We study the phenomenology of a hypercharge-zero SU(2) triplet scalar whose
existence is motivated by two-step electroweak symmetry-breaking. We consider
both the possibility that the triplets are stable and contribute to the dark
matter density, or that they decay via mixing with the standard model Higgs
boson. The former is constrained by disappearing charged track searches at the
LHC and by dark matter direct detection experiments, while the latter is
constrained by existing multilepton collider searches. We find that a two-step
electroweak phase transition involving a stable triplet with a negative
quadratic term is ruled out by direct detection searches, while an unstable
triplet with a mass less than 230Â GeV is excluded at 95%
confidence level.Comment: 31 pages, 10 figures. Updated to match version published in JHE