Galois covers of N=2 BPS spectra and quantum monodromy

Understand

The BPS spectrum of many 4d N=2 theories may be seen as the (categorical) Galois cover of the BPS spectrum of a different 4d N=2 model.

• The Galois group G acts as a physical symmetry of the covering N=2 model.
• The simplest instance is SU(2) SQCD with N_f=2 quarks, whose BPS spectrum is a Z_2-cover of the BPS spectrum of pure SYM.
• More generally, N=2 SYM with simply--laced gauge group admits Z_k-covers for all k; e.g.