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Abstract
Searches are presented for heavy resonances decaying into a Z boson and another boson (a W boson, a Z boson, or a heavy Higgs boson H) using data from proton-proton collisions at a center-of-mass energy of 13 TeV. The data were collected with the ATLAS detector in 2015 and 2016 at the Large Hadron Collider and correspond to a total integrated luminosity of 36.1 inverse-fb. The searches are performed in the llqq final state wherein the Z boson decays into a pair of electrons or muons (l = e, u), and the other boson decays into two quarks (q). The decay product of the hadronically decaying boson is reconstructed as either two jets with small radii or a single large-radius jet, depending on the transverse momentum of the boson. No evidence for resonant production of ZH, ZW, or ZZ pairs is observed. Upper limits are derived at 95% confidence level on the product of the production cross section of a resonance and its decay branching ratio(s) for the selected benchmark signal models. The results are interpreted in the context of extensions of the Standard Model with additional Higgs bosons (e.g. a two-Higgs-doublet model), heavy vector triplets (a model independent framework for the interpretation of spin-1 resonances), and the Randall-Sundrum framework with a warped extra dimension.