Two-Point Functions of Composite Twist Fields in the Ising Field Theory

Abstract: All standard measures of bipartite entanglement in one-dimensional quantum field theories can be expressed in terms of correlators of branch point twist fields, here denoted by $\mathcal{T}$ and $\mathcal{T}^\dagger$. These are symmetry fields associated to cyclic permutation symmetry in a replica theory and having the smallest conformal dimension at the critical point. Recently, other twist fields (composite twist fields), typically of higher dimension, have been shown to play a role in the study of a new measure of entanglement known as the symmetry resolved entanglement entropy. In this paper we give an exact expression for the two-point function of a composite twist field that arises in the Ising field theory. In doing so we extend the techniques originally developed for the standard branch point twist field in free theories as well as an existing computation due to Horv\'ath and Calabrese of the same two-point function which focused on the leading large-distance contribution. We study the ground state two-point function of the composite twist field $\mathcal{T}\mu$ and its conjugate $\mathcal{T}\mu^\dagger$. At criticality, this field can be defined as the leading field in the operator product expansion of $\mathcal{T}$ and the disorder field $\mu$. We find a general formula for $\log \langle \mathcal{T}\mu(0) \mathcal{T}^\dagger\mu(r)\rangle$ and for (the derivative of) its analytic continuation to positive real replica numbers greater than 1. We check our formula for consistency by showing that at short distances it exactly reproduces the expected conformal dimension