  We construct a top-down holographic model of Weyl semimetal states using
$(3+1)$-dimensional $\mathcal{N}=4$ supersymmetric $SU(N_c)$ Yang-Mills theory,
at large $N_c$ and strong coupling, coupled to a number $N_f \ll N_c$ of
$\mathcal{N}=2$ hypermultiplets with mass $m$. A $U(1)$ subgroup of the
R-symmetry acts on the hypermultiplet fermions as an axial symmetry. In the
presence of a constant external axial gauge field in a spatial direction, $b$,
we find the defining characteristic of a Weyl semi-metal: a quantum phase
transition as $m/b$ increases, from a topological state with non-zero anomalous
Hall conductivity to a trivial insulator. The transition is first order.
Remarkably, the anomalous Hall conductivity is independent of the
hypermultiplet mass, taking the value dictated by the axial anomaly. At
non-zero temperature the transition remains first order, and the anomalous Hall
conductivity acquires non-trivial dependence on the hypermultiplet mass and
temperature.
