42 research outputs found
Proton charge radius from electron scattering
The rms-radius of the proton charge distribution is a fundamental
quantity needed for precision physics. This radius, traditionally determined
from elastic electron-proton scattering via the slope of the Sachs form factor
extrapolated to momentum transfer =0, shows a large scatter. We
discuss the approaches used to analyze the e-p data, partly redo these analyses
in order to identify the sources of the discrepancies, and explore alternative
parameterizations. The problem lies in the model dependence of the
parameterized needed for the extrapolation. This shape of
is closely related to the shape of the charge density at large radii
, a quantity which is ignored in most analyses. When using our {\em physics}
knowledge about this large- density together with the information contained
in the high- data, the model dependence of the extrapolation is reduced and
different parameterizations of the pre-2010 data yield a consistent value for
. This value disagrees with the more precise value
determined from the Lamb shift in muonic hydrogen.Comment: To be published in special issue of Atoms "High precision mesurements
of fundamental constants
Precise determination of low-Q nucleon electromagnetic form factors and their impact on parity-violating e-p elastic scattering
The extraction of the strangeness form factors from parity violating elastic
electron-proton scattering is sensitive to the electromagnetic form factors at
low Q^2. We provide parameterizations for the form factors and uncertainties,
including the effects of two-photon exchange corrections to the extracted EM
form factors. We study effect of the correlations between different form
factors, in particular as they impact the parity violating asymmetry and the
extraction of the strangeness form factors. We provide a prescription to
extract the strangeness form factors from the asymmetry that provides an
excellent approximation of the full two-photon correction. The corrected form
factors are also appropriate as input for other low-Q analyses, although the
effects of correlations and two-photon exchange corrections may be different.Comment: 9 pages, 4 figures, to be submitted to Phys. Rev.
Coulomb distortion in high-Q^2 elastic e-p scattering
Recently, there has been a significant amount of activity to try and
understand the discrepancy between Rosenbluth and polarization transfer
measurements of the proton form factors. It has been suggested that the
standard use of plane-wave Born approximation in extracting the form factors is
insufficient, and that higher-order terms must also be included. Of the
corrections missing in standard prescriptions, Coulomb distortion is the most
well understood. In this paper, we examine the effect of Coulomb distortion on
the extraction of the proton form factors.Comment: 3 pages, 5 figures, submitted to Phys. Rev.
Troubles with the Proton rms-Radius
Due to the peculiar shape of the proton charge density ρ(r) the value of the rms-radius r rms determined from electron scattering data depends strongly on the density ρ(r) at large radii, which is not fixed by scattering data. Supplementing the data with the large-r shape of ρ(r) resulting from the Fock components (n+π, . . .) dominating the large-r behavior produces a more reliable value. This radius agrees with the one we previously extracted, but disagrees with the one recently obtained from muonic Hydrogen. The origin of the discrepancy is not understoo
Spin asymmetries for confined Dirac particles
We study the Bjorken `x' (or equivalently Nachtmann `xi') dependence of the
virtual photon spin asymmetry in polarized deep inelastic scattering of
electrons from hadrons. We use an exactly solved relativistic potential model
of the hadron, treating the constituents as independent massless Dirac
particles confined to an infinitely massive force center. The importance of
including the p-wave components of the Dirac wave function is demonstrated.
Comparisons are made to the observed data on the proton by taking into account
the observed flavor dependence of the valence quark distribution functions.Comment: 4 pages, 4 figues; submitted to Phys. Lett.