Hopping parameter series construction for models with non-trivial vacuum Original Research Article

Hopping parameter expansions are convergent power series. Under general conditions they allow for the quantitative investigation of phase transition and critical behaviour. The critical information is encoded in the high order coefficients. Recently, 20th order computations have become feasible and used for a large class of lattice field models both in finite and infinite volume. They have been applied to quantum spin models and field theories at finite temperature. The models considered are subject to a global Z2 symmetry or to an even larger symmetry group such as O(N) with N ⩾ 2. In this paper we are concerned with the technical details of series computations to allow for a non-trivial vacuum expectation value 〈ϱ(x)〉 ≠ 0, which is typical for models that break a global Z2 symmetry. Examples are scalar fields coupled to an external field, or manifestly gauge invariant effective models of Higgs field condensates in the electroweak theory, even in the high temperature phase. A non-vanishing tadpole implies an enormous proliferation of graphs and limits the graphical series computation to the 10th order. To achieve the hopping parameter series to comparable order as in the Z2 symmetric case, the graphical expansion is replaced by an expansion into new algebraic objects called vertex structures. In this way the 18th order becomes feasible.