One of the simplest (1, 0) supersymmetric theories in six dimensions lives on the world volume of one M5 brane at a D type singularity C 2 /D k . The low energy theory is given by an SQCD theory with Sp(k − 4) gauge group, a precise number of 2k flavors which is anomaly free, and a scale which is set by the inverse gauge coupling. The Higgs branch at finite coupling H f is a closure of a nilpotent orbit of D 2k and develops many more flat directions as the inverse gauge coupling is set to zero (violating a standard lore that wrongly claims the Higgs branch remains classical). The quaternionic dimension grows by 29 for any k and the Higgs branch stops being a closure of a nilpotent orbit for k > 4, with an exception of k = 4 where it becomes min E 8 , the closure of the minimal nilpotent orbit of E 8 , thus having a rare phenomenon of flavor symmetry enhancement in six dimensions. Geometrically, the natural inclusion of H f ⊂ H ∞ fits into the Brieskorn Slodowy theory of transverse slices, and the transverse slice is computed to be min E 8 for any k > 3. This is identified with the well known small E 8 instanton transition where 1 tensor multiplet is traded with 29 hypermultiplets, thus giving a physical interpretation to the geometric theory. By the analogy with the classical case, we call this the Kraft Procesi transition.
arXiv:1801.01129v3 [hep-th] 6 Jul 20181 There exist other examples of Higgs branch flows in 6d in which n T , n H , and n V all change. An example of this is a theory of N M5-branes on C 2 /D k discussed in section 3 of this paper. The number of tensor multiplets at a generic point of the tensor branch is n T = 2N −1, whereas there are N − 1 tensor multiplets left at the end of the Higgs branch flow [5]. In this theory, 29n T + n H − n V is equal to 30(N − 1) + dim(SO(2k)) + 1, which is proportional to the anomaly coefficient δ given by (3.3).