ABC Resonance Structure in the Double-Pionic Fusion to 4He

TL;DR

This study reports the observation of the ABC resonance structure in the double-pionic fusion to helium-4, revealing a correlation between the ABC effect and a resonance-like energy dependence in the cross section, supported by high-statistics measurements.

Contribution

First high-statistics, exclusive measurements of the double pionic fusion to helium-4 showing the ABC effect linked to a resonance structure at 2.37 GeV.

Findings

Resonance at 2.37 GeV observed in the total cross section.

The resonance width is approximately 160 MeV.

Differential cross sections support a $pn$ resonance formation hypothesis.

Abstract

Exclusive and kinematically complete high-statistics measurements of the double pionic fusion reaction $dd \to ^4$He$\pi^0\pi^0$ have been performed in the energy range 0.8 - 1.4 GeV covering thus the region of the ABC effect, which denotes a pronounced low-mass enhancement in the $\pi\pi$-invariant mass spectrum. The experiments were carried out with the WASA detector setup at COSY. Similar to the observation in the basic $pn \to d \pi^0\pi^0$ reaction, the data reveal a correlation between the ABC effect and a resonance-like energy dependence in the total cross section. The maximum occurs at m=2.37 GeV + 2$m_N$, i.e. at the same position as in the basic reaction. The observed resonance width $\Gamma \approx$ 160 MeV can be understood from broadening due to Fermi motion of the nucleons in initial and final nuclei together with collision damping. Differential cross sections are described equally well by the hypothesis of a $pn$ resonance formation during the reaction process.