Search

Wide-angle exclusive Compton scattering and single-pion photoproduction from the proton have been investigated via measurement of the polarization transfer from a circularly polarized photon beam to the recoil proton. The wide-angle Compton scattering polarization transfer was analyzed at an incident photon energy of 3.7 GeV at a proton scattering angle of θcmp=70°. The longitudinal transfer KLL, measured to be 0.645±0.059±0.048, where the first error is statistical and the second is systematic, has the same sign as predicted for the reaction mechanism in which the photon interacts with a single quark carrying the spin of the proton. However, the observed value is ~3 times larger than predicted by the generalized-parton-distribution-based calculations, which indicates a significant unknown contribution to the scattering amplitude. [ABSTRACT FROM AUTHOR]

First measurement of the beam asymmetry Σ for Compton scattering off the proton in the energy range Ey = 0.85-1.25 GeV is presented. The data reveal two narrow structures at Er = 1.036 and Ey = 1.119 GeV. They may signal narrow resonances with masses near 1.68 and 1.72 GeV, or they may be generated by the subthreshold K A and cop production. Their decisive identification requires additional theoretical and experimental efforts. [ABSTRACT FROM AUTHOR]

A measurement of the electroproduction of photons off protons in the deeply inelastic regime was performed at Jefferson Lab using a nearly 6 GeV electron beam, a longitudinally polarized proton target, and the CEBAF Large Acceptance Spectrometer. Target-spin asymmetries for ep → e'p'γ events, which arise from the interference of the deeply virtual Compton scattering and the Bethe-Heitler processes, were extracted over the widest kinematics in Q2,xB,t, and ϕ, for 166 four-dimensional bins. In the framework of generalized parton distributions, at leading twist the t dependence of these asymmetries provides insight into the spatial distribution of the axial charge of the proton, which appears to be concentrated in its center. These results also bring important and necessary constraints for the existing parametrizations of chiral-even generalized parton distributions. [ABSTRACT FROM AUTHOR]

The article discusses the experimental developments in nuclear physics. It presents the study which examined the Virtual Compton scattering (VCS) on the proton using the exclusive electroproduction reaction ep → epϒ. Parts of the study were conducted at the Mainz Microton (MAMI), the Thomas Jefferson Accelerator Facility (JLab) and Massachusetts Institute of Technology (MIT)-Bates.

Virtual Compton scattering on the proton has been investigated at three yet unexplored values of the four-momentum transfer Q²: 0.10, 0.20, and 0.45 GeV², at the Mainz Microtron. Fits performed using either the low-energy theorem or dispersion relations allowed the extraction of the structure functions PLL-PTT/ε and PLT, as well as the electric and magnetic generalized polarizabilities αE1(Q²) and βM1(Q²). These new results show a smooth and rapid falloff of αE1(Q²), in contrast to previous measurements at Q²=0.33 GeV², and provide for the first time a precise mapping of βM1(Q²) in the low-Q² region. [ABSTRACT FROM AUTHOR]

In the past two decades, deeply virtual Compton scattering of electrons has been successfully used to advance our knowledge of the partonic structure of the free proton and investigate correlations between the transverse position and the longitudinal momentum of quarks inside the nucleon. Meanwhile, the structure of bound nucleons in nuclei has been studied in inclusive deep-inelastic lepton scattering experiments off nuclear targets, showing a significant difference in longitudinal momentum distribution of quarks inside the bound nucleon, known as the EMC effect. In this Letter, we report the first beam spin asymmetry (BSA) measurement of exclusive deeply virtual Compton scattering off a proton bound in 4He. The data used here were accumulated using a 6 GeV longitudinally polarized electron beam incident on a pressurized 4He gaseous target placed within the CLAS spectrometer in Hall-B at the Thomas Jefferson National Accelerator Facility. The azimuthal angle (ϕ) dependence of the BSA was studied in a wide range of virtual photon and scattered proton kinematics. The Q², xB, and t dependencies of the BSA on the bound proton are compared with those on the free proton. In the whole kinematical region of our measurements, the BSA on the bound proton is smaller by 20% to 40%, indicating possible medium modification of its partonic structure. [ABSTRACT FROM AUTHOR]