The polarization observables T, E, P, H, and G in photoproduction of η mesons off protons are measured for photon energies from threshold to W=2400MeV (T), 2280 MeV (E), 1620 MeV (P, H), or 1820 MeV (G), covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, JüBo, and BnGa partial-wave analyses. A refit within the BnGa approach including further data yields precise branching ratios for the Nη decay of nucleon resonances. A Nη-branching ratio of 0.33± 0.04 for N(1650)1/2^- is found, which reduces the large and controversially discussed Nη-branching ratio difference of the two lowest mass J^P=1/2^--resonances significantly.

The polarization observables T, E, P, H, and G in photoproduction of eta mesons off protons are measured for photon energies from threshold to W = 2400 MeV (T), 2280 MeV (E), 1620 MeV (P, H), or 1820 MeV (G), covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, JuBo, and BnGa partial-wave analyses. A refit within the BnGa approach including further data yields precise branching ratios for the N eta decay of nucleon resonances. A N eta-branching ratio of 0.33 +/- 0.04 for N(1650)1/2(-) is found, which reduces the large and controversially discussed N eta-branching ratio difference of the two lowest mass J(P) = 1/2(-) -resonances significantly. (C) 2020 The Authors. Published by Elsevier B.V.

. High-statistics measurements of the photon asymmetry Σ for the γp→π^0p reaction have been made in the center-of-mass energy range W=1214-1450 MeV. The data were measured with the MAMI A2 real photon beam and Crystal Ball/TAPS detector systems in Mainz, Germany. The results significantly improve the existing world data and are shown to be in good agreement with previous measurements, and with the MAID, SAID, and Bonn-Gatchina predictions. We have also combined the photon asymmetry results with recent cross-section measurements from Mainz to calculate the profile functions, Σ̌ (= σ_0Σ) , and perform a moment analysis. Comparison with calculations from the Bonn-Gatchina model shows that the precision of the data is good enough to further constrain the higher partial waves, and there is an indication of interference between the very small F -waves and the N(1520) 3/2^- and N(1535) 1/2^- resonances.

High-statistics measurements of the photon asymmetry for the reaction have been made in the center-of-mass energy range MeV. The data were measured with the MAMI A2 real photon beam and Crystal Ball/TAPS detector systems in Mainz, Germany. The results significantly improve the existing world data and are shown to be in good agreement with previous measurements, and with the MAID, SAID, and Bonn-Gatchina predictions. We have also combined the photon asymmetry results with recent cross-section measurements from Mainz to calculate the profile functions, , and perform a moment analysis. Comparison with calculations from the Bonn-Gatchina model shows that the precision of the data is good enough to further constrain the higher partial waves, and there is an indication of interference between the very small F -waves and the and resonances.

Total cross sections, angular distributions, and invariant-mass distributions have been measured for the photoproduction of \( \pi^{0}\pi^{0}\) pairs off free protons and off nucleons bound in the deuteron. The experiments were performed at the MAMI accelerator facility in Mainz using the Glasgow photon tagging spectrometer and the Crystal Ball/TAPS detector. The accelerator delivered electron beams of 1508 and 1557MeV, which produced bremsstrahlung in thin radiator foils. The tagged photon beam covered energies up to 1400MeV. The data from the free proton target are in good agreement with previous measurements and were only used to test the analysis procedures. The results for differential cross sections (angular distributions and invariant-mass distributions) for free and quasi-free protons are almost identical in shape, but differ in absolute magnitude up to 15%. Thus, moderate final-state interaction effects are present. The data for quasi-free neutrons are similar to the proton data in the second resonance region (final-state invariant masses up to \( \approx 1550\) MeV), where both reactions are dominated by the \( N(1520)3/2^{-} \rightarrow \Delta(1232)3/2^{+}\pi\) decay. At higher energies, angular and invariant-mass distributions are different. A simple analysis of the shapes of the invariant-mass distributions in the third resonance region is consistent with strong contributions of an \( N^{\star}\rightarrow N\sigma\) decay for the proton, while the reaction is dominated by a sequential decay via a \( \Delta\pi\) intermediate state for the neutron. The data are compared to predictions from the Two-Pion-MAID model and the Bonn-Gatchina coupled-channel analysis.

Photoproduction of πη-pairs from nucleons has been investigated from threshold up to incident photon energies of ≈ 1.4 GeV. The quasi-free reactions γp→pπ0η, γn→nπ0η, γp→nπ+η, and γn→pπ−η were for the first time measured from nucleons bound in the deuteron. The corresponding reactions from a free-proton target were also studied to investigate final-state interaction effects (for neutral pions the free-proton results could be compared to previous measurements; the γp→nπ+η reaction was measured for the first time). For the π0η final state coherent production via the γd→dπ0η reaction was also investigated. The experiments were performed at the tagged photon beam of the Mainz MAMI accelerator using an almost 4π coverage electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. The total cross sections for the four different final states obey the relation σ(pπ0η)≈σ(nπ0η)≈2σ(pπ−η)≈2σ(nπ+η) as expected for a dominant contribution from a Δ⋆→ηΔ(1232)→πηN reaction chain, which is also supported by the shapes of the invariant-mass distributions of nucleon–meson and π–η pairs. The experimental results are compared to the predictions from an isobar reaction model.

An experimental study of ω photoproduction on the proton was conducted by using the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. The γp→ωp differential cross sections are measured from threshold to the incident-photon energy Eγ=1.40 GeV (W=1.87 GeV for the center-of-mass energy) with 15-MeV binning in Eγ and full production-angle coverage. The quality of the present data near threshold gives access to a variety of interesting physics aspects. As an example, an estimation of the ωN scattering length αωp is provided.

Data on the polarization observables T, P, and H for the reaction gamma p -> p pi(0) are reported. Compared to earlier data from other experiments, our data are more precise and extend the covered range in energy and angle substantially. The results were extracted from azimuthal asymmetries measured using a transversely polarized target and linearly polarized photons. The data were taken at the Bonn electron stretcher accelerator ELSA with the CBELSA/TAPS detector. Within the Bonn-Gatchina partial wave analysis, the new polarization data lead to a significant narrowing of the error band for the multipoles for neutral-pion photoproduction. (C) 2015 The Authors. Published by Elsevier B.V.

Data on the polarization observables T, P, and H for the reaction γp→pπ0 are reported. Compared to earlier data from other experiments, our data are more precise and extend the covered range in energy and angle substantially. The results were extracted from azimuthal asymmetries measured using a transversely polarized target and linearly polarized photons. The data were taken at the Bonn electron stretcher accelerator ELSA with the CBELSA/TAPS detector. Within the Bonn-Gatchina partial wave analysis, the new polarization data lead to a significant narrowing of the error band for the multipoles for neutral-pion photoproduction.

An experimental study of omega photoproduction on the proton was conducted by using the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. The gamma p -> omega p differential cross sections are measured from threshold to the incident-photon energy E-gamma = 1.40 GeV (W = 1.87 GeV for the center-of-mass energy) with 15-MeV binning in E-gamma and full production-angle coverage. The quality of the present data near threshold gives access to a variety of interesting physics aspects. As an example, an estimation of the omega N scattering length alpha(omega p) is provided.

We report the first large-acceptance measurement of polarization transfer from a polarized photon beam to a recoiling nucleon. The measurement pioneers a novel polarimetry technique, which can be applied to many other nuclear and hadron physics experiments. The commissioning reaction of 1H(γ, p)π0 in the range 0.4<Eγ<1.4  GeV validates the technique and provides essential new data to constrain the excitation spectrum of the nucleon.

We report the first large-acceptance measurement of polarization transfer from a polarized photon beam to a recoiling nucleon. The measurement pioneers a novel polarimetry technique, which can be applied to many other nuclear and hadron physics experiments. The commissioning reaction of 1H((gamma) over bar, (p) over bar)pi(0) in the range 0.4 < E-gamma < 1.4 GeV validates the technique and provides essential new data to constrain the excitation spectrum of the nucleon.

Citation for published version: Sikora, MH, Watts, DP, Glazier, DI, Aguar-Bartolome, P, Akasoy, LK, Annand, JRM, Arends, HJ, Bantawa, K, Beck, R, Bekrenev, VS, Berghaeuser, H, Braghieri, A, Branford, D, Briscoe, WJ, Brudvik, J, Cherepnya, S, Codling, RFB, Demissie, BT, Downie, EJ, Drexler, P, Fil'kov, LV, Freehart, B, Gregor, R, Hamilton, D, Heid, E, Hornidge, D, Howdle, DA, Jaegle, I, Jahn, O, Jude, TC, Kashevarov, VL, Keshelashvili, I, Kondratiev, R, Korolija, M, Kotulla, M, Koulbardis, AA, Kruglov, SP, Krusche, B, Lisin, V, Livingston, K, MacGregor, IJD, Maghrbi, Y, Manley, DM, Marinides, Z, Martinez, M, McGeorge, JC, McKinnon, B, McNicoll, EF, Mekterovic, D, Metag, V, Micanovic, S, Middleton, DG, Mushkarenkov, A, Nefkens, BMK, Nikolaev, A, Novotny, R, Ostrick, M, Otte, PB, Oussena, B, Pedroni, P, Pheron, F, Polonski, A, Prakhov, S, Robinson, J, Rosner, G, Rostomyan, T, Schumann, S, Sober, DI, Starostin, A, Strakovsky, II, Suarez, IM, Supek, I, Thiel, M, Thomas, A, Unverzagt, M, Werthmueller, D, Workman, RL, Zamboni, I, Zehr, F & A2 Collaboration MAMI 2014, 'Measurement of the H-1((gamma)over-right-arrow,(p)over-right-arrow)pi(0) Reaction Using a Novel Nucleon Spin Polarimeter' Physical Review Letters, vol 112, no. 2, 022501., 10.1103/PhysRevLett.112.022501

The helicity dependences of the differential cross sections for the semi-inclusive γ He → πX and γ He → πX reactions have been measured for the first time in the energy region 200 < Eγ < 450 MeV. The experiment was performed at the tagged photon beam facility of the MAMI accelerator in Mainz using a longitudinally polarised high-pressure He gas target. Hadronic products were measured with the large acceptance Crystal Ball detector complemented with additional devices for charged particle tracking and identification. Unpolarised differential cross sections and their helicity dependence are compared with theoretical calculations using the Fix-Arenhövel model. The effect of the intermediate excitation of the ∆(1232) resonance can be clearly seen from this comparison, especially for the polarised case, where nuclear effects are relatively small. The model provides a better theoretical description of the unpolarised charged pion photoproduction data than the neutral pion channel. It does significantly better in describing the helicity-dependent data in both channels. These comparisons provide new information on the mechanisms involved in pion photoproduction on He and suggest that a polarised He target can provide valuable information on the corresponding polarised quasi-free neutron reactions. PACS. 13.60.Le Meson production – 14.20.Gk Baryon resonances with S = 0 – 25.20.Lj Photoproduction reactions 2 S. Costanza et al.: Helicity dependence of the γ He → πX reactions in the ∆(1232) resonance region