Instanton Corrections and Emergent Strings

TL;DR

This paper investigates how instanton effects influence the approach to infinite distance limits in 4d $ abla$=2 string compactifications, revealing a connection between instantons, emergent strings, and duality in moduli spaces.

Contribution

It demonstrates the role of instantons in driving emergent tensionless strings and clarifies the duality between instanton effects and string tensions in moduli space limits.

Findings

Lightest D-brane string becomes tensionless faster due to instantons.

Dominant instantons prevent other strings from becoming tensionless.

Duality relates the lightest string to a fundamental type IIB string.

Abstract

We study limits of infinite distance in the moduli space of 4d $\mathcal{N} = 2$ string compactifications, in which instanton effects dominate. We first consider trajectories in the hypermultiplet moduli space of type IIB Calabi-Yau compactifications. We observe a correspondence between towers of D-brane instantons and D-brane 4d strings, such that the lighter the string the more relevant the instanton effects are. The dominant instantons modify the classical trajectory such that the lightest D-brane string becomes tensionless even faster, while the other strings are prevented to go below the fundamental string tension. This lightest string is dual to a fundamental type IIB string and realises the Emergent String Conjecture. We also consider the vector multiplet moduli space of type I string theory on $K3 \times T^2$ , where quantum corrections can also become significant. Naively, we only find trajectories that correspond to decompactification limits, in apparent contradiction with the picture obtained in some dual setup.