A lead-glass hodoscope calorimeter that was constructed for use in the Jefferson Lab Real Compton Scattering experiment is described. The detector provides a measurement of the coordinates and the energy of scattered photons in the GeV energy range w
ith resolutions of 5 mm and 6%/sqrt(E{gamma} [GeV]). Features of both the detector design and its performance in the high luminosity environment during the experiment are presented.
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 scatterin
g polarization transfer was analyzed at an incident photon energy of 3.7~GeV at a proton scattering angle of cma$= 70^circ$. The longitudinal transfer KLL, measured to be $0.645 pm 0.059 pm 0.048$, 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 $sim$3~times larger than predicted by the generalized-parton-distribution-based calculations, which indicates a significant unknown contribution to the scattering amplitude.
Three experimental concepts investigating possible anisotropy of the speed of light are presented. They are based on i) beam deflection in a 180 degree magnetic arc, ii) narrow resonance production in an electron-positron collider, and iii) the ratio
of magnetic moments of an electron and a positron moving in opposite directions.
We propose an experiment to search for a new gauge boson, A$^prime$, in $e^+e^-$ annihilation by means of a positron beam incident on a gas hydrogen target internal to the VEPP-3 storage ring. The search method is based on a missing mass spectra in t
he reaction $e^+e^-rightarrow gamma$ A$^prime$. It allows observation of the A$^prime$ signal independently of its decay modes and life time. The projected result of this experiment corresponds to an upper limit on the square of coupling constant $|f_{etext{A}{^prime}}|^2=1cdot 10^{-8}$ with a signal-to-noise ratio of five to one at an A$^prime$ mass of 15 MeV.
We present a search at Jefferson Laboratory for new forces mediated by sub-GeV vector bosons with weak coupling $alpha$ to electrons. Such a particle $A$ can be produced in electron-nucleus fixed-target scattering and then decay to an $e^+e^-$ pair,
producing a narrow resonance in the QED trident spectrum. Using APEX test run data, we searched in the mass range 175--250 MeV, found no evidence for an $Ato e^+e^-$ reaction, and set an upper limit of $alpha/alpha simeq 10^{-6}$. Our findings demonstrate that fixed-target searches can explore a new, wide, and important range of masses and couplings for sub-GeV forces.
The u- and d-quark contributions to the elastic nucleon electromagnetic form factors have been determined using experimental data on GEn, GMn, GpE, and GpM. Such a flavor separation of the form factors became possible up to 3.4 GeV2 with recent data
on GEn from Hall A at JLab. At a negative four-momentum transfer squared Q2 above 1 GeV2, for both the u- and d-quark components, the ratio of the Pauli form factor to the Dirac form factor, F2/F1, was found to be almost constant, and for each of F2 and F1 individually, the d-quark portions of both form factors drop continuously with increasing Q2.
The electric form factor of the neutron was determined from studies of the reaction He3(e,en)pp in quasi-elastic kinematics in Hall A at Jefferson Lab. Longitudinally polarized electrons were scattered off a polarized target in which the nuclear pola
rization was oriented perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons that were registered in a large-solid-angle detector. More than doubling the Q2-range over which it is known, we find GEn = 0.0225 +/- 0.0017 (stat) +/- 0.0024 (syst), 0.0200 +/- 0.0023 +/- 0.0018, and 0.0142 +/- 0.0019 +/- 0.0013 for Q2 = 1.72, 2.48, and 3.41 GeV2, respectively.
