Search for an additional, heavy Higgs boson in the $$H\rightarrow ZZ$$ H → Z Z decay channel at $$\sqrt{s} = 8\;\text{ TeV }$$ s = 8 TeV in $$pp$$ p p collision data with the ATLAS detector

A search is presented for a high-mass Higgs boson in the $$H\rightarrow ZZ\rightarrow \ell ^+\ell ^-\ell ^+\ell ^-$$ , $$H\rightarrow ZZ\rightarrow \ell ^+\ell ^-\nu \bar{\nu }$$ , $$H\rightarrow ZZ\rightarrow \ell ^+\ell ^- q \bar{q}$$ , and $$H\rightarrow ZZ\rightarrow \nu \bar{\nu } q \bar{q}$$ decay modes using the ATLAS detector at the CERN Large Hadron Collider. The search uses proton–proton collision data at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb $$^{-1}$$ . The results of the search are interpreted in the scenario of a heavy Higgs boson with a width that is small compared with the experimental mass resolution. The Higgs boson mass range considered extends up to $$1~\mathrm{TeV}$$ for all four decay modes and down to as low as 140  $$\mathrm{GeV}$$ , depending on the decay mode. No significant excess of events over the Standard Model prediction is found. A simultaneous fit to the four decay modes yields upper limits on the production cross-section of a heavy Higgs boson times the branching ratio to $$Z$$  boson pairs. 95 % confidence level upper limits range from 0.53 pb at $$m_{H} =195$$  GeV to 0.008 pb at $$m_{H} =950$$  GeV for the gluon-fusion production mode and from 0.31 pb at $$m_{H} =195$$  GeV to 0.009 pb at $$m_{H} =950$$  GeV for the vector-boson-fusion production mode. The results are also interpreted in the context of Type-I and Type-II two-Higgs-doublet models.