WW as the discovery mode for a light Higgs boson

The production cross section for amH ≈ 115 GeV, SM Higgs boson in weak boson fusion at the LHC is sizable. However, the branching fraction for H → WW is expected to be relatively small. The signal, with its two forward jets, is sufficiently different from the main backgrounds that a signal to background ratio of better than 1:1 can nevertheless be obtained, with large enough rate to allow for a 5σ signal with 35 fb of data. The H → WW signal in weak boson fusion may thus prove to be the discovery mode for the Higgs boson at the LHC. After the end of LEP II running, the search for the Higgs boson and, hence, for the origin of electroweak symmetry breaking and fermion mass generation, remains one of the premier tasks of present and future high energy physics experiments. With the exclusion of a SM Higgs boson of mass mH < 113.5 GeV and some evidence for mH ≈ 115 GeV [1], the mass range now preferred by supersymmetry [2,3], 113 GeV< ∼ mh ∼ 130 GeV, will likely remain the focus of investigations until the beginning of data taking at the CERN LHC. Previously we have shown that a somewhat heavier Higgs boson, in the range 130 GeV< ∼ mH ∼ 190 GeV, will give a highly significant H → WW → eμp /T signal in weak boson fusion (WBF) at the LHC [4,5]. For a smaller Higgs boson mass, the branching ratio of the H → WW mode quickly decreases, making the observation of this mode more difficult. Only a marginal signal was expected for mH ≈ 115 GeV. However, this earlier analysis was optimized for a Higgs boson mass near W -pair threshold and can clearly be improved for the significantly smaller masses favored at the end of LEP II running. This situation motivates a complete reanalysis of possible H → WW signals in WBF at the LHC, and of possible backgrounds, with the goal of optimizing the significance of a signal for mH ≈ 115 GeV. At the same time a better signal to background (S/B) ratio, higher signal acceptance and higher accuracy in the determination of Bσ(qq → qqH, H → WW ) would significantly improve the extraction of Higgs boson properties such as its total width or the HWW coupling constant [6]. In this letter we summarize the results of this reanalysis, and show that even for a SM Higgs of