with the ATLAS Detector Royal Institute of Technology, KTH, Stockholm, Sweden E-mail: jostran@kth.se Higgs boson searches in the H WW ( ) lνlν (l = e, µ) and the H WW ( ) lνqq decay modes, using. fb of proton-proton collision data delivered by the large hadron collider (LHC) at a centre-of-mass energy of 7 TeV collected with the ATLAS detector, are presented. An upper bound is placed on the Higgs boson production cross-section as a function of m H. The H WW ( ) lνlν analysis excludes a Higgs boson with a mass in the range from 58 GeV to 86 GeV at 95% confidence level, while the expected Higgs boson mass exclusion range is m H 86 GeV. An excess of events in data corresponding to more than σ significance is observed for the Higgs boson mass range from 6 GeV to 58 GeV. The H WW ( ) lνqq analysis place limits on the Higgs boson cross section that are between. to times the expected standard model cross section in the mass range from GeV to 6 GeV. PoS(EPS-HEP)5 The Europhysics Conference on High Energy Physics-HEP, July -7, Grenoble, Rhône-Alpes France Speaker. for the ATLAS Collaboration c Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/
. Introduction The Higgs boson is the only particle in the standard model (SM) which has not been experimentally verified. This particle, which appears as a consequence of the breaking of electroweak symmetry, is responsible for giving masses to all other massive particles. Direct searches at LEP and the Tevatron have excluded, at 95% Confidence Level (CL), a Higgs boson with a mass below. GeV and in the region 58 < m H < 75 GeV [] []. Indirect limits of m H < 85 GeV at 95% CL have also been set using global fits to electroweak precision results [3]. In these proceedings, two Higgs boson searches in the H WW ( ) lνlν (l = e, µ) and H WW ( ) lνqq(l=e, µ) channels are presented. For a more detailed description, see [, 5, 6].. Samples, Event and Object Selections The analyses are based on a data sample corresponding to. fb of pp collisions at s= 7 TeV recorded with the ATLAS detector. The sample has been collected with unprescaled single lepton triggers requiring presence of an electron with E T of at least GeV or a muon with p T of at least 8 GeV, or GeV if it is reconstructed using only the muon spectrometer in the barrel region. The events are further required to have a primary vertex with at least three tracks that is consistent with the beam spot position. The basic signature of a H WW ( ) lνlν candidate event is the presence of two high p T opposite sign leptons and large ET miss (missing transverse momentum), with little jet activity inside the detector acceptance. Candidate events are required to have exactly two leptons, with the leading lepton fulfilling p T > 5 GeV and the sub-leading electron (muon) fulfilling p T > GeV (p T > 5 GeV). The leptons are isolated from nearby tracks with requirements on the scalar sum of the transverse momenta of nearby charged tracks and of the calorimeter energy deposits within R=. of the candidate. Further, the dilepton invariant mass is required to be m ll > 5() GeV for the ee and µµ channels (eµ channel). An additional requirement of m Z m ll > 5 GeV is performed to suppress the Z boson background while the QCD and Drell-Yan background events, defined as Emiss T sin φ if φ < π/. Here, φ is the absolute value of the difference in the azimuthal angle φ between the ET miss and the nearest lepton or jet. The ET,rel miss is requiered to be > GeV (ee and µµ channels) and > 5 GeV (eµ channel). To reduce the top quark pair production background, the multiplicity of reconstructed jets is required to be less than two. The jet multiplicity distribution after the ET,rel miss selection is shown in Fig. (left). Using this sample, the analysis is performed separately for events with no jets, referred to as H+ jets channel, and events with exactly one jet, referred to as the H+ jets channel. In the H+ jets analysis, it is required that the transverse momentum of the dilepton system, P ll T, is at least 3 GeV. In the H+ jet analysis the following selections are made instead: Remove events where the jet tagged as originating from a b-quark, require that the total p T of the Higgs boson plus jet system (defined as the magnitude of the vector sum P tot T = Pl T + Pl T + Pj T + Pmiss T ) is less than 3 GeV and reject Z ττ events by requiring that the ττ invariant mass m ττ (reconstructed using the approximation that the neutrinos are collinear with the visible products of the corresponding τ decays) does not fulfill m ττ m Z <5 GeV. Finally the following selections are made in both the H+ jets and the H+ jet channels: are suppressed using E miss T,rel if φ π/ and E miss T PoS(EPS-HEP)5
The dilepton invariant mass is required to satisfy m ll < 5 GeV or m ll < 65 GeV for predicted Higgs boson masses in the regions m H < 7 GeV and m H 7 GeV, respectively. The dilepton opening angle in the transverse plane, Φ ll, is required to be less than.3 (.8) radians for m H < 7 GeV (m H 7 GeV). The transverse mass, m T, is required to satisfy.75 < m T /m H <, where transverse mass is defined as m T = (ET ll+ Emiss T ) (P ll T + Pmiss T ), where ET ll = (P ll T ) + m ll, Pmiss T = ET miss and P ll T is the transverse momentum of the dilepton system. The basic signature of a H WW ( ) lνqq candidate event is the presence of one high p T leptons, large ET miss and two jets. Candidate events are required to have exactly one electron or muon with p T > 3 GeV. In order to ensure that this analysis is statistically independent from the H WW ( ) lνlν analysis, events are vetoed if there are any additional leptons with p T > GeV. The events are further required to have ET miss > 3 GeV and to have exactly two or three jets. The event is rejected if any of the jets are tagged as orgiginating from a b-quark. Analogously to the H WW ( ) lνlν analysis, the events are further split into two channels, the H+ (H+ ) jets channel for events with exactly two (three) jets. An approximate invariant mass for the Higgs boson candidate is reconstructed by solving the mass constraint equation M lν = M W for the unmeasured z-momentum of the neutrino. The hadronically decaying W candidate is reconstructed by selecting the pair of jets whose dijet invariant mass is closest to the W mass. A maximum likelihood fit is then performed to this distribution to normalize the background to data and to extract the signal. The fit models the background as a sum of two falling exponential functions. The determination of the background normalization in the fit is dominated by the sidebands in M lνqq, the mass of the Higgs boson candidate. 3. Results Figure (middle and right) shows the transverse mass m T distribution in the H+ and H+ jet analyses after all the cuts except for the cut on the m T itself, for a selection of a Higgs boson with m H = 5 GeV. Table shows the expected numbers of signal and background events as well as the number of events observed in data after applying all cuts in. fb of integrated luminosity for the H+ jets and H+ jet analyses. The dominant contributions to the background in both channels comes from WW production. In the H+ (H+ ) jet channel the second largest background originates from W +jets (top pair) production. The normalizations of the major backgrounds are obtained with data-driven techniques, as described in more detail in []. PoS(EPS-HEP)5 Table : The expected numbers of signal (m H = 5 GeV) and background events for the H WW ( ) lνlν H+ and H+ jet analyses in. fb of integrated luminosity together with the number of observed events in data. Channel Signal WW W+jets Z/γ +jets t t tw/tb/tqb W Z/ZZ/W γ Total Bkg. Obs. H+ jets ± 6±3.9±.9 ±.6±..7±..6±. 33±5 9 H+ jets 7.±.6 6.±.3 ±.9.±.6.9±.7.3±.7.3±.6 5±3 3
Entries Data SM (sys stat) 8 6, H WW lνlν WW WZ/ZZ/Wγ t t Single Top H [5] Entries / GeV 35 3 5 5 5, H WW lνlν + jet Data SM (sys stat) WW WZ/ZZ/Wγ t t Single Top H [5] Entries / GeV 6 Data stat) SM (sys, WW WZ/ZZ/Wγ t Single Top t H WW lνlν + jet H [5] 8 6.5 6 8 N jets.5 6 8 6 8 [GeV] M T.5 6 8 6 8 [GeV] M T Figure : Multiplicity of jets with p T > 5 GeV after the cut on E miss T,rel (left) and the transverse mass m T distribution in the H+ (middle) and H+ (right) jet analyses after all the cuts except for the cut on the m T itself, for a selection of a Higgs boson with m H = 5 GeV. Figure (left) shows the observed and expected limits at 95% confidence level for the combined H WW ( ) lνlν H+ jet and H+ jet analyses. The procedure used to compute exclusion limits is based on the modified frequentist method known as CLs [7]. A Higgs boson with a mass in the range from 58 GeV to 86 GeV is excluded at 95% confidence level, while the expected Higgs boson mass exclusion range is m H 86 GeV. The observed exclusion limit is more than σ larger than the expected exclusion limit in the Higgs boson mass range from 6 GeV to 5 GeV. Assuming the nominal standard model Higgs boson hypothesis, the expected and observed signal significances as functions of the Higgs boson mass are shown in Fig. (right). An excess of events in data corresponding to more than σ significance is observed for the Higgs boson mass range from 6 GeV to 58 GeV, with the largest deviation being.7σ for a Higgs boson mass of 3 GeV. 95% CL Limit on σ/σ SM ± σ ± σ (*) H WW lνlν Ldt =. fb Statistical significance 8 6 ± σ ± σ (*) H WW lνlν Ldt =. fb PoS(EPS-HEP)5 6 8 M H [GeV] 6 8 M H [GeV] Figure : The expected (dashed) and observed (solid) 95% CL upper limits on the cross-section normalized to the SM cross-section (left) and the signal significance (right) as a function of the Higgs boson mass in the H WW ( ) lνlν channel. The invariant mass distributions in the H WW ( ) lνqq analysis for the H+ and H+ jet channels are shown in Fig. 3 (left and middle). The shape of a potential signal at m H = GeV is also shown multiplied by a factor of one hundred. No significant H WW ( ) lνqq excess is observed, so limits are extracted, as show in Fig. 3 (right).
Events / GeV 35 3 5 5 5 Signal (x ) m H = GeV Data top W/Z+jets Multi-jet Dibosons H lν jj + jet ATLAS 3 5 6 7 m(lνjj) [GeV] Events / GeV 8 6 8 6 Signal (x ) m H = GeV Data top W/Z+jets Multi-jet Dibosons H lν jj + jet ATLAS 3 5 6 7 m(lνjj) [GeV] 95% CL limit on σ/σ(sm) ATLAS H lν jj + / jet s=7 TeV L dt=. fb ± σ ± σ 5 3 35 5 5 55 6 m H [GeV] Figure 3: The distribution of the invariant mass of Higgs candidates (M lνqq ), for the H+ jet analysis (left) and for the H+ jet analysis (middle) as well as the expected (dashed) and observed (solid) 95% CL upper limits on the SM Higgs boson cross section in the H WW ( ) lνqq analysis (right).. Conclusion Higgs boson searches in the H WW ( ) lνlν (l=e, µ) and the H WW ( ) lνqq decay modes, using. fb of proton-proton collision data at a centre-of-mass energy of 7 TeV collected with the ATLAS detector, have been presented. An upper bound is placed on the Higgs boson production cross-section as a function of m H. The H WW ( ) lνlν analysis excludes a Higgs boson with a mass in the range from 58 GeV to 86 GeV at 95% confidence level, while the expected Higgs boson mass exclusion range is m H 86 GeV. An excess of events in data corresponding to more than σ significance is observed for the Higgs boson mass range from 6 GeV to 58 GeV. The H WW ( ) lνqq analysis place limits on the Higgs boson cross section that are between. to times the expected standard model cross section in the mass range from GeV to 6 GeV. References [] G. Abbiendi et al., Phys. Lett. B565 (3) 6. [] T. Altonen et al., Phys. Rev. Lett. () 68. [3] ALEPH DELPHI L3 OPAL SLD CDF D Coll., The LEP Tevatron SLD Electroweak Working Group, Cern-ph-ep--95,, arxiv:.367 [hep-ex]. [] ATLAS Collaboration, ATLAS-CONF--5. [5] ATLAS Collaboration, ATLAS-CONF-. [6] ATLAS Collaboration, Phys. Rev. Lett. 7 () 38. [7] A. L. Read, J. Phys. G. Nucl. Part. Phys. 8 () 693. PoS(EPS-HEP)5 5