J. D. Carroll

Toward a resolution of the proton size puzzle

We show that off-mass-shell effects arising from the internal structure of the proton provide a new proton polarization mechanism in the Lamb shift, proportional to the lepton mass to the fourth power. This effect is capable of resolving the current puzzle regarding the difference in the proton radius extracted from muonic compared with electronic hydrogen experiments. These off-mass-shell effects could be probed in several other experiments. A significant ambiguity appearing in dispersion relation evaluations of the proton polarizability contribution to the Lamb shift is noted.

Quark-Meson Coupling Model, Nuclear Matter Constraints and Neutron Star Properties

We explore the equation of state for nuclear matter in the quark-meson coupling model, including full Fock terms. The comparison with phenomenological constraints can be used to restrict the few additional parameters appearing in the Fock terms which are not present at the Hartree level. Because the model is based upon the in-medium modification of the quark structure of the bound hadrons, it can be readily extended to include hyperons and to calculate the equation of state of dense matter in

Nonperturbative relativistic calculation of the muonic hydrogen spectrum

\ensuremath{\beta}

Phase Transition from QMC Hyperonic Matter to Deconfined Quark Matter

equilibrium. This leads naturally to a study of the properties of neutron stars, including their maximum mass, radii, and density profiles.

Nuclear quasielastic electron scattering limits nucleon off-mass shell properties

We investigate the muonic hydrogen 2P{sub 3/2}{sup F=2} to 2S{sub 1/2}{sup F=1} transition through a precise, nonperturbative numerical solution of the Dirac equation including the finite-size Coulomb force and finite-size vacuum polarization. The results are compared with earlier perturbative calculations of (primarily) [E. Borie, Phys. Rev. A 71, 032508 (2005); E. Borie and G. A. Rinker, Rev. Mod. Phys. 54, 67 (1982); E. Borie, Z. Phys. A 275, 347 (1975) and A. P. Martynenko, Phys. Rev. A 71, 022506 (2005); A. Martynenko, Phys. At. Nucl. 71, 125 (2008), and K. Pachucki, Phys. Rev. A 53, 2092 (1996)] and experimental results recently presented by Pohl et al.[Nature (London) 466, 213 (2010)], in which this very comparison is interpreted as requiring a modification of the proton charge radius from that obtained in electron scattering and electronic hydrogen analyses. We find no significant discrepancy between the perturbative and nonperturbative calculations, and we present our results as confirmation of the perturbative methods.

The Radius of the Proton: Size Does Matter

We investigate the possibility and consequences of phase transitions from an equation of state (EOS) describing nucleons and hyperons interacting via mean fields of {sigma}, {omega}, and {rho} mesons in the recently improved quark-meson coupling (QMC) model to an EOS describing a Fermi gas of quarks in an MIT bag. The transition to a mixed phase of baryons and deconfined quarks, and subsequently to a pure deconfined quark phase, is described using the method of Glendenning. The overall EOS for the three phases is calculated for various scenarios and used to calculate stellar solutions using the Tolman-Oppenheimer-Volkoff equations. The results are compared with recent experimental data, and the validity of each case is discussed with consequences for determining the species content of the interior of neutron stars.

The Quark‐Meson Coupling model as a description of dense matter

[...] The use of quasielastic electron nucleus scattering is shown to provide significant constraints on models of the proton electromagnetic form factor of off-shell nucleons. Such models can be constructed to be consistent with constraints from current conservation and low-energy theorems, while also providing a contribution to the Lamb shift that might potentially resolve the proton radius puzzle in muonic hydrogen. However, observations of quasielastic scattering limit the overall strength of the off-shell form factors to values that correspond to small contributions to the Lamb shift.

QMC and the nature of dense matter: written in the stars?

The measurement by Pohl et al. [1] of the 2S1/2F = 1 to 2P3/2F = 2 transition in muonic hydrogen and the subsequent analysis has led to a conclusion that the rms charge radius of the proton differs from the accepted (CODATA [2]) value by approximately 4%, leading to a 4.9σ discrepancy. We investigate the muonic hydrogen spectrum relevant to this transition using bound‐state QED with Dirac wave‐functions and comment on the extent to which the perturbation‐theory analysis which leads to the above conclusion can be confirmed.

Neutral pion decay into nu anti-nu in dense skyrmion matter

Quantum Hadrodynamics provides a useful framework for investigating dense matter, yet it breaks down easily when strangeness carrying baryons are introduced into the calculations, as the baryon effective masses become negative due to large meson field potentials. The Quark‐Meson Coupling model overcomes this issue by incorporating the quark structure of the nucleon, thus allowing for a feedback between the the nuclei and the interaction with the meson fields. With the inclusion of this feature, QMC provides a successful description of finite nuclei and nuclear matter. We present the latest parameterization of QMC and discuss the predictions for dense nuclear matter and ‘neutron’ stars.

Resolution of the proton radius puzzle via off-shell form factors

We discuss the recent progress in calculating the properties of ‘hybrid stars’ (stellar objects similar to neutron stars, classified by the incorporation of non‐nucleonic degrees of freedom, including but not limited to hyperons and/or a quark‐matter core) using the octet‐baryon Quark‐Meson Coupling (QMC) model. The version of QMC used is a recent improvement which includes the in‐medium modification of the quark‐quark hyperfine interaction.