Surface defects and chiral algebras

We investigate superconformal surface defects in four-dimensional N=2$$ \mathcal{N}=2 $$ superconformal theories. Each such defect gives rise to a module of the associated chiral algebra and the surface defect Schur index is the character of this module. Various natural chiral algebra operations such as Drinfeld-Sokolov reduction and spectral flow can be interpreted as constructions involving four-dimensional surface defects. We compute the …