The pion-nucleon σ term from pionic atoms

Earlier work suggested that the in-medium pi N threshold isovector amplitude b(1)(rho) gets renormalized in pionic atoms by similar to 30% away from its rho = 0 free-space value, relating such renormalization to the leading low-density decrease of the in-medium quark condensate < <(q)over bar>q > and the pion decay constant f(pi) in terms of the pion-nucleon sigma term sigma(pi N). Accepting the validity of this approach, we extracted sigma(pi N) from a large-scale fit of pionic-atom level shift and width data across the periodic table. Our fitted value sigma(pi N) = 57 +/- 7 MeV is robust with respect to variation of pi N interaction terms other than the isovector s-wave term with which sigma(pi N) was associated. Higher order corrections to the leading order in density involve some cancellations, suggesting thereby only a few percent overall systematic uncertainty. The value of sigma(pi N) derived here agrees with values obtained in several recent studies based on near-threshold pi N phenomenology, but sharply disagrees with values obtained in recent direct lattice QCD calculations. (C) 2019 The Authors. Published by Elsevier B.V.