Double-Pionic Fusion of Nuclear Systems and the ABCEffect -- Aproaching a Puzzle by Exclusive and Kinematically Complete Measurements

TL;DR

This study presents exclusive measurements of the double pionic fusion reaction pn → d π^0π^0, revealing the ABC effect as a resonance phenomenon linked to a ΔΔ system, and offers a simple s-channel resonance explanation.

Contribution

First exclusive, kinematically complete measurements of the pn → d π^0π^0 reaction, clarifying the ABC effect's association with a resonance and ΔΔ formation.

Findings

ABC effect linked to a resonance-like energy dependence

Formation of a ΔΔ system in the intermediate state

Simple s-channel resonance model describes data well

Abstract

The ABC effect - a puzzling low-mass enhancement in the $\pi\pi$ invariant mass spectrum - is well-known from inclusive measurements of two-pion production in nuclear fusion reactions. Here we report on first exclusive and kinematically complete measurements of the most basic double pionic fusion reaction $pn \to d \pi^0\pi^0$ at 1.03 and 1.35 GeV. The measurements, which have been carried out at CELSIUS-WASA, reveal the ABC effect to be a $(\pi\pi)_{I=L=0}$ channel phenomenon associated with both a resonance-like energy dependence in the integral cross section and the formation of a $\Delta\Delta$ system in the intermediate state. A corresponding simple s-channel resonance ansatz provides a surprisingly good description of the data.