39 research outputs found
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 : Probing Color Forces
Double-spin asymmetries and absolute cross sections were measured at large
Bjorken (0.25 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 He target.
In this dedicated experiment, the spin structure function on He was
determined with precision at large , and the neutron twist-three matrix
element was measured at \left of 3.21 and 4.32
GeV/, with an absolute precision of about . Our results are
found to be in agreement with lattice QCD calculations and resolve the
disagreement found with previous data at \left = 5 GeV/.
Combining and a newly extracted twist-four matrix element, , 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
Recoil Polarization Measurements of the Proton Electromagnetic Form Factor Ratio to Q^2 = 8.5 GeV^2
Among the most fundamental observables of nucleon structure, electromagnetic
form factors are a crucial benchmark for modern calculations describing the
strong interaction dynamics of the nucleon's quark constituents; indeed, recent
proton data have attracted intense theoretical interest. In this letter, we
report new measurements of the proton electromagnetic form factor ratio using
the recoil polarization method, at momentum transfers Q2=5.2, 6.7, and 8.5
GeV2. By extending the range of Q2 for which GEp is accurately determined by
more than 50%, these measurements will provide significant constraints on
models of nucleon structure in the non-perturbative regime
Measurements of Non-Singlet Moments of the Nucleon Structure Functions and Comparison to Predictions from Lattice QCD for
We present extractions of the nucleon non-singlet moments utilizing new
precision data on the deuteron structure function at large Bjorken-
determined via the Rosenbluth separation technique at Jefferson Lab
Experimental Hall C. These new data are combined with a complementary set of
data on the proton previously measured in Hall C at similar kinematics and
world data sets on the proton and deuteron at lower measured at SLAC and
CERN. The new Jefferson Lab data provide coverage of the upper third of the
range, crucial for precision determination of the higher moments. In contrast
to previous extractions, these moments have been corrected for nuclear effects
in the deuteron using a new global fit to the deuteron and proton data. The
obtained experimental moments represent an order of magnitude improvement in
precision over previous extractions using high data. Moreover, recent
exciting developments in Lattice QCD calculations provide a first ever
comparison of these new experimental results with calculations of moments
carried out at the physical pion mass, as well as a new approach which first
calculates the quark distributions directly before determining moments