COMPUTING MASSES FROM EFFECTIVE TRANSFER-MATRICES

We study the use of effective transfer matrices for the numerical comp utation of masses (or correlation lengths) in lattice spin models. The effective transfer matrix has a strongly reduced number of components . Its definition is motivated by a renormalization-group transformatio n of the full model onto a one-dimensional spin model. The matrix elem ents of the effective transfer matrix can be determined by Monte Carlo simulation. We show that the mass gap can be recovered exactly from t he spectrum of the effective transfer matrix. As a first step towards application we performed a Monte Carlo study for the two-dimensional I sing model. For the simulations in the broken phase we employed a mult imagnetic demon algorithm. The results for the tunneling correlation l ength are particularly encouraging.