/ hep-ex CERN-PH-EP-2015-026

Determination of the off-shell Higgs boson signal strength in the high-mass $ZZ$ and $WW$ final states with the ATLAS detector

Pages: 35

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.

Note: 35 pages plus author list + cover pages (52 pages total), 12 figures, 6 tables, submited to The European Physical Journal C, All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2014-10

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