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5d Yang-Mills theory with an Sp (N) gauge group admits a discrete analog of the θ parameter. We describe the origin of this parameter in N = 1 theories from Type I’ string theory, and study its effect on the 5d superconformal fixed point theories with an Sp (1) = SU (2) gauge group by computing the superconformal index. Our result confirms the lack of global symmetry enhancement in the so-called {E₁} theory.
Bergman et al. (Wed,) studied this question.