Point-like Instantons and the Spin(32)/Z2 Heterotic String

TL;DR

This paper explores heterotic string compactifications on K3 surfaces, revealing new instanton types linked to gauge symmetry enhancements and massless tensors, analyzed through F-theory.

Contribution

It introduces a new type of point-like instanton associated with the generalized second Stiefel-Whitney class and studies their role in gauge symmetry enhancement.

Findings

New instanton type linked to Sp(4) gauge symmetry

Massless tensor fields emerge from instanton coalescence

Large gauge groups with multiple tensor multiplets can form

Abstract

We consider heterotic string theories compactified on a K3 surface which lead to an unbroken perturbative gauge group of Spin(32)/Z2. All solutions obtained are combinations of two types of point-like instanton --- one ``simple type'' as discovered by Witten and a new type associated to the ``generalized second Stiefel-Whitney class'' as introduced by Berkooz et al. The new type of instanton is associated to an enhancement of the gauge symmetry by Sp(4) and the addition of a massless tensor supermultiplet. It is shown that if four simple instantons coalesce at an orbifold point in the K3 surface then a massless tensor field appears which may be used to interpolate between the two types of instanton. By allowing various combinations of point-like instantons to coalesce, large gauge groups (e.g., rank 128) with many massless tensor supermultiplets result. The analysis is done in terms of F-theory.