The nuclear S-wave pion production is described by the axial charge density operator of the twonucleon system. It is shown that the short range axial exchange charge operator implied by the nucleon-nucleon interaction enhances the predicted cross section for the reaction pp-+pp7t° near threshold by factors 3-5. This suffices to explain most of the underprediction obtained with the single-nucleon and S-wave pion rescattering operator. The result implies that the cross section for pp -+ ppn° can provide direct information on the short range components of the nucleon-nucleon interaction.PACS numbers: 13.75. Cs, 11.40.Fy, 21.30.+y, 25.40.Qa Nuclear S-wave pion absorption and production reactions are conventionally described by a single nucleon and a pion rescattering mechanism, according to which the pion scatters off one nucleon by an S-wave collision, and is absorbed (produced) on a second nucleon [1][2][3][4]. The key element of this description is a phenomenological effective Hamiltonian //=4;rm n X 2 >• 0y/+4;r-^V T * f^ny/(1) m:that describes the rescattering vertex. The two coupling constants ^i and X 2 are determined by the S\\ and 531 pion nucleon scattering lengths a\ and a 3 as , _ -1 1+-m" m N (<2j+2#3) , X 2 sss -m Jl o 1 + m n m N (a\ -a?,) . (2a) (2b) With the values a\-0.245 fm and #3--0.143 fm of Hohler et al [5], one obtains \\ =0.005, ^2 = 0.05, and with Arndt, Ford, and Roper [6] values a\ =0.23 fm and a 3 = -0.11 fm, one obtains ^1 = -0.0013 and >. 2 =0.047. The notable feature of the effective interaction Eq.(1) is the smallness of the coefficient X\. The pion rescattering mechanism induced by the second larger term in Eq.(1) has an antisymmetric isospin dependence T'XT 2 , and hence does not contribute to the pp-^ppx 0 reaction. The contribution of the S-wave rescattering mechanism to the rate for pp-*ppx 0 is therefore very small. This is borne out by detailed calculations [7,8], which show that the predicted cross section for pp-^ppn 0 near threshold is 4-5 times smaller than the empirical values [9] when described by this rescattering amplitude combined with the amplitude for absorption on a single nucleon. We here show that most of this underprediction is removed when the short range two-nucleon mechanisms that are implied by the nucleon-nucleon interaction are taken into account. The result implies that the reaction of pp-*ppn° near threshold may prove useful for discrimination between different nucleon-nucleon interaction models.The most coherent framework for taking into account the short range exchange contributions is to describe the pion-nucleus interaction by the direct extension of Weinberg's effective pion-nucleon interaction [10] to nuclei:where 0 is the isovector pion field operator and A^ is the isovector axial current of the nuclear system. This extension of the Weinberg Lagrangian appears naturally, e.g., in the Skyrme model [11], and reduces the calculation of matrix elements for nuclear pion production to the construction of the axial current operator, which is formed of ...

The total cross section for the near threshold η production in proton-proton collisions has been investigated with the assumption that the production mechanism is due to the emission of a meson x(π, η, σ) from one of the nucleons followed by a xN → ηN transition on the second one. The xN → ηN amplitudes are generated from the unitary multi-channel multi-resonance model which has recently been constructed in analyzing the πN scattering and πN → ηN reaction. The initial and final pp distortions are calculated from a coupled-channel πN N model which describes the N N scattering data up to about 2 GeV. With the x − N N vertex functions taken from the Bonn potential, the predicted total cross sections of threshold pp → ηpp reaction are in good agreement with the data. In contrast to previous works, we find that the η-exchange plays an important role. The effect of the two-pion exchange, simulated by σ-exchange, is found to be significant but not as dominant as the vector meson-exchange introduced in previous works.The η production in proton-proton collisions has attracted a lot of interest in the past decade. A theoretical understanding of this two-nucleon process near the production threshold is needed for exploring the N * (S 11 (1535)) dy-

A three-channel, multi-resonance, unitary model developed in 1995 is used to determine the πN → ηN and ηN → ηN amplitudes by re-analyzing the available data on πN elastic scattering and the weighted data for the πN → ηN total and differential cross sections. The input πN elastic scattering amplitude in the S11 channel has been improved, following suggestions of G. Höhler. Our new result of ηN scattering length, aηN = (0.717±0.030)+i · (0.263±0.025) fm, suggests that the ηd system is unbound or loosely bound.