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It has been commonly assumed that low-energy neutral pion photoproductionfrom the proton can be described accounting only for S and P waves, and thathigher partial waves are irrelevant. We have found that this assumption is notcorrect and that the inclusion of D waves is necessary to obtain a reliableextraction of the $E_{0+}$ multipole from experimental data. This is due inlarge measure to the spontaneous breaking of chiral symmetry in QCD which leadsto very small S-wave contributions. This makes the usual partial wave expansionless accurate and although D waves are small, their contribution is enhancedthrough the interference with P waves, which compromises the S-wave extractionfrom data if D waves are not taken into account. In our work we have used HeavyBaryon Chiral Perturbation Theory to one loop, and up to ${\cal O}(q^4)$, toaccount for the S and P waves, while D waves are added in an almostmodel-independent way using standard Born terms and vector mesons. We also showthat higher partial waves do not play an important role.

The unexpected role of D waves in low-energy neutral pion photoproduction