Abstract
We study the predictions of isospin invariance in πN backward data from threshold to 13 GeV/ c. Below 1.5 GeV/ c especially below 1 GeV/ c the present published experimental data systematically saturate and even violate the triangle inequality implied by isospin invariance. Conventional electromagnetic corrections or mass and width differences in formed resonances (as e.g. the Δ ++ and Δ 0) are an order of magnitude too small to explain the discrepancy. Thus either there are large systematic errors in the data or isospin invariance is violated, inherently or by strongly enhanced electromagnetic effects. Above 1.5 GeV/ c the inequality is satisfied by the data and we discuss the expected high-energy behaviour from the Regge model of Δ and N α dominance. Duality and exchange degeneracy predictions of phase relations between isospin amplitudes are used to speculate on the possible connection between duality and the fact that the data lie close to the limit of the inequality.
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