Electromagnetic Form Factors and Charge Densities From Hadrons to Nuclei

A simple exact covariant model in which a scalar particle is modeled as a bound state of two different particles is used to elucidate relativistic aspects of electromagnetic form factors. The model form factor is computed using an exact covariant calculation of the lowest-order triangle diagram and shown to be the same as that obtained using light-front techniques. The meaning of transverse density is explained using coordinate space variables, allowing us to identify a true mean-square transverse size directly related to the form factor. We show that the rest-frame charge distribution is generally not observable because of the failure to uphold current conservation. Neutral systems of two charged constituents are shown to obey the lore that the heavier one is generally closer to the transverse origin than the lighter one. It is argued that the negative central charge density of the neutron arises, in pion-cloud models, from pions of high longitudinal momentum. The non-relativistic limit is defined precisely and the ratio of the binding energy to that of the mass of the lightest constituent is shown to govern the influence of relativistic effects. The exact relativistic formula for the form factor reduces to the familiar one of the three-dimensional Fourier transform of a square of a wave function for a very limited range of parameters. For masses that mimic the quark-di-quark model of the nucleon we find substantial relativistic corrections for any value of $Q^2$. A schematic model of the lowest s-states of nuclei is used to find that relativistic effects decrease the form factor for light nuclei but increase the form factor for heavy nuclei. Furthermore, these states are strongly influenced by relativity.Comment: 18 pages, 11 figure

Charge Density of the Neutron

A model-independent analysis of the infinite-momentum-frame charge density of partons in the transverse plane is presented for the nucleon. We find that the neutron parton charge density is negative at the center, so that the square of the transverse charge radius is positive, in contrast with many expectations. Additionally, the proton's central u quark charge density is larger than that of the d quark by about 70 %. The proton (neutron) charge density has a long range positively (negatively) charged component.Comment: 7 pages, three figures The replacement mainly concerns correcting an error made in computing the proton up and down quark densities from the correctly computed proton and neutron charge densities. The proton central u quark density is now larger than that of the d quar

On the relation between the Deuteron Form Factor at High Momentum Transfer and the High Energy Neutron-Proton Scattering Amplitude

A non-relativistic potential-model version of the factorization assumption, used in perturbative QCD calculations of hadronic form factors, is used, along with the Born approximation valid at high energies, to derive a remarkably simple relationship between the impulse approximation contribution to the deuteron form factor at high momentum transfer and the high energy neutron-proton scattering amplitude. The relation states that the form factor at a given value of $Q^2$ is proportional to the scattering amplitude at a specific energy and scattering angle. This suggests that an accurate computation of the form factors at large $Q^2$ requires a simultaneous description of the phase-shifts at a related energy, a statement that seems reasonable regardless of any derivation. Our form factor-scattering amplitude relation is shown to be accurate for some examples. However, if the potential consists of a strong short distance repulsive term and a strong longer ranged attractive term, as typically occurs in many realistic potentials, the relation is found to be accurate only for ridiculously large values of $Q$. More general arguments, using only the Schroedinger equation, suggest a strong, but complicated, relationship between the form factor and scattering amplitude. Furthermore, the use of recently obtained soft potentials, along with an appropriate current operator, may allow calculations of form factors that are consistent with the necessary phase shifts.Comment: 14 pages, 4 figures, The discussion has been extended by including numerical examples and general argument

The Inclusive-Exclusive Connection and the Neutron Negative Central Charge Density

We find an interpretation of the recent finding that the central charge density of the neutron is negative by using models of generalized parton distributions at zero skewness to relate the behavior of deep inelastic scattering quark distributions, evaluated at large values of Bjorken x, to the transverse charge density evaluated at small distances. The key physical input of these models is the Drell-Yan-West relation We find that the d quarks dominate the neutron structure function for large values of Bjorken x, where the large longitudinal momentum of the struck quark has a significant impact on determining the center-of-momentum of the system, and thus the "center" of the nucleon in the transverse position plane.Comment: 17 pages, 8 figures. Text of an invited talk presented by G. A. Miller at the 2008 Division of Nuclear Physics Meeting in Oakland. Prepared for Int. Journ. Mod. Phys.

Color Transparency at COMPASS energies

Pionic quasielastic knockout of protons from nuclei at 200 GeV show very large effects of color transparency as -t increases from 0 to several GeV^2. Similar effects are expected for quasielastic photoproduction of vector mesons.Comment: 9 pages, 4 figure

Impact of soil erosion on soil productivity

To understand the effect that soil characteristics have on soil productivity (and the impact of erosion on soil productivity), producers need a working knowledge of soil properties. Iowa\u27s soils range in thickness from a few inches to several feet. Each soil has a unique combination of properties; however, some properties are common to all soils. All soils consist of solid materials and pores. Soil solids consist of mineral particles and organic matter. The pores are filled with air, water, or both. The ratio of soil solids to pores in a defined volume is a measure of soil density

Governor kicks off conservation milestones campaign

Governor Tom Vilsack helped kick off a new conservation campaign called Conservation Milestones at the Millennium. The kickoff was held in conjunction with a riparian demonstration field day held recently at the vonLackum/Mack farm in Grundy County. The Grundy County event was the first of 10 statewide events, each celebrating a different conservation milestone. The statewide campaign will celebrate conservation achievements on Iowa\u27s private lands, while raising awareness that more can be done. At the Grundy County milestone, special recognition was given to the 100,000 acres of conservation buffers that are now improving water quality and wildlife habitat in Iowa