Systematic effects in the low-energy behavior of the current SAID solution for the pion-nucleon system

Understand

We investigate the description of the pion-nucleon experimental data at low energy (i.e., below pion laboratory kinetic energy of $100$ MeV) on the basis of the current SAID solution (WI08).

• We demonstrate that, in a self-consistent analysis scheme, the scale factors of the fits based on the Arndt-Roper formula come out independent of the beam energy and `cluster' around the expectation value of $1$.
• We report systematic effects in the low-energy behavior of the WI08 solution.
• These effects indicate that at least one of the three following assumptions, underlying the WI08 analysis, does \emph{not} hold, namely that: a) the bulk of the data is reliable, b) the electromagnetic effects are correctly accounted for, and c) the isospin invariance in the strong interactions of the $\pi N$ system is valid.