Thermal effective action for the $O(N)$ vector model

We compute the leading coefficients of the thermal effective action for the critical O(N) vector model in three dimensions, in the large-N limit in presence of non vanishing angular twist. At high temperature, the partition function on a product of a two-dimensional spatial manifold and a thermal circle admits a Kaluza-Klein reduction to a local effective action on the spatial slice, whose coefficients encode universal CFT data such as the Casimir energy and the response to a Kaluza-Klein gauge field. We determine these coefficients through two independent computations: an evaluation of the twisted partition function on the two-sphere in the high-temperature limit, and a direct path-integral computation on a generic weakly curved background. The two methods yield consistent results, providing a non-trivial check of the thermal effective action framework.