The isovector potential parameter (b1) of the s-wave pion-nucleus interaction was deduced to be b1 = -0.116-0.017+0.015mπ-1 by using the binding energy and width of the recently observed deeply bound 1s π- state in 205Pb combined with the known 1s states of light pionic atoms. An enhancement of b1 over the free-πN value (|-0.090|) is indicated, which may infer that the chiral order parameter (squared pion decay constant, fπ2) is reduced by a factor of 0.78-0.09+0.13 in the nuclear medium of 205Pb where the bound π- is located. The isoscalar part is accounted for as arising mainly from a double-scattering effect, without invoking a large density-quadratic term. © 2002 Elsevier Science B.V. All rights reserved.
Geissel, H., Gilg, H., Gillitzer, A., Hayano, R. S., Hirenzaki, S., Itahashi, K., … Yoneyama, T. (2002). Experimental indication of a reduced chiral order parameter from the 1s π- state in 205Pb. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 549(1–2), 64–71. https://doi.org/10.1016/S0370-2693(02)02864-2