Speaker
Description
In heavy nuclei, the distribution of neutrons extends out further than the proton distribution forming a so-called “neutron skin”. An accurate experimental determination of the neutron skin thickness of heavy nuclei would provide a unique constraint on the symmetry energy of the nuclear Equation Of State which strongly depends on poorly constrained three-body forces. Photons have an advantage over other nuclear probes for this purpose since they can interact with the whole volume of the nucleus without any Coulomb scattering effects.
Coherent neutral pion photoproduction on nuclei (γ + A → π_0 + A, where A is a nucleus in its ground state) is an attractive approach to obtain information on the distribution of neutrons. The spin-zero nuclei are of special interest since in this case protons and neutrons contribute with the same amplitude to the coherent photoproduction in the incident photon energy range from threshold to 350 MeV. Therefore, the reaction is sensitive to the distribution of nucleons and the measured cross section is proportional to the nuclear mass form-factor [1]. Information on the neutron distribution can be extracted by comparing the diffraction pattern of the measured cross section with theoretical calculations. Recently, the skin thickness in 208Pb was measured in this way by the A2 collaboration at MAMI [2].
The method of coherent pion photoproduction provides an efficient tool to study the neutron skin however requires a reliable theoretical model. Because the cross section is strongly affected by final-state interactions of the pion on the way out of the nucleus, this effect has to be accounted for in the model calculations [2].
In this work, we develop a new coherent pion photoproduction reaction code in the framework of the distorted wave impulse approximation in the momentum space. To reliably account for the pion-nucleus final-state interaction we devise a second-order pion-nucleus optical potential which involves analysis of pion-nucleus elastic scattering as a solution of the Lippmann-Schwinger equation. The optical potential is constructed on the base of the individual pion-nucleon scattering amplitudes extracted from SAID [3] and the harmonic oscillator shell model is used to develop its second-order part. Finally, we estimate optimal energy-independent parameters of the optical potential by a global fit of pion-carbon total, reaction and differential elastic cross sections.
[1] D. Drechsel et al., Nucl. Phys. A 660, 423 (1999).
[2] C. M. Tarbert et al., Phys. Rev. Lett. 112, 242502 (2014).
[3] R. L. Workman et al., Phys. Rev. C 86, 035202 (2012).
| Topic | Nuclear Structure and Nuclear Astrophysics |
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