267 research outputs found
On timelike and spacelike deeply virtual Compton scattering at next to leading order
We study timelike and spacelike virtual Compton scattering in the generalized
Bjorken scaling regime at next to leading order in the strong coupling
constant, in the medium energy range which will be studied intensely at JLab12
and in the COMPASS-II experiment at CERN. We show that the Born amplitudes get
sizeable O(\alpha_s) corrections and, even at moderate energies, the gluonic
contributions are by no means negligible. We stress that the timelike and
spacelike cases are complementary and that their difference deserves much
special attention
Measurement of the Generalized Forward Spin Polarizabilities of the Neutron
The generalized forward spin polarizabilities and of
the neutron have been extracted for the first time in a range from 0.1 to
0.9 GeV. Since is sensitive to nucleon resonances and
is insensitive to the resonance, it is expected that the
pair of forward spin polarizabilities should provide benchmark tests of the
current understanding of the chiral dynamics of QCD. The new results on
show significant disagreement with Chiral Perturbation Theory
calculations, while the data for at low are in good agreement
with a next-to-lead order Relativistic Baryon Chiral Perturbation theory
calculation. The data show good agreement with the phenomenological MAID model.Comment: 5 pages, 2 figures, corrected typo in author name, published in PR
Measurement of the Proton and Deuteron Spin Structure Functions g2 and Asymmetry A2
We have measured the spin structure functions g2p and g2d and the virtual
photon asymmetries A2p and A2d over the kinematic range 0.02 < x < 0.8 and 1.0
< Q^2 < 30(GeV/c)^2 by scattering 38.8 GeV longitudinally polarized electrons
from transversely polarized NH3 and 6LiD targets.The absolute value of A2 is
significantly smaller than the sqrt{R} positivity limit over the measured
range, while g2 is consistent with the twist-2 Wandzura-Wilczek calculation. We
obtain results for the twist-3 reduced matrix elements d2p, d2d and d2n. The
Burkhardt-Cottingham sum rule integral - int(g2(x)dx) is reported for the range
0.02 < x < 0.8.Comment: 12 pages, 4 figures, 1 tabl
New Measurement of Parity Violation in Elastic Electron-Proton Scattering and Implications for Strange Form Factors
We have measured the parity-violating electroweak asymmetry in the elastic
scattering of polarized electrons from the proton. The result is A = -15.05 +-
0.98(stat) +- 0.56(syst) ppm at the kinematic point theta_lab = 12.3 degrees
and Q^2 = 0.477 (GeV/c)^2. The measurement implies that the value for the
strange form factor (G_E^s + 0.392 G_M^s) = 0.025 +- 0.020 +- 0.014, where the
first error is experimental and the second arises from the uncertainties in
electromagnetic form factors. This measurement is the first fixed-target parity
violation experiment that used either a `strained' GaAs photocathode to produce
highly polarized electrons or a Compton polarimeter to continuously monitor the
electron beam polarization.Comment: 8 pages, 4 figures, Tex, elsart.cls; revised version as accepted for
Phys. Lett.
Q^2 Evolution of the Neutron Spin Structure Moments using a He-3 Target
We have measured the spin structure functions and of He in a
double-spin experiment by inclusively scattering polarized electrons at
energies ranging from 0.862 to 5.07 GeV off a polarized He target at a
15.5 scattering angle. Excitation energies covered the resonance and
the onset of the deep inelastic regions. We have determined for the first time
the evolution of ,
and for the neutron in the range 0.1 GeV 0.9 GeV with good precision. displays a smooth
variation from high to low . The Burkhardt-Cottingham sum rule holds
within uncertainties and is non-zero over the measured range.Comment: 5 pages, 2 figures, submitted to Phys. Rev. Lett.. Updated Hermes
data in Fig. 2 (top panel) and their corresponding reference. Updated the low
x extrapolation error Fig. 2 (middle panel). Corrected references to ChiPT
calculation
Exclusive Neutral Pion Electroproduction in the Deeply Virtual Regime
We present measurements of the ep->ep pi^0 cross section extracted at two
values of four-momentum transfer Q^2=1.9 GeV^2 and Q^2=2.3 GeV^2 at Jefferson
Lab Hall A. The kinematic range allows to study the evolution of the extracted
hadronic tensor as a function of Q^2 and W. Results will be confronted with
Regge inspired calculations and GPD predictions. An intepretation of our data
within the framework of semi-inclusive deep inelastic scattering has also been
attempted
The Q^2 evolution of the generalized Gerasimov-Drell-Hearn integral for the neutron using a He-3 target
We present data on the inclusive scattering of polarized electrons from a
polarized He-3 target at energies from 0.862 to 5.06 GeV, obtained at a
scattering angle of 15.5 degrees. Our data include measurements from the
quasielastic peak, through the resonance region, to the beginning of the deep
inelastic regime, and were used to determine the spin difference in the virtual
photoabsorption cross section. We extract the extended Gerasimov-Drell-Hearn
integral for the neutron in the range of 4-momentum transfer squared Q^2 of
0.1-0.9 GeV.Comment: 14 pages of text when TeXed in preprint format with figures embedded.
RevTeX format. Three eps figure
Measurements of the -Dependence of the Proton and Neutron Spin Structure Functions g1p and g1n
The structure functions g1p and g1n have been measured over the range 0.014 <
x < 0.9 and 1 < Q2 < 40 GeV2 using deep-inelastic scattering of 48 GeV
longitudinally polarized electrons from polarized protons and deuterons. We
find that the Q2 dependence of g1p (g1n) at fixed x is very similar to that of
the spin-averaged structure function F1p (F1n). From a NLO QCD fit to all
available data we find at
Q2=5 GeV2, in agreement with the Bjorken sum rule prediction of 0.182 \pm
0.005.Comment: 17 pages, 3 figures. Submitted to Physics Letters
Precision Determination of the Neutron Spin Structure Function g1n
We report on a precision measurement of the neutron spin structure function
using deep inelastic scattering of polarized electrons by polarized
^3He. For the kinematic range 0.014<x<0.7 and 1 (GeV/c)^2< Q^2< 17 (GeV/c)^2,
we obtain at an average . We find relatively large negative
values for at low . The results call into question the usual Regge
theory method for extrapolating to x=0 to find the full neutron integral
, needed for testing quark-parton model and QCD sum rules.Comment: 5 pages, 3 figures To be published in Phys. Rev. Let
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