Abstract
A short overview over the ongoing few-body program at WASA@COSY is given with special emphasis on the ABC phenomenon, which is known since 50 years without having found a consistent explanation. We show that this phenomenon is linked with a resonant energy dependence in the total cross section of isoscalar double-pionic fusion processes. This resonance structure is connected with the baryon-baryon system, in particular with the pn and ∆∆ systems, however, possesses a mass, which is 90 MeV below the nominal ∆∆ threshold and has a width of only 50 MeV, i. e., five times smaller than expected from a conventional t-channel ∆∆ process.
Highlights
The experimental program of WASA-at-COSY focuses on production and decay of meson and baryon systems, which arise from initial nucleon-nucleon collisions
These include decays of η, ω and η′, where experimental results reveal substantial discrepancies to NNLO ChPT predictions [1], as well as isospin breaking in the dd → απ0 reaction, η-mesic nuclei and the ABC-effect. The latter has been a puzzle since 50 years. It deals with the observation of a low-mass enhancement in the ππ invariant mass in double-pionic fusion reactions leading to few-nucleon systems
This contribution concentrates on the ABC-effect in the most basic system, the pn → dπ0π0 reaction, where new exclusive high-statistics measurements have been carried out with WASA@COSY
Summary
The experimental program of WASA-at-COSY focuses on production and decay of meson and baryon systems, which arise from initial nucleon-nucleon collisions. It deals with the observation of a low-mass enhancement in the ππ invariant mass in double-pionic fusion reactions leading to few-nucleon systems.
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