We adapt the conjectural local Langlands parameterization to split
metaplectic groups over local fields. When $\tilde G$ is a central extension of
a split connected reductive group over a local field (arising from the
framework of Brylinski and Deligne), we construct a dual group $\mathbf{\tilde
G}^\vee$ and an L-group ${}^L \mathbf{\tilde G}^\vee$ as group schemes over
${\mathbb Z}$.
Brylinski and Deligne have provided a framework to study central extensions
of reductive groups by K2 over a field F. Such central extensions can be used
to construct central extensions of p-adic groups by finite cyclic groups,
including the metaplectic groups. Particularly interesting is the observation
of Brylinski and Deligne that a central extension of a reductive group by K2,
over a p-adic field, yields a family of central extensions of reductive groups
by the multiplicative group over the residue field, indexed by the points of
the building.
The local Langlands conjectures imply that to every generic supercuspidal
irreducible representation of $G_2$ over a $p$-adic field, one can associate a
generic supercuspidal irreducible representation of either $PGSp_6$ or$PGL_3$.
We prove this conjectural dichotomy, demonstrating a precise correspondence
between certain representations of $G_2$ and other representations of $PGSp_6$
and $PGL_3$ when $p \neq 2$. This correspondence arises from theta
correspondences in $E_6$ and $E_7$, analysis of Shalika functionals, and spin
L-functions.