Unstable Instantons in A-model Localization

We apply localization techniques to $A$-twisted $\mathcal{N}=(2,2)$ theories of vector multiplets on $S^{2}$. We derive formulae for $A$-model partition functions and correlators as integrals along a real contour, as opposed to a complex one. Using the correlator formula, we successfully recover the unstable instanton partition function of pure two-dimensional Yang-Mills theory from the vacuum expectation value of an $A$-model operator.