The status of the ATLAS Higgs boson search using about 5 fb^-1 of data
recorded in 2011 is summarized. Proceedings from Physics at the LHC 2012
(Vancouver).Comment: 6 pages. Proceedings from Physics at the LHC 2012 (Vancouver

Snyder, Scott

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arXiv

S., Snyder

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                                                                                                                   BNL-98505-2012-CP      Higgs Boson search at ATLAS   Scott Snyder on behalf of the ATLAS Collaboration  Brookhaven National Laboratory, Upton, 11973, USA   Presented at the Physics at the LHC (PLHC12) Conference Triumf, Vancouver, Canada June 4-9, 2012   September 2012     Physics/High Energy/ATLAS  Brookhaven National Laboratory      U.S. Department of Energy      Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.  This preprint is intended for publication in a journal or proceedings.  Since changes may be made before publication, it may not be cited or reproduced without the author’s permission.                                                                                                                    DISCLAIMER  This report was prepared as an account of work sponsored by an agency of the United States Government.  Neither the United States Government nor any agency thereof, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or any third party’s use or the results of such use of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof or its contractors or subcontractors.  The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.                                   Higgs boson search at ATLASScott Snyder on behalf of the ATLAS CollaborationBrookhaven National LaboratoryUpton, NY, 11973, USA1 IntroductionThe 2011 run of the Large Hadron Collider (LHC) at CERN was very successful,with experiments accumulating nearly 5 fb−1 of data. Of particular interest was thesearch for a Standard Model (SM) Higgs boson, the remaining component of the SMthat has not yet been definitively observed. This note summarizes the status of thissearch at the ATLAS experiment [1] using the full 2011 data set of (4.6–4.9) fb−1.2 Higgs boson search channelsThe Higgs boson search is carried out for many different final states. At the LHC,the dominant production mode for a Higgs boson is gluon-gluon fusion (gg → H).For a Higgs boson with mass mH above about 135 GeV, the dominant decay is into apair of W or Z gauge bosons; such decays that include leptons in the final state aredistinctive enough to search for directly. For lower Higgs boson masses, the dominantdecay is into two b-quarks. The very large heavy-flavour multijet background makes itinfeasible to search for this decay directly. One can look for rarer but more distinctivedecays of the Higgs boson, such as H → γγ or H → ττ , for diboson decays in whichone of the gauge bosons is off-shell, or for a Higgs boson produced in association witha W or Z boson. The best sensitivity for mH ≈ 125 GeV is obtained from H → γγ,followed by H → ZZ(∗) → ℓℓℓ′ℓ′.2.1 H → γγ channelThe diphoton decay of the Higgs boson is relatively rare (with a branching ratio∼ 0.2%). In order to have good sensitivity in this channel [2], one must have verygood mγγ resolution as well as good control over non-photon backgrounds. ATLASachieves a mass resolution of about 1.7% at mH = 120 GeV, and the fraction ofnon-photon background is less than 30%.To optimize the expected sensitivity, the analysis is split into nine subchannelswith differing mass resolution and expected signal fraction, depending on the kine-matics of the photons and whether they converted. The background is modeled with1an exponential fit to the data. Results are shown in Figure 1. The mH regions(113–115) GeV and (134.5–136) GeV are excluded at 95% CL. A small excess is seenaround mH = 126.5 GeV, with a local significance of 2.8σ, or a global significance of1.5σ when the look-elsewhere effect is taken into account over the mH range (110–150) GeV. [GeV]γγm100 110 120 130 140 150 160Events / GeV100200300400500600700800 Data 2011Total background=125 GeV, 1 x SMHmATLAS-1 Ldt = 4.9 fb∫ = 7 TeV, sγγ→H [GeV]Hm110 115 120 125 130 135 140 145 1500Local p-410-310-210-110110 0Observed p0 expected pγγ →SM H  (with energy scale uncertainty)0Observed p = 7 TeVsData 2011,  -1Ldt = 4.9 fb∫ATLASσ1σ2σ3Figure 1: Results from the H → γγ channel [2]. Left: Invariant mass distributionfor the entire sample, overlaid with the fitted total background. The expectation fora mH = 125 GeV SM Higgs boson is also shown. Right: Local probability p0 for thebackground to fluctuate to the observed number of events or higher. The dashed lineshows the expected median local p0 for the signal hypothesis when tested at mH .2.2 H → ZZ(∗) → ℓℓℓ′ℓ′ channelIn this channel [3], one looks for two pairs of opposite-sign, same-flavour leptons (e orµ), with one pair having an invariant mass close to the Z boson mass. This channelhas a quite small background and a fully-reconstructed Higgs boson decay, whichallows it to have good sensitivity over a wide range of Higgs boson masses, from600 GeV down to 110 GeV. The background is primarily ZZ diboson production,with smaller contributions from Z + jets and tt.The results are shown in Figure 2, Row 1. The mH regions (134–156) GeV, (182–233) GeV, (256–265) GeV, and (268–415) GeV are excluded at 95% CL. Small ex-cesses are seen around 125 GeV, 244 GeV, and 500 GeV with local significances of2.1σ, 2.2σ, and 2.1σ, respectively. The excess at 125 GeV corresponds to three eventsthat cluster within the mH mass resolution of 2%: two 2e2µ events at 123.6 and124.3 GeV, and one 4µ event at 124.3 GeV.2.3 H → WW (∗) → ℓνℓν channelThis channel [4] is also sensitive over a wide mass range. The event selection requirestwo isolated, opposite-sign leptons and a large missing transverse energy (EmissT ). The2 [GeV]llllm100 120 140 160 180 200 220 240Events / 5 GeV024681012 Data 2011Total background=125 GeV, 1 x SMHmATLAS-1 Ldt = 4.8 fb∫ = 7 TeV, s4l→(*)ZZ→H [GeV]llllm0 100 200 300 400 500 600Events / 10 GeV02468101214Data 2011Total background=125 GeV, 1 x SMHmATLAS-1 Ldt = 4.8 fb∫ = 7 TeV, s4l→(*)ZZ→H [GeV]Hm200 300 400 500 600SMσ/σ95% CL limit on -110110210sCLObserved sCLExpected σ 1 ±σ 2 ±ATLAS 4l→(*) ZZ→H-1Ldt = 4.8 fb∫=7 TeVs110 [GeV]Tm60 80 100 120 140 160 180 200 220 240Events / 10 GeV0102030405060708090100-1 Ldt = 4.7 fb∫ = 7 TeV, sATLASData 2011=125 GeV, 1 x SMHmTotal background+0jνlνl→WW→H [GeV]Tm60 80 100 120 140 160 180 200 220 240Events / 10 GeV051015202530-1 Ldt = 4.7 fb∫ = 7 TeV, sATLAS Data 2011=125 GeV, 1xSMHmTotal background+1jνlνl→WW→H [GeV]Hm100 200 300 400 500 600SMσ/σ95% CL Limit on -110110210Obs. Exp. σ1 ±σ2 ± = 7 TeVs     -1 Ldt = 4.7 fb∫ATLAS νlνl→(*)WW→H  [GeV]bb m0 50 100 150 200 250 Events / 10 GeV 020406080100120 Data 2011 5×Signal =120 GeV)H(mTotal BGTopZ+jetsDibosonATLAS = 7 TeVs,  -1 L dt = 4.7 fb∫bb-l+ l→ZH  Events / 10 GeV   [GeV]bb m0 50 100 150 200 250 Events / 10 GeV 050100150200250300350400450 Data 2011 5×Signal =120 GeV)H(mTotal BGTopZ+jetsW+jetsDibosonMultijetATLAS = 7 TeVs,  -1 L dt = 4.7 fb∫bbν l→WH  Events / 10 GeV  [GeV]H m110 115 120 125 130SMσ/σ 95% C.L. limit on 024681012Observed (CLs)Expected (CLs)σ 1±σ 2±), combinedbVH(bATLAS-1 Ldt = 4.6-4.7 fb∫ = 7 TeV,  s [GeV]ττm50 100 150 200 250 300Events / 20 GeV050100150200250Data 2011Total background=125 GeV, 10 x SMHmATLAS-1 Ldt = 4.7 fb∫ = 7 TeV, s + 1jlepτlepτ→H [GeV]MMCm0 50 100 150 200 250 300 350 400Events / 10 GeV020040060080010001200140016001800-1 Ldt = 4.7 fb∫ = 7 TeV, sATLASData 2011=125 GeV, 10xSMHmTotal background + 0/1jhadτlepτ→H [GeV]Hm100 110 120 130 140 150SMσ/σ 95% CL Limit on 02468101214161820ττ →H sCLObserved sCLExpected σ 2±σ 1±ATLAS-1 4.7 fb≈ Ldt ∫ = 7 TeVsFigure 2: Selected results. Left two columns: data compared with expected back-ground for (Row 1) m4ℓ from the H → ZZ(∗) → ℓℓℓ′ℓ′ channel for 100 GeV < mH <250 GeV and for the complete range [3]; (Row 2) mT from the H → WW(∗) → ℓνℓνchannel with 0/1 jets [4]; (Row 3) mbb from the ZH and WH channels [5]; (Row 4)mττ from the H → τlepτlep + 1j and H → τlepτhad + 0/1j channels [6]. An example ofthe expected signal is also shown (scaled up for some channels to make it more visible).Right: Corresponding exclusion plots, showing the expected (dashed) and observed(solid) 95% CL upper limits on SM Higgs boson production. The dark (green) andlight (yellow) bands indicate the expected limits with ±1σ and ±2σ fluctuations,respectively.3analysis is divided into subsamples with 0, 1, and ≥ 2 jets, as the background compo-sition is quite different for these cases: for the 0-jet case, the background is dominatedby the WW and Z + jets processes, while for the 2-jet case, tt dominates. As thereare two neutrinos in the final state, the Higgs boson mass cannot be reconstructed;the transverse mass mT is used instead. Results are shown in Figure 2, Row 2. Noexcess is seen, and the mH region (133–261) GeV is excluded at 95% CL.2.4 Other low-mH channelsTwo additional channels are sensitive to a low-mass Higgs boson. In the V (H → bb)channel [5], one searches for a Higgs boson produced in association with a vectorgauge boson, with the Higgs boson decaying into a bb pair. The event selection forthis channel requires a gauge boson decay to leptons/neutrinos (one of W → ℓν,Z → ℓℓ, or Z → νν) and exactly two b-tagged jets. The analysis is subdivided inbins of pT(V ). The H → ττ analysis [6] is divided into 2ℓ4ν, ℓτhad3ν, and 2τhad2νsubchannels; these are then further subdivided according to the number of extra jetsin the event. Since there are multiple neutrinos in the final state, the invariant massmττ is estimated either using the collinear approximation (assuming the neutrino to becollinear with the visible decay products) or the “missing mass calculator” technique(incorporating the probability distribution for the opening angle in τ decays).Selected results are shown in Figure 2, Rows 3 and 4. Exclusion limits for thesechannels range from about 2.5 to 12 times the SM Higgs boson production crosssection over the mH range (100–150) GeV.2.5 Other high-mH (diboson) channelsThe remaining diboson channels are sensitive to higher Higgs boson masses. Forthe H → ZZ → ℓℓνν channel [7], events with a Z → ℓℓ decay and large EmissT areselected. Different selection requirements are made for mH above or below 280 GeV.For H → ZZ → ℓℓqq [8], events are selected with one Z → ℓℓ decay, one Z → jjdecay, and small EmissT . As the fraction of heavy-flavour in the final state is quitedifferent for signal and background, this channel is divided into “tagged” (with twoidentified b-jets) and “untagged” subchannels. The principal backgrounds for thesetwo channels are Z + jets, tt, and ZZ. For the H → WW → ℓνqq channel [9], eventsare selected with an isolated lepton, large EmissT , and a W → jj decay. This analysisis divided into subchannels according to the number of additional jets in the event.Results are shown in Figure 3. The ZZ → ℓℓνν channel excludes at 95% CL themH range (320–560) GeV, while the ZZ → ℓℓqq channel excludes the ranges (300–322) GeV and (353–410) GeV. Exclusions from the WW → ℓνqq channel are 2–10times the SM cross section.4 [GeV]Tm200 300 400 500 600 700 800 900Events / 50 GeV0102030405060708090Data 2011Total background=350 GeV, 1 x SMHmATLAS-1 Ldt = 4.7 fb∫ = 7 TeV, sννll→ZZ→H   [GeV]llbbm200 300 400 500 600 700 800Events / 18 GeV12345678Data 2011Total background=350 GeV, 1 x SMHmATLAS-1 Ldt = 4.7 fb∫ = 7 TeV, sllbb→ZZ→H [GeV]jjνlm300 400 500 600 700 800 900 1000Events / 10 GeV020406080100120Data 2011Total background=350 GeV, 1 x SMHmATLAS-1 Ldt = 4.7 fb∫ = 7 TeV, sqq+0jνl→WW→H [GeV]Hm200 250 300 350 400 450 500 550 600SMσ/σ 95% CL limit on 012345ObservedExpectedσ 1±σ 2± 2011ATLASννll→ZZ→H=7TeVs, -1Ldt=4.7fb∫ [GeV]Hm200 300 400 500 600SMσ/σ95% CL limit on 0246810 ObservedExpectedσ 1 ±σ 2 ±ATLAS llqq→ ZZ→H=7 TeVs, -1Ldt = 4.7 fb∫[GeV]HM300 350 400 450 500 550 600SMσ/σ95% C.L. limit on -110110210Expectedσ 1±σ 2±Observed=7 TeVs-1 L dt=4.7 fb∫ATLAS  jjν l→ WW→H+0/1/2j, HFigure 3: Selected results from the remaining diboson channels. Top: data andexpected background for H → ZZ → ℓℓνν, showing mT [7] (left); H → ZZ → ℓℓqq,showing mℓℓbb for the high-mH , tagged subchannel [8] (middle); and H → WW →ℓνqq, showing mℓνjj for the 0-jet subchannel [9] (right). The expectations for a Higgsboson of mH = 350 GeV are also shown. Bottom: Corresponding exclusion plots (seeFigure 2).3 Combination of all channelsFor the final results, all these channels are combined [10]. Systematic uncertaintiesare taken to be either 100% correlated or uncorrelated between channels. The resultsare shown in Figure 4. The mH ranges (111.4–116.6) GeV, (119.4–122.1) GeV, and(129.2–541) GeV are excluded at 95% CL, and the mH range (130.7–506) GeV isexcluded at 99%. An excess is seen around mH = 126 GeV, with a local significance of3.0σ. The global significance is approximately 15% over the full range (100–600) GeVand (5–7)% over the range (110–146) GeV, corresponding to the range at low-mH notexcluded by the previous LHC SM Higgs boson combined search at 99% CL [11]. Ifthe excess is interpreted as a SM Higgs boson, the corresponding cross section ratioat mH = 126 GeV is µ = σobs/σSM = 1.1 ± 0.4, consistent with the SM value of 1.0.The excess is observed only in the H → γγ and H → ℓℓℓ′ℓ′ channels; however, noneof the other channels are inconsistent with a SM Higgs boson at this mass.4 SummaryFrom the 2011 data, ATLAS excludes at 95% CL the mH range (111.4–541) GeV,except for the regions (116.6–119.4) GeV and (122.1–129.2) GeV. An excess is seenin the H → γγ and H → ℓℓℓ′ℓ′ channels at mH ≈ 126 GeV which is consistent with5 [GeV]Hm100 200 300 400 500 600SMσ/σ95% CL Limit on -110110Obs. Exp. σ1 ±σ2 ±  = 7 TeVs     -1 Ldt = 4.6-4.9 fb∫ATLAS  2011 2011 DataCLs Limits [GeV]Hm110 115 120 125 130 135 140 145 150SMσ/σ95% CL Limit on -110110 Obs. Exp. σ1 ±σ2 ±  = 7 TeVs     -1 Ldt = 4.6-4.9 fb∫ATLAS  2011 2011 DataCLs Limits [GeV]Hm110 115 120 125 130 135 140 145 1500Local p-310-210-110110Exp. Comb.Obs. Comb.γγ →Exp. H γγ →Obs. H  llll→ ZZ →Exp. H  llll→ ZZ →Obs. H νlν l→ WW →Exp. H νlν l→ WW →Obs. H  bb→Exp. H  bb→Obs. H ττ →Exp. H ττ →Obs. H ATLAS 2011 -1 L dt ~ 4.6-4.9 fb∫  = 7 TeVsσ1 σ2 σ3 Figure 4: Left, middle: Combined exclusion limits for the full mass range and formH < 150 GeV [10] (see Figure 2). Right: Local probability p0 for the background tofluctuate to the observed number of events or higher, by channel, for mH < 150 GeV.Dashed lines show the expected median local p0 for the signal hypotheses at mH .a SM Higgs boson. The chance of this being due to a background fluctuation is 15%over the full mass range, or (5–7)% over (110–146) GeV.This work is supported in part by the U.S. Department of Energy under contractDE-AC02-98CH10886 with Brookhaven National Laboratory.References[1] ATLAS Collaboration, JINST 3, S08003 (2008).[2] ATLAS Collaboration, Phys. Rev. Lett. 108, 111803 (2012) [arXiv:1202.1414].[3] ATLAS Collaboration, Phys. Lett. B 710, 383 (2012) [arXiv:1202.1415].[4] ATLAS Collaboration, arXiv:1206.0756.[5] ATLAS Collaboration, arXiv:1207.0210.[6] ATLAS Collaboration, arXiv:1206.5971.[7] ATLAS Collaboration, arXiv:1205.6744.[8] ATLAS Collaboration, arXiv:1206.2443.[9] ATLAS Collaboration, arXiv:1206.6074.[10] ATLAS Collaboration, Phys. Rev. D 86, 032003 (2012) [arXiv:1207.0319].[11] ATLAS and CMS Collaborations, ATLAS-CONF-2011-157, CMS-PAS-HIG-11-023 (2011) [https://cdsweb.cern.ch/record/1399599/].6

Higgs Boson search at ATLAS

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Higgs boson search at ATLAS

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