Taxonomy of Instanton Corrections in Infinite Distance Limits

Using the BPS-protected higher derivative $R^4$-term as an exactly solvable example, we analyze which instanton corrections are generated by a one-loop Schwinger integral over the light towers of states that arise in infinite distance limits in moduli space. We find that the Schwinger integral fully captures precisely those instantons whose action lies parametrically in the window $(\Lambda_{\rm sp}/M_{\rm light})^{-1}\le {\rm S}_{\rm inst}\le \Lambda_{\rm sp}/M_{\rm light}$, that is, instantons whose action is bounded by the ratio of the gravity cutoff and the mass scale of the lightest tower. This proposal is supported by considering the entire moduli space of toroidal compactifications in eight dimensions, together with a number of limits in seven dimensions. In each case, integrating out the light towers via the Schwinger integral reproduces the complete contribution of the instantons within the above window. We further recast the proposal in terms of the taxonomy classification, allowing us to determine the emergent instantonic spectrum associated with any infinite distance limit.