Anomalies for compact simple Lie groups via $G$-kernels for loop group nets

Abstract

Let $G$ be a compact simply connected simple Lie group and $k \geq 1$. Jump loops give an outer homomorphism $θ_k$ from $G$ to the outer automorphisms of the local factor of the level-$k$ loop group net, which is the hyperfinite type $\mathrm{III}_1$ factor. We show that its Sutherland obstruction is $k$ times the positive basic class in $H^3_{\mathrm{Borel}}(G, \mathbb{T}) \cong H^{4}(BG, \mathbb{Z})$ and that $θ_k$ is faithful, so it is a $G$-kernel. Central elements act by the outer automorphisms of the simple currents. In types A, D, E the level-one class is computed by comparing maximal-torus jump loops with the lattice $α$-induction construction of the companion paper on both current and charged generators, followed by integral degree-four Cartan restriction. Higher levels follow by tensor additivity, and the remaining types by restriction along index-one embeddings into simply laced groups. As a consequence, for every compact simply connected semisimple $G$, every class in $H^4(BG, \mathbb{Z})$ is the obstruction of a $G$-kernel on the hyperfinite type $\mathrm{III}_1$ factor.