On the symmetry of real-space renormalisation

A geometric structure, arising from the embedding into a Hilbert space of the parametrised probability measure for a given lattice model, is applied here to study the symmetry properties of real-space renormalisation group (RG) flow. In the projective state space this flow is shown to have two contributions: a gradient term, which generates a projective automorphism of the state space for each given length scale; and a correction term due to the scale change. We argue that this structure implies the absence of any symmetry of a geodesic type for the RG how when restricted to the parameter space submanifold of the state space. This is demonstrated explicitly via a study of the one-dimensional Ising model in an external field, In this example we construct exact expressions for the beta functions associated with the Row induced by infinitesimal rescaling. These constitute a generating vector field for RG diffeomorphisms on the parameter space manifold, and we analyse the symmetry properties of this transformation. The results indicate an approximate conformal Killing symmetry near the critical point, but no generic symmetry of the RG flow globally on the parameter space. (C) 1998 Elsevier Science B.V.