The cross section for inclusive multipion production in the pp->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 quasi-particle is in the 1S0 state. At the three higher energies the missing mass Mx spectra show a strong enhancement at low Mx, corresponding to an ABC 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 Mx 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.
@article{arxiv.0902.0715,
title = {Observation of an ABC effect in proton-proton collisions},
author = {S. Dymov and M. Hartmann and A. Kacharava and A. Khoukaz and V. Komarov and P. Kulessa and A. Kulikov and V. Kurbatov and G. Macharashvili and S. Merzliakov and M. Mielke and S. Mikirtychiants and M. Nekipelov and M. Nioradze and H. Ohm and F. Rathmann and H. Ströher and D. Tsirkov and Yu. Uzikov and Yu. Valdau and C. Wilkin and S. Yaschenko and B. Zalikhanov},
journal= {arXiv preprint arXiv:0902.0715},
year = {2009}
}