Chiral symmetry probed with pionic atoms

Study of pionic atom has been revisited by a recent discovery of deeply bound pionic states in the context of possible sensitivity to the partial chiral restoration in the nuclear medium. Precisely measured binding energies and widths of the deeply bound π− Is states in 115,119,123Sn in 116,120,124Sn(d,3He) reactions at recoil-free conditions are used to deduce the medium-modified isovector scattering amplitude as b1 = 0.115 ± 0.007mπ−1. The observed enhancement of b1 over the free πN value infers 34% reduction of \(\left\langle {\bar qq} \right\rangle \) at the normal nuclear density, ρ = 0.17 fm−3.