title: Mass spectrum of the two-dimensional O(3) sigma model with a theta term.
abstract: Form factor perturbation theory is applied to study the spectrum of the O(3) nonlinear sigma model with the topological term in the vicinity of theta=pi. Its effective action near this value is given by the nonintegrable double sine-Gordon model. Using previous results by Affleck and the explicit expressions of the form factors of the exponential operators e(+/-isqrt[8pi]phi(x)), we show that the spectrum consists of a stable triplet of massive particles for all values of theta and a singlet state of higher mass. The singlet is a stable particle only in an interval of values of theta close to pi, whereas it becomes a resonance below a critical value theta(c).
authors.affiliation: Institute for Theoretical Physics, Valckenierstraat 65, 1018XE Amsterdam, The Netherlands.
authors.forename: D
authors.initials: D
authors.lastname: Controzzi
authors.forename: G
authors.initials: G
authors.lastname: Mussardo