Morse theory and stable pairs

We study the Morse theory of the Yang-Mills-Higgs functional on the space of pairs $(A,\Phi)$, where $A$ is a unitary connection on a rank 2 hermitian vector bundle over a compact Riemann surface, and $\Phi$ is a holomorphic section of $(E, d_A")$. We prove that a certain explicitly defined substratification of the Morse stratification is perfect in the sense of $\G$-equivariant cohomology, where $\G$ denotes the unitary gauge group. As a consequence, Kirwan surjectivity holds for pairs. It also follows that the twist embedding into higher degree induces a surjection on equivariant cohomology. This may be interpreted as a rank 2 version of the analogous statement for symmetric products of Riemann surfaces. Finally, we compute the $\G$-equivariant Poincar\'e polynomial of the space of $\tau$-semistable pairs. In particular, we recover an earlier result of Thaddeus. The analysis provides an interpretation of the Thaddeus flips in terms of a variation of Morse functions.