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Register a new userFirst Measurement of the $Sigma$ Beam Asymmetry in $eta^{prime}$ Photoproduction off the Proton near Threshold
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The $Sigma$ beam asymmetry in $eta^{prime}$ photoproduction off the proton was measured at the GrAAL polarised photon beam with incoming photon energies of 1.461 and 1.480 GeV. For both energies the asymmetry as a function of the meson production angle shows a clear structure, more pronounced at the lowest one, with a change of sign around 90$^o$. The observed behaviour is compatible with P-wave D-wave (or S-wave F-wave) interference, the closer to threshold the stronger. The results are compared to the existing state-of-the-art calculations that fail to account for the data.
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Measurements of the linearly-polarized photon beam asymmetry $Sigma$ for photoproduction from the proton of $eta$ and $eta^prime$ 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 $gamma p to eta p$ reaction for incident photon energies from 1.070 to 1.876 GeV, and from 1.516 to 1.836 GeV for the $gamma p to eta^prime p$ reaction. For $gamma p to eta p$, 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 $gamma p to eta^prime p$, 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.
We report on the measurement of the $gamma p rightarrow J/psi p$ cross section from $E_gamma = 11.8$ 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 $dsigma /dt$ has an exponential slope of $1.67 pm 0.39$ 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 $mathcal{B}(P_c^+ rightarrow J/psi p)$ and cross sections $sigma(gamma p to P_c^+)timesmathcal{B}(P_c^+ to J/psi p) $.
Results are presented for the first measurement of the double-polarization helicity asymmetry E for the $eta$ photoproduction reaction $gamma p rightarrow eta p$. Data were obtained using the FROzen Spin Target (FROST) with the CLAS spectrometer in Hall B at Jefferson Lab, covering a range of center-of-mass energy W from threshold to 2.15 GeV and a large range in center-of-mass polar angle. As an initial application of these data, the results have been incorporated into the Julich model to examine the case for the existence of a narrow $N^*$ resonance between 1.66 and 1.70 GeV. The addition of these data to the world database results in marked changes in the predictions for the E observable using that model. Further comparison with several theoretical approaches indicates these data will significantly enhance our understanding of nucleon resonances.
We report measurements of the photon beam asymmetry $Sigma$ for the reactions $vec{gamma}pto ppi^0$ and $vec{gamma}pto peta $ from the GlueX experiment using a 9 GeV linearly-polarized, tagged photon beam incident on a liquid hydrogen target in Jefferson Labs Hall D. The asymmetries, measured as a function of the proton momentum transfer, possess greater precision than previous $pi^0$ measurements and are the first $eta$ 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.
The $Sigma$ beam asymmetry in the photoproduction of negative pions from quasi-free neutrons in a deuterium target was measured at Graal in the energy interval 700 - 1500 MeV and a wide angular range, using polarized and tagged photons. The results are compared with recent partial wave analyses.
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