The cross section for inclusive multipion production in the $pp\to ppX$ reaction was measured at COSY-ANKE at four beam energies, 0.8, 1.1, 1.4, and 2.0 GeV, for low excitation energy in the final $pp$ system, such that the diproton quasiparticle is in the ${}^{1}S_0$ state. At the three higher energies, the missing-mass $M_X$ spectra show a strong enhancement at low $M_X$, corresponding to an Abashian-Booth-Crowe effect that moves steadily to larger values as the energy is increased. Despite the missing-mass structure looking very different at 0.8 GeV, the variation with $M_X$ and beam energy are consistent with two-pion production being mediated through the excitation of two $\Delta (1232)$ isobars, coupled to $S$ and $D$ states of the initial $pp$ system. There is no sign of any resonancelike structure in the energy dependence of the type recently observed for the $pn\to d{\pi }^0{\pi }^0$ total cross section.

• Received 4 February 2009

DOI:https://doi.org/10.1103/PhysRevLett.102.192301