Orbifold Indices in Four Dimensions

We introduce supersymmetric indices for four-dimensional gauge theories defined on $\mathscr O \times S^1$, where $\mathscr O$ is a circle bundle over the weighted complex projective line informally known as spindle. Trivial fibrations yield a four-dimensional version of the spindle index, which we obtain by applying localization to partition functions of theories on the direct product of a spindle and a two-dimensional torus. Conversely, non-trivial fibrations lead to the branched lens index, which we compute by localizing theories on the direct product of a circle and a branched covering of the lens space, possibly endowed with conical singularities. The branched lens index encompasses the maximally refined four-dimensional lens index as a special case.