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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 treelevel analysis is insufﬁcient, and one must
go to at least one loop. At this level, one ﬁnds that predictions
depend also on the topquark mass and the Higgsboson
mass, since these particles appear in loops. In fact, a range
for the topquark 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 Higgsboson 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 topquark mass, there are the masses of
all the other quarks and leptons, as well as the elements of the
CabbiboKobayashiMaskawa (CKM) quarkmixing matrix
and the MakiNakagawaSakata leptonmixing 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 singletop 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.
 Fall '13
 Energy

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