Reversible Higgs Bundles

We study reversible Higgs bundles on a compact Riemann surface $\mathrm{\Sigma }$ of genus $g\ge 2$, that is, $G$-Higgs bundles $(E,\mathrm{\Phi })$ with $(E,\mathrm{\Phi })\cong (E,-\mathrm{\Phi })$. For a complex reductive group $G$ realized as a self-adjoint subgroup of $GL(n,\mathbb{C})$, we show that if $(E,\mathrm{\Phi })$ corresponds to a representation $\rho$ of ${\pi }_1(\mathrm{\Sigma })$, then $(E,-\mathrm{\Phi })$ corresponds to $({\bar{\rho }}^t{)}^{-1}$. As a consequence, the moduli space of reversible Higgs bundles is the fixed locus of an involution of the character variety and carries the structure of a real affine algebraic set. For a connected semisimple group and every topological type, we prove that the reversible locus in the smooth moduli space is a $(B,A,A)$-brane. We then describe reversibility through spectral data: in even rank, for a smooth spectral curve, reversible Higgs bundles correspond to line bundles invariant under the involution $\eta \mapsto -\eta$; in odd rank, under natural genericity assumptions for which the spectral curve is nodal, we give a gluing criterion together with an explicit polystability condition. Finally, for $SL(2n,\mathbb{C})$ we construct a relative Fourier–Mukai transform over the regular locus of the Hitchin base and prove that it maps the structure sheaf of the reversible locus to an object whose support is exactly the regular degree-zero $PSp(2n,\mathbb{C})$-locus in the Langlands dual $PGL(2n,\mathbb{C})$-moduli space. This support is of type $(B,B,B)$.