257 research outputs found
Experimental determination of the evolution of the Bjorken integral at low Q^2
We extract the Bjorken integral Gamma^{p-n}_1 in the range 0.17 < Q^2 < 1.10
GeV^2 from inclusive scattering of polarized electrons by polarized protons,
deuterons and 3He, for the region in which the integral is dominated by nucleon
resonances. These data bridge the domains of the hadronic and partonic
descriptions of the nucleon. In combination with earlier measurements at higher
Q^2, we extract the non-singlet twist-4 matrix element f_2.Comment: Quoted world data updated. Minor change in some results, Minor
rephrasin
Experimental Study of Isovector Spin Sum Rules
We present the Bjorken integral extracted from Jefferson Lab experiment EG1b for 0.05\u3c Q2 2. The integral is fit to extract the twist-4 element f p−n2 which appears to be relatively large and negative. Systematic studies of this higher twist analysis establish its legitimacy at Q2 around 1 GeV2. We also performed an isospin decomposition of the generalized forward spin polarizability γ0. Although its isovector part provides a reliable test of the calculation techniques of chiral perturbation theory, our data disagree with the calculations
Precise Measurement of the Pi+ -> Pi0 e+ nu Branching Ratio
Using a large acceptance calorimeter and a stopped pion beam we have made a
precise measurement of the rare Pi+ -> Pi0 e+ Nu,(pi_beta) decay branching
ratio. We have evaluated the branching ratio by normalizing the number of
observed pi_beta decays to the number of observed Pi+ -> e+ Nu, (pi_{e2})
decays. We find the value of Gamma(Pi+ -> Pi0 e+ Nu)/Gamma(total) = [1.036 +/-
0.004(stat.) +/- 0.004(syst.) +/- 0.003(pi_{e2})] x 10^{-8}$, where the first
uncertainty is statistical, the second systematic, and the third is the pi_{e2}
branching ratio uncertainty. Our result agrees well with the Standard Model
prediction.Comment: 4 pages, 5 figures, 1 table, revtex4; changed content; updated
analysi
Determination of the Proton Spin Structure Functions for 0.05 \u3c Q\u3csup\u3e2\u3c/sup\u3e \u3c5GEV\u3csup\u3e2\u3c/sup\u3e Using CLAS
We present the results of our final analysis of the full data set of gp1 Q2, the spin structure function of the proton, collected using CLAS at Jefferson Laboratory in 2000-2001. Polarized electrons with energies of 1.6, 2.5, 4.2, and 5.7 GeV were scattered from proton targets 15NH3 dynamically polarized along the beam direction) and detected with CLAS. From the measured double spin asymmetries, we extracted virtual photon asymmetries Ap1 and Ap2 and spin structure functions g p1 and gp2 over a wide kinematic range (0.05 GeV2 \u3c Q2 \u3c 5 GeV2 and 1.08 GeV\u3c W \u3c 3 GeV) and calculated moments of gp1. We compare our final results with various theoretical models and expectations, as well as with parametrizations of the world data. Our data, with their precision and dense kinematic coverage, are able to constrain fits of polarized parton distributions, test pQCD predictions for quark polarizations at large x, offer a better understanding of quark-hadron duality, and provide more precise values of higher twist matrix elements in the framework of the operator product expansion
Differential cross sections for pion charge exchange on the proton at 27.5 MeV
We have measured pion single charge exchange differential cross sections on
the proton at 27.5 MeV incident kinetic energy in the center of
momentum angular range between and . The extracted cross
sections are compared with predictions of the standard pion-nucleon partial
wave analysis and found to be in excellent agreement.Comment: ReVTeX v3.0 with aps.sty, 23 pages in e-print format, 7 PostScript
Figures and 4 Tables, also available via anonymous ftp at
ftp://helena.phys.virginia.edu/pub/preprints/scx.p
A New Measurement of the Radiative Decay Width
High precision measurements of the differential cross sections for
photoproduction at forward angles for two nuclei, C and Pb, have
been performed for incident photon energies of 4.9 - 5.5 GeV to extract the
decay width. The experiment was done at Jefferson
Lab using the Hall B photon tagger and a high-resolution multichannel
calorimeter. The decay width was extracted by
fitting the measured cross sections using recently updated theoretical models
for the process. The resulting value for the decay width is . With the 2.8% total uncertainty, this result is a factor of 2.5 more
precise than the current PDG average of this fundamental quantity and it is
consistent with current theoretical predictions.Comment: 4 pages, 5 figure
Measurement of the Proton and Deuteron Spin Structure Function g_1 in the Resonance Region
We have measured the proton and deuteron spin structure functions g_1^p and
g_1^d in the region of the nucleon resonances for W^2 < 5 GeV^2 and and GeV^2 by inelastically scattering 9.7 GeV polarized
electrons off polarized and targets. We observe
significant structure in g_1^p in the resonance region. We have used the
present results, together with the deep-inelastic data at higher W^2, to
extract . This is the first
information on the low-Q^2 evolution of Gamma toward the Gerasimov-Drell-Hearn
limit at Q^2 = 0.Comment: 7 pages, 2 figure
Light Vector Mesons in the Nuclear Medium
The light vector mesons (, , and ) were produced in
deuterium, carbon, titanium, and iron targets in a search for possible
in-medium modifications to the properties of the meson at normal nuclear
densities and zero temperature. The vector mesons were detected with the CEBAF
Large Acceptance Spectrometer (CLAS) via their decays to . The rare
leptonic decay was chosen to reduce final-state interactions. A combinatorial
background was subtracted from the invariant mass spectra using a
well-established event-mixing technique. The meson mass spectrum was
extracted after the and signals were removed in a nearly
model-independent way. Comparisons were made between the mass spectra
from the heavy targets () with the mass spectrum extracted from the
deuterium target. With respect to the -meson mass, we obtain a small
shift compatible with zero. Also, we measure widths consistent with standard
nuclear many-body effects such as collisional broadening and Fermi motion.Comment: 15 pages, 18 figures, 3 table
Determination of the proton spin structure functions for 0.05 \u3c Q(2) \u3c 5GeV(2) using CLAS
We present the results of our final analysis of the full data set of g(1)(p) (Q(2)), the spin structure function of the proton, collected using CLAS at Jefferson Laboratory in 2000-2001. Polarized electrons with energies of 1.6, 2.5, 4.2, and 5.7 GeV were scattered from proton targets ((NH3)-N-15 dynamically polarized along the beam direction) and detected with CLAS. From the measured double spin asymmetries, we extracted virtual photon asymmetries A(1)(p) and A(2)(p) and spin structure functions g(1)(p) and g(2)(p) over a wide kinematic range (0.05 GeV2 \u3c Q(2) \u3c 5 GeV2 and 1.08 GeV\u3c W \u3c 3 GeV) and calculated moments of g(1)(p). We compare our final results with various theoretical models and expectations, as well as with parametrizations of the world data. Our data, with their precision and dense kinematic coverage, are able to constrain fits of polarized parton distributions, test pQCD predictions for quark polarizations at large x, offer a better understanding of quark-hadron duality, and provide more precise values of higher twist matrix elements in the framework of the operator product expansion
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