F-theory and the classification of little strings

Little string theories (LSTs) are UV complete nonlocal six-dimensional (6D) theories decoupled from gravity in which there is an intrinsic string scale. In this paper, we present a systematic approach to the construction of supersymmetric LSTs via the geometric phases of F-theory. Our central result is that all LSTs with more than one tensor multiplet are obtained by a mild extension of 6D superconformal field theories in which the theory is supplemented by an additional, nondynamical tensor multiplet, analogous to adding an affine node to an ADE quiver, resulting in a negative semidefinite Dirac pairing. We also show that all 6D superconformal field theories naturally embed in a LST. Motivated by physical considerations, we show that in geometries where we can verify the presence of two elliptic fibrations, exchanging the roles of these fibrations amounts to T-duality in the 6D theory compactified on a circle.