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

Abstract The isovector potential parameter ( b 1 ) of the s -wave pion–nucleus interaction was deduced to be b 1 =−0.116 +0.015 −0.017 m π −1 by using the binding energy and width of the recently observed deeply bound 1 s π − state in 205 Pb combined with the known 1 s states of light pionic atoms. An enhancement of b 1 over the free- πN value (|−0.090|) is indicated, which may infer that the chiral order parameter (squared pion decay constant, f π 2 ) is reduced by a factor of 0.78 +0.13 −0.09 in the nuclear medium of 205 Pb 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.