Energy dependence of forward 1S0 diproton production in the pp→ppπ0 reaction

The pp→{pp}sπ0 differential cross section has been measured with the ANKE spectrometer at COSY-Jülich for seven proton beam energies Tp between 0.51 and 1.97 GeV. By selecting proton pairs with an excitation energy of less than 3 MeV it is ensured that the final {pp}s system is in the S01 state. In the measured region of θppcm≲18°, the data reveal a forward dip for Tp⩽1.4 GeV whereas a forward peaking is seen at 1.97 GeV. The energy dependence of the forward cross section shows a broad peak in the 0.6-0.8 GeV region, probably associated with Δ(1232) excitation, and a minimum at 1.4 GeV. Some of these features are similar to those observed for the spin-isospin partner reaction, pp→dπ+. However, the ratio of the forward differential cross sections of the two reactions shows a significant suppression of single pion production associated with a spin-singlet final nucleon pair.