Little String Defects and Bala-Carter Theory

We give a physical realization of the Bala-Carter labels that classify nilpotent orbits of semi-simple Lie algebras, for the case $\mathfrak{g}=A,D,E$. We start from type IIB string theory compactified on an $ADE$ singularity and study the six-dimensional (2,0) $\mathfrak{g}$-type little string on a Riemann surface with punctures. The defects are introduced as D-branes wrapping the 2-cycles of the singularity. At low energies, the little string becomes the (2,0) conformal field theory of type $\mathfrak{g}$. As an application, we derive the full list of $E_n$ little string defects, and their Bala-Carter label in the CFT limit. Furthermore, we investigate new relations between the quiver gauge theory describing the D-brane defects at low energies, and the weighted Dynkin diagrams of $\mathfrak{g}$. We also give a physical version of the dimension formula of a nilpotent orbit based on its weighted Dynkin diagram.

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