Search results for "quantum electrodynamics"
showing 10 items of 809 documents
The Pion Velocity in Dense Skyrmion Matter
2003
We have developed a field theory formalism to calculate $in$-$medium$ properties of hadrons within a unified approach that exploits a single Lagrangian to describe simultaneously both matter background and meson fluctuations. In this paper we discuss the consequences on physical observables of a possible phase transition of hadronic matter taking place in the chiral limit. We pay special attention to the pion velocity $v_\pi$, which controls, through a dispersion relation, the pion propagation in the hadronic medium. The $v_\pi$ is defined in terms of parameters related to the matrix element in matter of the axial-vector current, namely, the in-medium pion decay constants, $f_t$ and $f_s$. …
Photon Scattering with the Lorentz Integral Transform Method
2011
The application of the Lorentz integral transform (LIT) method to photon scattering off nuclei is presented in general. As an example, elastic photon scattering off the deuteron in the unretarded dipole approximation is considered using the LIT method. The inversion of the integral transform is discussed in detail paying particular attention to the high-energy contributions in the resonance term. The obtained E1-polarizabilities are compared to results from the literature. The corresponding theoretical cross section is confronted with experimental results confirming, as already known from previous studies, that the E1-contribution is the most important one at lower energies.
Photoproduction of pions on 3He and 3H
1969
Abstract In this paper the photoproduction of pions on nuclei with three nucleons is considered in the “elastic” channels. The calculation is done using the impulse approximation and neglecting rescattering effects.
Violation of Siegert's theorem in the electro-excitation of the (1−, T = 0)-level at 7.117 MeV in 16O
1989
Abstract Cross sections are measured for the electro-excitation of the first 1−-level in 16O at extremely small momentum transfer. The model-independent interpretation with a Fourier-Bessel expansion of the transition charge density reveals an incompatibility with the life-time measurement at the photon point, i.e. a violation of Siegert's theorem.
Extended Thomas - Fermi expansion from nuclear linear response
1996
The coefficients of the successive orders of the extended Thomas - Fermi expansion of the kinetic energy density are obtained to the leading order in the density inhomogeneities. This is done by comparing the corresponding static polarizability with the exact value in two cases: a non-interacting nuclear infinite system and symmetric nuclear matter with a Skyrme interaction.
Threshold electro production of neutral pions at low q2 off the proton and off the deuteron
2000
Abstract The A1 Collaboration performs an extend program to measure threshold production of the neutral pion as test of Heavy Baryon Chiral Perturbation Theory. Starting with direct threshold measurements to extract the s wave amplitudes at threshold we continued with out of plane measurements with polarized beam to access the small p wave amplitudes. Coherent production off the deuteron is the first attempt to access the neutron amplitude in electro production.
Proton spin and the cheshire cat principle
1991
Abstract We discuss the proton matrix element of the flavor-singlet axial current (FSAC), often referred to as proton spin, in terms of the chiral bag model with due account of the axial anomaly and quantum number fractionation. We conclude that the contribution due to gluons is significant for large bag radii and that it is crucial in order to establish the Cheshire cat principle.
Polarized structure functions of the nucleon in the resonance region
2000
Abstract We present predictions for the spin structure functions of the proton in the framework of a unitary isobar model for one-pion photo- and electroproduction. Our results are compared with recent experimental data from SLAC. The first moments of the calculated structure functions fullfil the Gerasimov-Drell-Hearn and Burkhardt-Cottingham sum rules within an error of typically 5–10%.
Vacuum polarization around stars: Nonlocal approximation
2004
We compute the vacuum polarization associated with quantum massless fields around stars with spherical symmetry. The nonlocal contribution to the vacuum polarization is dominant in the weak field limit, and induces quantum corrections to the exterior metric that depend on the inner structure of the star. It also violates the null energy conditions. We argue that similar results also hold in the low energy limit of quantum gravity. Previous calculations of the vacuum polarization in spherically symmetric spacetimes, based on local approximations, are not adequate for newtonian stars.
Skyrme-force parametrization: Least-squares fit to nuclear ground-state properties.
1986
We investigate systematically the possibilities and the limits of the Skyrme force for reproducing nuclear ground-state properties in a spherical Hartree-Fock calculation. This investigation is performed by means of least-squares fits of the force parameters to the measured binding energy, diffraction radius, and surface width of eight selected nuclei. Particular emphasis is put on the density dependence of the interaction, which turns out to be determined mainly by the surface width. The least-squares fitting procedure yields the best-fit parameters together with uncertainties on them, and it also allows one to estimate the uncertainties of an extrapolation to other fields, e.g., nuclear m…