Nucleon Correlations Seen in Pion Double Charge‐Exchange Reactions

The position of one nucleon in a nucleus is assumed to affect the location of another, but such spatial correlations are hard to demonstrate. Many hoped they would appear in pion double charge‐exchange reactions once the meson factories were built in the 1970s. Unfortunately, any correlations were masked by other complex interactions at the normal operating energies of the meson factories. Only recently—in new data taken at energies as low as 35 MeV—have nucleon correlations begun to surface. They show up in pion double charge exchange because these reactions necessarily involve two nucleons: The ingoing positive pion has successive charge‐exchange scatterings with two neutrons in a nucleus, changing them into protons in the process. At low energies the cross sections for pion double charge‐exchange transitions to certain states are larger than expected, and they are peaked in the forward direction. Theorists have explained these features and others in terms of nucleon correlations. Their work indicates that the pion interacts with two valence nucleons outside a closed shell that are within one femtometer (about a proton radius) of each other.