1 Introduction

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HZZHighMassPaper.bib \addbibresourceatlas-paper.bib \addbibresourceATLAS.bib \addbibresourceCMS.bib \addbibresourcePubNotes.bib \addbibresourceConfNotes.bib \AtlasTitleSearch for an additional, heavy Higgs boson in the \htozz decay channel at in collision data with the ATLAS detector \AtlasRefCodeHIGG-2013-20 \PreprintIdNumberCERN-PH-EP-2015-154 \AtlasJournalEPJC \AtlasAbstractA search is presented for a high-mass Higgs boson in the \htollll, \htollvv, \htollqq, and \htovvqq 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. 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 for all four decay modes and down to as low as , 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  boson pairs. 95% confidence level upper limits range from 0.53 pb at  \gev to 0.008 pb at  \gev for the gluon-fusion production mode and from 0.31 pb at  \gev to 0.009 pb at  \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. \AtlasCoverSupportingNote\htollqqhttps://cds.cern.ch/record/1693159 \AtlasCoverSupportingNote\htollvvhttps://cds.cern.ch/record/1693161 \AtlasCoverSupportingNote\htollllhttps://cds.cern.ch/record/1693487 \AtlasCoverSupportingNote\htovvqqhttps://cds.cern.ch/record/1692942 \AtlasCoverSupportingNoteCombinationhttps://cds.cern.ch/record/1995509 \AtlasCoverCommentsDeadline25 June 2015 \AtlasCoverAnalysisTeam S. H. Abidi, C. Anastopoulos, S. Angelidakis, L. Aperio Bella, G. Artoni, M. Baak, P. Clark, F. Conventi, R. Di Nardo, T. Donszelmann Cuhadar, N. Edwards, D. Fassouliotis, G. Garrillo Montoya, F. Garay Walls, C. Gwilliam, M. Hoffmann, R. Harrington, X. Ju, L.S. Kaplan, D. Kyriazopoulos, A. Mehta, E. Mountricha, R. Nicolaidou, S. Oda, H. Okawa, K. Parker, A. Sanchez Pineda, J. Price, M. Rescigno, C. Savarese, F. Lo Sterzo, S. Snyder, J. Voosebeld, C. Wang, S.M. Wang, S.L. Wu, L. Xu, L. Zhang, Z. Zhao, B. Zhou, G. Zurzolo \AtlasCoverEdBoardMemberM. Vos (chair) \AtlasCoverEdBoardMemberV. Cavaliere \AtlasCoverEdBoardMemberP. Maettig \AtlasCoverEdBoardMemberD. Whiteson \AtlasCoverEgroupEditorsatlas-HIGG-2013-20-editors@cern.ch \AtlasCoverEgroupEdBoardatlas-HIGG-2013-20-editorial-board@cern.ch \nolinenumbers

Contents:

1 Introduction

In 2012, a Higgs boson with a mass of was discovered by the ATLAS and CMS collaborations at the LHC [HIGG-2012-27, CMS-HIG-12-028]. One of the most important remaining questions is whether the newly discovered particle is part of an extended scalar sector as postulated by various extensions to the Standard Model (SM) such as the two-Higgs-doublet model (2HDM) [Branco:2011iw] and the electroweak-singlet (EWS) model [Hill:1987ea]. These predict additional Higgs bosons, motivating searches at masses other than .

This paper reports four separate searches with the ATLAS detector for a heavy neutral scalar decaying into two SM bosons, encompassing the decay modes , , , and , where stands for either an electron or a muon. These modes are referred to, respectively, as , , , and .

It is assumed that additional Higgs bosons would be produced predominantly via the gluon fusion (ggF) and vector-boson fusion (VBF) processes but that the ratio of the two production mechanisms is unknown in the absence of a specific model. For this reason, results are interpreted separately for ggF and VBF production modes. For Higgs boson masses below , associated production (\VH, where stands for either a  or a  boson) is important as well. In this mass range, only the decay mode is considered. Due to its excellent mass resolution and high signal-to-background ratio, the decay mode is well-suited for a search for a narrow resonance in the range ; thus, this search covers the range down to . The search includes channels sensitive to \VHproduction as well as to the VBF and ggF production modes. The and searches, covering ranges down to and respectively, consider ggF and VBF channels only. The search covers the range down to and does not distinguish between ggF and VBF production. Due to their higher branching ratios, the , , and decay modes dominate at higher masses, and contribute to the overall sensitivity of the combined result. The range for all four searches extends up to .

The ggF production mode for the search is further divided into four channels based on lepton flavour, while the search includes four channels, corresponding to two lepton flavours for each of the ggF and VBF production modes. For the and searches, the ggF production modes are divided into two subchannels each based on the number of -tagged jets in the event. For Higgs boson masses above , jets from  boson decay are boosted and tend to be reconstructed as a single jet; the ggF search includes an additional channel sensitive to such final states.

For each channel, a discriminating variable sensitive to is identified and used in a likelihood fit. The and searches use the invariant mass of the four-fermion system as the final discriminant, while the and 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. Additional distributions from background-dominated control regions also enter the fit in order to constrain nuisance parameters. Unless otherwise stated, all figures show shapes and normalizations determined from this fit. All results are interpreted in the scenario of a new Higgs boson with a narrow width, as well as in Type-I and Type-II 2HDMs.

The ATLAS collaboration has published results of searches for a Standard Model Higgs boson decaying in the \llll, \llqq, and \llvv modes with 4.7–4.8

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