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Background: Photoproduction of pion pairs allows to study sequential decays of nucleon resonances via excited intermediate states. Such decays are important e.g. for states which in the quark model have both oscillators excited and de-excite them in a two-step process. However, analyses of multi-meson final states is difficult and requires more than unpolarized cross section measurements. Purpose: Experimental study and model analysis in view of resonant contributions of target and beam-target polarization observables for the reaction \\(\\gamma p \\to \\pi^0 \\pi^0 p\\). Methods: Investigated were target (single) and beam-target (double) polarization asymmetries in dependence of several parameters. The experiments were performed at the Mainz Microtron (MAMI) using circularly polarized photon beams and transversally polarized solid-state butanol targets. The reaction products were analyzed with the Crystal Ball and TAPS detectors. Results: Studied were the polarization observables Py (unpolarized beam, target polarized in y direction) and Px (circularly polarized beam, target polarized in x direction), which are similar to T (target asymmetry) and F (beam-target asymmetry) for single pion production. The asymmetries were analyzed with three independent methods, revealing systematic uncertainties.

The double-polarization observable \\(E\\) and helicity-dependent cross sections \\(\\sigma_{1/2}\\), \\(\\sigma_{3/2}\\) have been measured for the photoproduction of \\(\\pi^0\\) pairs off quasi-free protons and neutrons at the Mainz MAMI accelerator with the Crystal Ball/TAPS setup. A circularly polarized photon beam was produced by bremsstrahlung from longitudinally polarized electrons and impinged on a longitudinally polarized deuterated butanol target. The reaction products were detected with an almost \\(4\\pi\\) covering calorimeter. The results reveal for the first time the helicity- and isospin-dependent structure of the \\(\\gamma N\\rightarrow N\\pi^0\\pi^0\\) reaction. They are compared to predictions from reaction models in view of nucleon resonance contributions and also to a refit of one model that predicted results for the proton and for the neutron target. The comparison of the prediction and the refit demonstrate the large impact of the new data.

We report a measurement of the spin polarisation of the recoiling neutron in deuterium photodisintegration, utilising a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured photon energy range of 300~--~700~MeV provides the first measurement of recoil neutron polarisation at photon energies where the quark substructure of the deuteron plays a role, thereby providing important new constraints on photodisintegration mechanisms. A very high neutron polarisation in a narrow structure centred around \\(E_{\\gamma}\\sim\\)~570~MeV is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition suggests this behaviour could be related to the excitation of the \\(d^*(2380)\\) hexaquark.

The \\(\\gamma n \\to \\pi^0 n\\) differential cross section evaluated for 27 energy bins span the photon-energy range 290-813 MeV (W = 1.195-1.553 GeV) and the pion c.m. polar production angles, ranging from 18 deg to 162 deg, making use of model-dependent nuclear corrections to extract pi0 production data on the neutron from measurements on the deuteron target. Additionally, the total photoabsorption cross section was measured. The tagged photon beam produced by the 883-MeV electron beam of the Mainz Microtron MAMI was used for the 0-meson production. Our accumulation of 3.6 x 10^6 \\(\\gamma n \\to \\pi^0 n\\) events allowed a detailed study of the reaction dynamics. Our data are in reasonable agreement with previous A2 measurements and extend them to lower energies. The data are compared to predictions of previous SAID, MAID, and BnGa partial-wave analyses and to the latest SAID fit MA19 that included our data. Selected photon decay amplitudes \\(N^* \\to \\gamma n\\) at the resonance poles are determined for the first time.

The spin polarizabilities of the nucleon describe how the spin of the nucleon responds to an incident polarized photon. The most model-independent way to measure the nucleon spin polarizabilities is through polarized Compton scattering. Double-polarized Compton scattering asymmetries on the proton were measured in the \\(\\Delta(1232)\\) region using circularly polarized incident photons and a transversely polarized proton target at the Mainz Microtron. Fits to asymmetry data were performed using a dispersion model calculation and a baryon chiral perturbation theory calculation, and a separation of all four proton spin polarizabilities in the multipole basis was achieved. The analysis based on a dispersion model calculation yields \\(\\gamma_{E1E1} = -3.5 \\pm 1.2\\), \\(\\gamma_{M1M1}= 3.16 \\pm 0.85\\), \\(\\gamma_{E1M2} = -0.7 \\pm 1.2\\), and \\(\\gamma_{M1E2} = 1.99 \\pm 0.29\\), in units of \\(10^{-4}\\) fm\\(^4\\).

Photoproduction of mesons off quasi-free nucleons bound in the deuteron allows to study the electromagnetic excitation spectrum of the neutron and the isospin structure of the excitation of nucleon resonances. The database for such reactions is much more sparse than for free proton targets. Single \\(\\pi^0\\) photoproduction off quasi-free nucleons from the deuteron was experimentally studied. Nuclear effects were investigated by a comparison of the results for free protons and quasi-free protons and used as a correction for the quasi-free neutron data. The experiment was performed at the tagged photon beam of the Mainz MAMI accelerator for photon energies between 0.45~GeV and 1.4~GeV, using an almost \\(4\\pi\\) electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. A complete kinematic reconstruction of the final state removed the effects of Fermi motion. Reaction model predictions and PWA for \\(\\gamma n\\rightarrow n\\pi^{0}\\), based on fits to data for the other isospin channels, disagreed between themselves and no model provided a good description of the new data. The results demonstrate clearly the importance of a measurement of the fully neutral final state for the isospin decomposition of the cross section. Model refits, for example from the Bonn-Gatchina analysis, show that the new and the previous data for the other three isospin channels can be simultaneously described when the contributions of several partial waves are modified. The results are also relevant for the suppression of the higher resonance bumps in total photoabsorption on nuclei, which are not well understood.

Quasi-free photoproduction of \\(\\eta\\)-mesons has been measured off nucleons bound in \\(^3\\)He nuclei for incident photon energies from the threshold region up to 1.4 GeV. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with an almost \\(4\\pi\\) covering electromagnetic calorimeter, combining the TAPS and Crystal Ball detectors. The \\(\\eta\\)-mesons were detected in coincidence with the recoil nucleons. This allowed a comparison of the production cross section off quasi-free protons and quasi-free neutrons and a full kinematic reconstruction of the final state, eliminating effects from nuclear Fermi motion. In the \\(S_{11}\\)(1535) resonance peak, the data agree with the neutron/proton cross section ratio extracted from measurements with deuteron targets. More importantly, the prominent structure observed in photoproduction off quasi-free neutrons bound in the deuteron is also clearly observed. Its parameters (width, strength) are consistent with the expectations from the deuteron results. On an absolute scale the cross sections for both quasi-free protons and neutrons are suppressed with respect to the deuteron target pointing to significant nuclear final state interaction effects.

The photoproduction of \\(\\eta\\)-mesons off nucleons bound in \\(^2\\)H and \\(^3\\)He has been measured in coincidence with recoil protons and recoil neutrons for incident photon energies from threshold up to 1.4 GeV. The experiments were performed at the Mainz MAMI accelerator, using the Glasgow tagged photon facility. Decay photons from the \\(\\eta\\rightarrow 2\\gamma\\) and \\(\\eta\\rightarrow 3\\pi^0\\) decays and the recoil nucleons were detected with an almost \\(4\\pi\\) electromagnetic calorimeter combining the Crystal Ball and TAPS detectors. The data from both targets are of excellent statistical quality and show a narrow structure in the excitation function of \\(\\gamma n\\rightarrow n\\eta\\). The results from the two measurements are consistent taking into account the expected effects from nuclear Fermi motion. The best estimates for position and intrinsic width of the structure are \\(W\\) = (1670\\(\\pm\\)5) MeV and \\(\\Gamma\\) =(30\\(\\pm\\)15) MeV. For the first time precise results for the angular dependence of this structure have been extracted.

High statistics measurements of the photon asymmetry \\(\\mathrm{\\Sigma}\\) for the \\(\\overrightarrow{\\gamma}\\)p\\(\\rightarrow\\pi^{0}\\)p 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, \\(\\check{\\mathrm{\\Sigma}}\\) (= \\(\\sigma_{0}\\mathrm{\\Sigma}\\)), 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.

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\\to\\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.