  Gauge singlet extensions of the Standard Model (SM) scalar sector may help
remedy its theoretical and phenomenological shortcomings while solving
outstanding problems in cosmology. Depending on the symmetries of the scalar
potential, such extensions may provide a viable candidate for the observed
relic density of cold dark matter or a strong first order electroweak phase
transition needed for electroweak baryogenesis. Using the simplest extension of
the SM scalar sector with one real singlet field, we analyze the generic
implications of a singlet-extended scalar sector for Higgs boson phenomenology
at the Large Hadron Collider (LHC). We consider two broad scenarios: one in
which the neutral SM Higgs and singlet mix and the other in which no mixing
occurs and the singlet can be a dark matter particle. For the first scenario,
we analyze constraints from electroweak precision observables and their
implications for LHC Higgs phenomenology. For models in which the singlet is
stable, we determine the conditions under which it can yield the observed relic
density, compute the cross sections for direct detection in recoil experiments,
and discuss the corresponding signatures at the LHC.
