Pionic double charge exchange

The pionic double charge exchange reaction on nuclei is reviewed. The obser ved systematics in the data points to a large sensitivity to short-range nu cleon-nucleon correlations in nuclei - as anticipated since long for this g enuine two-nucleon process. This is particularly true for non-analog transi tions, which do not contain the mean-field driven analog route. in non-anal og transitions the resonant Delta Delta process dominates at high energies, whereas at low energies another resonance-like narrow structure appears, w hich has led to the hypothesis of a very special correlation, the formation of a pi NN resonance decoupled from the NN-channel, the so-called d' reson ance.