Empirical deduction of a medium-modified chiral order parameter from 1s π− states in heavy nuclei

We propose a model-independent procedure to deduce empirically the isovector strength b1 of the pion–nucleus interaction, which is related to possible modification of the pion-decay constant fπ, and thus of the chiral order parameter, in the nuclear medium. The first procedure is to compare the 1s binding energies of π− in two nuclei which have different (N−Z)/A values, such as 205Pb and 115Sn. The second, more direct way is to deduce b1 from an observed isotope shift of the 1s binding energy, such as in 115Sn and 123Sn.