  We present an infinite-dimensional hyperk\"ahler reduction that extends the
classical moment map picture of Fujiki and Donaldson for the scalar curvature
of K\"ahler metrics. We base our approach on an explicit construction of
hyperk\"ahler metrics due to Biquard and Gauduchon. The construction is
motivated by how one can derive Hitchin's equations for harmonic bundles from
the Hermitian Yang-Mills equation, and yields a system of moment map equations
which modifies the constant scalar curvature K\"ahler (cscK) condition by
adding a "Higgs field" to the cscK equation. In the special case of complex
curves, we recover previous results of Donaldson, while for higher-dimensional
manifolds the system of equations has not yet been studied. We study the
existence of solutions to the system in some special cases. On a Riemann
surface, we extend an existence result for Donaldson's equation to our system.
We then study the existence of solutions to the moment map equations on a class
of ruled surfaces which do not admit cscK metrics, showing that adding a
suitable Higgs term to the cscK equation can stabilize the manifold. Lastly, we
study the system of equations on abelian and toric surfaces, taking advantage
of a description of the system in symplectic coordinates analogous to Abreu's
formula for the scalar curvature.
