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