Abstract
Measurements of the linearly-polarized photon beam asymmetry Σ for photoproduction from the proton of η and η′ mesons are reported. A linearly-polarized tagged photon beam produced by coherent bremsstrahlung was incident on a cryogenic hydrogen target within the CEBAF Large Acceptance Spectrometer. Results are presented for the γp→ηp reaction for incident photon energies from 1.070 to 1.876 GeV, and from 1.516 to 1.836 GeV for the γp→η′p reaction. For γp→ηp, the data reported here considerably extend the range of measurements to higher energies, and are consistent with the few previously published measurements for this observable near threshold. For γp→η′p, the results obtained are consistent with the few previously published measurements for this observable near threshold, but also greatly expand the incident photon energy coverage for that reaction. Initial analysis of the data reported here with the Bonn–Gatchina model strengthens the evidence for four nucleon resonances – the N(1895)1/2−, N(1900)3/2+, N(2100)1/2+ and N(2120)3/2− resonances – which presently lack the “four-star” status in the current Particle Data Group compilation, providing examples of how these new measurements help refine models of the photoproduction process.
Highlights
Much effort in nuclear physics at present is aimed at obtaining a quantum-chromodynamic description of the nucleon in terms of its quark constituents
The results presented here for the photon beam asymmetry provide a check on prior measurements for both these reactions, and extend the measurements of to considerably higher energies than previously reported, thereby providing access with this observable to the details of higher-lying resonances
The photon beam asymmetry results obtained here for η and η are shown in Figs. 2 to 5
Summary
Much effort in nuclear physics at present is aimed at obtaining a quantum-chromodynamic description of the nucleon in terms of its quark constituents. Our current knowledge of nucleon resonances [1,2,3] has come from analyses of the results of experiments primarily with π N, ηN, K , and K final states. These analyses have identified (with varying degrees of certainty) a large number of excited states over the past several decades To better isolate specific contributions to the nucleon excitation spectrum, studies using the electromagnetic interaction have proven to be powerful, since the features of that interaction are well understood in terms of quantum electrodynamics and since photons potentially might have large couplings to resonances that have escaped detection in previous analyses of reactions using pion beams. The reactions γ p → ηp and γ p → η p have been seen to be quite advantageous in probing the nucleon since those reactions provide an “isospin filter” on the nucleon resonance spectrum: the final states ηp and η p can only be accessed in one-step decays of isospin
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