Exclusive ρ0-meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, in ...the kinematic region 1.0 GeV2<Q2< 7.0 GeV2, 3.0 GeV <W< 6.3 GeV, and -t′< 0.4 GeV2. Using an unbinned maximum-likelihood method, 25 parameters are extracted. These determine the real and imaginary parts of the ratios of several helicity amplitudes describing ρ0-meson production by a virtual photon. The denominator of those ratios is the dominant amplitude, the nucleon-helicity-non-flip amplitude F012012, which describes the production of a longitudinal ρ0-meson by a longitudinal virtual photon. The ratios of nucleon-helicity-non-flip amplitudes are found to be in good agreement with those from the previous HERMES analysis. The transverse target polarization allows for the first time the extraction of ratios of a number of nucleon-helicity-flip amplitudes to F012012. Results obtained in a handbag approach based on generalized parton distributions taking into account the contribution from pion exchange are found to be in good agreement with these ratios. Within the model, the data favor a positive sign for the π-ρ transition form factor. By also exploiting the longitudinal beam polarization, a total of 71 ρ0 spin-density matrix elements is determined from the extracted 25 parameters, in contrast to only 53 elements as directly determined in earlier analyses.
We report on the results of the E06-014 experiment performed at Jefferson Lab in Hall A, where a precision measurement of the twist-3 matrix element d(2) of the neutron (d(2)(n)) was conducted. The ...quantity d(2)(n) represents the average color Lorentz force a struck quark experiences in a deep inelastic electron scattering event off a neutron due to its interaction with the hadronizing remnants. This color force was determined from a linear combination of the third moments of the He-3 spin structure functions, g(1) and g(2), after nuclear corrections had been applied to these moments. The structure functions were obtained from a measurement of the unpolarized cross section and of double-spin asymmetries in the scattering of a longitudinally polarized electron beam from a transversely and a longitudinally polarized He-3 target. The measurement kinematics included two average Q(2) bins of 3.2 GeV2 and 4.3 GeV2, and Bjorken-x 0.25 <= x <= 0.90 covering the deep inelastic and resonance regions. We have found that d(2)(n) is small and negative for < Q(2)> = 3.2 GeV2, and even smaller for < Q(2)> = 4.3 GeV2, consistent with the results of a lattice QCD calculation. The twist-4 matrix element f(2)(n) was extracted by combining our measured d(2)(n) with the world data on the first moment in x of g(1)(n), Gamma(n)(1). We found f(2)(n) to be roughly an order of magnitude larger than d(2)(n). Utilizing the extracted d(2)(n) and f(2)(n) data, we separated the Lorentz color force into its electric and magnetic components, F-E(y,n) and F-B(y,n), and found them to be equal and opposite in magnitude, in agreement with the predictions from an instanton model but not with those from QCD sum rules. Furthermore, using the measured double-spin asymmetries, we have extracted the virtual photon-nucleon asymmetry on the neutron A(1)(n), the structure function ratio g(1)(n)/F-1(n), and the quark ratios (Delta u + Delta(u) over bar)/(u + (u) over bar) and (Delta d + Delta(d) over bar)/(d + (d) over bar). These results were found to be consistent with deep-inelastic scattering world data and with the prediction of the constituent quark model but at odds with the perturbative quantum chromodynamics predictions at large x.
Exclusive
ρ
0
-meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, in ...the kinematic region 1.0 GeV
2
<
Q
2
<
7.0 GeV
2
, 3.0 GeV
<
W
<
6.3 GeV, and
-
t
′
<
0.4 GeV
2
. Using an unbinned maximum-likelihood method, 25 parameters are extracted. These determine the real and imaginary parts of the ratios of several helicity amplitudes describing
ρ
0
-meson production by a virtual photon. The denominator of those ratios is the dominant amplitude, the nucleon-helicity-non-flip amplitude
F
0
1
2
0
1
2
, which describes the production of a longitudinal
ρ
0
-meson by a longitudinal virtual photon. The ratios of nucleon-helicity-non-flip amplitudes are found to be in good agreement with those from the previous HERMES analysis. The transverse target polarization allows for the first time the extraction of ratios of a number of nucleon-helicity-flip amplitudes to
F
0
1
2
0
1
2
. Results obtained in a handbag approach based on generalized parton distributions taking into account the contribution from pion exchange are found to be in good agreement with these ratios. Within the model, the data favor a positive sign for the
π
-
ρ
transition form factor. By also exploiting the longitudinal beam polarization, a total of 71
ρ
0
spin-density matrix elements is determined from the extracted 25 parameters, in contrast to only 53 elements as directly determined in earlier analyses.
We report the first measurement of the parity-violating elastic electron scattering asymmetry on ^{27}Al. The ^{27}Al elastic asymmetry is A_{PV}=2.16±0.11(stat)±0.16(syst) ppm, and was measured at ...⟨Q^{2}⟩=0.02357±0.00010 GeV^{2}, ⟨θ_{lab}⟩=7.61°±0.02°, and ⟨E_{lab}⟩=1.157 GeV with the Q_{weak} apparatus at Jefferson Lab. Predictions using a simple Born approximation as well as more sophisticated distorted-wave calculations are in good agreement with this result. From this asymmetry the ^{27}Al neutron radius R_{n}=2.89±0.12 fm was determined using a many-models correlation technique. The corresponding neutron skin thickness R_{n}-R_{p}=-0.04±0.12 fm is small, as expected for a light nucleus with a neutron excess of only 1. This result thus serves as a successful benchmark for electroweak determinations of neutron radii on heavier nuclei. A tree-level approach was used to extract the ^{27}Al weak radius R_{w}=3.00±0.15 fm, and the weak skin thickness R_{wk}-R_{ch}=-0.04±0.15 fm. The weak form factor at this Q^{2} is F_{wk}=0.39±0.04.
The Q sub(weak) experimental apparatus Allison, T; Anderson, M; Androic, D ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
05/2015, Letnik:
781
Journal Article
Recenzirano
The Jefferson Lab experiment determined the weak charge of the proton by measuring the parity-violating elastic scattering asymmetry of longitudinally polarized electrons from an unpolarized liquid ...hydrogen target at small momentum transfer. A custom apparatus was designed for this experiment to meet the technical challenges presented by the smallest and most precise asymmetry ever measured. Technical milestones were achieved at Jefferson Lab in target power, beam current, beam helicity reversal rate, polarimetry, detected rates, and control of helicity-correlated beam properties. The experiment employed 180 mu A of 89% longitudinally polarized electrons whose helicity was reversed 960 times per second. The electrons were accelerated to 1.16 GeV and directed to a beamline with extensive instrumentation to measure helicity-correlated beam properties that can induce false asymmetries. Moeller and Compton polarimetry were used to measure the electron beam polarization to better than 1%. The electron beam was incident on a 34.4 cm liquid hydrogen target. After passing through a triple collimator system, scattered electrons between 5.8 degree and 11.6 degree were bent in the toroidal magnetic field of a resistive copper-coil magnet. The electrons inside this acceptance were focused onto eight fused silica Cherenkov detectors arrayed symmetrically around the beam axis. A total scattered electron rate of about 7 GHz was incident on the detector array. The detectors were read out in integrating mode by custom-built low-noise pre-amplifiers and 18-bit sampling ADC modules. The momentum transfer Q super(2)=0.025 GeV super(2) was determined using dedicated low-current measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted; see image) Hard exclusive electroproduction of ... mesons is studied with the HERMES spectrometer at the DESY laboratory by scattering ...27.6 GeV positron and electron beams off a transversely polarized hydrogen target. The amplitudes of five azimuthal modulations of the single-spin asymmetry of the cross section with respect to the transverse proton polarization are measured. They are determined in the entire kinematic region as well as for two bins in photon virtuality and momentum transfer to the nucleon. Also, a separation of asymmetry amplitudes into longitudinal and transverse components is done. These results are compared to a phenomenological model that includes the pion pole contribution. Within this model, the data favor a positive ... transition form factor.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted; see image).Exclusive electroproduction of ... mesons on unpolarized hydrogen and deuterium targets is studied in the kinematic region ...of ... GeV..., 3.0 GeV ... 6.3 GeV, and ... GeV... Results on the angular distribution of the ... meson, including its decay products, are presented. The data were accumulated with the HERMES forward spectrometer during the 1996-2007 running period using the 27.6 GeV longitudinally polarized electron or positron beam of HERA. The determination of the virtual-photon longitudinal-to-transverse cross-section ratio reveals that a considerable part of the cross section arises from transversely polarized photons. Spin density matrix elements are presented in projections of ... or ... Violation of ...-channel helicity conservation is observed for some of these elements. A sizable contribution from unnatural-parity-exchange amplitudes is found and the phase shift between those amplitudes that describe transverse ... production by longitudinal and transverse virtual photons, ... and ..., is determined for the first time. A hierarchy of helicity amplitudes is established, which mainly means that the unnatural-parity-exchange amplitude describing the ... transition dominates over the two natural-parity-exchange amplitudes describing the ... and ... transitions, with the latter two being of similar magnitude. Good agreement is found between the HERMES proton data and results of a pQCD-inspired phenomenological model that includes pion-pole contributions, which are of unnatural parity.
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