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Nuclear Inst. and Methods in Physics Research, A 599 (2009) 28-46. doi:10.1016/j.nima.2008.10.028Received by publisher: 2008-06-26Harvest Date: 2016-01-04 12:21:43DOI: 10.1016/j.nima.2008.10.028Page Range: 28-4

Recent calculations of s-wave pion production have severely underestimated the accurately known pp → ppπ 0 total cross section near threshold. In these calculations, only the single-nucleon axial-charge operator is considered. We have calculated, in addition to the one-body term, the two-body contributions to this reaction that arise from the exchange of mesons. We find that the inclusion of the scalar σ-meson exchange current (and lesser contributions from other mesons) increases the cross section by about a factor of five, and leads to excellent agreement with the data. The results are neither very sensitive to changes in the distorting potential that generates the N N wave function, nor to different choices for the meson-nucleon form factors. We argue that pp → ppπ 0 data provide direct experimental evidence for meson- * Electonic communication to charlie@venus.iucf.indiana.edu 1 exchange contributions to the axial current.

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The total cross section for the reaction pp-*ppn Q was measured at nine center-of-mass energies from 1.5 to 23 MeV above threshold. The experiment was carried out with the Indiana Cooler, a recently constructed storage ring. The experimental advantages of an electron-cooled proton beam were utilized. The data cover an energy range where only the lowest possible angular momentum state contributes in the exit channel. The measured energy dependence of the total cross section is not compatible with that predicted by models of s-wave pion production and rescattering. PACS numbers: 25.10,+s, 13.75.Cs, 21.30.+y, 29.25.Fb Meson-exchange models of the nucleon-nucleon (AW) interaction above the pion threshold rely on detailed information about the strongly coupled inelastic channels which must be treated coherently with the elastic interaction. Information on pion production in the NN system is also required in models of pion production or absorption in nuclei. Customarily, the partial waves in the exit channel of NN-* NNK reactions are labeled by Ll, where L is the angular momentum of the nucleon pair and / is the angular momentum of the pion with respect to the nucleon pair. l Within 100 MeV of threshold, because of the short range of the interaction, only Ss, Sp, Ps, and Pp final states contribute significantly; within 25 MeV the Ss configuration should dominate. In addition, the Sp final state is forbidden in the pp-* ppn° reaction.In the pp-^ppK° reaction, the normally dominant pion production via an intermediate A7V system is suppressed, since the N and the A cannot be in a relative S state. The rescattering contribution is also small because the dominant isovector part of low-energy nN scattering cannot contribute. Thus, near threshold, the pp-*ppn° reaction is dominated by the direct-production Born term. 2 The energy dependence of the cross section is customarily expressed in terms of n, the largest possible center-of-mass pion momentum (with nucleons at rest relative to each other) divided by the pion mass. Based on phase-space arguments, the assumption of r/ o)-This should greatly simplify the theoretical interpretation of the data. The experiment described here is the first to cover this energy range. Previous experiments were hampered by the fact that the cross section (due to the suppressed resonant production) is small and competes with background caused by nuclei other than hydrogen in the path of the beam. Because of the properties of the novel experimental environment described below, it becomes feasible to measure the four-momenta of both outgoing protons, making it possible to comple...