Chiral expansion of nucleon PDF at xsim m_π/M_N

Based on the chiral perturbation theory, we investigate the low-energy dynamics of nucleon parton distributions. We show that in different regions of the momentum fraction $x$ the chiral expansion is significantly different. For nucleon parton distributions these regions are characterized by $x\sim1$, $x\sim m_\pi/M_N$ and $x\sim (m_\pi/M_N)^2$. We derive extended counting rules for each region and obtain model-independent results for the nucleon parton distributions down to $x\gtrsim m^2_\pi/M^2_N \approx 10^{-2}$.