Abstract
A search is presented for a high-mass Higgs boson in the Hrightarrow ZZrightarrow ell ^+ell ^-ell ^+ell ^-, Hrightarrow ZZrightarrow ell ^+ell ^-nu bar{nu }, Hrightarrow ZZrightarrow ell ^+ell ^- q bar{q}, and Hrightarrow ZZrightarrow 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.
Highlights
Background samplesMonte Carlo simulations are used to model the shapes of distributions from many of the sources of SM backgroundPage 5 of 42 45 to these searches
This paper reports four separate searches with the ATLAS detector for a heavy neutral scalar H decaying into two SM Z bosons, encompassing the decay modes Z Z → + − + −, Z Z → + −νν, Z Z → + −qq, and Z Z → ννqq, where stands for either an electron or a muon
These samples are generated at leading order (LO) in αS using MCFM 6.1 [35] ( ) or gg2vv 3.1.3 [36,37] but corrected to nextto-next-to-leading order (NNLO) as a function of mass of the Z boson (m Z) Z [38] using the same procedure as described in Ref. [6]
Summary
(SM) such as the two-Higgs-doublet model (2HDM) [3] and the electroweak-singlet (EWS) model [4]. The ννqq search covers the mH range down to 400 GeV and does not distinguish between ggF and VBF production Due to their higher branching ratios, the qq, νν, and ννqq decay modes dominate at higher masses, and contribute to the overall sensitivity of the combined result. For Higgs boson masses above 700 GeV, jets from Z boson decay are boosted and tend to be reconstructed as a single jet; the ggF qq search includes an additional channel sensitive to such final states. The and qq searches use the invariant mass of the four-fermion system as the final discriminant, while the νν and ννqq searches use a transverse mass distribution Distributions of these discriminants for each channel are combined in a simultaneous likelihood fit which estimates the rate of heavy Higgs boson production and simultaneously the nuisance parameters corresponding to systematic uncertainties.
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