OPE in a generally covariant form

Source abstract

Abstract We discuss the general covariance of operator product expansion in $D$-dimensional Euclidean 
conformal field theories. We propose to organise the expansion in powers of geodesic distance between two insertion points and 
to use the tangent vector to the geodesic for contractions with tensor operators. For conformally flat manifolds we show by explicit calculation that 
certain curvature terms arise in the OPE. For example for $D>2$ the leading term of this type in the identity channel of OPE of two scalar primaries is proportional to the 
Schouten tensor. We further argue that the terms we found are present for a general metric and are thus universal but there may be curvature terms at higher order in the expansion whose coefficients are not determined by the flat space OPE.
The curvature terms we discuss are of practical interest in conformal perturbation theory calculations on curved spaces.