In the Higgs search at the LHC, a light Higgs boson (115 GeV <~ M_H <~ 130 GeV) is not excluded by experimental data. In this mass range, the width of the Standard Model Higgs boson is more than four orders of magnitude smaller than its mass. The zero-width approximation is hence expected to be an excellent approximation. We show that this is not always the case. The inclusion of off-shell contributions is essential to obtain an accurate Higgs signal normalisation at the 1% precision level. For gg (-> H) -> VV, V= W,Z, O(10%) corrections occur due to an enhanced Higgs signal in the region M_VV > 2 M_V, where also sizable Higgs-continuum interference occurs. We discuss how experimental selection cuts can be used to exclude this region in search channels where the Higgs invariant mass cannot be reconstructed. We note that the H -> VV decay modes in weak boson fusion are similarly affected.Comment: 26 pages, 18 figures, 6 tables; added references, expanded introduction, version to appear in JHE

Pair production of W bosons constitutes an important background to Higgs boson and new physics searches at the Large Hadron Collider LHC. We have calculated the loop-induced gluon-fusion process gg → W * W * → leptons, including intermediate light and heavy quarks and allowing for arbitrary invariant masses of the W bosons. While formally of next-to-next-to-leading order, the gg → W * W * → leptons process is enhanced by the large gluon flux at the LHC and by experimental Higgs search cuts, and increases the next-to-leading order W W background estimate for Higgs searches by about 30%. We have extended our previous calculation to include the contribution from the intermediate topbottom massive quark loop and the Higgs signal process. We provide updated results for cross sections and differential distributions and study the interference between the different gluon scattering contributions. We describe important analytical and numerical aspects of our calculation and present the public GG2WW event generator.

Vector-boson pair production is an important background for Higgs boson and new physics searches at the Large Hadron Collider LHC. We have calculated the loop-induced gluon-fusion process gg → W W → leptons, allowing for arbitrary invariant masses of the intermediate W bosons. This process contributes at O(α 2 s ) relative to quarkantiquark annihilation, but its importance is enhanced by the large gluon flux at the LHC and by experimental cuts employed in Higgs boson searches. We find that gg → W W provides only a moderate correction (ca. 5%) to the inclusive W -pair production cross section at the LHC. However, after taking into account realistic experimental cuts, the gluon-fusion process becomes significant and increases the theoretical W W background estimate for Higgs searches in the pp → H → W W → leptons channel by approximately 30%.

The production cross section for a m H ≈ 115 GeV, SM Higgs boson in weak boson fusion at the LHC is sizable. However, the branching fraction for H → W + W − 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 −1 of data. The H → W W 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 m H < 113.5 GeV and some evidence for m H ≈ 115 GeV [1], the mass range now preferred by supersymmetry [2,3], 113 GeV < ∼ m h < ∼ 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 < ∼ m H < ∼ 190 GeV, will give a highly significant H → W + W − → e ± µ ∓ p / T signal in weak boson fusion (WBF) at the LHC [4,5]. For a smaller Higgs boson mass, the branching ratio of the H → W + W − mode quickly decreases, making the observation of this mode more difficult. Only a marginal signal was expected for m H ≈ 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 → W + W − signals in WBF at the LHC, and of possible backgrounds, with the goal of optimizing the significance of a signal for m H ≈ 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 → W W ) would significantly improve the extraction of Higgs boson properties such as its total width or the HW W coupling constant [6].In this letter we summarize the results of this reanalysis, and show that even for a SM Higgs of 1

Many highly developed Monte Carlo tools for the evaluation of cross sections based on tree matrix elements exist and are used by experimental collaborations in high energy physics. As the evaluation of one-loop matrix elements has recently been undergoing enormous progress, the combination of one-loop matrix elements with existing Monte Carlo tools is on the horizon. This would lead to phenomenological predictions at the next-toleading order level. This note summarises the discussion of the next-to-leading order multileg (NLM) working group on this issue which has been taking place during the workshop on Physics at TeV colliders at Les Houches, France, in June 2009. The result is a proposal for a standard interface between Monte Carlo tools and one-loop matrix element programs.Dedicated to the memory of, and in tribute to, Thomas Binoth, who led the effort to develop this proposal for Les Houches 2009. Thomas led the discussions, set up the subgroups, collected the contributions, and wrote and edited this paper. He made a promise that the paper would be on the arXiv the first week of January, and we are faithfully fulfilling his promise. In his honour, we would like to call this the Binoth Les Houches Accord.