ABC Effect and Resonance Structure in the Double-Pionic Fusion to $^3$He

TL;DR

This study investigates the ABC effect and associated resonance structure in double-pionic fusion to helium-3, revealing a correlation with a resonance-like energy dependence and supporting the $d^*$ resonance hypothesis through experimental data.

Contribution

It provides the first comprehensive measurement of the ABC effect in double-pionic fusion to $^3$He, demonstrating the resonance's role alongside conventional mechanisms.

Findings

Observation of low-mass enhancement in $ ext{pi} ext{pi}$ spectrum

Correlation between ABC effect and resonance-like energy dependence

Differential cross sections support $d^*$ resonance formation

Abstract

Exclusive and kinematically complete measurements of the double pionic fusion to $^3$He have been performed in the energy region of the so-called 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 observations in the basic $pn \to d \pi^0\pi^0$ reaction and in the $dd \to ^4$He$\pi^0\pi^0$ reaction, the data reveal a correlation between the ABC effect and a resonance-like energy dependence in the total cross section. Differential cross sections are well described by the hypothesis of $d^*$ resonance formation during the reaction process in addition to the conventional $t$-channel $\Delta\Delta$ mechanism. The deduced $d^*$ resonance width can be understood from collision broadening due to Fermi motion of the nucleons in initial and final nuclei.