Two- and three-nucleon emission reactions measured using photon and electron probes

Nuclear mean-field models [1] are highly successful in describing a wide range of properties of atomic nuclei, including their structure, binding energies, and the spins and parities of their quantum states. However, by their very nature, mean-field models cannot account for the effects of residual two- and three-body forces between nucleons. In particular they over-predict the measured occupancy of valence nucleon orbits in a wide range of nuclei and under-predict the occupancies of states above the Fermi surface [2]. It has long been thought that this transfer of occupation from valence to higher-lying energy levels may be due to interactions, or correlations, between nucleons, which are not described by mean-field models. The electromagnetic interaction is well understood [3]. Hence electron and photon probes provide an excellent means of studying the internal structure of nuclei and the interactions between nucleons. This thesis describes an extensive programme of photon- and electron-induced experiments, most of which were led by the author, which studied the ejection of two, or three, correlated nucleons from atomic nuclei, in order to deduce information about the interactions between them. This work was carried out over a period of many years, primarily at the electron Microtron facility (MAMI) of the University of Mainz, Germany [4]. The author was assisted in this work by six Research Associates and ten Research Students working on different aspects of the programme. A wide range of experiments was carried out on several light nuclei: 3He, 4He, 6Li, 12C and 16O. Real photon experiments used the tagged-photon technique [A1] to measure the energy of incident photons. Many different aspects of two-nucleon emission reactions were studied, including their photon energy dependence, missing energy dependence, recoil momentum dependence, angular dependence, dependence on kinematic conditions, isospin dependence and their dependence on photon linear polarisation. The work was extended to study the contribution of three-body interactions in 3He and 12C by looking at the simultaneous emission of three nucleons from light nuclei, as well as the emission of proton-deuteron pairs. Collaborations were formed with nuclear theoreticians working in Valencia, Ghent and Pavia in order to provide a detailed interpretation of the data obtained. This involved filtering the predictions of theory calculations through the physical acceptance of the experimental apparatus to allow meaningful comparisons with measured observables. The author joined the CLAS collaboration at the Jefferson Lab 6.0 GeV electron accelerator at Newport News, Virginia, USA in 2009. The Jefferson Lab facilities allowed nucleon-nucleon correlations to be studied with higher energy probes, permitting electron scattering measurements to be carried out at large values of fourmomentum transfer and at values of Bjoerken-x greater than 1. This latter condition explicitly requires the participation of more than one nucleon. Several key results from this work have advanced our understanding of nucleon-nucleon correlations. The previously-noted strong isospin dependence of nucleon-nucleon correlations was observed to persist at higher energies, even though the detailed mechanisms evolve with energy transfer. In addition, the strong observed charge dependence of the highmomentum fraction of nucleons within the nucleus has been related to the isospin dependence of the correlations. Finally, evidence has been found which supports a connection between short-range correlations (SRC) and the “EMC effect”, in which the structure function for Deep Inelastic Scattering (DIS) of leptons on nucleons in heavy nuclei is strongly suppressed compared to the same reaction in light nuclei. The author led the programme of two- and three-nucleon emission studies at Mainz from the mid-1980s onwards, writing and presenting seven experiment proposals [5-11] which were approved by the Mainz International Programme Advisory Committee. These proposals provide a strong rationale for undertaking these investigations. Throughout his Mainz work, the author worked closely with academic colleagues from the University of Glasgow, as well as physicists from Mainz, Edinburgh and Tübingen. The Glasgow research group designed, constructed, tested and subsequently upgraded, two tagged photon spectrometers which underpinned and enabled the photon-induced experiments described in this thesis. The author took responsibility for the design, production, installation and testing of two “trigger” proton detector hodoscopes. The first detector array was used in experiments at photon energies up to 180 MeV, while the second was used in experiments at higher energies. Correlated neutrons and protons were detected in a separate “time-of-flight” scintillation detector array [12], developed jointly by physicists from Glasgow and Tübingen universities. The author directed the analysis of the majority of the data obtained from the experiments at Mainz, while other Glasgow research staff developed data collection and analysis software. Colleagues from Tübingen, Mainz and Edinburgh contributed cryogenic targets and analysed the data from the remaining experiments. The author’s work on equipment development is detailed in section A of this thesis. The author contributed to the design, installation and testing of the initial tagged photon spectrometer at the Mainz laboratory, before leading the development and production of two detector hodoscopes used to detect protons from photon-induced reactions. He also made contributions to studies of diamond radiators which were used to produce linearly polarised photons used in some of the later experiments. Section B forms the central part of this thesis. The papers in this section describe the results and interpretation of an extended programme of two- and three-nucleon emission reaction studies carried out at Mainz under the leadership of the author. In addition to directing the experimental work, the author led the drafting, revision and production of the majority of the papers in this section. He initiated close working relationships with three different groups of theoretical physicists in order to carry out calculations to interpret the experimental data. Some of these theoreticians are coauthors on particular papers. The papers in Section C report results from a small number of selected experiments at the higher energy 6.0 GeV Jefferson Laboratory electron accelerator. This work provides valuable additional insight into correlations between nucleons in atomic nuclei and shows how this field has developed with the availability of higher energy electromagnetic probes. The author was co-investigator on a major Jefferson Lab grant [13] which laid the foundations for the analyses reported in this section. Section D contains a recent conference review paper by the author which provides a concise and succinct summary of the most important work included in this thesis