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

Predictions of all other electroweak processes can be

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Unformatted text preview: ctroweak processes can be calculated, at least at tree level. The level of precision of electroweak measurements is such that a tree-level analysis is insufficient, and one must go to at least one loop. At this level, one finds that predictions depend also on the top-quark mass and the Higgs-boson mass, since these particles appear in loops. In fact, a range for the top-quark mass was correctly predicted by precision electroweak data before the top quark was discovered, and the measured mass falls into this range. We are now following the same tack with the Higgs boson. Remarkably, the precision electroweak data imply that the Higgs-boson mass is not far above the experimental lower bound of mH > 114 GeV=c2 , which means that it may be accessible at the Tevatron as well as the LHC. The electroweak interaction has many other parameters as well. Along with the top-quark mass, there are the masses of all the other quarks and leptons, as well as the elements of the Cabbibo-Kobayashi-Maskawa (CKM) quark-mixing matrix and the Maki-Nakagawa-Sakata lepton-mixing matrix. Most of these mixing parameters are not measured at the energy frontier, with one exception: the CKM element Vtb that describes the coupling of a W boson to a top and bottom quark. The only direct measurement of this parameter comes from electroweak production of the top quark at hadron colliders via a process known as single-top production, as discussed in Sec. IV.C. B. Electroweak symmetry breaking The strong and electroweak forces are gauge theories, based on the groups SUð3ÞC and Uð1ÞEM , respectively. The associated gauge bosons, the gluon and the photon, are massless as a consequence of the gauge symmetry. We know that the interactions of electroweak bosons with fermions as well as with themselves are also governed by a gauge theory, with gauge group SUð2ÞL  Uð1ÞY . Why, then, are the Rev. Mod. Phys., Vol. 84, No. 4, October–December 2012 1479 electroweak bosons W Æ and Z not massless, as would be expected of gauge bosons? In the SM, the answer is that the electroweak symmetry is spontaneously broken, and that the electroweak gauge bosons acquire mass through the Higgs mechanism. This is the most plausible explanation of why the interactions appear to be those of a gauge theory, despite the fact that the gauge bosons are not massless. However, this argument leaves completely open the question of how (and why) the electroweak symmetry is broken. The simplest model of electroweak symmetry breaking, which is also the original proposal, is based on a fundamental scalar field that is an electroweak doublet carrying hypercharge Y ¼ 1=2. The potential for this scalar field is chosen such that its minimum is at nonzero field strength. This breaks the electroweak symmetry to Uð1ÞEM , as desired. This simple model, which can be criticized on several grounds, has withstood the test of time. It predicts that there is a scalar particle, dubbed the Higgs boson, of unknown mass...
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This document was uploaded on 09/28/2013.

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