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

Aad, G. and Abbott, B. and Abdallah, J. and Abdinov, O. and Aben, R. and Krasznahorkay, Attila and Nagai, Yoshikazu and Pásztor, Gabriella and Tóth, József (2015) Constraints on the off-shell Higgs boson signal strength in the high-mass ZZ and WW final states with the ATLAS detector. EUROPEAN PHYSICAL JOURNAL C, 75 (7). ISSN 1434-6044

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Abstract

Measurements of the ZZ and WW final states in the mass range above the 2mZ and 2mW 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 nor- malised to the Standard Model prediction (signal strength) in the ZZ → 4`, ZZ → 2`2ν and WW → 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 at a collision energy of √s = 8 TeV. Using the CLs 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 → ZZ and gg → 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 ΓH /ΓSM H 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 → 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.

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