Pion-nucleon elastic scattering

Abstract

Pion-Nucleon scattering is reviewed. Positive results include: 1) good agreement between the πN resonance parameters of the KarlsruheHelsinki K-H, and Carnegie-Mellon-Berkeley, CMU-LBL, partial wave analyses; 2) no sizable isospin violation near the Roper resonance; 3) a 1° – 2° isospin violation in the region of the Δ (1232); 4) the u- and d- quark mass difference is small, with m d > m u; 5) forward dispersion relation predictions agree with experiment up to 300 GeV/c; 6)there are no good candidates for πN hermaphrodites; 7) the Isqur-Karl quark model calculations are useful up to m ⋍ 1800 MeV . Negative findings are: 1) there is urgent need for spin rotation measurements to verify the correct use of theoretical constraints in a11 partial wave analyses; 2) existing data is insufficient to establish the fate of 24 πN resonance candidates; 3) 3 strong and 7 weak πN resonances with mass above 1800 MeV predicted by the Isgur-Karl quark model are not seen; 4) the experimental result for the πN σ-term is 64 ± 8 MeV while the QCD inspired prediction is 35 ± 5 MeV. Finally, there is some evidence for clustering of πN resonance poles in the complex energy plane, it is unknown if this is accidental or new physics.