[en] During the almost 20 year period of this grant research was carried out on atomic nuclei and their constituents using both photons and electrons. Research was carried out at the electron accelerator facility of the Netherlands Institute for Nuclear and High Energy Physics (NIKHEFK, Amsterdam) until the electron accelerator facility was closed in 1998. Subsequently, research was carried out at the Laser-Electron Gamma Source (LEGS) of the National Synchrotron Light Source (NSLS) located at the Brookhaven National Laboratory (BNL) until the LEGS was closed at the end of 2006. During the next several years research was carried out at both the Thomas Jefferson National Accelerator Facility (JLAB) and the High Intensity Gamma Source (HIGS) of the Tri-Universities Nuclear Laboratory (TUNL) located on the campus of Duke University. Since approximately 2010 the principal focus was on research at TUNL, although analysis of data from previous research at other facilities continued. The principal early focus of the research was on the role of pions in nuclei. This was studied by studying the production of pions using both photons (at LEGS) and electrons (at NIKHEF-K and JLAB). Measurements of charged pion photoproduction from deuterium at LEGS resulted in the most interesting result of these two decades of work. By measuring the production of a charged pion (π ⁺) in coincidence with an emitted photon we observed structures in the residual two-nucleon system. These indicated the existence of long-lived states not explicable by standard nuclear theory; they suggest a set of configurations not explicable in terms of a nucleon-nucleon pair. The existence of such “exotic” structures has formed the foundation for most of the work that has ensued.

[en] The work discussed here is an extension of work previously funded by U.S. Department of Energy Grant DE-FG02-97ER41025. Measurements of charged pion photoproduction from deuterium using the Laser Electron Gamma Source (LEGS) at the Brookhaven National Laboratory previously made by us, as members of the LEGS Collaboration, resulted in the most interesting result of two decades of work. By measuring the production of a charged pion (π⁺) in coincidence with an emitted photon we observed structures in the residual two-nucleon system. These indicated the existence of rare, long-lived states not explicable by standard nuclear theory; they suggested a set of configurations not explicable in terms of a nucleon-nucleon pair. The existence of such “exotic” structures has formed the foundation for most of the work that has ensued. Several measurements at various laboratories have supported, but not proved, the existence of these exotic states. The rarity of these states made their existence undetectable in most previous measurements. Only by observing characteristic signatures of such states (i.e., decay photons), by using very specific kinematics which isolate certain reaction products, or by measuring polarization-dependent observables. During the period of this grant we pursued and made progress on the development of experiments to be performed at the High Intensity Gamma Source (HIGS) of the Tri Universities Nuclear Laboratory (TUNL). Our understanding of photon- and electron-induced nuclear reactions depends on understanding of the basic electron and photon interaction. Recently, the issue of two-photon contributions has arisen in the context of deeply inelastic electron scattering. One way to address this is to measure asymmetries in the Bethe-Heitler ee process. We also made progress in developing the detectors required to measure these asymmetries at HIGS. During the last several years the apparent discrepancy between the size of the proton as measured using electrons and that as measured using muons has received a great deal of attention. Working with colleagues at the Jefferson Laboratory (JLAB) we showed that the apparent discrepancy was almost surely the result of mistakes in the statistical analysis of electron scattering data, that there is almost surely no discrepancy.

[en] We analyze the amplitude of radiative charged pion photoproduction within the framework of heavy baryon chiral perturbation theory and discuss the best experimental setup for the extraction of the charged pion polarizabilities from the differential cross section. We find that the contributions from two unknown low-energy constants in the πN chiral Lagrangian at order p³ are comparable with the contributions of the charged pion polarizabilities. As a result, it is necessary to take the effects of these low-energy constants into account. Furthermore, we discuss the applicability of the extrapolation method and conclude that this method is applicable only if the polarization vector of the incoming photon is perpendicular to the scattering plane in the center-of-mass frame of the final γ-π system.

[en] Compton backscattering polarimetry provides a fast measurement of the polarization of an electron beam in a storage ring. Since the method is non-destructive, the polarization of the electrons can be monitored during internal target experiments. At NIKHEF a Compton polarimeter has been constructed to measure the polarization of the longitudinally polarized electrons stored in the AmPS ring. First results obtained with the polarimeter, the first Compton polarimeter to measure the polarization of a stored longitudinally polarized electron beam, are presented in this paper

[en] The charge form factor of ⁴He has been extracted in the range 29 fm^-2 <= Q² <= 77 fm^-2 from elastic electron scattering, detecting ⁴He nuclei and electrons in coincidence with the High Resolution Spectrometers of the Hall A Facility of Jefferson Lab. The results are in qualitative agreement with realistic meson-nucleon theoretical calculations. The data have uncovered a second diffraction minimum, which was predicted in the Q² range of this experiment, and rule out conclusively long-standing predictions of dimensional scaling of high-energy amplitudes using quark counting

[en] An experimental study of the 16O(e, e'K+)16N_#Lambda# reaction has been performed at Jefferson Lab. A thin film of falling water was used as a target. This permitted a simultaneous measurement of the p(e, e'K+)Λ,Σ₀ exclusive reactions and a precise calibration of the energy scale. A ground-state binding energy of 13.76 ± 0.16 MeV was obtained for 16N_#Lambda# with better precision than previous measurements on the mirror hypernucleus 16O_#Lambda#. Precise energies have been determined for peaks arising from a Lambda in s and p orbits coupled to the p1/2 and p3/2 hole states of the 15N core nucleus.