Abstract
We investigate effects of explicit chiral symmetry breaking on the ( γ, π 0) amplitude at threshold (the photoproduction “sigma”- term) using a chiral bag model of the nucleon. We point out a strong connection between this term and the flavor singlet axial coupling constant g A 0 which measures the spin fraction carried by the quarks inside the nucleon. A comparison is made with recent experimental data which show deviations from the standard low energy theorem for photoproduction. The possible origin of this discrepancy is discussed on the basis of our calculations.
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