132 research outputs found
An Equivalent Gauge and the Equivalence Theorem
I describe a novel covariant formulation of massive gauge theories in which
the longitudinal polarization vectors do not grow with the energy. Therefore in
the present formalism, differently from the ordinary one, the energy and
coupling power-counting is completely transparent at the level of individual
Feynman diagrams, with obvious advantages both at the conceptual and practical
level.
Since power-counting is transparent, the high-energy limit of the amplitudes
involving longitudinal particles is immediately taken, and the Equivalence
Theorem is easily demonstrated at all orders in perturbation theory. Since the
formalism makes the Equivalence Theorem self-evident, and because it is based
on a suitable choice of the gauge, we can call it an "Equivalent Gauge".Comment: 36 pages, 2 figures. In v2: references added and typos corrected;
comparison with arXiv:1106.5537 adde
Orbifold resolutions and fermion localization
We study the Dirac equation of chiral fermions on a regularized version of
the two-dimensional T^2/Z_2 orbifold, where the conical singularities are
replaced by suitable spherical caps with constant curvature. This study shows
how localized and bulk fermions arise in the orbifold as the resolved space
approaches the orbifold limit.
Our analysis also shows that not all possible fermion configurations on
T^2/Z_2 admit such a simple resolution. We focus our study to a fermion coupled
to a U(1) gauge field. It is explicitly shown how a resolution of the orbifold
puts severe constraints on the allowed chiralities and U(1) charges of the
massless four dimensional fermions, localized or not, that can be present in
the orbifold.
The limit in which T^2/Z_2 (and its corresponding resolved space) collapses
to S^1/Z_2 is also studied in detail.Comment: 37 pages, 5 figures; v2: minor improvements and references adde
Baryon physics in a five-dimensional model of hadrons
We review the procedure to calculate baryonic properties using a recently
proposed five-dimensional approach to QCD. We show that this method give
predictions to baryon observables that agree reasonable well with the
experimental data.Comment: Contribution to "The Multi-facet of Skyrmions" edited by G. Brown and
M. Rho for World Scientific Publishing Co, 30
Baryon Physics in Holographic QCD
In a simple holographic model for QCD in which the Chern-Simons term is
incorporated to take into account the QCD chiral anomaly, we show that baryons
arise as stable solitons which are the 5D analogs of 4D skyrmions. Contrary to
4D skyrmions and previously considered holographic scenarios, these solitons
have sizes larger than the inverse cut-off of the model, and therefore they are
predictable within our effective field theory approach. We perform a numerical
determination of several static properties of the nucleons and find a
satisfactory agreement with data. We also calculate the amplitudes of
``anomalous'' processes induced by the Chern-Simons term in the meson sector,
such as omega -> pi gamma and omega -> 3pi. A combined fit to baryonic and
mesonic observables leads to an agreement with experiments within 16%.Comment: 18 pages, version to appear in Nucl. Phys.
Heavy Vector Triplets: Bridging Theory and Data
We introduce a model-independent strategy to study narrow resonances which we
apply to a heavy vector triplet of the Standard Model (SM) group for
illustration. The method is based on a simplified phenomenological Lagrangian
which reproduces a large class of explicit models. Firstly, this allows us to
derive robust model-independent phenomenological features and, conversely, to
identify the peculiarities of different explicit realizations. Secondly, limits
on cross-section times BR can be converted into bounds on a few relevant
parameters in a fully analytic way, allowing for an interpretation in any given
explicit model. Based on the available 8 TeV LHC analyses, we derive current
limits and interpret them for vector triplets arising in weakly coupled (gauge)
and strongly coupled (composite) extensions of the SM. We point out that a
model-independent limit setting procedure must be based on purely on-shell
quantities, like a cross-section times BR. Finite width effects altering the
limits can be considerably reduced by focusing on the on-shell signal region.
We illustrate this aspect with a study of the invariant mass distribution in
di-lepton searches and the transverse mass distribution in lepton-neutrino
final states. In addition to this paper we provide a set of online tools
available at a dedicated webpage.Comment: 53 pages, 10 figures; references added, typos corrected; published
versio
On the Tuning and the Mass of the Composite Higgs
We analyze quantitatively the tuning of composite Higgs models with partial
compositeness and its interplay with the predicted Higgs mass. In this respect
we identify three classes of models, characterized by different quantum numbers
of the fermionic colored resonances associated with the top quark, the
so-called top partners. The main result of this classification is that in all
models with moderate tuning a light Higgs, of 125 GeV mass, requires the
presence of light top partners, around 1 TeV. The minimal tuning is comparable
to the one of the most attractive supersymmetric models in particular the ones
realizing Natural SUSY. This gives further support to an extensive program of
top partners searches at the LHC that can already probe the natural region of
composite Higgs models.Comment: 32 pages, 14 figures; v2: version published in JHE
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