Pion-Nucleus Elastic Scattering, DCX and Sub-Threshold Resonances

TL;DR

This paper investigates how pionic atom states and nuclear instabilities influence elastic scattering and resonance structures in low-energy positive pion interactions with carbon-12 nuclei.

Contribution

It introduces a mechanism explaining oscillation structures in scattering cross sections via nuclear instability effects near the DCX reaction threshold.

Findings

Resonance structures are linked to pionic atom states near the DCX threshold.

Nuclear instability significantly affects resonance shape and strength.

The proposed mechanism explains observed oscillations in experimental data.

Abstract

In the scattering of positive pions by nuclei the double-charge-exchange (DCX) reaction creates the possibility of formation of pionic atom states in the vicinity of the threshold of this reaction. These quasi stationary states can manifest themselves as resonances in the elastic scattering cross section. The strength and shape of these resonances is strongly affected by an instability of the nucleus created in the DCX channel. It is shown that this mechanism can explain oscillation structures in the excitation function observed in scattering of low-energy positive pions from $^{12}\text{C}$.