EFT strings and emergence

Abstract: We revisit the Emergence Proposal in 4d ${\cal N}=2$ vector multiplet sectors that arise from type II string Calabi--Yau compactifications, with emphasis on the role of axionic fundamental strings, or EFT strings. We focus on large-volume type IIA compactifications, where EFT strings arise from NS5-branes wrapping internal four-cycles, and consider a set of infinite-distance moduli-space limits that can be classified in terms of a scaling weight $w=1,2,3$. It has been shown before how one-loop threshold effects of an infinite tower of BPS particles made up of D2/D0-branes generate the asymptotic behaviour of the gauge kinetic functions along limits with $w=3$. We extend this result to $w=2$ limits, by taking into account D2-brane multi-wrapping numbers. In $w=1$ limits the leading tower involves EFT string oscillations, and one can reproduce the behaviour of both weakly and strongly-coupled $U(1)$'s independently on whether the EFT string is critical or not, by assuming that charged modes dominate the light spectrum.