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 2mZ2mZ and 2mW2mW thresholds provide a unique opportunity to measure the off-shell coupling strength of the Higgs boson. This paper presents constraints on the off-shell Higgs boson event yields normalised to the Standard Model prediction (signal strength) in the ZZ→4ℓZZ→4ℓ , ZZ→2ℓ2νZZ→2ℓ2ν and WW→eνμνWW→eνμν 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 −1−1 at a collision energy of s√=8s=8 TeV. Using the CLsCLs 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→ZZgg→ZZ and gg→WWgg→WW background K-factor from higher-order quantum chromodynamics 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 boson production, a combination with the on-shell measurements yields an observed (expected) 95 %% CL upper limit on ΓH/ΓSMHΓH/ΓHSM 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→VVgg→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.