π−p atom in ChPT: strong energy-level shift

The general formula of the π−p atom strong energy-level shift in the 1s state is derived in the next-to-leading order in the isospin breaking, and in all orders in chiral expansion. Isospin-breaking corrections to the level shift are explicitly evaluated at order p2 in ChPT. The results clearly demonstrate the necessity to critically reaccess the values of the πN scattering lengths, extracted from the energy-level shift measurement by means of the potential model-based theoretical analysis.