In the absence of nucleons, we use partial wave unitarity, to show that the chiral expansion parameter must be close to $p^2/4\pi f_{ \pi}^2$ rather than $p^2/16\pi^2 f_{ \pi}^2$ as previously suggested, where $p$ is a typical pion momentum and $f_{ \pi}$ the pion decay constant. When nucleons are included, we apply the Tani-Foldy-Wouthuysen (TFW) transformation to the pion-nucleon effective Lagrangian to obtain an expansion in powers of $1/m_N$ (inverse nucleon mass). The results are presented up to order ${ \cal O} (1/m_N^3)$, corresponding to ${ \cal O} (p^4)$ in the momentum. In this case partial wave-unitarity is also lost in about the same range of momenta.
Categories: hep-ph