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Register a new userInterference effects for H --> WW/ZZ --> l anti-nu_l anti-l nu_l searches in gluon fusion at the LHC
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Nikolas Kauer
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WW/ZZ interference for Higgs signal and continuum background as well as signal-background interference is studied for same-flavour l anti-nu_l anti-l nu_l final states produced in gluon-gluon scattering at the LHC for light and heavy Higgs masses with minimal and realistic experimental selection cuts. For the signal cross section, we find WW/ZZ interference effects of O(5%) at M_H = 126 GeV. For M_H >= 200 GeV, we find that WW/ZZ interference is negligible. For the gg continuum background, we also find that WW/ZZ interference is negligible. As general rule, we conclude that non-negligible WW/ZZ interference effects occur only if at least one weak boson of the pair is dominantly off-shell due to kinematic constraints. The subdominant weak boson pair contribution induces a correction to the signal-background interference, which is at the few percentage point level before search selection cuts. Optimised selection cuts for M_H >~ 600 GeV are suggested.
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Signal-background interference effects are studied for H --> WW and H --> ZZ searches in gluon fusion at the LHC. More specifically, the interference in the channels with semileptonic weak boson pair decay is analysed for light and heavy Higgs masses with minimal and realistic experimental selection cuts. In the semileptonic decay modes, the interference is affected by tree-level background contributions enhanced by 1/e^2 relative to the gluon-fusion continuum background in the fully leptonic decay modes. We find that for both light and heavy Higgs masses the interference with the loop-induced weak-boson pair background dominates over the interference with the tree-level weak-boson plus jets background for a range of selection cuts. We therefore conclude that higher-order background contributions can induce leading interference effects. With appropriate background suppression cuts the interference can be reduced to the 10% level for heavy Higgs masses, and to the per mille level for the light SM Higgs.
A calculation of the loop-induced gluon-fusion process gg --> Z(photon)Z(photon) --> l anti-l l anti-l is presented, which provides an important background for Higgs boson searches in the H --> ZZ channel at the LHC. We find that the photon contribution is important for Higgs masses below the Z-pair threshold and that the gg-induced process yields a correction of about 15% relative to the NLO QCD prediction for the q anti-q-induced process when only a M(l anti-l), M(l anti-l) > 5 GeV cut is applied.
We compute the QCD form factor resumming threshold logarithms in B --> X_c + l + nu_l decays to next-to-leading logarithmic approximation. We present an interpolation formula including soft as well as collinear effects softened by the non-vanishing charm mass.
Using 13.53/fb of CLEO data, we have measured the ratios of the branching fractions R+(e) = BF(D+ --> K*0bar e+ nu_e) / BF(D+ --> K- pi+ pi+), R+(mu) = BF(D+ --> K*0bar mu+ nu_mu) / BF(D+ --> K- pi+ pi+) and the combined branching fraction ratio R+(l) = BF(D+ --> K*0bar l+ nu_l) / BF(D+ --> K- pi+ pi+). We find R+(e) = 0.74 +- 0.04 +- 0.05, R+(mu) = 0.72 +- 0.10 +- 0.06 and R+(l) = 0.74 +- 0.04 +- 0.05, where the first and second errors are statistical and systematic respectively. The known branching fraction BF(D+ --> K- pi+ pi+) leads to: BF(D+ --> K*0bar e+ nu_e) = (6.7 +- 0.4 +- 0.5 +- 0.4)%, BF(D+ --> K*0bar mu+ u_mu) = (6.5 +- 0.9 +- 0.5 +- 0.4)% and BF(D+ --> K*0bar l+ nu_l) = (6.7 +- 0.4 +- 0.5 +- 0.4)%, where the third error is due to the uncertainty in BF(D+ --> K- pi+ pi+).
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 -> WW -> leptons, allowing for arbitrary invariant masses of the intermediate W bosons. This process contributes at O(alpha_s^2) relative to quark-antiquark 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 -> WW 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 WW background estimate for Higgs searches in the pp -> H -> WW -> leptons channel by approximately 30%.
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