  Relativistic light-front bound-state equations for double-heavy mesons,
baryons and tetraquarks are constructed in the framework of supersymmetric
light front holographic QCD. Although heavy quark masses strongly break
conformal symmetry, supersymmetry and the holographic embedding of
semiclassical light-front dynamics still holds. The theory, derived from
five-dimensional anti-de Sitter space, predicts that the form of the confining
potential in the light-front Hamiltonian is harmonic even for heavy quarks.
Therefore, the basic underlying supersymmetric mechanism, which transforms
meson-baryon and baryon-tetraquark wave functions into each other, can also be
applied to the double-heavy sector; one can then successfully relate the masses
of the double-heavy mesons to double-heavy baryons and tetraquarks. The
dependence of the confining potential on the hadron mass scale agrees
completely with the one derived in heavy light systems from heavy quark
symmetry. We also make predictions for higher excitations of the charmonium and
bottomonium states. In particular, the remarkable equality of the Regge slopes
in the orbital angular momentum, $L$, and the principal quantum number, $n$, is
predicted to remain valid.
