Carl E. Carlson

Partonic calculation of the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer

We estimate the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer through the scattering off a parton in the proton. We relate the two-photon exchange amplitude to the generalized parton distributions which appear in hard exclusive processes. We find that when taking the polarization transfer determinations of the form factors as input, adding in the 2-photon correction does reproduce the Rosenbluth cross sections.

Bounding noncommutative QCD

Abstract Jurco, Moller, Schraml, Schupp, and Wess have shown how to construct noncommutative SU(N) gauge theories from a consistency relation. Within this framework, we present the Feynman rules for noncommutative QCD and compute explicitly the most dangerous Lorentz-violating operator generated through radiative corrections. We find that interesting effects appear at the one-loop level, in contrast to conventional noncommutative U(N) gauge theories, leading to a stringent bound. Our results are consistent with others appearing recently in the literature that suggest collider limits are not competitive with low-energy tests of Lorentz violation for bounding the scale of spacetime noncommutativity.

The Proton Radius Puzzle

Abstract The proton size, specifically its charge radius, was thought known to about 1% accuracy. Now a new method probing the proton with muons instead of electrons finds a radius about 4% smaller, and to boot gives an uncertainty limit of about 0.1%. We review the different measurements, some of the calculations that underlie them, some of the suggestions that have been made to resolve the conflict, and give a brief overview new related experimental initiatives. At present, however, the resolution to the problem remains unknown.

Two-photon exchange contribution to elastic electron-nucleonscattering at large momentum transfer

We estimate the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer by using a quark-parton representation of virtual Compton scattering. We thus can relate the two-photon exchange amplitude to the generalized parton distributions which also enter in other wide angle scattering processes. We find that the interference of one- and two-photon exchange contribution is able to substantially resolve the difference between electric form factor measurements from Rosenbluth and polarization transfer experiments. Two-photon exchange has additional consequences which could be experimentally observed, including nonzero polarization effects and a positron-proton/electron-proton scattering asymmetry. The predicted Rosenbluth plot is no longer precisely linear; it acquires a measurable curvature, particularly at large laboratory angle.

Nucleon electromagnetic and gravitational form factors from holography

The electromagnetic form factors of nucleons are calculated using an anti-de Sitter (AdS)/QCD model by considering a Dirac field coupled to a vector field in the five-dimensional AdS space. We also calculate a gravitational or energy-momentum form factor by perturbing the metric from the static AdS solution. We consider both the hard-wall model where the AdS geometry is cut off at z{sub 0} and the soft-wall model where the geometry is smoothly cut off by a background dilaton field.

Gravitational form factors of vector mesons in an AdS/QCD model

We calculate gravitational form factors of vector mesons using a holographic model of QCD. These provide restrictions on the generalized parton distributions of vector mesons, via the sum rules connecting stress tensor form factors to generalized parton distributions. We concentrate on the traceless part of the stress tensor, which suffices to fix the momentum and angular momentum sum rules. The vector mesons appear noticeably more compact measured by the gravitational form factors than by the charge form factor.

Noncommutative gauge theory without Lorentz violation

The most popular noncommutative field theories are characterized by a matrix parameter

Higher-order proton structure corrections to the Lamb shift in muonic hydrogen

{\ensuremath{\theta}}^{\ensuremath{\mu}\ensuremath{\nu}}

Phenomenology of the Pentaquark Antidecuplet

that violates Lorentz invariance. We consider the simplest algebra in which the

Empirical transverse charge densities in the nucleon and the nucleon-to-Delta transition

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