Deeply bound pionic systems and the pion-nucleon interaction in the medium

Deeply bound pionic states have been studied in a series of experiments at the GSI fragment separator using the (d, 3He) reaction at incident d kinetic energies of 500 MeV and 600 MeV, close to the condition of recoil free kinematics. The most recent experiment was devoted to the measurement of the isotope dependence of pionic 1s states in Sn isotopes, where the isotopic shift of deeply bound pionic states was observed for the first time. The isotopic shift of the pionic binding energy allows to deduce the s-wave isovector strength of the pion-nucleon interaction in the nuclear medium which is of fundamental importance due to its relation to the chiral order parameter fπ. (author)