Role of a triangle singularity in the $\pi N(1535)$ contribution to $\gamma p \to p \pi^0 \eta$

Abstract

We have studied the $\gamma p \to p \pi^0 \eta$ reaction paying attention to the two main mechanisms at low energies, the $\gamma p \to \Delta(1700) \to \eta \Delta(1232)$ and the $\gamma p \to \Delta(1700) \to \pi N(1535)$. Both of them are driven by the photoexcitation of the $\Delta(1700)$ and the second one involves a mechanism that leads to a triangle singularity. We are able to evaluate quantitatively the cross section for this process and show that it agrees with the experimental determination. Yet, there are some differences with the standard partial wave analysis which does not include explicitly the triangle singularity. The exercise also shows the convenience to explore possible triangle singularities in other reactions and how a standard partial analysis can be extended to accommodate them.