. 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.

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

Information on the size and shape of the neutron skin on (208)Pb is extracted from coherent pion photoproduction cross sections measured using the Crystal Ball detector together with the Glasgow tagger at the MAMI electron beam facility. On exploitation of an interpolated fit of a theoretical model to the measured cross sections, the half-height radius and diffuseness of the neutron distribution are found to be c(n)=6.70±0.03(stat.)  fm and a(n)=0.55±0.01(stat.)(-0.03)(+0.02)(sys.)  fm, respectively, corresponding to a neutron skin thickness Δr(np)=0.15±0.03(stat.)(-0.03)(+0.01)(sys.)  fm. The results give the first successful extraction of a neutron skin thickness with an electromagnetic probe and indicate that the skin of (208)Pb has a halo character. The measurement provides valuable new constraints on both the structure of nuclei and the equation of state for neutron-rich matter.

. Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of mixed-charge pion pairs in the reactions γ p→ nπ^0π^+ off free protons and γ d → (p)pπ^0π^- and γ d→ (n)nπ^0π^+ off quasi-free nucleons bound in the deuteron for incident photon energies up to 1.4GeV. Circularly polarized photons were produced from bremsstrahlung of longitudinally polarized electrons and tagged with the Glasgow-Mainz magnetic spectrometer. The charged pions, recoil protons, recoil neutrons, and decay photons from π^0 mesons were detected in the 4π electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. Using a complete kinematic reconstruction of the final state, excellent agreement was found between the results for free and quasi-free protons, suggesting that the quasi-free neutron results are also a close approximation of the free-neutron asymmetries. A comparison of the results to the predictions of the Two-Pion-MAID reaction model shows that the reaction mechanisms are still not well understood, in particular at low incident photon energies in the second nucleon-resonance region.

Differential and total cross sections for the quasifree reactions gamma p -> eta p and gamma n -> eta n have been determined at the MAMI-C electron accelerator by 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 eta -> 2 gamma and eta -> 3 pi(0) -> 6 gamma and coincident recoil nucleons were detected in a combined setup of the Crystal Ball and the TAPS calorimeters. The eta-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 S-11(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 Gamma approximate to 30 MeV.

The Dalitz decay eta -> e(+) e(-) gamma has been measured in the gamma p -> eta p reaction with the Crystal Ball and TAPS multiphoton spectrometers, together with the photon-tagging facility at the Mainz Microtron MAMI. The experimental statistic used in this work is one order of magnitude greater than in any previous measurement of eta -> e(+) e(-) gamma. The value obtained for the slope parameter Lambda(-2) of the eta transition form factor, Lambda(-2) = (1.95 +/- 0.15(stat) +/- 0.10(syst)) GeV-2, is in good agreement with recent measurements conducted in eta -> e(+) e(-) gamma and eta -> mu(+) mu(-) gamma decays, as well as with recent form-factor calculations. The uncertainty obtained in the value of Lambda(-2) is lower compared to results from previous measurements of the eta -> e(+) e(-) gamma decay.

A new precise determination of the \( \eta\) meson mass is presented. It is based on a measurement of the threshold for the \( \gamma p\rightarrow p\eta\) reaction using the tagger focal-plane microscope detector at the MAMI-B facility in Mainz. The tagger microscope has a higher energy resolution than the standard tagging spectrometer and, hence, allowed an improvement in the accuracy compared to the previous \( \eta\) mass measurement at MAMI-B. Special emphasis was put on a very careful energy calibration of the electron beam and the tagging device, which reduced considerably the systematic uncertainty compared to the previous MAMI experiment. The result \( m_{\eta}=(547.865 \pm 0.031_{stat.}\pm 0.062_{syst.})\) MeV agrees very well with the precise values of the NA48, KLOE, CLEO and COSY-ANKE Collaborations and deviates by \( 6\sigma\) from the smaller value obtained by the GEM Collaboration at COSY.