Giant Gravitons, Fermionic Forms and Vertex Algebras

We investigate the mathematical and physical content of the giant graviton expansion of three-dimensional $\mathcal{N}=4$ superconformal field theories in a simplifying limit. We uncover an interesting relation between the coefficients in this expansion, the Hilbert series of certain quiver varieties and the representation theory of vertex algebras. In particular, for the worldvolume theory of $N$ M2-branes at the tip of a toric hyper-Kähler four-fold cone: $X_4={\mathbb{C}}^2\mathrm{/}{\mathbb{Z}}_L\times {\mathbb{C}}^2\mathrm{/}{\mathbb{Z}}_K$, we derive an explicit expression for the coefficients in terms of affine fermionic forms and show that they coincide with characters of a direct sum of parafermionic W-algebras.