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

Abstract

The isovector potential parameter (b1) of the s-wave pion–nucleus interaction was deduced to be b1=−0.116+0.015−0.017mπ−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+0.13−0.09 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.