Search for ZZ resonances in the $\ell\ell qq$ final state in pp collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

This note reports a search for heavy resonances decaying into a pair of $Z$ bosons, using 3.2 fb$^{-1}$ of proton--proton collision data at a centre-of-mass energy of $\sqrt{s}=13$ TeV recorded with the ATLAS detector at the Large Hadron Collider. The search is performed in the final state in which one $Z$ boson decays to a pair of charged leptons (electrons or muons) and the other $Z$ boson decays hadronically. The hadronic $Z$ boson candidate is reconstructed either as a pair of small-radius jets or as one large-radius jet. The analysis searches for resonance structures in the invariant mass distributions of the $ZZ$ final state. No evidence for the production of resonances decaying to a $ZZ$ pair is observed. Upper bounds on the production cross sections at $\sqrt{s}=13$ TeV times their decay branching ratios to $ZZ$ pairs ($\sigma\times{\rm BR}$) are derived for a heavy Higgs boson and a spin-2 Randall-Sundrum graviton. The observed (expected) 95% CL limit on $\sigma\times{\rm BR}$ varies from 5.6 (3.3) pb at 300 GeV to 0.21 (0.11) pb at 1000 GeV for the Higgs boson and from 0.62 (0.52) pb at 500 GeV to 0.20 (0.11) pb at 1000 GeV for the graviton.