The discovery potential of the Standard Model Higgs boson with the ATLAS experiment at the Large Hadron Collider is reviewed for the HEP-MAD'09 conference. The Higgs decay channels bb, tau-tau, gamma-gamma, WW(*) and ZZ(*) are discussed. The statistical procedure used for the combination of the individual analyses to obtain the discovery potential and exclusion limits is also presented

Fleischmann, P

CERN Document Server

ATL-PHYS-SLIDE-2009-24508September2009HEP-MAD 21st-28th August 2009Discovery potential of the SM Higgs with ATLASPhilipp FleischmannUniversität WürzburgOn behalf of the ATLAS Collaboration 2Outline● Introduction● SM Higgs at the LHC● Higgs Decay Channels discussed here– H → bb– H → – H → – H → WW– H → ZZ● Conclusions and outlook 3IntroductionarXiv:0903.4001v1 [hep-ex]● Higgs is missing keystone in the Standard Model● Higgs discovery is one of the main goals for the LHC● Results of direct searches so far:– LEP: excluded (@95%CL) mH<114GeV– Tevatron: exclude (@95%CL) 160GeV<mH<170GeV● Results presented here correspond to– centre-of-mass energy √S=14 TeV● More details available at–  arXiv:0901.0512 [hep-ex] 4Higgs Production at the LHC 5Higgs Decay Modes● H → bb– high BR, but difficult to select● H → – important at low mH● H → – important at low mH ● H → WW– important over a large range● H → ZZ– „golden mode“● H → tt (not discussed here)– diffcult to selectAt LHC several decay modes accessible,with different significance at various regions of mH  6ttH  ttbb→● At LHC dominated by gluon fusion (~90%)● Top quark decays predominantly to Wb● W decay used to classify final states– All-hadronic: difficult to trigger– Fully-leptonic: low BR and includes – Semi-leptonic: BR of ~28% (without )● Main background contributions– Reducible: tt+jets– Irreducible: ttbb (QCD and EW)● More important for coupling measurements than for discovery 7VBF H  → pseudorapidity of the jet with highest pT ● Poor mH resolution, no inclusive analysis● VBF shows two characteristics– two high pT forward jets– no jet activity in between (rapidity gap)● Three decay modes– H  → hh(~12%)– H  → lh(~46%)– H  → ll(~42%)● Signal– -pair from resonance– Missing ET pseudorapidity of the jet with 2nd-highest pT  8H  → ● Background:– Reducible: W+jets, tt+jets– Irreducible: Z+jets (Z→)– Estimate from data needed due to uncertainties● mH reconstruction– Assuming collinear  decays– Resolution limited by missing ET (8-10 GeV) and reconstruction (10-13 GeV)● Significance– No pile-up included– H  → hhnot included due to large QCD background uncertainty (study on-going) 9H  → ● Clean signal with 2 high pT photons● Small BR (~0.2%)● Large background– Reducible: +jet, di-jets– Irreducible: , q ● Good mass resolution requires– Good photon energy calibration– Good photon direction– Good understanding of -conversion 10H  → Higgs + 1 jetHiggs + 2 jets● Beside inclusive analysis also exclusive production in association with hadronic jets– In case of 2 jets also 3rd-jet-veto applied (VBF)● Expected significance for inclusive and combination of exclusive analyses (H+0,1,2 jets) 11H  WW*→● Large BR which peaks around 2MW<mH<2MZ ● Channels studied– No jets (gluon fusion) and 2 jets (VBF)– WW e→ µ (other studies on-going)● No mass peak due to neutrinos, but excess above backgroundNo jets (GF dominated) Two jets (VBF dominated) 12H  WW*→Expected significance for the GF and VBF process ● Signal– 2 leptons + missing ET–No jets (GF) or 2 forward jets + rapidity gap (VBF)● Background– tt, tWb: reducable by jet veto–WW, WZ, ZZ: reducable by kinematical cuts● Good knowledge of background needed–Measurement from data required– Techniques for normalisation from data established● Correlated W spins– Leptons go preferentially in same directionTransverse opening angle of the two leptons 13H  ZZ*  llll→ →● „Golden channel“ for mH>130GeV–Both Z on-shell for mH>2MZ● Clean signature–Two l+l- pairs forming Z– leptons from primary vertex● Background–Reducible: Zbb, tt– Irreducible: ZZ● mH fully reconstructed 14H  ZZ*  llll→ →● Background suppression– Large theoretical uncertainties on Zbb and tt production cross-section● Reduce well below irreducible ZZ– Use differences between leptons from Z and heavy quarks● Impact parameter● Lepton isolation (in the tracker and calorimeter)● Issues– Good single lepton reconstruction and trigger efficiency need: 4l~ l4 – Lepton energy resolutionReconstructed 4-lepton mass for signal and background for mH=130, 180 and 400 GeV  15H  ZZ*  llll→ →● Analysed decay modes– ZZ  4e→– ZZ  4→ µ– ZZ  2e2→ µSelection efficiency for each decay mode as a function of mHExpected significance as a function of mH for all combined decay modes 16Statistical Combinations of Channels● To increase sensitivity over the full mass range– Systematics taken into account using profile likelihood method– Channels used: H→, H→, H WW* and H ZZ*→ →– Currenlty fixed-mass hypothesis (no „look-elsewhere-effect“ in combination, but evaluated in individual analyses)● Discovery– Generated data for s+b hypothesis– Test b-only hypothesis● Exclusion– Generated data for b-only hypothesis– Test s+b hypothesis 17Statistical Combinations of ChannelsApproximations used in combination not always accurate for L<2 fb-1, but considered conservativeSignificance Exclusion 18Conclusions & Outlook● The search for the Standard Model Higgs Boson with ATLAS looks promising● Final states require good level of understanding of...– Detector performance and calibration– Reconstruction of leptons, photons, jets, missing ET,...● Mesurement of SM backgrounds from data will be essential– Many efforts to evaluate backgrounds from data on-going● Statistical combination of channels challenging, but improves significance– Not yet all channels included● Feasability depends on LHC parameters...– Results shown here were using 14TeV– At 10TeV factor of about 2 more integrated luminosity needed– If we are below 10TeV... 19Additional Slides 20The Large Hadron Collider● (design) CM energy: 14 TeV● (design) Luminosity: 1034 cm-2s-1 ● Bunch crossing: 25 ns● Protons per bunch: ~1011 ● Beam radius: 16.7 µmATLASLHCbALICECMS 21The LHC Schedule● Incident on September 19th 2008 brought LHC to a standstill● „The LHC is a much better understood machine than it was a year ago“● On August 6th 2009 it has been decided...– LHC will restart in November 2009 with a centre-of-mass energy of 7 TeV– After „significant data sample has been collected“ the energy will be increased to 10 TeV– At the end of 2010 LHC will be operated with lead ions– Afterwards THC will be shutdown for preparation of run at the design energy of 14 TeV 22The ATLAS DetectorG. Aad et al., (The ATLAS Collaboration) J. Instrum. 3 (2008) S08003● Magnets– Solenoid: 2T– Toroid (barrel): 0.5T– Toroid (endcap): 1T● Tracker– (pT)/pT≈ 0.05% pT⊕1%● ECAL– (E)/E ≈ 10% /√E ⊕0.7%● HCAL– (E)/E ≈ 50% /√E ⊕3%● Muon Spectrometer– (pT)/pT≈ 2%  @50GeV– (pT)/pT≈ 10%  @1TeV● Trigger– L1 + HLT(L2+EF): 200Hz 23Statistical Model● Consider histogram of mH with numbers of entries denoted by n=(n1,...nN)● The values are modeled as Poisson variable with mean value         E[ni]=µLiiB+bi=µsi+bi● Where µ is channel independent strength parameter          µ=0 : background-only          µ=1 : signal and background● The Standard Model expectations can be written as          si = stot ∫bin i ƒs(x, s)dx          bi = btot ∫bin i ƒb(x, )dx● Where ƒ is probability density function and  is set of nuisance parameters● Parametrisation of ƒ can be obtained from MC simulation or data control sample 24Profile Likelihood Ratio● The likelihood function is the product of Poisson probabilities for all bins● The full likelihood function is derived from those of each search channel● To test a hypothesised value of µ the profile likelihood ratio is● Or equivalently                                     → large values of qµ correspond to poor agreement                                        between data and hypothesized µdata in signal histogram data in control histogramMaximised for specified µMaximised for all µ and  25p-value of a hypothesised value µ● Probability pµ under the assumption of µ to see data with equal or greater incompatibility as measured by qµ relative to the data obtained● f(qµ|µ) : distribution of qµ for data               generated with the same value               of µ as was used to derive qµ ● Wilks' theorem: for large data sample                         f(qµ|µ)  → 2(1 df )● f(qµ|µ' ) : distribution of qµ for data                generated with the same value                of µ as was used to derive qµ ● Significance Z: Number of standard deviations at which a Gaussian random variable of zero mean would give a one-sided tail area equal to p

Discovery potential of the SM Higgs with ATLAS

The discovery potential of the Standard Model Higgs boson with the ATLAS experiment at the Large Hadron Collider is reviewed for the HEP-MAD'09 conference. The Higgs decay channels bb, tautau, gammagamma, WW(*) and ZZ(*) are discussed. The statistical procedure used for the combination of the individual analyses to obtain the discovery potential and exclusion limits is also presented

ATL-PHYS-PROC-2009-13306November2009Discovery potential of the SM Higgs with ATLAS ∗P. Fleischmann†On behalf of the ATLAS Collaboration1st October 2009AbstractThe discovery potential of the Standard Model Higgs boson with the ATLAS experiment at theLarge Hadron Collider is reviewed for the HEP-MAD’09 conference. The Higgs decay channelsb¯b, τ+τ−, γγ , WW (∗) and ZZ(∗) are discussed. The results of the combination of the individualanalyses to obtain the discovery potential and exclusion limits are also presented.1 IntroductionThe ATLAS detector [1] is a general purpose detector at the Large Hadron Collider (LHC), the newparticle accelerator at CERN in Geneva. The first collisions of protons are scheduled for the end of2009. The design centre-of-mass energy for the two proton beams is 14 TeV. One of the prime tasksfor the ATLAS experiment is the search for the Standard Model (SM) Higgs boson. The search will beperformed at different ranges of possible Higgs masses using various decay channels [2]. In additionto each individual analysis the obtained results will also be combined to enhance the significance.The Higgs boson is the missing keystone of the Standard Model of particle physics. A lowerbound to the mass of this particle has been set by the LEP experiments [3]. They exclude at a 95%confidence level the mass region below 114.4 GeV, where most of the region is excluded at an evenhigher confidence level. More recently the experiments at the Tevatron have published results thatexclude an additional region between 160 GeV and 170 GeV [4].All results of the ATLAS Collaboration presented here have been obtained using Monte Carlosimulations corresponding to the LHC’s design centre-of-mass energy of 14 TeV. In the current runschedule it is foreseen to start with a lower energy of about 7 TeV, that will be increased during thefirst year of operations to 10 TeV and afterwards to the design value. In some first on-going studies ithas been found, that the amount of integrated luminosity as mentioned in the results of the analyses at14 TeV have to be multiplied by a factor of about 2 to get similar results for a centre-of-mass energyof the two proton beams of 10 TeV.In section 2 a brief overview of the production and decay channels for a SM Higgs boson asexpected at the LHC is given. In section 3 the prepared analyses of the decay channels most relevantfor the discovery of the SM Higgs boson are described. Section 4 summarises the effort to combinethe results of various analyses in order to increase the significance of a potential signal or exclusionlimits.∗Talk presented at the conference HEP-MAD’09, 21-28th August 2009, Antananarivo, Madagascar†Universita¨t Wu¨rzburg12 The SM Higgs boson at the LHCThe production cross-section of the SM Higgs boson in proton-proton collisions depends on the Higgsmass and centre-of-mass energy. Figure 1(a) shows the cross-sections for five production channels atthe LHC. The dominant process at the LHC is the production via gluon fusion followed by the VectorBoson Fusion (VBF). At much lower rates the associated production with a Vector Boson (HW andHZ) or a t ¯t-pair contributes to the total production cross-section.The search for the SM Higgs boson is performed at the LHC in various decay channels. The signalrates and the signal to background ratios of each channel depend on the Higgs mass. Figure 1(b) showsthe branching ratios for decay modes of a SM Higgs boson as a function of the Higgs mass. At lowmasses of the Higgs boson the decay H → b¯b has the highest branching ratio, but also the decaysinto τ+τ− or γγ plays an important role. At intermediate and higher Higgs masses the relevance ofthe decays H →WW (∗) → ℓνℓν and H → ZZ(∗) → 4ℓ increases. The SM Higgs decay into t ¯t is notconsidered here since the experimental results up to now favour a not too high Higgs mass.3 Decay channels for the SM Higgs boson searchIn this section the most relevant decay channels for a discovery of the SM Higgs boson are overviewed.A more detailed description of each analysis can be found elsewhere [2].3.1 t ¯tH → t ¯tb¯bThe dominant decay mode for a low mass Higgs (mH < 130 GeV) is the decay H → b¯b. However itis very difficult to measure, since the expected signal sits on top of a large QCD background (see Fig-ure 2(a)). It is therefore important to look for a noticeable signature. In the ATLAS Collaboration theprocess t ¯tH → t ¯tb¯b has been investigated, where the top quark predominantly decays into Wb. Thus,the final state contains at least four b-jets. Figure 2(b) shows an example of a Feynman diagram forthe t ¯tH channel, where one of the W bosons decays leptonically, while the other decays hadronically.Due to the challenges in the decay mode, this channel is not considered the most important channelfor the discovery of the SM Higgs boson. However this channel grants access to some parameters ofthe Higgs boson which will be important to measure, once it has been discovered. In addition ithas recently been found [5], that this decay mode is reinstated as a promising search channel if oneperforms an exclusive analysis of the high pT region of the HW and HZ production.3.2 H → τ+τ−The Higgs decay into a pair of τ leptons is another important channel in the low mass region. Thoughit has a lower branching ratio than the decay H → b¯b, it provides a clean signature in the VBFproduction channel. The characteristics of the VBF are two high pT jets at large pseudo-rapiditywith only little hadronic activity in the central region (rapidity gap). Figure 3 shows the pseudo-rapidity distributions for the two jets with the highest pT. In this analysis three decay modes ofthe τ+τ−-pair are considered: hadron-hadron (hh-channel), lepton-hadron (ℓh-channel) and lepton-lepton (ℓℓ-channel). The signal events are selected using the VBF signature characteristics as wellas the requirement of a τ+τ−-pair from a resonance and missing transverse energy EmissT . The Higgsmass is reconstructed using a collinear approximation for the τ decays, which assumes, that the decayproducts are emitted in the same direction as the τ .The dominant background for this process is Z+jets with the Z decaying into a pair of τ leptons.The Higgs mass peak is sitting in the tail of the Z → τ+τ− distribution as shown in Figure 4(a).Since the resolution of the reconstructed Higgs mass is limited by the knowledge of the EmissT andthe τ-reconstruction, it is essential to know the shape of the background distribution. A data-driven2method has been established to estimate the shape of the mZ→ττ distribution by using Z → µ+µ−data where the µ is replaced by simulated τ leptons. The expected sensitivity using only the ℓℓ- andthe ℓh-channel is presented in Figure 4(b). The hh-channel is not used in the combination as it suffersfrom uncertainties on the large QCD background.3.3 H → γγAnother Higgs decay mode relevant at low masses of the Higgs boson is the decay into a pair ofphotons. It has a rather small branching ratio, however it has a distinctive signature of two high pTphotons forming a resonance on top of a smooth background (Figure 5). The biggest contributionto the large background comes from the irreducible direct γγ production. A good rejection power isneeded to keep the fraction of γ + jet events low, where at least one particle is misidentified as photon.To achieve this a cut on shower shape parameters is applied to reject pions which are the main sourceof fake photons. In addition track isolation cuts are applied to further reduce fake background.Beside the inclusive analysis also exclusive searches for the H → γγ decay in association with no,one or two jets are performed. Figure 6(a) shows an examples for the invariant mass distribution forthe analysis requiring two jets. It has been found, that the combination of the results of the exclusiveanalyses shows a higher significance than the result of the inclusive search (Figure 6(b)).3.4 H →WW (∗) → ℓνℓνThe SM Higgs boson decay into a pair of W bosons has a large branching ratio over a wide rangeof mH . Two separate analyses are being performed for the gluon fusion and the VBF. In contrastto the other Higgs decay channels presented here one cannot reconstruct the Higgs boson due tothe neutrinos in the decay WW (∗) → ℓνℓν , thus only the transverse mass is accessible (Figure 7).Currently only the decay into eνµν is used in the combination of the different Higgs searches, butother studies are on-going. The analyses are based on event counting and rely on a good understandingof the background shape and normalisation. The main contributions to the background originate fromproduction of WW , t ¯t and W+jets. Due to the correlated spins of the two W bosons the decay leptonsgo preferentially in the same direction. This fact can be used to suppress the WW background and toselect a background-enhanced control sample to determine the background normalisation (Figure 8).The t ¯t background can be suppressed by a jet veto in the central region. Cuts on the lepton isolationcan be used to reduce the amount of W +jets events.3.5 H → ZZ(∗) → 4ℓThe Higgs decay into four leptons is often referred to as the golden channel for the search of a SMHiggs boson with a mass above 130 GeV. It has a smaller branching ratio as the H →WW (∗) channel,but due to its clean experimental signature it is considered the most promising channel except for aregion around mH ≈ 2MW where the decay H→WW (∗) dominates the other modes. The signal can beobserved as a narrow resonance on top of a broad background, which is dominated by the ZZ produc-tion. Figure 9 shows two example of the expected invariant mass distribution for two different valuesof the Higgs mass. Beside the ZZ background there are also contributions from Zb¯b and t ¯t. There arecurrently large theoretical uncertainties on their production cross sections. Therefore it is importantto suppress these processes by selection criteria to a rate well below the irreducible ZZ background.Due to the longer lifetime of the heavy quarks, leptons from the Zb¯b and t ¯t background originatefrom secondary vertices and can thus be suppressed by cuts on the impact parameter significance. Inaddition requirements on the lepton track isolation in the tracker and calorimeter can further reducethe amount of these background processes.This study is being performed in three sub-channels: 4e, 4µ and 2e2µ . The Higgs mass can befully reconstructed. However with four leptons in the final state a good single lepton reconstruction3and trigger efficiency is essential. Figure 10 shows the expected selection efficiency in the threechannels as well as the signal significance as a function of the Higgs mass.4 Higgs discovery potential and exclusion limitsSome of the presented decay channels will allow to reach a 5σ discovery already stand-alone in partsof the mass range. In order to further increase the sensitivity over a wider range of Higgs mass valuesseveral of the individual results are combined using a method based on the profile likelihood ratio.The channels currently used are H → τ+τ−, H → γγ , H →WW (∗) and H → ZZ(∗). The resultspresented here use a fixed-mass hypothesis, however studies where the Higgs mass is regarded as afree parameter are on-going. Figure 11 shows the expected signal significance and exclusion limitsfor different Higgs masses and different values of integrated luminosity. One can see that in someregions of the Higgs mass it is expected to be able to reject it or see a signal already with 2fb−1.5 AcknowledgementsThe author would like to thank all ATLAS collaborators. Special thanks to Ketevi Assamagan, BillMurray and other colleagues of the Higgs Working Group for their useful comments and suggestions.References[1] ATLAS Collaboration, G. Aad et al., The ATLAS Experiment at the CERN Large HadronCollider, JINST 3 (2008) S08003.[2] ATLAS Collaboration, G. Aad et al., Expected Performance of the ATLAS Experiment -Detector, Trigger and Physics, arXiv:0901.0512 [hep-ex].[3] LEP Higgs boson search Collaboration, R. Barate et al., Search for the standard model Higgsboson at LEP, Phys. Lett. B565 (2003) 61–75, arXiv:0306033 [hep-ex].[4] TEVNPH Collaboration, Combined CDF and DZero Upper Limits on Standard ModelHiggs-Boson Production with up to 4.2 fb-1 of Data, arXiv:0903.4001 [hep-ex].[5] ATLAS Sensitivity to the Standard Model Higgs in the HW and HZ Channels at High TransverseMomenta, Tech. Rep. ATL-PHYS-PUB-2009-088. ATL-COM-PHYS-2009-345, CERN,Geneva, Aug, 2009.4 [GeV]Hm210 310Cross-section [pb]-110110210ATLAS H→gg qqHWHZHttH(a) [GeV]Hm210 310Branching Ratio-310-210-1101ATLASbbττγ γWWZZtt(b)Figure 1: (a) Cross-sections for the five production channels of the SM Higgs boson at the LHC as afunction of its mass. (b) Branching ratios for the most relevant decay modes of the SM Higgs bosonas a function of its mass.(a)qq0bbbb` W+W ttHgttg(b)Figure 2: (a) Reconstructed invariant mass spectrum for signal and backgrounds in the t ¯tH → t ¯tb¯bchannel. (b) One of the Feynman diagrams for the t ¯tH production where the Higgs boson decays intob¯b. Here one top quark decays hadronically, while the other decays leptonically.5η-5 -4 -3 -2 -1 0 1 2 3 4 5Arbitrary Units00.010.020.030.040.050.060.070.080.090.1µµ →-τ+τ→VBF H(120)) +jetsµµ→Z() +jetsµµ→(ttATLAS(a)η-5 -4 -3 -2 -1 0 1 2 3 4 5Arbitrary Units00.010.020.030.040.050.060.070.080.090.1µµ →-τ+τ→VBF H(120)) +jetsµµ→Z() +jetsµµ→(ttATLAS(b)Figure 3: Pseudo-rapidity of the (a) highest pT and (b) the second highest pT jets in the VBF processH → ττ → µµ and backgrounds.60 80 100 120 140 160 18002468101214 (GeV)ττmEvents / ( 5 GeV )ATLASlh→ττ→VBF H(120)-1 = 14 TeV,   30 fbs(a) (GeV)Hm105 110 115 120 125 130 135 140)σExpected Significance (012345678910ATLAS-1 = 14 TeV, 30 fbsll-channellh-channelcombined(b)Figure 4: (a) Example of the reconstructed invariant mass spectrum for signal and backgrounds in theH → ττ channel for a SM Higgs boson of mass mH = 120 GeV based on an integrated luminosity of30fb−1. (b) Expected signal significance for the H → ττ channel as a function of the Higgs mass.6 [GeV]γγM100 105 110 115 120 125 130 135 140Arbitrary units020040060080010001200140016001800200022002400γAt least 1 converted 0.01) GeV±Mean=(119.720.01) GeV±=(1.42σATLAS(a) [GeV]γγ M110 115 120 125 130 135 140 145 150 [fb/GeV]γγ/dMσd50100150200250300 [fb/GeV]γγ/dMσdSignalIrreducible bkgReducible bkgATLAS(b)Figure 5: (a) Invariant mass distribution for photon pairs from Higgs boson decays with mH =120 GeV after trigger and identification cuts. The shaded histogram correspond to events with atleast one converted photon. (b) Invariant mass distribution for signal and backgrounds in the inclu-sive H → γγ channel.(a)Higgs boson mass [GeV]120 125 130 135 140Signal Significance012345678Inclusive, number countingcombined, number counting-1L = 10 fb∫ATLAS(b)Figure 6: (a) Invariant mass distribution for signal and backgrounds in the exclusive H → γγ channelrequiring two jets. (b) Expected signal significance for a Higgs boson using the H → γγ decayfor 10fb−1 of integrated luminosity as a function of the mass. The solid circles correspond to thesensitivity of the inclusive analysis while the open circles display the significance when the Higgsboson plus jet analyses are included.7(GeV)TM0 100 200 300 400 500 600Events / ( 10 )051015202530354045Events / ( 10 ) W+jetsWWttSignalPseudodataATLAS-1 L dt=10 fb∫(a)(GeV)TM0 100 200 300 400 500 600Events / ( 10 )024681012Events / ( 10 ) -1 L dt=10 fb∫Pseudodata, Best-fit Signal2-lepton BackgroundFake BG (W+jets)ATLAS(b)Figure 7: Expected distribution of the transverse mass MT for signal and backgrounds in the H →WW ∗ channel requiring (a) no jets (b) two jets for an integrated luminosity of 10fb−1.llφ∆0 0.5 1 1.5 2 2.5 3Normalized to 100.020.040.060.080.10.12SignalQCD WWATLAS(a)(GeV)HM130 140 150 160 170 180 190Median Significance02468101214161820µ e→ WW→H+2j, Hµ e→ WW→H+0j, HCombinedATLAS-1 L dt = 10 fb∫(b)Figure 8: (a) Normalised distribution of the opening angle ∆φℓℓ between the two leptons in signal andbackground events of the H →WW ∗ channel. (b) Expected signal significance as a function of theHiggs mass for the H →WW ∗ channel based on 10fb−1 for the individual exclusive analyses as wellas for the combined study. [GeV]4lm100 110 120 130 140 150 160 170 180 190 200Events/(2.5 GeV)02468101214161820Events/(2.5 GeV)4l→ZZ*→HZZZbbtt-1 L=30 fb∫ATLAS(a) [GeV]4lm140 150 160 170 180 190 200 210 220 230 240Events/(2.5 GeV)0510152025Events/(2.5 GeV)4l→ZZ*→HZZZbbtt-1 L=30 fb∫ATLAS(b)Figure 9: Expected invariant mass distribution for signal and backgrounds in the H → 4ℓ channelbased on 30fb−1 for two different Higgs mass values: (a) mH = 130 GeV (b) mH = 180 GeV8Higgs Mass [GeV]      120 130 140 150 160 170 180Selection Efficiency (%)01020304050 H->ZZ*->4eµH->ZZ*->4µH->ZZ*->2e2ATLAS(a)Higgs mass [GeV]    100 200 300 400 500 600Signal significance110Profile likelihood ratioPoisson probability -1 L=30 fb∫ATLAS(b)Figure 10: (a) Expected selection efficiency for H → 4ℓ events as a function of the Higgs mass forthree different decay sub-channels. (b) Comparison of two different methods to retrieve the signalsignificance for the H → 4ℓ channel as a function of the Higgs mass based on 30fb−1mH [GeV]Luminosity [fb−1 ]significance  ATLASH → γγH → ZZ* → 4lH → ττH → WW → eνµν120 140 160 180 200 220 240 260 280 30001234567891012345678910111213(a)0.990.950.900.80mH [GeV]Luminosity [fb−1 ]Combined Exclusion CL  ATLASH → γγH → ZZ* → 4lH → ττH → WW → eνµν110 115 120 125 130 135 1400123456789100.550.620.700.750.800.850.900.930.950.970.98(b)Figure 11: (a) Significance contours for different Standard Model Higgs masses and integrated lumi-nosities. The thick curve represents the 5σ discovery contour. The median significance is shown witha colour according to the legend. The hatched area below 2fb−1 indicates the region where the ap-proximations used in the combination are not accurate, although they are expected to be conservative.(b) The expected luminosity required to exclude a Higgs boson with a mass mH at a confidence levelgiven by the corresponding colour. The hatched area below 2fb−1 indicates the region where the ap-proximations used in the combination are not accurate, although they are expected to be conservative.9

Discovery potential of the SM Higgs with ATLAS

Abstract

Similar works

Full text

Available Versions

CERN Document Server

CERN Document Server