LHC constraints on large scalar multiplet models with a $Z_2$ symmetry

We study the LHC search constraints on models that extend the Standard Model (SM) with an inert, complex scalar electroweak multiplet, $Σ$, with isospin T=5/2 (sextet) or T=7/2 (octet) and identical hypercharge to the SM Higgs doublet. Imposing a global $Z_2$ symmetry under which $Σ\to -Σ$, the lightest member of $Σ$ is stable and we require that it be neutral ($ζ^{0,r}$) to avoid cosmological constraints from charged relics. Pair production of scalars by electroweak interactions followed by cascade decays to $ζ^{0,r}$ through W and Z emission produces signatures similar to those of supersymmetric electroweak gauginos, and we constrain the models by recasting a collection of such searches made with data from the 8 TeV run of the LHC. We find that there is no sensitivity from these searches to the compressed spectrum regime, in which the mass splittings between the lightest and heaviest states in $Σ$ are less than about 20 GeV. In the remaining parameter space, we find significant exclusions for $m_{ζ^{0,r}} \sim$ 80-180 GeV in the sextet model, and $m_{ζ^{0,r}} \sim$ 80-120 GeV in the octet model.

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