Nuclear mass form factors from coherent photoproduction of πmesons

Abstract

Abstract. Data for coherent photoproduction of π 0 mesons from nuclei ( 12 C, 40 Ca, 93 Nb, nat Pb), recently measured with the TAPS detector at the Mainz MAMI accelerator, have been analyzed in view of the mass form factors of the nuclei. The form factors have been extracted in plane wave approximation of the A(γ, π 0 )A reaction and corrected for final state interaction effects with the help of distorted wave impulse approximations. Nuclear mass rms-radii have been calculated from the slope of the form factors for q 2 → 0. Furthermore, the Helm model (hard sphere form factor folded with Gaussian) was used to extract diffraction radii from the zeroes of the form factor and skin thicknesses from the position and height of its first maximum. The diffraction radii from the Helm model agree with the corresponding charge radii obtained from electron scattering experiments within their uncertainties of a few per cent. The rms-radii from the slope of the form factors are systematically lower by up to 5% for PWIA and up to 10% for DWIA. Also the skin thicknesses extracted from the Helm model are systematically smaller than their charge counter parts.PACS. 13.60.Le meson production -25.20.Lj photoproduction reactions -21.10.Gv mass distributions

Production of η and η′ mesons on nucleons and nuclei

“…The amplitudes of the elementary reactions are then evaluated at W (E γ , q). Up to this point there is analogy to the much better studied coherent π 0 photoproduction off (mostly J = 0) nuclei [144,145,146,147]. In this case the spin-and isospin-independent components of the elementary amplitude from all nucleons must be coherently added.…”

“…However, with few exceptions, the typical cross sections are small and the experiments are demanding (in most cases coherent reactions are difficult to identify in the presence of much larger contributions from breakup reactions). Detailed studies are so far only available for π 0 mesons to investigate the in-medium behaviour of the ∆(1232) resonance (see e.g., [144,145]) and, in a completely different context, for the extraction of nuclear mass form factors [146,147,148].…”

Neutron skin studies in heavy nuclei with coherent p0 photo-production

“…One powerful method to measure the neutron distribution using electromagnetic probes is coherent π 0 photoproduction [15][16][17][18]. Here, a photon beam impinges on a nucleus with a mass number A, probing the entire volume.…”

“…The protons and neutrons of the target material contribute equally to the total cross section for the coherent π 0 photoproduction, resulting in a sensitivity to the matter distribution of the nucleus [17]. In the simplest theoretical description, the plane wave impulse approximation (PWIA), the differential cross section for the coherent π 0 photoproduction for nuclei with spin zero can be written as:…”

“…m N is the nucleon mass, q(E γ , θ π ) the momentum transfer to the nucleus and F(q) is the nuclear matter form factor. √ s is the total energy of the photon-nucleon pair, E * γ , k * are the photon energy and the momentum, respectively, both in the photon-nucleon CM, Θ * π , q * π are the pion angle and momentum, respectively, also in the photon-nucleon CM [17].…”