The Self-Duality Equations on a Riemann Surface and Four-Dimensional Chern-Simons Theory

We construct a Lagrangian formulation of Hitchin's self-duality equations on a Riemann surface $\Sigma$ using potentials for the connection and Higgs field. This two-dimensional action is then obtained from a four-dimensional Chern-Simons theory on $\Sigma\times \mathbb{CP}^1$ with an appropriate choice of meromorphic 1-form on $\mathbb{CP}^1$ and boundary conditions at its poles. We show that the symplectic structure induced by the four-dimensional theory coincides with the canonical symplectic form on the Hitchin moduli space in the complex structure corresponding to the moduli space of Higgs bundles. We further provide a direct construction of Hitchin Hamiltonians in terms of the four-dimensional gauge field. Exploiting the freedom in the choice of the meromorphic one-form, we construct a family of four-dimensional Chern-Simons theories depending on a $\mathbb{CP}^1$-valued parameter. Upon reduction to two dimensions, these descend to a corresponding family of two-dimensional actions on $\Sigma$ whose field equations are again Hitchin's equations. Furthermore, we obtain a family of symplectic structures from our family of four-dimensional theories and show that they agree with the hyperk\"ahler family of symplectic forms on the Hitchin moduli space, thereby identifying the $\mathbb{CP}^1$-valued parameter with the twistor parameter of the Hitchin moduli space. Our results place Hitchin's equations and their integrable structure within the framework of four-dimensional Chern-Simons theory and make the role of the twistor parameter manifest.