Point-like instantons and the Spin(32)/ℤ2 heterotic string
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. © 1997 Elsevier Science B.V.
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