncertainties shown.The estimated systematic uncertainties are indicated by gray histograms.Other experimental results from the LEPS [5], CLAS [7], and CBELSA/TAPS [6] collaborations are depicted as magenta squares, green triangles, and blue inverted triangles, respectively.The blue solid lines, green dashed lines, black dotteddashed lines, and magenta dotted lines represent the partial wave analysis (PWA) calculations by the Bonn-Gatchina2019 [11,12], EtaMAID2018 [8,9], SAID2009 [10], and ANL-Osaka2016 [13,14] models, respectively Photon beam energy was evaluated to the recoil electrons with the tagging counter.The tagged photon energy range was from 1.3 to 2.4 GeV and this energy resolution was 12.1 MeV.The linearly polarized photons are injected into a 54-mm-long liquid hydrogen target for the η photoproduction measurement.To identify γ p → ηp → γ γ p reaction, we measured all particles in the final state.Two γ 's were detected using the egg-shape electromagnetic calorimeter (BGOegg).The BGOegg consists of 1320 BGO crystals covering the laboratory polar angle at 24 • -144 • .Recoil protons were detected by the BGOegg calorimeter if emitted laterally or by the Drift Chamber (DC) if emitted forward.The distance between the DC and the liquid hydrogen target was 1.6 m, and the DC covered the laboratory polar angle at up to 21 • forward.To reduce background events, we performed the kinematic fit under the constraints of energy and momentum conservation.The kinematic variables are the 4-momenta of two γ 's, the polar and azimuthal angles of the recoil proton, the photon beam energy, and a vertex z-position.The recoil proton momentum was treated as an unmeasured variable.After applying a 99% confidence level cut in the kinematic fit, the typical background contamination was less than 5%.We compared our results with the Partial Wave Analysis (PWA) calculations by Bonn-Gatchina2019 [11,12] (blue solid lines), EtaMAID2018 [8,9] (green dashed lines), SAID2009 [10] (black dotted-dashed lines), and ANL-Osaka2016 [13,14] (magenta dotted lines).The Bonn-Gatchina2019 results reproduces the enhancement of differential cross sections at the backward angles, but the strength is larger than our results.The measured data are consistent with the EtaMAID2018 results in W < 2.2 GeV, while the EtaMAID2018 does not reproduce our data in cos θ η c.m. < −0.9 in W > 2.2 GeV.The SAID and ANL-Osaka calculations do not reproduce either the energy or angular dependence of our results.

Photon Beam Asymmetry
When the photon beam is linearly polarized, the differential cross section has azimuthal dependence and is presented by the following equation:  [15], CLAS [16], and CBELSA/TAPS [17] collaborations are shown as magenta squares, green triangles, and blue inverted triangles, respectively.The blue solid, green dashed, black dash-dotted, magenta dotted, and red dashed curves represent the partial wave analysis (PWA) results from the Bonn-Gatchina2019 [11], EtaMAID2018 [8], SAID2009 [10], ANL-Osaka [13], and Julich-Bonn [18] models, respectively where dσ 0 d is the differential cross section using unpolarized photon beam, P γ is the degree of linear polarization of the photon beam, is the photon beam asymmetry, and is the azimuthal angle between the linear polarization direction of the photon beam and the reaction plane in the γ p → ηp reaction.Since the variation of the differential cross section is proportional to P γ , a highly polarized photon beam is necessary to measure the photon beam asymmetries precisely.The backward Compton scattering photon is highly polarized in higher energy region, and this polarization degree reaches 90% at the Compton edge.
Figure 2 shows the photon beam asymmetries as a function of cos θ η c.m. measured by the LEPS2/BGOegg experiment and the other experiments (GRAAL, CLAS, and CBELSA/TAPS).The BGOegg results are shown by red circles with statistical uncertainties.The other experimental results from the GRAAL [15], CLAS [16], and CBELSA/TAPS [17] collaborations are shown by the magenta squares, green triangles, and blue inverted triangles.The experimental results are plotted using energy values that closely match the energy bins of the individual analyses.The results demonstrate statistical agreement with other experimental findings within the overlapping energy range below W = 2.1 GeV.A dip structure has been observed around cos θ η c.m. = −0.2 in W > 1.9 GeV.It has been suggested that this structure is caused by the helicity couplings for N (1720)3/2 + and N (1900)3/2 + [16].We precisely measure the values in −1.0 < cos θ η c.m. < 0.6 for the first time above W = 2.1 GeV. Figure 2 shows also the existing PWA results calculated by the EtaMAID2018 [8], Bonn-Gatchina2019 [11], SAID2009 [10], ANL-Osaka [13], and Jülich-Bonn [18] models.The ANL-Osaka and Jülich-Bonn results are limited to the total energy ranges below 1.95 and 2.1 GeV, respectively.The ANL-Osaka results do not reproduce the experimental data across all energy regions due to the absence of heavy-meson contributions in the coupled-channel calculation.The SAID calculations do not reproduce the dip structure above W = 1.90 GeV.The EtaMAID2018, Bonn-Gatchina2019, and Jülich-Bonn models agree with the results in W < 2.0 GeV.In the region above 2.0 GeV, in which the experimental data is scarce, no PWA calculations reproduce our experimental results and the discrepancy between the PWA results.Our data is useful to limit PWA calculations above W = 2.0 GeV.

Discussion
In Fig. 1, an enhancement is visible in the differential cross sections for the γ p → ηp reaction in cos θ η c.m. < −0.6 in 2.0 < W < 2.3 GeV.The possible cause of this enhancement is a resonance with high orbital angular momentum.The enhancement is found only in the η backward region.To explain this, a resonant state with high orbital angular momentum is required.The differential cross sections for π 0 and ω photoproduction were derived from the same data set utilized in the current analysis.These results are reported in Refs.[19,20], respectively.In Fig. 3, the energy dependence of the differential cross sections is depicted at cos θ c.m. = −0.95,−0.85, and −0.75 for the processes of η, π 0 and ω photoproduction.These measurements were conducted by the LEPS2/BGOegg collaboration.The differential cross section distributions of the π 0 photoproduction show a decreasing trend from 1.8 to 2.1 GeV with a small enhancement seen above 2.1 GeV at the angle cos θ π 0 c.m. = −0.95,but no visible bump structure is observed.The differential cross sections of ω photoproduction also lack any notable structures above 1.9 GeV.In contrast, only the η photoproduction exhibits a distinct bump structure at total energies exceeding 2.0 GeV.This observation can be attributed to the composition of the mesons within the framework of flavor SU(3) quark models.The η meson comprises an s s quark pair, while the π 0 and ω mesons solely consist of u ū and d d quark configurations.Consequently, the observed bump structure in the differential cross sections of η photoproduction is likely linked to nucleon resonances characterized by a significant s s component and a strong coupling to the ηN channel.

Summary
The measurements of differential cross sections and photon beam asymmetries were performed for the reaction γ p → ηp.The kinematic bins covered total energies ranging from 1.82 to 2.32 GeV and η polar angles within the range of −1.0 < cos θ η c.m. < 0.6.Within the cross sections, a distinctive bump structure emerges at W = 2.0-2.3GeV when cos θ η c.m. < −0.6, and its magnitude increases when the η particle is emitted at backward angles.Notably, this bump structure is absent in the differential cross sections of the reactions γ p → π 0 p and γ p → ωp.These findings suggest that the observed bump structure in the differential cross sections of η photoproduction is likely associated with nucleon resonances that possess a significant s s component and exhibit strong coupling to the ηN channel.Additionally, we have conducted measurements of the photon beam asymmetries for η photoproduction above W = 2.1 GeV, which have not been previously explored.None of the existing partial wave analysis (PWA) calculations reproduce our results in this higher energy range.These new findings provide further constraints for the comprehension of baryon resonances through PWAs.

Figure 1
Figure 1 shows the energy-dependent differential cross sections measured by the our experiment (LEPS2/BGOegg) and other experiments (LEPS, CLAS, and CBELSA/TAPS) for some specific cos θ η c.m. bins.The BGOegg results are shown by red circles with statistical uncertainties.The LEPS, CLAS, and CBELSA/TAPS results results are shown by magenta diamonds, green triangles, and blue inverted triangles.As shown in Fig. 1, we observe a bump structure in cos θ η c.m. < −0.6 in W > 2.0 GeV.The peak of this structure varies with the η emission angle.It is suggested the existence of multi-resonances contribution.Similar bump structures have been observed in all other experiments, but their strength and behaviors differ from each other.We compared our results with the Partial Wave Analysis (PWA) calculations by Bonn-Gatchina2019[11,12] (blue solid lines), EtaMAID2018[8,9] (green dashed lines), SAID2009 [10] (black dotted-dashed lines), and ANL-Osaka2016 [13,14] (magenta dotted lines).The Bonn-Gatchina2019 results reproduces the enhancement of differential cross sections at the backward angles, but the strength is larger than our results.The measured data are consistent with the EtaMAID2018 results in W < 2.2 GeV, while the EtaMAID2018 does not reproduce our data in cos θ

Fig. 2
Fig.2The photon beam asymmetries are plotted as a function of cos θ η c.m. for the reaction γ p → ηp.The results obtained by the BGOegg collaboration are shown as red circles, with statistical uncertainties indicated.The associated systematic uncertainties are represented by gray histograms.Other experimental results from the GRAAL[15], CLAS[16], and CBELSA/TAPS[17] collaborations are shown as magenta squares, green triangles, and blue inverted triangles, respectively.The blue solid, green dashed, black dash-dotted, magenta dotted, and red dashed curves represent the partial wave analysis (PWA) results from the Bonn-Gatchina2019[11], EtaMAID2018[8], SAID2009 [10], ANL-Osaka [13], and Julich-Bonn[18] models, respectively