Symmetry enhancements via 5d instantons, qW-algebrae and (1;0) superconformal index

We explore N = (1; 0) superconformal six-dimensional theories arising from M5 branes probing a transverse Ak singularity. Upon circle compactication to 5 dimen- sions, we describe this system with a dual pq-web of ve-branes and propose the spectrum of basic ve-dimensional instanton operators driving global symmetry enhancement. For a single M5 brane, we nd that the exact partition function of the 5d quiver gauge theory matches the 6d (1; 0) index, which we compute by letter counting. We nally show that S-duality of the pq-web implies new relations among vertex correlators of qW algebrae.