The search for the Higgs boson is one of the most important tasks of two multi-purpose experiments: CMS and ATLAS. Both experiments are expected to start taking data in 2008, when the LHC begins its operation at 14 TeV. The most recent results from the CMS and ATLAS Standard Model Higgs boson discovery potential are presented, obtained using the most recent full simulation and reconstruction software as well as with detailed treatment of systematic uncertainties. Results show that the whole Standard Model Higgs boson mass range will be covered with about 10 fb^-1

Drozdetskiy, A

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Available on CMS information server CMS CR 2007/022THE STANDARD MODEL HIGGS BOSON ATLHC: RECENT DEVELOPMENTSA. Drozdetskiy1)on behalf of CMS and ATLAS collaborationsAbstractThe search for the Higgs boson is one of the most important tasks of two multi-purpose experiments:CMS and ATLAS. Both experiments are expected to start taking data in 2008, when the LHC begins itsoperation at 14 TeV. The most recent results from the CMS and ATLAS Standard Model Higgs bosondiscovery potential are presented, obtained using the most recent full simulation and reconstructionsoftware as well as with detailed treatment of systematic uncertainties. Results show that the wholeStandard Model Higgs boson mass range will be covered with about ∼ 10fb−1.23 May 20071) University of Florida, Gainesville, FL, USAFigure 1: Left: Higgs production cross sections at the LHC for the various production mechanisms as a functionof the Higgs mass. The full QCD-corrected results for the gluon fusion gg → H , vector-boson fusion qq →V V qq → Hqq, vector-boson bremsstrahlung qq¯ → V ∗ → HV and associated production gg, qq¯ → Htt¯ areshown. Right: Branching ratios of the dominant decay modes of the SM Higgs particle. All relevant higher-ordercorrections are taken into account.There are four main experiments at the Large Hadron proton-proton Collider (LHC) [1] at CERN [2], two of themare general purpose detectors: CMS (Compact Muon Solenoid) [3] and ATLAS [4] (A Toroidal Lhc ApparatuS).One of the most important tasks for these experiments is to search for the Higgs boson, the last missing piece ofthe Standard Model (SM) that is still not confirmed experimentally.1 The Standard Model Higgs boson production and decayThe main production channels for the SM Higgs boson are: gluon fusion via top loop, vector boson (W/Z) fusion,W/Z associated production, tt¯ fusion. The production cross section at the LHC as a function of the the Higgs bosonmass is shown in Fig. 1 [5] (right). In the Higgs boson search analyses, NLO corrections for signal and backgroundprocesses were used when available. At LHC, 14 TeV proton-proton collider, the dominant production mechanismis gluon fusion, whereas at Tevatron, proton-anti-proton 2 TeV collider, the most relevant mechanism is W/Zassociated production, see e.g. Ref. [6].The Higgs decay modes (see Fig. 1, left plot) can be divided into two different mass ranges. ForMH ≤ 135GeV/c2the Higgs boson mainly decays into bb¯ and τ+τ− pairs with branching ratios of about 85% and 8% respectively.Both bb¯ and τ+τ− Higgs boson decay modes (as well as modes into cc¯ and gluon pairs) are [almost] impossiblefor discovery due to overwhelming background level. The most useful Higgs decays in this mass range at the LHCis the decay into photon pairs. It reaches a branching fraction of up to 2 × 10−3. Its importance lies in the cleanenvironment and controllable background, which makes it an important discovery channel for small Higgs bosonmasses.For Higgs masses above 135 GeV/c2 the main decay modes are those into WW and ZZ pairs. These are veryimportant discovery channels, in particular H → ZZ(∗) → 4µ is called the “golden mode” for its very clean (4muons) final state, which provides a low/medium integrated luminosity level for discovery of the Higgs boson forall possible Higgs boson masses in consideration (∼ 115− 600GeV/c2).2 Searches for the SM Higgs bosonThis section discusses the detailed analyses available for many of the SM Higgs boson production and decaychannels from both CMS and ATLAS collaborations. They include:• inclusive H → γγ [7, 8],• inclusive H →WW/ZZ [9, 10],2• qqH production mode with Higgs decaying into ττ, γγ,WW/ZZ [11, 12],• W/Z+H production mode with Higgs decaying into γγ,WW [13],• tt¯H production mode with Higgs decaying into bb¯, γγ [14, 15].2.1 Inclusive H → γγThe distinct feature of this analysis is the assignment of a weight to every event depending on the “quality” ofreconstructed photon candidates. One can find the signal-background spectrum for different photon categories inFig. 2 [16]. There is a clear gain in exploiting this feature: getting the most for significance from events with lessbackground contamination. This analysis also took advantage of optimizing the signal vs. background significancevia Neural Network techniques, which decreased the integrated luminosity level needed for 5σ discovery roughlyby factor of 2.2.2 Inclusive H → ZZIn the H → ZZ(∗) → 4µ analysis counting and Log-Likelihood ratio techniques were used as well as geneticalgorithm techniques [17] to optimize the outcome of the analysis, which led to M4µ-dynamic cuts and about 1σimprovement in terms of signal to background significance all over the M4µ spectrum. Effort was made to makeanalysis as close as possible to “as one would do it with real data”: lepton reconstruction and isolation techniquesfrom data were developed and tested.A detailed study of systematic uncertainties was performed in this and many other analyses by CMS and ATLAS.Some of the main systematic uncertainties come from the major background cross sections (for many analyses)and NLO k-factors as well as possible contribution from unaccounted backgrounds (including processes, whichbecome important when fake leptons, for example, are taken into account). Also estimations were made to rescalethe local excess of significance due to the fact that we look for a narrow resonance in a broad range of MH .2.3 Inclusive H → WWIn H → γγ analysis, one uses sidebands of huge, but smooth background in Mγγ-spectrum; the, inclusive H →WW analysis uses control regions in phase space where signal “contamination” is minimized (see Fig. 3 [16],top). Signal and irreducible WW background events were also re-weighted with dynamic NLO k-factors (forpWWT distribution) in order to properly calculate the discovery potential for this important channel (the procedureis proposed in Ref. [18] and it improves significance considerably).2.4 Vector boson fusion H → WWAs in the previous one, this analysis is also a counting experiment and uses signal and control parts of phase spacefor the discovery region and background control (see Fig. 3 [12], bottom). The advantage in this analysis is theability to tag two additional forward jets.2.5 Discovery reach summaryFigure 4 (left) [16] shows the signal significance as a function of the Higgs boson mass for 30 fb−1 of integratedluminosity for the different Higgs boson production and decay channels with CMS. Similar results for ATLAS areshown on Fig. 4 (right) [12].Figure 5 (left) [16] shows the integrated luminosity needed for the 5σ discovery of the inclusive Higgs bosonproduction pp → H + X with the Higgs boson decay modes H → γγ, H → ZZ → 4`, and H → WW → 2`2ν– the three front runners among all possible decay channels. H → γγ dominates the discovery reach up tomH ∼ 130GeV/c2, similarly H → WW → 2`2ν dominates for ∼ 150 − 180 GeV and H → ZZ → 4` – in therest of the possible Higgs boson masses. Figure 5 (right) shows the Higgs boson mass measurements precision,∆MH/MH , as a function of the Higgs boson mass for 30 fb−1 of integrated luminosity for the different Higgsboson production and decay channels with CMS.3Figure 2: Inclusive H → γγ analysis. Invariant mass spectrum for four photon categories. Events are normalizedto an integrated luminosity of 1fb−1 and the Higgs signal, shown for different masses, is scaled by a factor 10.Figure 3: Inclusive H → WW analysis. Top: The angle between the leptons in the transverse plane is shownfor the signal and the different backgrounds and a luminosity of 10fb−1, left plot is for the signal cuts and rightplot is for the WW background normalization region where all signal cuts are applied, except the one on the leptoninvariant mass. VBF H → WW analysis. Bottom: Distribution of the transverse mass MT for Higgs bosonmasses of 160 GeV/c2 is shown (left - is for “signal-like” part of the phase space, and right - is for “background-like” part of the phase space).4Figure 4: Left: The signal significance as a function of the Higgs boson mass for 30 fb−1 of the integratedluminosity is shown for the different Higgs boson production and decay channels for analyses with CMS. Right:The corresponding summary plot for ATLAS is shown. The difference in reach curves mainly comes from NLOvs. LO cross sections and full simulation vs. fast simulation for CMS vs. ATLAS results. (There is an updateexpected for ATLAS official results.)Figure 5: Left: The integrated luminosity needed for the 5σ discovery of the inclusive Higgs boson productionpp→ H+X is shown for the Higgs boson decay modesH→ γγ, H→ ZZ→ 4`, andH→WW→ 2`2ν. Right:Higgs boson mass measurements precision as a function of the Higgs boson mass for 30 fb−1 of the integratedluminosity is shown for the different Higgs boson production and decay channels for analyses with CMS.53 SummaryBoth collaborations, CMS and ATLAS, show that discoveries may be expected already at integrated luminosity,about 1fb−1. The SM Higgs boson, if it exists, is expected to be discovered by the time we reach L ≈ 10fb−1. Bythe time we get L ≈ 30fb−1 we should be able to measure the Higgs boson mass with precision of about 0.1%.References[1] LHC Project, http://lhc.web.cern.ch/lhc/, http://lhc-machine-outreach.web.cern.ch/lhc-machine-outreach/lhc in pictures.htm[2] CERN, http://www.cern.ch/[3] CMS Collaboration, http://cms.cern.ch/[4] ATLAS Collaboration, http://atlas.web.cern.ch/Atlas/index.html[5] M. Spira and P. Zerwas Electroweak symmetry breaking and Higgs physics, CERN-TH-97-379, DESY-97-261, Dec 1997. 69pp. hep-ph/9803257[6] M. Spira Higgs boson production and decay at the Tevatron, DESY-98-159, Oct 1998. 12pp. hep-ph/9810289[7] Pieri M. et al., Inclusive Search for the Higgs Boson in the H → γγ Channel, CMS Note 2006/112.[8] L. Carminati (on behalf of ATLAS collaboration), Search for a Standard Model Higgs boson in the H → γγchannel with the ATLAS detector, ATL-PHYS-CONF-2006-018 (and references therein).[9] G. Davatz et al., Standard Model Higgs Discovery Potential of CMS in H → WW → lνlν Channel, CMSNote 2006/047.[10] S. Abdullin et al., Search Strategy for the Standard Model Higgs Boson in the H → ZZ(∗) → 4µ DecayChannel using M(4µ)-Dependent Cuts, CMS Note 2006/122; S. Baffioni et al., Discovery Potential for theSM Higgs Boson in the H → ZZ(∗) → 4e Decay Channel, CMS Note 2006/115; D. Futyan et al., Searchfor the Standard Model Higgs Boson in the Two-Electron and Two-Muon Final State with CMS, CMS Note2006/136.[11] M. Dubinin et al., Vector Boson Fusion Production with H → γγ, CMS Note 2006/097; S. Kunori et al.,Search for a Standard Model Higgs Boson in CMS via Vector Boson Fusion in the H → WW → lνlνChannel, CMS NOTE 2006/091; H. Pi et al., Search for Standard Model Higgs Boson via Vector BosonFusion in the H → W+W− → l±νjj with 120 ¡ mH ¡ 250 GeV/c2, CMS Note 2006/092; C. Foudas etal., Observation of the Standard Model Higgs boson via H → ττ → lepton + jet Channel, CMS Note2006/088.[12] S. Asai et al., Prospects for the Search for a Standard Model Higgs Boson in ATLAS using Vector BosonFusion, Eur.Phys.J. C32S2 (2004) 19-54, hep-ph/0402254.[13] M. Lethuillier et al., Search for a Neutral Higgs Boson with WH/ZH, H → γγ, CMS Note 2006/110; C. De-laere Study of associated WH production with H →WW ∗ in the 3 leptons final state, CMS Note 2006/053.[14] S. Cucciarelli et al., Search for H → bb¯ in Association with a ttbar Pair at CMS, CMS Note 2006/119.[15] Cammin J. et al., The ATLAS discovery potential for the channel tt¯H , H → bb¯, ATLAS-PHYS-2003-024.[16] M.Della Negra, A. Petrilli, A. Ball, L. Foa et al., CMS Physics Technical Design Report, volume II, J. Phys.G: Nucl. Part. Phys., vol34 (2007), 995-1579.[17] S. Abdullin, A. Drozdetskiy et al., GARCON: Genetic Algorithm for Rectangular Cuts OptimizatioN. User’smanual for version 2.0, hep-ph/0605143, http://drozdets.home.cern.ch/drozdets/home/genetic/[18] G. Davatz et al., Effective K-factors for gg → H → WW → lνlν at the LHC, JHEP 0405 (2004) 009,arXiv:hep-ph/0402218v26

The Standard Model Higgs Boson at LHC: Recent Developments

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