Evangeline Downie

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.

The Compton double-polarization observable ${\mathrm{\Sigma }}_{2z}$ has been measured for the first time in the $\mathrm{\Delta }\left(1232\right)$ resonance region using a circularly polarized photon beam incident on a longitudinally polarized proton 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 either a dispersion relation calculation or a baryon chiral perturbation theory calculations with the weighted average of these two fits resulting in ${\gamma }_{E1E1}=-2.87\pm 0.52, {\gamma }_{M1M1}=2.70\pm 0.43, {\gamma }_{E1M2}=-0.85\pm 0.72$, and ${\gamma }_{M1E2}=2.04\pm 0.43$, in units of ${10}^{-4}{\mathrm{fm}}^4$.

We report a measurement of the spin polarization of the recoiling neutron in deuterium photodisintegration, utilizing 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 polarization 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 polarization in a narrow structure centered around $E_{\gamma }\sim 570\mathrm{MeV}$ is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition suggests this behavior could be related to the excitation of the $d^{*}(2380)$ hexaquark.

Differential cross sections for elastic Compton scattering from ${}^4\mathrm{He}$ have been measured with high statistical precision at the High Intensity $\gamma$-ray Source at laboratory scattering angles of ${55}^{\circ }$, ${90}^{\circ }$, and ${125}^{\circ }$ using a quasi-monoenergetic photon beam with a weighted mean energy value of 81.3 MeV. The results are compared to previous measurements and similar fore-aft asymmetry in the angular distribution of the differential cross sections is observed. This experimental work is expected to strongly motivate the development of effective-field-theory calculations of Compton scattering from ${}^4\mathrm{He}$ to fully interpret the data.

The first experimental investigation of the near-threshold cross section for incoherent ${\pi }^{-}$ photoproduction on the deuteron $\gamma d\to {\pi }^{-}pp$ is presented. The experimental technique involved detection of the $\approx 131$ MeV $\gamma$ ray resulting from the radiative capture of photoproduced ${\pi }^{-}$ in the target. The total cross section was measured using an unpolarized tagged-photon beam, a liquid-deuterium target, and three very large NaI(Tl) spectrometers. The data are compared to theoretical models that give insight into the elementary reaction $\gamma n\to {\pi }^{-}p$ and pion-nucleon and nucleon-nucleon final-state interactions.

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 ${18}^{\circ }$ to ${162}^{\circ }$, making use of model-dependent nuclear corrections to extract ${\pi }^0$ 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 ${\pi }^0$-meson production. Our accumulation of $3.6\times {10}^6$ $\gamma n\to {\pi }^{0}n$ 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 with 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.