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ggHVHg(a)g(b)q¯q¯qHHV ∗q(c)qq(d)VFigure 2.7: Feynman diagrams for Higgs production processes at the LHC - (a) gluonfusion, (b) vector boson fusion, (c) associated production with heavyquarks, (d) associated production with vector bosons.to the Higgs boson. While the Hb¯b cross-section is actually larger than the Ht¯t crosssectionfor most of the m H range where these channels are relevant, the Hb¯b finalstate is hard to isolate experimentally and so is usually considered irrelevant for initialdiscovery.Decay Channels and Experimental SignaturesFigure 2.6 shows the branching ratios for Standard Model Higgs boson decays. Forlow masses (m H < 135 GeV), the dominant decay channels are into a pairs of b quarksor τ leptons, with a combined BR of about 93%. These channels will be unobservablebecause there is a huge background of similar final states from QCD processes. Thisleaves the H → γγ process as the most promising channel in this mass region, eventhough its branching ratio is only on the order of 10 −3 . Decays into pairs of W orZ bosons dominate intermediate to high mass ranges. Contributions from t¯t becomesignificant for m H > 300 GeV, but these suffer from similar detection problems as theb¯b states and will not be considered further. The region of 2M W < m H < 2M Z mustbe considered separately because Z pair production is significantly less likely therethan W pair production.H → γγ Analysis A discovery through the H → γγ channel is experimentally challengingbecause the backgrounds from direct photon production are much larger thanthe expected Higgs signal (by a factor of about 10 6 ). Nevertheless, at m H < 130 GeV,it remains the most promising option because the vector boson decays are very rare.Backgrounds come both from “irreducible” sources which produce two photons11