K-Points and Type IIB/Heterotic Duality with NS5-Branes

TL;DR

The paper analyzes type II0 degenerations in Type IIB on Calabi–Yau threefolds, showing that, despite their non-mirror-describability, the corresponding heterotic dual contains a weakly coupled string sector with a decoupled non-perturbative field-theory sector arising from spacetime-filling NS5-branes on K3×T^2. The leading prepotential acquires exponential dependence on the heterotic dilaton, consistent with Emergent String behavior, and K-points can be viewed via Higgsing as transitions to a one-parameter model while preserving the NS5-brane imprint. The results support the Distance and Emergent String Conjectures in these limits by identifying the light towers with heterotic string excitations and clarifying how non-perturbative sectors decouple from gravity. Overall, the work reconciles K-points with current swampland notions and highlights NS5-brane physics as a natural source of non-perturbative corrections in the heterotic frame.

Abstract

We revisit type II$_0$ limits in the vector multiplet moduli space of compactifications of Type IIB string theory on Calabi-Yau threefolds. These limits are special because they cannot be described using mirror symmetry. While we showed in previous work that the gravitational duality frame emerging in such limits corresponds to a weakly coupled heterotic string compactified on K3 times a string-sized torus, our focus here is on the additional field theory sectors that decouple from gravity in these asymptotic regimes. Concretely, we argue that, in the dual heterotic description, these field theory sectors correspond to spacetime-filling NS5-branes wrapping the heterotic torus. The existence of such non-perturbative field theory sectors manifests itself in an exponential dependence of the prepotential on the heterotic dilaton at leading order. Applied to K-point limits in one-parameter moduli spaces, our results imply that these limits are in perfect agreement with the Distance and Emergent String Conjectures. More precisely, the light tower of states predicted by the Distance Conjecture arises from excitations of a weakly coupled heterotic string compactified to four dimensions on a background containing spacetime-filling NS5-branes.

Figures (1)

• Figure 1: An overview of the different duality frames that emerge along the various singular divisors discussed in this work. In addition, we have included the local expansions for the prepotential $F$, as well as the number of perturbative / non-perturbative and gravitationally decoupled electric $U(1)$s in each regime. The non-gravitational $U(1)$ is part of a Seiberg--Witten like subsector with an associated W-boson of mass $m_W$ and a monopole of mass $m_M$. We have also highlighted in purple the intersection of the $\mathrm{II}_0$ and conifold ($\mathrm{I}_1$) divisors, where the transition to the K-point is performed by Higgsing the decoupled magnetic U(1)$_M$.