We report on the measurement of the γp → J/ψp cross section from Eγ = 11.8 GeV down to the threshold at 8.2 GeV using a tagged photon beam with the GlueX experiment. We find the total cross section falls toward the threshold less steeply than expected from two-gluon exchange models. The differential cross section dσ/dt has an exponential slope of 1.67 ± 0.39 GeV −2 at 10.7 GeV average energy. The LHCb pentaquark candidates P + c can be produced in the s-channel of this reaction. We see no evidence for them and set model-dependent upper limits on their branching fractions B(P + c → J/ψp).

Differential and total cross sections for the quasifree reactions γp → ηp and γn → ηn have been determined at the MAMI-C electron accelerator using a liquid deuterium target. Photons were produced via bremsstrahlung from the 1.5 GeV incident electron beam and energy-tagged with the Glasgow photon tagger. Decay photons of the neutral decay modes η → 2γ and η → 3π 0 → 6γ and coincident recoil nucleons were detected in a combined setup of the Crystal Ball and the TAPS calorimeters. The η-production cross sections were measured in coincidence with recoil protons, recoil neutrons, and in an inclusive mode without a condition on recoil nucleons, which allowed a check of the internal consistency of the data. The effects from nuclear Fermi motion were removed by a kinematic reconstruction of the final-state invariant mass and possible nuclear effects on the quasifree cross section were investigated by a comparison of free and quasifree proton data. The results, which represent a significant improvement in statistical quality compared to previous measurements, agree with the known neutron-to-proton cross-section ratio in the peak of the S11(1535) resonance and confirm a peak in the neutron cross section, which is absent for the proton, at a center-of-mass energy W = (1670 ± 5) MeV with an intrinsic width of Γ ≈ 30 MeV.

The recent precise experimental results for the photoproduction of η-mesons off the neutron measured with the Crystal Ball/TAPS calorimeter at the MAMI accelerator have been investigated in detail in the framework of the Bonn-Gatchina coupled channel model. The main result is that the narrow structure observed in the excitation function of γn → nη can be reproduced fully with a particular interference pattern in the J P = 1/2 − partial wave. Introduction of the narrow resonance N (1685) with the properties reported in earlier publications deteriorates the quality of the fit.

Coherent photoproduction of η-mesons off 3 He, i.e. the reaction γ 3 He → η 3 He, has been investigated in the near-threshold region. The experiment was performed at the Glasgow tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball -TAPS detector. Angular distributions and the total cross section were measured using the η → γγ and η → 3π 0 → 6γ decay channels. The observed extremely sharp rise of the cross section at threshold and the behavior of the angular distributions are evidence for a strong η 3 He final state interaction, pointing to the existence of a resonant state. The search for further evidence of this state in the excitation function of π 0 -proton back-to-back emission in the γ 3 He → π 0 pX reaction revealed a very complicated structure of the background and could not support previous conclusions.

We report measurements of the photon beam asymmetry Σ for the reactions γp → pπ 0 and γp → pη from the GLUEX experiment using a 9 GeV linearly-polarized, tagged photon beam incident on a liquid hydrogen target in Jefferson Lab's Hall D. The asymmetries, measured as a function of the proton momentum transfer, possess greater precision than previous π 0 measurements and are the first η measurements in this energy regime. The results are compared with theoretical predictions based on t-channel, quasi-particle exchange and constrain the axial-vector component of the neutral meson production mechanism in these models.