We present a calculation of ${\pi }^0$ photoproduction on the proton in manifestly Lorentz-invariant baryon chiral perturbation theory up to and including chiral order $q^4$. With the results we analyze the latest ${\pi }^0$ photoproduction data in the threshold region obtained at the Mainz Microtron. In the calculation of observables and the fit of the low-energy constants, we take $S$, $P$, and $D$ waves into account. We compare the results for the multipoles with the corresponding single-energy analysis. Furthermore, we also fit the $O\left(q^4\right)$ heavy-baryon chiral perturbation theory calculation and compare both results. We provide predictions for several polarization observables for future experiments. Finally, we discuss the $\beta$ parameter of the unitarity cusp which is related to the breaking of isospin symmetry.

• Received 23 January 2013

DOI:https://doi.org/10.1103/PhysRevC.87.045204