Upgraded photon calorimeter with integrating readout for Hall A Compton Polarimeter at Jefferson Lab

The photon arm of the Compton polarimeter in Hall A of Jefferson Lab has been upgraded to allow for electron beam polarization measurements with better than 1% accuracy. The data acquisition system (DAQ) now includes an integrating mode, which eliminates several systematic uncertainties inherent in the original counting-DAQ setup. The photon calorimeter has been replaced with a Ce-doped GSO crystal, which has a bright output and fast response, and works well for measurements using the new integrating method at electron beam energies from 1 to 6 GeV.Comment: 16 pages, 15 figure

A Search for Sigma^0_5, N^0_5 and Theta^++ Pentaquark States

A high-resolution (sigma_instr. = 1.5 MeV) search for narrow states (Gamma < 10 MeV) with masses of M_x approx 1500-1850 MeV in ep -> e'K^+ X, e'K^- X and e' pi^+ X electroproduction at small angles and low Q^2 was performed. These states would be candidate partner states of the reported Theta^+(1540) pentaquark. No statistically significant signal was observed in any of the channels at 90% C.L. Upper limits on forward production were determined to be between 0.7% and 4.2% of the Lambda(1520) production cross section, depending on the channel and the assumed mass and width of the state.Comment: 5 pages, 5 figures, to appear in Phys. Rev. C, update with responses to referee suggestion

Polarization transfer in wide-angle Compton scattering and single-pion photoproduction from the proton

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 θpcm=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

Compton Scattering Cross Section on the Proton at High Momentum Transfer

Cross-section values for Compton scattering on the proton were measured at 25 kinematic settings over the range s = 5-11 and -t = 2-7 GeV2 with statistical accuracy of a few percent. The scaling power for the s-dependence of the cross section at fixed center of mass angle was found to be 8.0 +/ 0.2, strongly inconsistent with the prediction of perturbative QCD. The observed cross-section values are in fair agreement with the calculations using the handbag mechanism, in which the external photons couple to a single quark.Comment: 5 pages, 5 figure

A Precision Measurement of the Neutron Twist-3 Matrix Element d2nd_2^n: Probing Color Forces

Double-spin asymmetries and absolute cross sections were measured at large Bjorken $x$ (0.25 $\le x \le$ 0.90), in both the deep-inelastic and resonance regions, by scattering longitudinally polarized electrons at beam energies of 4.7 and 5.9 GeV from a transversely and longitudinally polarized $^3$He target. In this dedicated experiment, the spin structure function $g_2$ on $^3$He was determined with precision at large $x$, and the neutron twist-three matrix element $d_2^n$ was measured at \left of 3.21 and 4.32 GeV$^2$/$c^2$, with an absolute precision of about $10^{-5}$. Our results are found to be in agreement with lattice QCD calculations and resolve the disagreement found with previous data at \left = 5 GeV$^2$/$c^2$. Combining $d_2^n$ and a newly extracted twist-four matrix element, $f_2^n$, the average neutron color electric and magnetic forces were extracted and found to be of opposite sign and about 30 MeV/fm in magnitude.Comment: Corrected a typo in the author list and Figure 1 legend. 6 pages, 2 figures, 2 table