Abstract
Massive QED for two fermion species in 1 + 1 dimensions is studied using Hamiltonian lattice techniques. The bound-state masses are obtained as strong-coupling expansions in inverse powers of the dimensionless coupling constant up to 16th order. Various Padé approximant methods for extracting the continuum limit are compared. In the non-relativistic regime (very massive fermions) our estimates are shown to converge to the correct continuum value. There is a phase transition as the fermion mass vanishes. The vacuum of the lattice theory changes from being Ising-like to being Heisenberg-like as chiral SU(2) symmetry is restored. We attempt to predict the strong-coupling behaviour by perturbing about two distinct Ising-like vacua. Our results are compared with those for the one-species (Schwinger) model.
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