  Although the 125 GeV Higgs boson discovered at the LHC is often heralded as
the origin of mass, it may not in fact be the origin of Yukawa couplings. In
alternative models, Yukawa couplings may instead arise from a seesaw type
mechanism involving the mixing of Standard Model (SM) chiral fermions with new
vector-like fermions, controlled by the vacuum expectation value (VEV) of a new
complex Higgs singlet field $\langle \Phi \rangle$. For example, the largest
third family $(t,b)$ quark Yukawa couplings may be forbidden by a $U(1)'$ gauge
or global symmetry, broken by $\langle \Phi \rangle$, and generated effectively
via mixing with a vector-like fourth family quark doublet $(T,B)$. Such
theories predict a new physical Higgs singlet $\phi$, which we refer to as the
Yukon, resulting from $\langle \Phi \rangle$, in the same way that the Higgs
boson $h^0$ results from $\langle H\rangle$. In a simplified model we discuss
the prospects for discovering the Yukon $\phi$ in gluon-gluon fusion
production, with $(t,b)$ and $(T,B)$ quarks in the loops, and decaying in the
channels $\phi\rightarrow \gamma\gamma, Z\gamma$ and $\phi\rightarrow
tT\rightarrow tth^0,ttZ$. The potential for discovery of the Yukon $\phi$ is
studied at present or future hadron colliders such as the LHC (Run 3), HL-LHC,
HE-LHC and/or FCC. For example, we find that a 300-350 GeV Yukon $\phi$ could
be accessed at LHC Run 3 in the di-photon channel in the global model,
providing a smoking gun signature of the origin of Yukawa couplings. The
$tth^0,ttZ$ channels are more involved and warrant a more sophisticated
analysis.
