Isospin violation in theπNsystem at low energies

The low-energy $\pi N$ interaction is investigated with the use of a relativistic isospin-symmetric $\pi N$ model based on scalar-isoscalar and vector-isovector exchanges in the $t$ channel, and the nucleon and Δ-isobar contributions in the $s$ and $u$ channels; the small contributions from the well-established $s$ and $p$ higher (baryon) 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 old) $\pi N$ measurements between (pion laboratory kinetic energy of) 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 $\pi N$ system.