422 research outputs found
Radiative Correction to the Transferred Polarization in Elastic Electron-Proton Scattering
Model independent radiative correction to the recoil proton polarization for
the elastic electron-proton scattering is calculated within method of electron
structure functions. The explicit expressions for the recoil proton
polarization are represented as a contraction of the electron structure and the
hard part of the polarization dependent contribution into cross-section. The
calculation of the hard part with first order radiative correction is
performed. The obtained representation includes the leading radiative
corrections in all orders of perturbation theory and the main part of the
second order next-to-leading ones. Numerical calculations illustrate our
analytical results.Comment: 14 pages, 4 figure
The ratio of proton's electric to magnetic form factors measured by polarization transfer
The ratio of the proton's elastic electromagnetic form factors was obtained
by measuring the transverse and longitudinal polarizations of recoiling protons
from the elastic scattering of polarized electrons with unpolarized protons.
The ratio of the electric to magnetic form factor is proportional to the ratio
of the transverse to longitudinal recoil polarizations. The ratio was measured
over a range of four-momentum transfer squared between 0.5 and 3.5 GeV-squared.
Simultaneous measurement of transverse and longitudinal polarizations in a
polarimeter provides good control of the systematic uncertainty. The results
for the ratio of the proton's electric to magnetic form factors show a
systematic decrease with increasing four momentum squared, indicating for the
first time a marked difference in the spatial distribution of charge and
magnetization currents in the proton.Comment: 5 pages, 2 figures, version of paper after corrections due to
referees comments and shortened by removing one figure for Physical Review
Letter
A Comparison of Polarization Observables in Electron Scattering from the Proton and Deuteron
Recoil proton polarization observables were measured for both the p(,e) and d(,en reactions at two values of Q using a newly commissioned proton
Focal Plane Polarimeter at the M.I.T.-Bates Linear Accelerator Center. The
hydrogen and deuterium spin-dependent observables and
, the induced polarization and the form factor ratio
were measured under identical kinematics. The deuterium and
hydrogen results are in good agreement with each other and with the plane-wave
impulse approximation (PWIA).Comment: 9 pages, 1 figure; accepted by Phys. Rev. Let
Recoil Polarization Measurements for Neutral Pion Electroproduction at Q^2=1 (GeV/c)^2 Near the Delta Resonance
We measured angular distributions of differential cross section, beam
analyzing power, and recoil polarization for neutral pion electroproduction at
Q^2 = 1.0 (GeV/c)^2 in 10 bins of W across the Delta resonance. A total of 16
independent response functions were extracted, of which 12 were observed for
the first time. Comparisons with recent model calculations show that response
functions governed by real parts of interference products are determined
relatively well near 1.232 GeV, but variations among models is large for
response functions governed by imaginary parts and for both increases rapidly
with W. We performed a nearly model-independent multipole analysis that adjusts
complex multipoles with high partial waves constrained by baseline models.
Parabolic fits to the W dependence of the multipole analysis around the Delta
mass gives values for SMR = (-6.61 +/- 0.18)% and EMR = (-2.87 +/- 0.19)% that
are distinctly larger than those from Legendre analysis of the same data.
Similarly, the multipole analysis gives Re(S0+/M1+) = (+7.1 +/- 0.8)% at
W=1.232 GeV, consistent with recent models, while the traditional Legendre
analysis gives the opposite sign because its truncation errors are quite
severe. Finally, using a unitary isobar model (UIM), we find that excitation of
the Roper resonance is dominantly longitudinal with S1/2 = (0.05 +/- 0.01)
GeV^(-1/2) at Q^2=1. The ReS0+ and ReE0+ multipoles favor pseudovector coupling
over pseudoscalar coupling or a recently proposed mixed-coupling scheme, but
the UIM does not reproduce the imaginary parts of 0+ multipoles well.Comment: 60 pages, 54 figure
Spin-Momentum Correlations in Quasi-Elastic Electron Scattering from Deuterium
We report on a measurement of spin-momentum correlations in quasi-elastic
scattering of longitudinally polarized electrons with an energy of 720 MeV from
vector-polarized deuterium. The spin correlation parameter was
measured for the reaction for missing
momenta up to 350 MeV/ at a four-momentum transfer squared of 0.21
(GeV/c). The data give detailed information about the spin structure of the
deuteron, and are in good agreement with the predictions of microscopic
calculations based on realistic nucleon-nucleon potentials and including
various spin-dependent reaction mechanism effects. The experiment demonstrates
in a most direct manner the effects of the D-state in the deuteron ground-state
wave function and shows the importance of isobar configurations for this
reaction.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Lett. for publicatio
Probing the high momentum component of the deuteron at high Q^2
The d(e,e'p) cross section at a momentum transfer of 3.5 (GeV/c)^2 was
measured over a kinematical range that made it possible to study this reaction
for a set of fixed missing momenta as a function of the neutron recoil angle
theta_nq and to extract missing momentum distributions for fixed values of
theta_nq up to 0.55 GeV/c. In the region of 35 (deg) <= theta_nq <= 45 (deg)
recent calculations, which predict that final state interactions are small,
agree reasonably well with the experimental data. Therefore these experimental
reduced cross sections provide direct access to the high momentum component of
the deuteron momentum distribution in exclusive deuteron
electro-disintegration.Comment: 5 pages, 2 figure
Measurement of GEp/GMp in ep -> ep to Q2 = 5.6 GeV2
The ratio of the electric and magnetic form factors of the proton, GEp/GMp,
was measured at the Thomas Jefferson National Accelerator Facility (JLab) using
the recoil polarization technique. The ratio of the form factors is directly
proportional to the ratio of the transverse to longitudinal components of the
polarization of the recoil proton in the elastic
reaction. The new data presented in this article span the range 3.5 < Q2 < 5.6
GeV2 and are well described by a linear Q2 fit. Also, the ratio QF2p/F1p
reaches a constant value above Q2=2 GeV2.Comment: 6 pages, 4 figures Added two names to the main author lis
Conceptualizing Communication Capital for a Changing Environment
With rapidly evolving technologies, boundaries between traditional modes of communication have blurred, creating an environment that scholars still describe from viewpoints as researchers in interpersonal, organizational or mass communication. This manuscript looks at the social capital literature and argues for conceptualizing “communication capital” to help understand the impact of communication phenomena in a changing environment. The literature has treated interpersonal communication variables as components of social capital and mass communication variables as factors affecting social capital, but scholars long ago recognized their reinforcing nature, leading us to develop a concept of communication capital merging symbolic activity across domains in its potential for impacting civic engagement, defined as persistent communication patterns that facilitate social problem solving in the community. Analysis of survey data shows that 4 dimensions of communication capital explain variance in civic engagement beyond that accounted for by traditional measures of social capital, media use, neighborhood communication, and efficacy
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
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