–1–
HIGGS BOSONS: THEORY AND SEARCHES
Updated May 2010 by G. Bernardi (LPNHE, CNRS/IN2P3,
U. of Paris VI & VII), M. Carena (Fermi National Accelerator
Laboratory and the University of Chicago), and T. Junk (Fermi
National Accelerator Laboratory).
I. Introduction
Understanding the mechanism that breaks electroweak sym
metry and generates the masses of all known elementary par
ticles is one of the most fundamental problems in particle
physics. The Higgs mechanism [1] provides a general framework
to explain the observed masses of the
W
±
and
Z
gauge bosons
by means of charged and neutral Goldstone bosons that end
up as the longitudinal components of the gauge bosons. These
Goldstone bosons are generated by the underlying dynamics
of electroweak symmetry breaking (EWSB). However, the fun
damental dynamics of the electroweak symmetry breaking are
unknown. There are two main classes of theories proposed in
the literature, those with weakly coupled dynamics  such as in
the Standard Model (SM) [2]  and those with strongly coupled
dynamics.
In the SM, the electroweak interactions are described by
a gauge ±eld theory based on the SU(2)
L
×
U(1)
Y
symmetry
group. The Higgs mechanism posits a selfinteracting complex
doublet of scalar ±elds, and renormalizable interactions are
arranged such that the neutral component of the scalar doublet
acquires a vacuum expectation value
v
≈
246 GeV, which
sets the scale of EWSB. Three massless Goldstone bosons are
generated, which are absorbed to give masses to the
W
±
and
Z
gauge bosons. The remaining component of the complex
doublet becomes the Higgs boson  a new fundamental scalar
particle. The masses of all fermions are also a consequence of
EWSB since the Higgs doublet is postulated to couple to the
fermions through Yukawa interactions. If the Higgs boson mass
m
H
is below
∼
180 GeV, all ±elds remain weakly interacting
up to the Planck scale,
M
Pl
.
The validity of the SM as an e²ective theory describing
physics up to the Planck scale is questionable, however, because
CITATION: K. Nakamura
et al.
(Particle Data Group), JPG
37
, 075021 (2010) (URL: http://pdg.lbl.gov)
July 30, 2010
14:34
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of the following “naturalness” argument. All fermion masses
and dimensionless couplings are logarithmically sensitive to the
scale Λ at which new physics becomes relevant. In contrast,
scalar squared masses are quadratically sensitive to Λ. Thus,
the observable SM Higgs mass has the following form:
m
2
H
=(
m
2
H
)
0
+
kg
2
Λ
2
16
π
2
,
where (
m
H
)
0
is a fundamental parameter of the theory. The
second term is a oneloop correction in which
g
is an elec
troweak coupling and
k
is a constant, presumably of
O
(1), that
is calculable within the lowenergy eFective theory. The two
contributions arise from independent sources and one would not
expect that the observable Higgs boson mass is signi±cantly
smaller than either of the two terms. Hence, if the scale of new
physics Λ is much larger than the electroweak scale, unnatural
cancellations must occur to remove the quadratic dependence
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 Spring '11
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