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The target asymmetry T , recoil asymmetry P , and beam-target double polarization observable H were determined in exclusive π 0 and η photoproduction off quasi-free protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the γ p → π 0 p and γ p → η p reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the η n system at W = 1.68 GeV . A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the S 11 ( 1535 ) and S 11 ( 1650 ) resonances within the S 11 -partial wave.

A novel geometry for a sampling calorimeter employing inorganic scintillators as an active medium is presented. To overcome the mechanical challenges of construction, an innovative light collection geometry has been pioneered, that minimises the complexity of construction. First test results are presented, demonstrating a successful signal extraction. The geometry consists of a sampling calorimeter with passive absorber layers interleaved with layers of an active medium made of inorganic scintillating crystals. Wavelength-shifting (WLS) fibres run along the four long, chamfered edges of the stack, transporting the light to photodetectors at the rear. To maximise the amount of scintillation light reaching the WLS fibres, the scintillator chamfers are depolished. It is shown herein that this concept is working for cerium fluoride (CeF3) as a scintillator. Coupled to it, several different types of materials have been tested as WLS medium. In particular, materials that might be sufficiently resistant to the High- Luminosity Large Hadron Collider radiation environment, such as cerium-doped Lutetium- Yttrium Orthosilicate (LYSO) and cerium-doped quartz, are compared to conventional plastic WLS fibres. Finally, an outlook is presented on the possible optimisation of the different components, and the construction and commissioning of a full calorimeter cell prototype is presented.

The target asymmetry T , recoil asymmetry P , and beam-target double polarization observable H were determined in exclusive $$\\pi ^0$$ π 0 and $$\\eta $$ η photoproduction off quasi-free protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the $$\\gamma p \\rightarrow \\pi ^0 p$$ γ p → π 0 p and $$\\gamma p \\rightarrow \\eta p$$ γ p → η p reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the $$\\eta n$$ η n system at $$W = 1.68\\ \\textrm{GeV}$$ W = 1.68 GeV . A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the $$S_{11}(1535)$$ S 11 ( 1535 ) and $$S_{11}(1650)$$ S 11 ( 1650 ) resonances within the $$S_{11}$$ S 11 -partial wave.

The target asymmetry T, recoil asymmetry P, and beam-target double polarization observable H were determined in exclusive $$\\pi ^0$$ π0 and $$\\eta $$ η photoproduction off quasi-free protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the $$\\gamma p \\rightarrow \\pi ^0 p$$ γp→π0p and $$\\gamma p \\rightarrow \\eta p$$ γp→ηp reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the $$\\eta n$$ ηn system at $$W = 1.68\\ \\textrm{GeV}$$ W=1.68GeV. A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the $$S_{11}(1535)$$ S11(1535) and $$S_{11}(1650)$$ S11(1650) resonances within the $$S_{11}$$ S11-partial wave.

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}\\).

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.

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.