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The spin-exotic hybrid meson \\(\\pi_{1}(1600)\\) is predicted to have a large decay rate to the \\(\\omega\\pi\\pi\\) final state. Using 76.6~pb\\(^{-1}\\) of data collected with the GlueX detector, we measure the cross sections for the reactions \\(\\gamma p \\to \\omega \\pi^+ \\pi^- p\\), \\(\\gamma p \\to \\omega \\pi^0 \\pi^0 p\\), and \\(\\gamma p\\to\\omega\\pi^-\\pi^0\\Delta^{++}\\) in the range \\(E_\\gamma =\\) 8-10 GeV. Using isospin conservation, we set the first upper limits on the photoproduction cross sections of the \\(\\pi^{0}_{1}(1600)\\) and \\(\\pi^{-}_{1}(1600)\\). We combine these limits with lattice calculations of decay widths and find that photoproduction of \\(\\eta'\\pi\\) is the most sensitive two-body system to search for the \\(\\pi_1(1600)\\).

The Compton double-polarization observable \\(\\Sigma_{2z}\\) has been measured for the first time in the \\(\\Delta(1232)\\) resonance region using a circularly polarized photon beam incident on a longitudinally polarized target at the Mainz Microtron. This paper reports these results, together with the model-dependent extraction of four proton spin polarizabilities from fits to additional asymmetry data using dispersion relation and chiral perturbation theory calculations, with the former resulting in: \\(\\gamma_{E1E1} = -3.18 \\pm 0.52\\), \\(\\gamma_{M1M1} = 2.98 \\pm 0.43\\), \\(\\gamma_{E1M2} = -0.44 \\pm 0.67\\) and \\(\\gamma_{M1E2} = 1.58 \\pm 0.43\\), in units of \\(10^{-4}~\\mathrm{fm}^{4}\\).

We measure the spin-density matrix elements (SDMEs) of the \\(\\Delta^{++}(1232)\\) in the photoproduction reaction \\(\\gamma p \\to \\pi^-\\Delta^{++}(1232)\\) with the GlueX experiment in Hall D at Jefferson Lab. The measurement uses a linearly--polarized photon beam with energies from \\(8.2\\) to \\(8.8\\)~GeV and the statistical precision of the SDMEs exceeds the previous measurement by three orders of magnitude for the momentum transfer squared region below \\(1.4\\) GeV\\(^2\\). The data are sensitive to the previously undetermined relative sign between couplings in existing Regge-exchange models. Linear combinations of the extracted SDMEs allow for a decomposition into natural and unnatural--exchange amplitudes. We find that the unnatural exchange plays an important role in the low momentum transfer region.

A new data set for the helicity-dependent differential cross section of the single-meson photoproduction reaction \\(\\gamma p \\to p \\pi^{0}\\) was obtained for the photon energy interval 150-400 MeV. The experiment was performed at the A2 tagged photon facility of the Mainz Microtron MAMI using a circularly polarized photon beam and a longitudinally polarized proton target. The reaction products were detected with the large acceptance Crystal Ball/TAPS calorimeter covering 97\\% of the full solid angle. These new results, obtained with a fine energy and polar angle binning, greatly increase both the existing quantity and quality of the data available for this observable. A moment analysis, based on a finite expansion in Legendre polynomials, was applied to these data by using a bootstrap-based fitting method to correctly account for their systematic uncertainties. From the resulting decomposition of the differential cross sections, the \\(E2/M1\\) ratio for the \\(N\\to \\Delta(1232)\\) transition was determined to be \\([-2.38 \\pm 0.16{\\hbox{ (stat.+sys.)}} \\pm 0.10 {\\hbox{ (model)}}] \\%\\). Combining this value with previous results also allowed us to evaluate the most precise available estimate of the \\(E2/M1\\) ratio to be used for all further reference and model comparisons.

We report the total and differential cross sections for \\(J/\\psi\\) photoproduction with the large acceptance GlueX spectrometer for photon beam energies from the threshold at 8.2~GeV up to 11.44~GeV and over the full kinematic range of momentum transfer squared, \\(t\\). Such coverage facilitates the extrapolation of the differential cross sections to the forward (\\(t = 0\\)) point beyond the physical region. The forward cross section is used by many theoretical models and plays an important role in understanding \\(J/\\psi\\) photoproduction and its relation to the \\(J/\\psi-\\)proton interaction. These measurements of \\(J/\\psi\\) photoproduction near threshold are also crucial inputs to theoretical models that are used to study important aspects of the gluon structure of the proton, such as the gluon Generalized Parton Distribution (GPD) of the proton, the mass radius of the proton, and the trace anomaly contribution to the proton mass. We observe possible structures in the total cross section energy dependence and find evidence for contributions beyond gluon exchange in the differential cross section close to threshold, both of which are consistent with contributions from open-charm intermediate states.

The GlueX experiment at Jefferson Lab studies photoproduction of mesons using linearly polarized \\(8.5\\,\\text{GeV}\\) photons impinging on a hydrogen target which is contained within a detector with near-complete coverage for charged and neutral particles. We present measurements of spin-density matrix elements for the photoproduction of the vector meson \\(\\rho\\)(770). The statistical precision achieved exceeds that of previous experiments for polarized photoproduction in this energy range by orders of magnitude. We confirm a high degree of \\(s\\)-channel helicity conservation at small squared four-momentum transfer \\(t\\) and are able to extract the \\(t\\)-dependence of natural and unnatural-parity exchange contributions to the production process in detail. We confirm the dominance of natural-parity exchange over the full \\(t\\) range. We also find that helicity amplitudes in which the helicity of the incident photon and the photoproduced \\(\\rho(770)\\) differ by two units are negligible for \\(-t<0.5\\,\\text{GeV}^{2}/c^{2}\\).

Photoproduction of \\(\\pi^0\\pi^{\\pm}\\)-pairs from quasifree nucleons bound in the deuteron has been investigated to study the helicity dependence of this reaction. Measurements with a liquid deuterium target were used to extract the unpolarized cross sections for reactions on protons and neutrons. A deuterated, longitudinally polarized solid-butanol target, together with a circularly polarized photon beam, determined the double polarization observable \\(E\\). From these results the spin-dependent cross sections \\(\\sigma_{1/2}\\) and \\(\\sigma_{3/2}\\), corresponding to the anti-parallel and parallel spin configurations of the beam photon and target nucleon, have been derived. The measurements were performed at the Mainz MAMI accelerator with tagged, circularly-polarized photon beams produced via bremsstrahlung from longitudinally polarized electron beams. The reaction products were detected with an almost \\(4\\pi\\) solid-angle covering calorimeter composed of the Crystal Ball and TAPS detectors, supplemented by plastic scintillation detectors for charged particle identification. The results are sensitive to sequential decays of nucleon resonances via intermediate states and also to the decay of nucleon resonances by emission of charged \\(\\rho\\) mesons, and are compared to recent model results.

We report the measurement of the helicity asymmetry \\(E\\) for the \\(p\\pi^0\\) and \\(n\\pi^+\\) final states using, for the first time, an elliptically polarized photon beam in combination with a longitudinally polarized target at the Crystal Ball experiment at MAMI. The results agree very well with data that were taken with a circularly polarized photon beam, showing that it is possible to simultaneously measure polarization observables that require linearly (e.g.~\\(G\\)) and circularly polarized photons (e.g.~\\(E\\)) and a longitudinally polarized target. The new data cover a photon energy range 270 - 1400 MeV for the \\(p\\pi^0\\) final state (230 - 842 MeV for the \\(n\\pi^+\\) final state) and the full range of pion polar angles, \\(\\theta\\), providing the most precise measurement of the observable \\(E\\). A moment analysis gives a clear observation of the \\(p\\eta\\) cusp in the \\(p\\pi^0\\) final state.

A first measurement of the polarisation transfer from a circularly-polarised photon to the final state neutron (\\(C^n_{x'}\\)) in deuterium photodisintegration has been carried out. This quantity is determined over the photon energy range 370~--~700~MeV and for neutron centre-of-mass breakup angles \\(\\sim45-120^{\\circ}\\). The polarisation of the final state neutrons was determined by an ancillary large-acceptance nucleon polarimeter, surrounding a cryogenic liquid deuterium target within the Crystal Ball detector at MAMI. The polarimeter characterised \\((n,p)\\) charge exchange of the ejected neutrons to determine their polarisation. The new \\(C^n_{x'}\\) data are also compared to a theoretical model based on nucleonic and nucleon resonance degrees of freedom constrained by the current world-database of deuterium photodisintegration measurements. Structures in \\(C^n_{x'}\\) observed in the region of the \\(d^*(2380)\\) could not be explained by conventional models of deuteron photodisintegration.

We report a first measurement of the double-polarisation observable, \\(C_{x'}\\), in \\(\\pi^+\\) photoproduction off the proton. The \\(C_{x'}\\) double-polarisation observable represents the transfer of polarisation from a circularly polarised photon beam to the recoiling neutron. The MAMI circularly polarised photon beam impinged on a liquid deuterium target cell, with reaction products detected in the Crystal Ball calorimeter. Ancillary apparatus surrounding the target provided tracking, particle identification and determination of recoil nucleon polarisation. The \\(C_{x'}\\) observable is determined for photon energies 800-1400 MeV, providing new constraints on models aiming to elucidate the spectrum and properties of nucleon resonances. This is the first determination of any polarisation observable from the beam-recoil group of observables for this reaction, providing a valuable constraint and systematic check of the current solutions of partial wave analysis based theoretical models.