RevModPhys.84.1477】Tests of the standard electroweak model at the energy frontier

2009a the sm cross section is proportional to jvtb j2

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Unformatted text preview: e-top crosssection measurements. The first row is the 95% C.L. result when no constraint is placed on jVtb j, and the second row is the result with the SM constraint 0 jVtb j 1. D0 jVtb j > 0:78 jVtb j ¼ 1:07 Æ 0:12 As discussed in Secs. II.A and II.B, the standard model, with the SUð2ÞL  Uð1ÞY symmetry spontaneously broken via the vacuum-expectation value of a single Higgs-doublet field, is consistent with precision electroweak data. These data are so precise that they are sensitive to the mass of the Higgs boson at one loop, despite the fact that the Higgs mass enters only logarithmically. Figure 49 shows the constraints on the Higgs-boson mass from both precision electroweak measurements (the wide shaded band) and direct searches (the shaded areas). The precision data depend on the extrapolation of the fine-structure constant from its measured value at low energy up to high energy, which suffers from an uncertainty associated with the contribution of low-energy QCD to the extrapolation. The solid line indicates the central value, and the shaded band takes into account all uncertainties. An alternative central value, associated with a different treatment of low-energy QCD, is indicated by the dashed line. The effect on the central value by including the NuTeV data, which has some tension with the other precision electroweak data, is shown by the dotted line. It is striking that the precision data strongly prefer a Higgsboson mass in the 100 GeV=c2 region, while a purely theoretical analysis accommodates a Higgs boson as heavy as about 700 GeV=c2 (Luscher and Weisz, 1988). Taken at face value, the precision data indicate the Higgs boson is not much heavier than the current lower bound of mH > 114 GeV=c2 . It is also remarkable that the Tevatron experiments succeeded in excluding the region 163 < mH < 166 GeV=c2 . B. Direct searches The experimental strategy and sensitivity to discover the standard model Higgs boson at the Tevatron through direct searches depends strongly on the value of mH . For direct searches, the sensitive range is limited to mH & 250 GeV=c2 6 mLimit = 157 GeV August 2009 Theory uncertainty (5) ∆αhad = 5 0.02758±0.00035 0.02749±0.00012 2 incl. low Q data 4 2 Combined, D0 [a] ∆χ BDT, D0 3 2 1 0 Excluded 30 Preliminary 100 300 mH [GeV] CDF jVtb j > 0:71 jVtb j ¼ 0:9 Æ 0:11 Rev. Mod. Phys., Vol. 84, No. 4, October–December 2012 FIG. 49 (color online). The Á2 as a function of assumed Higgs mass resulting from fits to data assuming the SM. The exclusion regions from direct searches are also shown. Hobbs, Neubauer, and Willenbrock: Tests of the standard electroweak model at . . . 1512 TABLE XXVIII. The production cross section and cross section times branching fraction for the low mass (mH < 135 GeV=c2 ) Higgs-boson search. Cross sections are in femtobarns. For the final states denoted ‘ and ‘‘, the branching fraction used is the sum of ‘ ¼ e. " " mH (GeV=c2 ) ðpp ! WH Þ ...
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