PDF《CERN-PH-EP-2012-019》

arXiv:1202.

1408v3 [hep-ex]

21 Mar

2012 CERN-PH-EP-2012-019 Combined search for the Standard Model Higgs boson using up to 4.9 fb?1 of pp collision data at √ s =

7 TeV with the ATLAS detector at the LHC The ATLAS Collaboration Abstract A combined search for the Standard Model Higgs boson with the ATLAS experiment at the LHC using datasets corresponding to integrated luminosities from 1.04 fb?1 to 4.9 fb?1 of pp collisions collected at √ s =

7 TeV is presented. The Higgs boson mass ranges 112.9C115.5 GeV, 131C238 GeV and 251C466 GeV are excluded at the 95% con?dence level (CL), while the range 124C519 GeV is expected to be excluded in the absence of a signal. An excess of events is observed around mH?126 GeV with a local signi?cance of 3.5 standard deviations (σ). The local signi?cance of H → γγ, H → ZZ(?) → ?+ ?? ?′+ ?′? and H → WW(?) → ?+ ν?′? ν, the three most sensitive channels in this mass range, are 2.8σ, 2.1σ and 1.4σ, respectively. The global probability for the background to produce such a ?uctuation anywhere in the explored Higgs boson mass range 110C600 GeV is estimated to be ?1.4% or, equivalently, 2.2σ. 1. Introduction The discovery of the mechanism for electroweak symmetry breaking (EWSB) is a major goal of the physics programme at the Large Hadron Collider (LHC). In the Standard Model (SM), EWSB is achieved by invoking the Higgs mechanism, which requires the existence of the Higgs boson [1C6]. In the SM, the Higgs boson mass, mH, is a priori unknown. However, for a given mH hypothesis, the production cross sec- tions and branching fractions of each decay mode are predicted, which enables a combined search with data from several decay channels. Direct searches at the CERN LEP e+ e? collider ex- cluded the production of a SM Higgs boson with mass below 114.4 GeV at the 95% CL [7]. The combined searches at the Fermilab Tevatron pp collider have ex- cluded the production of a Higgs boson with mass be- tween

156 GeV and

177 GeV at the 95% CL [8]. In 2011, the LHC delivered to ATLAS an integrated luminosity of 5.6 fb?1 of pp collisions at

7 TeV centre- of-mass energy. The ATLAS experiment collected and analysed an integrated luminosity corresponding to up to 4.9 fb?1 of data ful?lling all the data quality require- ments to search for the SM Higgs boson. In this Letter a combined search using six distinct channels, covering the mass range

110 GeV to

600 GeV, is presented. The Higgs boson is produced primarily through the gluon fu- sion process and the following decay modes are consid- ered: H → γγ, H → ZZ(?) → ?+ ?? ?′+ ?′? , H → ZZ → ?+ ?? qq, H → ZZ → ?+ ?? νν, H → WW(?) → ?+ ν?′? ν, and H → WW → ?νqq′ , where ? denotes an electron or a muon. New limits on SM Higgs boson production are established and the signi?cance of an excess of events observed in the low mass region around mH=126 GeV is quanti?ed. Preprint submitted to Physics Letters B March 22,

2012 2. Search Channels All search analyses are described in their respec- tive references [9C14] and therefore only the main fea- tures relevant to the statistical combination of the vari- ous channels are summarised here. Two channels, the H → ZZ → ?+ ?? qq and H → ZZ → ?+ ?? νν, have been updated to a data sample corresponding to an inte- grated luminosity larger than that used in the previously published results and are described in more detail. The H → γγ search is carried out for mH hypotheses between

110 GeV and

150 GeV and uses an integrated luminosity of 4.9 fb?1 [9]. The analysis in this chan- nel separates events into nine independent categories of varying sensitivity. The categorisation is based on the direction of each photon and whether it was re- constructed as a converted or unconverted photon, to- gether with the momentum component of the dipho- ton system transverse to the thrust axis. The dipho- ton invariant mass mγγ is used as a discriminating vari- able to distinguish signal and background, to take ad- vantage of the mass resolution of approximately 1.4% for mH?120 GeV. The distribution of mγγ in the data is ?t to a smooth function to estimate the background. The inclusive invariant mass distribution of the observed candidates, summing over all categories, is shown in Fig. 1(a). The search in the H → ZZ(?) → ?+ ?? ?′+ ?′? chan- nel is performed for mH hypotheses in the full