No abstract

The low-energy N interaction is investigated with the use of a relativistic isospin-symmetric N model based on scalar-isoscalar and vector-isovector exchanges in the t channel, and the nucleon and ⌬-isobar contributions in the s and u channels; the small contributions from the well-established s and p higher ͑baryon͒ resonances are also taken into account. In the region of elasticity, the model provides a firm basis for analyzing the experimental data. The analysis of all ͑recent and old͒ N measurements between ͑pion laboratory kinetic energy of͒ 20 and 100 MeV has been achieved with the implementation of robust statistics. Provided the correctness of the bulk of the experimental data and the completeness of the electromagnetic corrections applied to the scattering problem, this work provides overwhelming evidence for isospin-symmetry breaking of the strong interaction in the N system. ͓S0556-2813͑97͒03212-3͔ PACS number͑s͒: 13.75.Gx, 11.30.Hv, 25.80.Dj *Electronic address: evangelos.matsinos@psi.ch; Tel.: ϩ41 56 310 32 77, FAX: ϩ41 56 310 43 62.

Using electromagnetic corrections previously calculated by means of a potential model, we have made a phase-shift analysis of the π ± p elastic-scattering data up to a pion laboratory kinetic energy of 100 MeV. The hadronic interaction was assumed to be isospin invariant. We found that it was possible to obtain self-consistent databases by removing very few measurements. A pion-nucleon model, based on sand u-channel diagrams with N and ∆ in the intermediate states, and σ and ρ t-channel exchanges, was fitted to the elastic-scattering database obtained after the removal of the outliers. The model-parameter values showed an impressive stability when the database was subjected to different criteria for the rejection of experiments. Our result for the pseudovector πN N coupling constant (in the standard form) is 0.0733 ± 0.0014. The six hadronic phase shifts up to 100 MeV are given in tabulated form. We also give the values of the s-wave scattering lengths and the p-wave scattering volumes. Big differences in the s-wave part of the interaction were observed when comparing our hadronic phase shifts with those of the current GWU solution. We demonstrate that the hadronic phase shifts obtained from the analysis of the elastic-scattering data cannot reproduce the measurements of the π − p charge-exchange reaction, thus corroborating past evidence that the hadronic interaction violates isospin invariance. Assuming the validity of the result obtained Preprint submitted to Elsevier Science within the framework of chiral perturbation theory, that the mass difference between the u-and the d-quark has only a very small effect on the isospin invariance of the purely hadronic interaction, the isospin-invariance violation revealed by the data must arise from the fact that we are dealing with a hadronic interaction which still contains residual effects of electromagnetic origin. PACS: 13.75.Gx; 25.80.Dj; 25.80.Gn

No abstract

In a previous paper, we reported the results of a partial-wave analysis (PWA) of the pion–nucleon (πN) differential cross-sections (DCSs) of the CHAOS Collaboration and came to the conclusion that the angular distribution of their π+p data sets is incompatible with the rest of the modern (meson factory) database. The present work, re-addressing this issue, has been instigated by a number of recent improvements in our analysis, namely regarding the inclusion of the theoretical uncertainties when investigating the reproduction of experimental data sets on the basis of a given "theoretical" solution, modifications in the parametrization of the form factors of the proton and of the pion entering the electromagnetic part of the πN amplitude, and the inclusion of the effects of the variation of the σ-meson mass when fitting the ETH model of the πN interaction to the experimental data. The new analysis of the CHAOS DCSs confirms our earlier conclusions and casts doubt on the value for the πN Σ term, which Stahov, Clement and Wagner have extracted from these data.