Generalisations of the Coleman-Thun mechanism and boundary reflection factors

We make a complete pole analysis of the reflection factors of the boundary scaling Lee-Yang model. In the process we uncover a number of previously un remarked mechanisms for the generation of simple poles in boundary reflecti on factors, which have implications for attempts to close the boundary boot strap in more general models. We also explain how different boundary condit ions can sometimes share the same fundamental reflection factor, by relatin g the phenomenon to potential ambiguities in the interpretation of certain poles. In the case discussed, this ambiguity can be lifted by specifying th e sign of a bulk-boundary coupling. While the recipe we employ for the asso ciation of poles with general on-shell diagrams is empirically correct, we stress that a justification on the basis of more fundamental principles rem ains a challenge for future work. (C) 1999 Elsevier Science B.V. All rights reserved.