Sigma-resonance and convergence of chiral perturbation theory

The dimensionless parameter $\xi' = M^2/(16 \pi^2 F^2)$, where $F$ is the pion decay constant in the chiral limit and $M$ is the pion mass at leading order in the quark mass, is expected to control the convergence of chiral perturbation theory applicable to QCD. Here we demonstrate that a strongly coupled lattice gauge theory model with the same symmetries as two-flavor QCD but with a much lighter $\sigma$-resonance is different. Our model allows us to study efficiently the convergence of chiral perturbation theory as a function of $\xi'$. We first confirm that the leading low energy constants appearing in the chiral Lagrangian are the same when calculated from the $\epsilon$-regime and the $p$-regime. However, $\xi' \lesssim 0.002$ is necessary before 1-loop chiral perturbation theory predicts the data within 1%. However, for $\xi' > 0.0035$ the data begin to deviate qualitatively from 1-loop chiral perturbation theory predictions. We argue that this qualitative change is due to the presence of a light $\sigma$-resonance in our model. Our findings may be useful for lattice QCD studies.