Search for the SM Higgs Boson in the Channel $WH \to l\nu b\overline{b}$ with the ATLAS Experiment at the LHC

One of the most important scientific challenges of ATLAS and CMS, multi-purpose de- tectors at CERN’s Large Hadron Collider (LHC), is the discovery or exclusion of the longly sought standard model Higgs boson predicted almost fifty years ago. In summer 2012, both ATLAS and CMS discovered a new particle. Its mass is determined to be 126 . 0 ± 0 . 4 (stat) ± 0 . 4 (sys) GeV (ATLAS) and 125 . 3 ± 0 . 4 (stat) ± 0 . 5 (sys) GeV (CMS) [ 1 , 2 ]. Its further properties are so far consistent with the predicted properties of a standard model Higgs boson within large uncertainties. Besides the Higgs search in the sensitive bosonic channels, H → γγ , H → ZZ , and H → WW , the fermionic channels H → ττ and H → b b contributed to the exclusion of a standard model Higgs boson below the observed excess and are essential for measuring the couplings of the new particle to fermions. In the analysis presented here, the associated Higgs production WH in the Higgs decay channel H → b b is studied on the collision data corresponding to an integrated lumino- sity of 4 . 7 fb − 1 recorded by ATLAS in 2011 at a proton-proton centre-of-mass energy of 7 TeV. A cut-based analysis selects events with the signature of interest, consisting of one lepton (electron, muon), missing transverse energy, and two b jets. Techniques for a data-driven estimate of the multijet background are developed, validated and applied. The different contributions from heavy and light flavour jets to the W +jets background are estimated by a data-driven method. Furthermore, the large background components from top and W +jets are normalized using collision data. The invariant mass of the two selected jets m b b is used for the determination of exclusion limits on the Higgs signal strength σ/σ WH in the mass window 110 GeV ≤ m H ≤ 130 GeV. For a hypothesized Higgs mass of m H = 110 GeV, the Higgs signal strength is excluded for values larger than 4.8 at a confidence level of 95 %.