448 research outputs found
New Measurement of Parity Violation in Elastic Electron-Proton Scattering and Implications for Strange Form Factors
Measurement of the Neutral Weak Form Factors of the Proton
We have measured the parity-violating electroweak asymmetry in the elastic
scattering of polarized electrons from the proton. The kinematic point
(theta_lab = 12.3 degrees and Q^2=0.48 (GeV/c)^2) is chosen to provide
sensitivity, at a level that is of theoretical interest, to the strange
electric form factor G_E^s. The result, A=-14.5 +- 2.2 ppm, is consistent with
the electroweak Standard Model and no additional contributions from strange
quarks. In particular, the measurement implies G_E^s + 0.39G_M^s = 0.023 +-
0.034 (stat) +- 0.022 (syst) +- 0.026 (delta G_E^n), where the last uncertainty
arises from the estimated uncertainty in the neutron electric form factor.Comment: 10 pages, 4 figures, submitted to Phys. Rev. Let
Parity-Violating Electron Scattering from (4)He and the Strange Electric Form Factor of the Nucleon
We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from 4He at an average scattering angle ⟨θlab⟩=5.7° and a four-momentum transfer Q2=0.091  GeV2. From these data, for the first time, the strange electric form factor of the nucleon GEs can be isolated. The measured asymmetry of APV=(6.72±0.84(stat)±0.21(syst))×10-6 yields a value ofGEs=-0.038±0.042(stat)±0.010(syst), consistent with zero
Preliminary Results from Integrating Compton Photon Polarimetry in Hall A of Jefferson Lab
A wide range of nucleon and nuclear structure experiments in Jefferson Lab's
Hall A require precise, continuous measurements of the polarization of the
electron beam. In our Compton polarimeter, electrons are scattered off photons
in a Fabry-Perot cavity; by measuring an asymmetry in the integrated signal of
the scattered photons detected in a GSO crystal, we can make non-invasive,
continuous measurements of the beam polarization. Our goal is to achieve 1%
statistical error within two hours of running. We discuss the design and
commissioning of an upgrade to this apparatus, and report preliminary results
for experiments conducted at beam energies from 3.5 to 5.9 GeV and photon rates
from 5 to 100 kHz.Comment: 6 pages, 7 figures. To appear in the Proceedings of the International
Nuclear Physics Conference (INPC 2010), July 4-9 2010, Vancouver, Canada
(Journal of Physics: Conference Series
Strangeness and Chiral Symmetry Breaking
The implications of chiral symmetry breaking and SU(3) symmetry breaking have
been studied in the chiral constituent quark model (CQM). The role of
hidden strangeness component has been investigated for the scalar matrix
elements of the nucleon with an emphasis on the meson-nucleon sigma terms. The
CQM is able to give a qualitative and quantitative description of the
"quark sea" generation through chiral symmetry breaking. The significant
contribution of the strangeness is consistent with the recent available
experimental observations.Comment: 10 pages, 1 table. To appear in Mod. Phys. Lett.
Proton strangeness form factors in (4,1) clustering configurations
We reexamine a recent result within a nonrelativistic constituent quark model
(NRCQM) which maintains that the uuds\bar s component in the proton has its
uuds subsystem in P state, with its \bar s in S state (configuration I). When
the result are corrected, contrary to the previous result, we find that all the
empirical signs of the form factors data can be described by the lowest-lying
uuds\bar s configuration with \bar s in P state that has its uuds subsystem in
state (configuration II). Further, it is also found that the removal of the
center-of-mass (CM) motion of the clusters will enhance the contributions of
the transition current considerably. We also show that a reasonable description
of the existing form factors data can be obtained with a very small probability
P_{s\bar s}=0.025% for the uuds\bar s component. We further see that the
agreement of our prediction with the data for G_A^s at low-q^2 region can be
markedly improved by a small admixture of configuration I. It is also found
that by not removing CM motion, P_{s\bar s} would be overestimated by about a
factor of four in the case when transition dominates over direct currents.
Then, we also study the consequence of a recent estimate reached from analyzing
the existing data on quark distributions that P_{s\bar s} lies between 2.4-2.9%
which would lead to a large size for the five-quark (5q) system, as well as a
small bump in both G^s_E+\eta G^s_M and G^s_E in the region of q^2 =< 0.1
GeV^2.Comment: Prepared for The Fifth Asia-Pacific Conference on Few-Body Problems
in Physics 2011 in Seoul, South Korea, 22-26 August 201
Moments of the Neutron \u3cem\u3eg\u3c/em\u3e\u3csub\u3e2\u3c/sub\u3e Structure Function at Intermediate \u3cem\u3eQ\u3c/em\u3e\u3csup\u3e2\u3c/sup\u3e
We present new experimental results for the 3He spin structure function g2 in the resonance region at Q2 values between 1.2 and 3.0(GeV/c)2. Spin dependent moments of the neutron were extracted. Our main result, the inelastic contribution to the neutron d2 matrix element, was found to be small at ⟨Q2⟩=2.4(GeV/c)2 and in agreement with the lattice QCD calculation. The Burkhardt-Cottingham sum rule for 3He and the neutron was tested with the measured data and using the Wandzura-Wilczek relation for the low x unmeasured region
First Measurement of Unpolarized Semi-Inclusive Deep-Inelastic Scattering Cross Sections From a He 3 Target
The unpolarized semi-inclusive deep-inelastic scattering (SIDIS) differential cross sections in 3He(e,e′π±)X have been measured for the first time in Jefferson Lab experiment E06-010 with a 5.9GeV e- beam on a 3He gas target. The experiment focuses on the valence quark region, covering a kinematic range 0.12\u3cxbj\u3c0.45,1\u3cQ2\u3c4(GeV/c)2,0.45\u3czh\u3c0.65, and 0.05\u3cPt\u3c0.55GeV/c. The extracted SIDIS differential cross sections of π± production are compared with existing phenomenological models while the 3He nucleus approximated as two protons and one neutron in a plane-wave picture, in multidimensional bins. Within the experimental uncertainties, the azimuthal modulations of the cross sections are found to be consistent with zero. © 2017 American Physical Society
E00-110 Experiment at Jefferson Lab Hall A: Deeply Virtual Compton Scattering Off the Proton at 6 GeV
We present final results on the photon electroproduction (→e p → epγ) cross section in the deeply virtual Compton scattering (DVCS) regime and the valence quark region from Jefferson Lab experiment E00-110. Results from an analysis of a subset of these data were published before, but the analysis has been improved, which is described here at length, together with details on the experimental setup. Furthermore, additional data have been analyzed, resulting in photon electroproduction cross sections at new kinematic settings for a total of 588 experimental bins. Results of the Q2 and xB dependencies of both the helicity-dependent and the helicity-independent cross sections are discussed. The Q2 dependence illustrates the dominance of the twist-2 handbag amplitude in the kinematics of the experiment, as previously noted. Thanks to the excellent accuracy of this high-luminosity experiment, it becomes clear that the unpolarized cross section shows a significant deviation from the Bethe-Heitler process in our kinematics, compatible with a large contribution from the leading twist-2 DVCS2 term to the photon electroproduction cross section. The necessity to include higher-twist corrections to fully reproduce the shape of the data is also discussed. The DVCS cross sections in this paper represent the final set of experimental results from E00-110, superseding the previous publication
Calculation of fermion loops for and nucleon scalar and electromagnetic form factors
The exact evaluation of the disconnected diagram contributions to the
flavor-singlet pseudoscalar meson mass, the nucleon sigma term and the nucleon
electromagnetic form factors, is carried out utilizing GPGPU technology with
the NVIDIA CUDA platform. The disconnected loops are also computed using
stochastic methods with several noise reduction techniques. Various dilution
schemes as well as the truncated solver method are studied. We make a
comparison of these stochastic techniques to the exact results and show that
the number of noise vectors depends on the operator insertion in the fermionic
loop.Comment: Version accepted for publication in Comp. Phys. Commun. References
added. 13 pages, 12 figure
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