Pion axioproduction: The Delta resonance contribution

The process of pion axioproduction, $aN\to\pi N$, with an intermediate $\Delta$ resonance is analyzed using baryon chiral parturbation theory. The $\Delta$ resonance is included in two ways: First, deriving the $a\Delta N$-vertices, the axion is brought into contact with the resonance, and, second, taking the results of $\pi N$ elastic scattering including the $\Delta$, it is implicitly included in the form of a pion rescattering diagram. As a result, the partial wave cross section of axion-nucleon scattering shows an enhancement in the energy region around the $\Delta$ resonance. Because of the isospin breaking, the enhancement is not as pronounced as previously anticipated. However, since the isospin breaking here is much milder than that for usual hadronic processes, novel axion search experiments might still exploit this effect.