Description
In this talk, I will discuss the physics of Type IIB and F-theory compactifications on Calabi–Yau 3- and 4-folds undergoing semi-stable degeneration. In the resulting 4d effective theory, these degenerations give rise to solitonic string solutions. I will describe how the worldsheet theory of these strings can be inferred from the degeneration geometry and how it encodes the effective physics in this regime of the 4d theory. For Type IIB compactifications on Calabi–Yau threefolds, the strings correspond to gravitational strings in a weakly coupled duality frame, in agreement with expectations from the classical low-energy effective action. In contrast, for F-theory compactifications on elliptic Calabi–Yau fourfolds, the strings arising from semi-stable degenerations do not admit an interpretation as gravitational strings in the weakly coupled dual frame predicted by the classical effective action. In the F-theory setting, these string solutions therefore provide evidence for perturbative and non-perturbative corrections to the classical effective action that become unsuppressed in the region of moduli space corresponding to the semi-stable degeneration.
