The one-loop QCD scale anomaly on ADE orbifolds as an index

We derive the one-loop scale anomaly of $\SU(N)$ Yang--Mills on the orbifold $\R^4/Γ$, with $Γ\subset \SU(2)$ a finite subgroup, from an equivariant index theorem on twistor space. The anomaly separates into the QCD $β$-function and a contribution localized at the orbifold fixed point. This localize