Bulk stabilization, the extra-dimensional Higgs portal and missing energy in Higgs events
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abstract

To solve the hierarchy problem, extra-dimensional models must explain why the
new dimensions stabilize to the right size, and the known mechanisms for doing
so require bulk scalars that couple to the branes. Because of these couplings
the energetics of dimensional stabilization competes with the energetics of the
Higgs vacuum, with potentially observable effects. These effects are
particularly strong for one or two extra dimensions because the bulk-Higgs
couplings can then be super-renormalizable or dimensionless. Experimental reach
for such extra-dimensional Higgs `portals' are stronger than for gravitational
couplings because they are less suppressed at low-energies. We compute how
Higgs-bulk coupling through such a portal with two extra dimensions back-reacts
onto properties of the Higgs boson. When the KK mass is smaller than the Higgs
mass, mixing with KK modes results in an invisible Higgs decay width,
missing-energy signals at high-energy colliders, and new mechanisms of energy
loss in stars and supernovae. Astrophysical bounds turn out to be complementary
to collider measurements, with observable LHC signals allowed by existing
constraints. We comment on the changes to the Higgs mass-coupling relationship
caused by Higgs-bulk mixing, and how the resulting modifications to the running
of Higgs couplings alter vacuum-stability and triviality bounds.