The study of the mechanism behind electroweak symmetry breaking is one of the
main goals of the Large Hadron Collider and of its general-purpose experiments,
ATLAS and CMS. This paper reviews some of the ongoing studies by these
collaborations and, when possible, highlights the differences between
equivalent channels in both experiments.Comment: Parallel talk at ICHEP08, Philadelphia, USA, July 2008. 4 pages,
  LaTeX, 8 eps figure

Goncalo, R.

English

arXiv

The study of the mechanism behind electroweak symmetry breaking is one of the main goals of the Large Hadron Collider and of its general-purpose experiments, ATLAS and CMS. This paper reviews some of the ongoing studies by these collaborations and, when possible, highlights the differences between equivalent channels in both experiments.The study of the mechanism behind electroweak symmetry breaking is one of the main goals of the Large Hadron Collider and of its general-purpose experiments, ATLAS and CMS. This paper reviews some of the ongoing studies by these collaborations and, when possible, highlights the differences between equivalent channels in both experiments

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ATL-PHYS-PROC-2008-06021November200834th International Conference on High Energy Physics, Philadelphia, 2008Standard Model Higgs Searches at the LHCR. Gonc¸aloRoyal Holloway, University of LondonOn behalf of the ATLAS and CMS collaborationsThe study of the mechanism behind electroweak symmetry breaking is one of the main goals of the Large HadronCollider and of its general-purpose experiments, ATLAS and CMS. This paper reviews some of the ongoing studies bythese collaborations and, when possible, highlights the differences between equivalent channels in both experiments.1. IntroductionThe Higgs mechanism [1] is central to the Standard Model of particle physics (SM). The existence of the Higgsfield maintains the theory weakly interacting up to high energy scales and prevents some processes from violatingunitarity. In the process of spontaneous breaking of electroweak symmetry, the weak bosons W± and Z0, as wellas leptons and quarks, acquire mass through interactions with the Higgs field. In the SM, the simplest form of theHiggs mechanism is assumed, which predicts the existence of a single scalar particle, the Higgs boson and a singlefree parameter, its mass (mH). The discovery of this particle would provide experimental evidence for the Higgsmechanism. The discovery and study of the mechanism of electroweak symmetry breaking is one of the main goalsof the ATLAS [2] and CMS [3] experiments, operating at the Large Hadron Collider (LHC). Many models of physicsbeyond the SM predict a more complex Higgs sector, covered elsewhere [4].Direct searches for the Higgs boson produced in association with a Z0 were performed in the Large ElectronPositron collider (LEP). These resulted in the exclusion of the Higgs boson in the mass range up to 114.4 GeV at95% confidence level [5]. On the other hand, precision fits [6] of electroweak observables, including data from the LEPand Tevatron colliders, provide an indirect estimate of the Higgs boson mass, assuming the SM scenario. The latestfit results give mH = 84+34−26GeV/c2, or the one-sided 95% confidence-level limit mH < 154GeV. Including the LEPdirect search results, this limit increases to 185GeV/c2. Recent combined results from the Tevatron experimentshave, for the first time, excluded the hypothesis of a Higgs boson mass around 170GeV [7] at 95% confidence level.Although the expected sensitivity of Tevatron experiments is not enough to make a 5σ discovery of the SM Higgsboson [8], it is enough to exclude it out up to mH ∼ 200GeV/c2 at 95% confidence level, or to make a 3σ observation.2. Higgs boson searches in ATLAS and CMSThe Higgs boson production at the LHC is dominated by the gluon-gluon fusion process, described at leading orderthrough a heavy-quark loop. The next-to-leading order cross section for this process is 37.6 pb, for mH = 120GeV/c2.The Higgs boson can also be produced by Vector Boson Fusion (VBF) with a cross section of 4.25 pb, or by associatedproduction with a W±, a Z0, or a tt¯ quark pair, with 3.19 pb for the three processes and mH = 120GeV/c2 (crosssections calculated at next-to-leading order using parton density function sets CTEQ6M and CTEQ6L1 [9]).The Higgs boson branching ratio is strongly dependent on its mass. At mH <∼ 135GeV/c2, the main decay modeis to a bb¯ pair (BR = 81%), followed by the decay to a τ+τ− pair (BR ∼ 8%). For a small but important intervalof mH , the Higgs boson decays to a pair of photons with a small branching ratio. At higher masses, the decay to apair of (possibly off-shell) W± or Z0 bosons becomes dominant.The most abundant signal topologies, containing bb¯ pairs are unfortunately hard to separate from the large QCDbackground. The following gives a summary of the discovery channels being investigated at the LHC. In all cases,full detector simulation was used, which included realistic descriptions of the material budget and detector geometry.The trigger response was also realistically simulated, and systematic uncertainties were estimated. Next-to-leadingorder cross sections were used whenever available, and used to normalise simulated event samples.134th International Conference on High Energy Physics, Philadelphia, 2008Figure 1: Left: ATLAS sensitivity in the H → 4l channel for 30 fb−1 [9] (background estimation systematic uncertaintiesare included); centre: four-lepton invariant mass distribution in the H → ZZ(∗) → 2e2µ channel in CMS [10]; right: CMSsensitivity in H → γγ channel for 30 fb−1 [10].2.1. Higgs boson decay to four leptons: H → ZZ(∗) → 4l (4e, 4µ or 2e2µ)This channel provides excellent sensitivity for a wide range ofmH above 130GeV/c2, except for the interval between2mW and 2mZ , where the Higgs boson branching ratio is dominated by H → W+W−. The main background ispp→ ZZ(∗) → 4l±. Other backgrounds, such as Zbb¯, ZW , tt¯, and Z +X are effectively suppressed by the analysisevent selection. Both CMS and ATLAS rely on selecting events which contain pairs of electrons or muons withopposite charge. At least one Z0 is expected to be on mass shell, and the two-lepton invariant mass is used toreject fake and misidentified leptons. A clear four-lepton mass peak above a flat background is expected in thischannel (see figure 1, left), thus allowing a good background estimation from the peak sidebands. Two factorsare especially significant for analyses of this channel: the lepton reconstruction efficiency and the invariant-massresolution. Both experiments show similar experimental sensitivities in this channel, with the total signal significancefor each experiment expected to be in excess of 10 for an integrated luminosity of 30 fb−1 (see figure 1, centre).2.2. Higgs boson decay to two photons: H → γγIn spite of the low Higgs boson branching ratio to two photons (around 0.2% for mH = 120GeV/c2), this channelshows very good sensitivity in a range of mH between the LEP limit and mH ∼ 140GeV/c2. The main irreduciblebackground is the direct γγ production. Signal appears as a sharp peak over a smoothly falling di-photon invariantmass distribution. Reducible instrumental backgrounds such as γ + jet and multijet production, where one or moreparticles are misidentified as photons, are important due to the large QCD cross sections. Both experiments need totake photon conversions into account, since ∼ 60% of Higgs events contain at least one converted photon.Both collaborations classify the selected events into several categories, according to the event kinematics or topol-ogy. In this way, the event selection can be optimized, in order to maximize the background discrimination withoutdegrading the efficiency for selecting signal events. In ATLAS, a significance of about 3.5σ is expected for 10 fb−1,assuming mH = 120GeV/c2. With an optimized analysis employing a neural network, CMS expects up to 10σ aftercollecting 30 fb−1 and a 5σ after accumulating 8 fb−1 (see figure 1, right). Analyses are also being developed totarget the associated production channels ZH , WH and tt¯H .2.3. Higgs boson decay to a tau pair: H → τ+τ−This channel currently provides one of the best sensitivities at low mH . Analyses rely on the topology of VBFevents to provide additional rejection against SM backgrounds. In this process, the Higgs boson is radiated by W±or Z0 bosons exchanged between the interacting partons. Due to the lack of colour flow between the two interactingpartons, the characteristic event topology consists of two relatively forward jets with a rapidity gap in between,containing little hadronic activity. Both ATLAS and CMS investigate this channel for all final states, in which at234th International Conference on High Energy Physics, Philadelphia, 2008Figure 2: Left: τ+τ− invariant mass distribution in the H → τ+τ− channel in ATLAS with one τ decaying hadronically andmH = 120 GeV/c2 [9]; centre: expected sensitivity for the H → τ+τ− channel in ATLAS (systematic uncertainties due to thebackground estimation are included) [10]; right: transverse mass in the H →W+W− → lνlν in the CMS analysis [10].least one tau decays to an electron or muon plus neutrinos. The final state where both taus decay hadronicallywas also investigated by ATLAS and proved to be feasible, but there is as yet no detailed sensitivity estimates. Thedominant background in all cases is Z+jets with the Z boson decaying to τ+τ−. Both collaborations have developeddata-driven methods to estimate these backgrounds. Other backgrounds are W +N jets, tt¯, and di-jet events.The τ+τ− invariant mass reconstruction requires an approximation, in which the τ± is assumed to be collinearwith the visible lepton (in τ → e or τ → µ decays). The resulting Higgs mass resolution can be observed in figure 2(left). The expected sensitivity for this channel, currently reaches up to 5σ for 30 fb−1.2.4. Higgs boson decay to a W boson pair: H →W+W− → lνlν (l = e± or µ±)The Higgs boson decay to a W+W− pair provides the most sensitive search channel in the mass range 2mW <mH < 2mZ , where this decay mode has a branching ratio above 95%. Analyses have concentrated on final statescontaining electrons or muons from the W± decay. Contrary to the remaining channels, though, the presence of hightransverse momentum neutrinos makes it unfeasible to obtain a Higgs mass peak. The transverse mass [10] can stillbe calculated and used in the event selection (see figure 2, right). Analyses in this channel then need to rely on avery good knowledge of the background shape and normalisation.The dominant backgrounds come from events containing a W-boson pair, most importantly W+W− and tt¯ pro-duction. The H → W+W− decay creates a correlation between the W spins in the Higgs reference frame whichtranslates into an angular correlation between the leptons emitted in the W-bosons decay. This is exploited tosuppress the W+W− background. The tt¯ background can be effectively suppressed by a veto on central jets (jetswith a small pseudorapidity value). Separate analyses are performed for the cases where there are no high transversemomentum jets present in the event and where there are two additional jets, directed at the gluon fusion and theVBF production modes, respectively. For a Higgs boson mass close 160GeV/c2 a sensitivity in excess of 10σ isexpected for each experiment for 30 fb−1 of integrated luminosity. Additional channels being explored are the VBFH →W+W− → lν + 2jets and the associated production channels W±H and tt¯H .3. Summary and OutlookThe expected sensitivity of ATLAS and CMS to a SM Higgs boson was evaluated by both collaborations. Analysesin both experiments show similar sensitivity in most channels. The sensitivity for SM Higgs discovery in the variouschannels is illustrated by figure 3 for the CMS case. An integrated luminosity of less than 10 fb−1 should be enough tomake a 5σ discovery in the mass range above the LEP limit. Preliminary ATLAS studies indicate that an integrated334th International Conference on High Energy Physics, Philadelphia, 2008Figure 3: SM Higgs discovery potential for CMS assuming an integrated luminosty of 30 fb−1 (left) and integrated luminosityneeded for a 5σ Higgs discovery, versus Higgs boson mass [10]).luminosity of 2 fb−1 would be enough to exclude the Higgs boson in the range 121GeV/c2 <∼mH <∼ 460GeV/c2 at95% confidence level. Other analyses, not described here, may contribute to further enhancements of the sensitivity.AcknowledgmentsThe author would like to express his thanks to ATLAS and CMS colleagues. Special thanks to Chiara Mariotti,Yves Sirois, Louis Fayard, Aleandro Nisati, Ketevi Assamagan, Eilam Gross, and Glen Cowan, and to many othercolleagues who have kindly contributed with work and material for this paper.References1 P.W. Higgs, Phys.Lett., 12 132 (1964); P.W. Higgs, Phys.Rev.Lett., 13 508 (1964); P.W. Higgs, Phys.Rev., 1451156 (1966); F.Englert and R.Brout, Phys.Rev.Lett. 13 321 (1964); G.S.Guralnik, C.R.Hagen and T.W.B.Kibble,Phys.Rev.Lett. 13 585 (1964).2 The ATLAS Collaboration, G. Aad et al., The ATLAS Experiment at the CERN Large Hadron Collider, JINST(2008) 3 S08003; M. Bosman, ATLAS Detector Status and Physics Startup Plans, these proceedings.3 The CMS Collaboration, G.L.Bayatian et al., CMS Technical Design Report, vol.I - Detector Performance &Software, CERN/LHCC 2006-001, February 2006; The CMS Collaboration, S. Chatrchyan et al., The CMSexperiment at the CERN LHC, JINST (2008) 3 S08004.4 S.Lowette, Beyond the Standard Model Higgs at LHC, these proceedings.5 The LEP Higgs Working Group, Search for the Standard Model Higgs Boson at LEP, Phys.Lett. B565 61 (2003);CERN-EP/2003-11.6 P. Renton, “Standard Model Electroweak fits”, these proceedings; The LEP Electroweak Working Group websitehttp://lepewwg.web.cern.ch/LEPEWWG/.7 M. Herndon, “Searches for the Higgs Boson”, these proceedings.8 S.So¨ldner-Rembold, Standard Model Higgs Searches, [arXiv:hep-ex/0803.1451v2].9 ATLAS Collaboration, Expected Performance of the ATLAS Experiment, Detector, Trigger and Physics, CERN-OPEN-2008-020, Geneva, 2008, to appear.10 The CMS Collaboration, G.L.Bayatian et al., CMS Technical Design Report, vol.II - Physics Performance, J.Phys.G (2007) 995, CERN/LHCC 2006-021, June 2006.4

Standard Model Higgs Searches at the LHC

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Standard Model Higgs Searches at the LHC

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