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Scientific Report 2007-2009
Theoretical physics
T4. The origin of Electroweak Symmetry Breaking
and New Physics at the Electroweak scale
More than a century of experimental results and theoretical
progress has led us to the formulation of an extremely
elegant and compact theory of the fundamental
interactions among particles. Despite their profoundly
different manifestations on macroscopic scales, the electromagnetic,
weak and strong forces are all described
within the same mathematical framework of gauge theories.
The electromagnetic and weak interactions are associated
to the same SU(2) L ×U(1) Y gauge invariance at
short distances, although only electromagnetism is experienced
as a long-range force. The rest of the electroweak
symmetry is hidden at large distances or low energies,
i.e. it is spontaneously broken by the vacuum. As a
matter of fact, despite the abundance of experimental
information, we do not know much about the dynamics
responsible for such spontaneous breaking. An important
clue comes from the results of the LEP experiments
at Cern, which show strong evidence, although not yet
conclusive, in favor of the existence of a light Higgs boson.
The Higgs mechanism of the Standard Model (SM)
certainly gives the most economical formulation of the
electroweak symmetry breaking (EWSB), as it requires
the existence of just one new elementary particle: the
Higgs boson. It has two main virtues: it is perturbative,
hence calculable, and it is insofar phenomenologically
successful, passing the LEP electroweak precision tests.
On the other hand, a light elementary Higgs boson is
highly unnatural in absence of a symmetry protection,
since its mass receives quantum corrections of the order
of the largest energy scale to which the theory can be
extrapolated, which is the Planck scale in the case of the
SM. In this sense the SM gives no explanation of why the
Higgs is light, nor does it really explain the dynamical
origin of the symmetry breaking. In fact, it should be
considered as a parametrization rather than a dynamical
description of the EWSB.
On the other hand, it is possible, and plausible in several
respects, that a light and narrow Higgs-like scalar
does exist, but that this particle be a bound state from
some strong dynamics not much above the weak scale.
Its being composite would solve the SM hierarchy problem,
as quantum corrections to its mass are now saturated
at the compositeness scale. As first pointed out
by Georgi and Kaplan in the eighties, the composite
Higgs boson can be naturally lighter than the other resonances
of the strong dynamics – as required by the LEP
precision tests – if it emerges as the (pseudo-)Nambu-
Goldstone boson of an enlarged global symmetry of the
strong dynamics. The phenomenology of these theoretical
constructions is far richer than that of the SM, since
an entire sector of resonances of the new strong dynamics
is predicted and can be discovered at the Large Hadron
Collider (LHC) experiments at Cern.
In the last years the Particle Theory Group (PTG) of
Rome has actively worked on the formulation of realistic
composite Higgs theories and on the investigation of
their phenomenology at present and future colliders.
Much of the recent theoretical progress on the model
building front has come from the intriguing connection
between gravity in higher-dimensional curved spacetimes
and strongly-coupled gauge theories. This correspondence
suggests that the strong dynamics that generates
the light Higgs could be realized by the bulk of an extra
dimension. The research of the PTG has led to the
formulation of the first realistic 5-dimensional composite
Higgs models, resolving the long-standing problems
of the original theories of Georgi and Kaplan. In the
newly proposed constructions the Higgs is realized as
the fifth component of a 5-dimensional gauge field and
its potential is calculable and predicted in terms of a
few parameters [1]. The phenomenology of these models
has also been studied in details. Particular attention
has been devoted to deriving the constraints implied by
Flavor Changing Neutral Current effects [2] and to derive
the best strategies to produce and observe the new
particles at the LHC [3].
Whatever the form of New Physics is, a crucial issue
that experiments should be able to settle is whether
the dynamics responsible for the symmetry breaking
is weakly or strongly coupled. If a light Higgs boson
is discovered at the LHC or at Tevatron, the most
important questions to address will be: what is its role
in the mechanism of electroweak symmetry breaking ?
Is it an elementary or a composite scalar ? Crucial
evidence will come from a precise measurement of
its couplings and a detailed study of the scattering
processes that the exchange of the SM Higgs is assumed
to unitarize, such as the scattering of two longitudinally
polarized vector bosons. To address the above issues
and thus unravel the origin of the electroweak symmetry
breaking, much of the recent research activity of the
PTG has focussed on the study of the properties of the
Higgs bosons, ranging from the identification of new
production channels at the LHC [4], to highlighting the
best strategies to extract its couplings.
References
1. R. Contino et al., Phys. Rev. D75, 055014 (2007).
2. K. Agashe et al., Phys. Rev. D80, 075016 (2009).
3. R. Contino et al., J. High Energy Phys. 0705, 074 (2007).
4. E. Gabrielli et al., Nucl. Phys. B781, 64 (2007).
Authors
R. Contino, E. Franco 1 , G. Martinelli, B. Mele 1 ,
L. Silvestrini 1
Sapienza Università di Roma 27 Dipartimento di Fisica