A coupled K-matrix description of the reactions pi+N -> pi+N, pi+N -> eta+N, gamma+N -> pi+N and gamma+N -> eta+N

Abstract

The coupled pi+N, eta+N, gamma+N systems are described by a K-matrix method. The parameters in this model are adjusted to get an optimal fit to pi+N -> pi+N, pi+N -> eta+N, gamma+N -> pi+N and gamma+N -> eta+N data in an energy range of about 100 MeV each side of the eta-threshold. The coupling of photons to the N(1535) state is extracted and also an alternative to the current S11(gamma+N -> pi+N) amplitudes suggested. Expansions are given for the eta-eta and gamma-eta amplitudes in terms of the eta-momentum. Effects of interference of this state with background potential interactions are discussed and experimental consequences are indicated.