Abstract
The ABC effect is an enhancement which appears at around 310 MeV in the missing-mass spectra of the inclusive reactions: pn → d , pd → 3 He , and dd → 4 He . For a long time it was regarded as evidence for a low-mass I = 0, J P = 0 + ππ resonance. But such states have never been seen in ordinary hadronic processes, like π N → N ππ . Most of the evidence now points to 3-3 resonance excitation in nuclei as the root cause of the effect. In this paper a number of possible mechanisms are examined. We find, on the basis of explicit calculations which neglect only the spins of the particles, that the one-pion-exchange model of Risser and Shuster provides a qualitative description of some of the data. The model of Anjos, Levy and Santoro is not substantiated by independent calculation, while the Brody model has little predictive power. A critical review of the various models is presented.
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