Anomaly Inflow and Gauge Group Topology in the 10d Sugimoto String Theory

We revisit the chiral spectra on charged 1- and 5-branes in the 10d non-supersymmetric $\mathfrak{s}\mathfrak{p}(16)$ string theory (also known as the Sugimoto model), and verify that they consistently cancel the anomaly inflow induced by a Green–Schwarz mechanism in the bulk. By analyzing the $\mathfrak{s}\mathfrak{p}(16)$ representations arising from quantizing the fermion zero modes on these branes as well as uncharged 4-branes, we find compelling evidence that the global structure of the gauge group is $Sp(16)\mathrm{/}{\mathbb{Z}}_2$. We further comment on a possible duality to non-supersymmetric heterotic strings, and explore bottom-up anomaly inflow constraints for 10d effective $Sp(16)\mathrm{/}{\mathbb{Z}}_2$ gauge theories coupled to gravity.