Experimental indication of a reduced chiral order parameter from the 1s π- state in 205Pb

The isovector potential parameter (b(1)) of the s-wave pion-nucleus interaction was deduced to be b(1) = -0.116(-0.017)(+0.015) m(pi)(-1) by using the binding energy and width of the recently observed deeply bound 1s pi(-) state in Pb-205 combined with the known 1s states of light picnic atoms. An enhancement of b(1) over the free-pi N value (\-0.090\) is indicated, which may infer that the chiral order parameter (squared pion decay constant, f(pi)(2)) is reduced by a factor of 0.78(-0.09)(+0.13) in the nuclear medium of Pb-205 where the bound pi(-) is located. The isoscalar part is accounted for as arising mainly from a double-scattering effect, without invoking a large density-quadratic term. (C) 2002 Elsevier Science B.V. All rights reserved.