The ABC Effect in Double-Pionic Nuclear Fusion and a pn Resonance as its Possible Origin

TL;DR

This study provides the first comprehensive measurements of the ABC effect in double-pionic fusion, revealing a possible s-channel resonance as its origin, characterized by a low-mass enhancement and a narrow resonance peak.

Contribution

It introduces the first exclusive, high-statistics measurements confirming the ABC effect's association with a narrow s-channel resonance in double-pionic fusion reactions.

Findings

Observation of a low-mass enhancement in the pion-pion invariant mass.

Identification of a resonance peak 90 MeV below the ΔΔ mass.

Resonance width measured at only 50 MeV, much narrower than conventional models.

Abstract

The ABC effect -- a long-standing puzzle in double-pionic fusion -- has been reexamined by the first exclusive and kinematically complete measurements of solid statistics for the fusion reactions $pn \to d\pi^0\pi^0$, $pd \to ^3$He$\pi\pi$ and $dd \to ^4$He$\pi\pi$ using the WASA detector, first at CELSIUS and recently at COSY -- the latter with a statistics increased by another two orders of magnitude. In all cases we observe a huge low-mass enhancement in the $\pi\pi$-invariant mass accompanied by a pronounced $\Delta\Delta$ excitation. For the most basic fusion reaction, the $pn \to d\pi^0\pi^0$ reaction, we observe in addition a very pronounced resonance-like energy dependence in the total cross section with a maximum 90 MeV below the $\Delta\Delta$ mass and a width of only 50 MeV, which is five times smaller than expected from a conventional $t$-channel $\Delta\Delta$ excitation. This reveals the ABC effect to be the consequence of a s-channel resonance with the formfactor of this dibaryonic state being reflected in the low-mass enhancement of the $\pi\pi$-invariant mass. From the fusion reactions to $^3$He and $^4$He we learn that this resonance is robust enough to survive even in nuclei.