Deeply bound 1s states of π − in 115,119,123 Sn were preferentially observed using the Sn(d, 3 He) pion-transfer reaction under the recoil-free condition. The 1s binding energies and widths were precisely determined, and were used to deduce the isovector parameter of the s-wave pion-nucleus potential to be b1 = −0.115 ± 0.007 m −1 π . The observed enhancement of |b1| over the free πN value (b free 1 /b1 = 0.78 ± 0.05) indicates a reduction of the chiral order parameter, f * π (ρ) 2 /f 2 π ≈ 0.64, at the normal nuclear density, ρ = ρ0.PACS numbers: 36.10. Gv,13.75.Gx,14.40.Aq,25.45.Hi Our work deals with characteristic properties of strongly interacting bound sytems, hadrons, consisting of light quarks (u and d). Their masses are nearly two orders of magnitude smaller (m u ≃ 5MeV, m d ≃ 8MeV) than typical hadron masses of ≈ 1 GeV. This extraordinary phenomenon is proposed to be produced by spontaneous breaking of chiral symmetry for massless quarks subject to the strong interaction [1,2,3]. It results in a ground state, the vacuum state, with a finite expectation value of quark-antiquark pairs, qq 0 ≈ −(250 MeV) 3 [4]. In such a scenario the hadrons are considered as quasi particle excitations of the condensate, qq , separated by an energy gap of ∼ 1 GeV. The lowest energy excitation modes of the condensate, so called Nambu-Goldstone bosons, are identified as pions. Their s-wave interaction with nucleons is predicted to vanish in its isoscalar part, and determined by the pion decay constant, f 2 π , in its isovector part [5,6] which is consistent with recent experimental values [7,8]. The f 2 π is also the order parameter of chiral symmetry breaking and directly connected to the magnitude of qq through the Gell-Mann-OaksRenner relation [4].We examine this chiral symmetry scenario by implanting a π − in a nuclear medium of density ρ [9, 10], where a new vacuum state with a reduced condensate, qq ρ , is proposed to be created [2]. This is the most basic case of a large effort to study the density and temperature dependence of the qq in relativistic heavy ion collisions.The density dependence of the quark condensate is expressed in the leading order by the pion-nucleon σ-term (σ N ≈ 45 MeV) in the following form [11]:which yields a reduction of about 0.65 for the normal nuclear density, ρ = ρ 0 = 0.17 fm −3 . Likewise, the pion decay constant in a medium (identified as the time component of the axial current) is reduced as [12],where the parameter α is predicted to be αρ 0 ≈ 0.45 from the chiral dynamics [13]. This reduction of the ratio is associated with the free and the in-medium isovector πN scattering length (b free 1 and b * 1 , respectively) as [14]Our program aims at measuring the isovector πN interaction parameter in the pion-nucleus potential (b * 1 (ρ)) [15] by studying deeply bound 1s states of π − in heavy N > Z nuclei [16,17,18,19,20]. Such states were predicted to be produced as discrete states by nuclear reactions [21,22,23,24,25] and to provide unique information on the s-wave interaction, where...