Reduction of the Chiral Order Parameter by a Nuclear Medium

We propose a model independent procedure to deduce from the 1sbinding energy of heavy, neutron rich pionic atoms, the isovector scattering length b1 of the pion nucleus interaction. It is related to the pion decay constant fπ, the order parameter of spontaneous chiral symmetry breaking and thus to the value of the chiral quark condensate. Based on the results with pionic 205Pb, we find with the assertion that only the isovector part of the pion-nucleus interaction be modified by a QCD effect, a reduction of the quark condensate by 30% in a 205Pb nucleus. Forthcoming experiments to measure pionic 1s-binding energies in Snisotopes, including isotope shifts, will yield decisive information on the quark condensate without assertion.