Null vectors, 3-point and 4-point functions in conformal field theory

TL;DR

This paper investigates how null states in W-algebra representations constrain 3-point and 4-point correlation functions in conformal field theories, revealing conditions under which these functions are uniquely determined or limited.

Contribution

It extends the understanding of correlation functions in W-algebra CFTs by analyzing the impact of null states and degeneracies on their form and determination.

Findings

3-point functions of W-descendants have arbitrary degrees unless null states are present.

Doubly-degenerate representations constrain 3-point functions to a single constant.

Two doubly-degenerate fields in 4-point functions limit intermediate channels and determine chiral blocks.

Abstract

We consider 3-point and 4-point correlation functions in a conformal field theory with a W-algebra symmetry. Whereas in a theory with only Virasoro symmetry the three point functions of descendants fields are uniquely determined by the three point function of the corresponding primary fields this is not the case for a theory with $W_3$ algebra symmetry. The generic 3-point functions of W-descendant fields have a countable degree of arbitrariness. We find, however, that if one of the fields belongs to a representation with null states that this has implications for the 3-point functions. In particular if one of the representations is doubly-degenerate then the 3-point function is determined up to an overall constant. We extend our analysis to 4-point functions and find that if two of the W-primary fields are doubly degenerate then the intermediate channels are limited to a finite set and that the corresponding chiral blocks are determined up to an overall constant. This corresponds to the existence of a linear differential equation for the chiral blocks with two completely degenerate fields as has been found in the work of Bajnok~et~al.