Constraints on the off-shell Higgs boson signal strength in the high-mass \(ZZ\) and \(WW\) final states with the ATLAS detector

Abstract

Measurements of the \(ZZ\) and \(WW\) final states in the mass range above the \(2m_Z\) and \(2m_W\) thresholds provide a unique opportunity to measure the off-shell coupling strength of the Higgs boson. This paper presents a determination of the off-shell Higgs boson event yields normalised to the Standard Model prediction (signal strength) in the \(ZZ \rightarrow 4\ell\), \(ZZ\rightarrow 2\ell2\nu\) and \(WW\rightarrow e\nu\mu\nu\) final states. The result is based on \(pp\) collision data collected by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of \(20.3\) fb\(^{\mathrm{-1}}\) at a collision energy of \(\sqrt{s}=8\) TeV. Using the \(CL_s\) method, the observed 95% confidence level (CL) upper limit on the off-shell signal strength is in the range 5.1--8.6, with an expected range of 6.7--11.0. In each case the range is determined by varying the unknown \(gg\rightarrow ZZ\) and \(gg\rightarrow WW\) background K-factor from higher-order QCD corrections between half and twice the value of the known signal K-factor. Assuming the relevant Higgs boson couplings are independent of the energy scale of the Higgs production, a combination with the on-shell measurements yields an observed (expected) 95% CL upper limit on \(\Gamma_H/\Gamma_H^{\mathrm{SM}}\) in the range 4.5--7.5 (6.5--11.2) using the same variations of the background K-factor. Assuming that the unknown \(gg\rightarrow VV\) background K-factor is equal to the signal K-factor, this translates into an observed (expected) 95% CL upper limit on the Higgs boson total width of 22.7 (33.0) MeV.