2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/214859We investigate the ground-state energy of the pi- p atom (pionic hydrogen) in the framework of QCD+QED. In particular, we evaluate the strong energy-level shift. We perform the calculation at next-to-leading order in the low-energy expansion in the framework of the relevant effective field theory. The result provides a relation between the strong energy shift and the pion-nucleon S-wave scattering lengths - evaluated in pure QCD - at next-to-leading order in isospin breaking and in the low-energy expansion. We compare our result with available model calculations.Version accepted for publication in Eur. Phys. J. C; Eq. (B8) corrected, comment after Eq. (B11) changed, additional minor changes in the text; preprint number addedHigh Energy Physics - Phenomenologytext