2,205 research outputs found
Beautiful mirrors for a pNGB Higgs
We consider one of the most significant deviations from the Standard Model:
the forward-backward asymmetry of the b-quark measured at leptonic colliders.
We investigate the possibility to solve this discrepancy by introducing new
physics at the TeV scale. We focus on models where the Higgs is a pseudo
Nambu-Goldstone boson of a new strongly coupled sector with a global SO(5)
symmetry broken spontaneously to SO(4). Besides the usual top partners, we
introduce bottom partners in the representations 16 and 4 of SO(5) and show
that they can improve significantly the fit by correcting the Zbb couplings. We
also estimate the corrections to the couplings at one-loop and obtain that the
tree-level ones dominate and can give a reliable estimation. We find that the
large shift required for Zb_Rb_R leads to light custodians associated to the
b-quark, similar to the top partners, as well as a rich phenomenology involving
neutral interactions in the bottom-sector.Comment: 31 pages, 2 figure
Generalised bottom-up holography and walking technicolour
In extradimensional holographic approaches the flavour symmetry is gauged in
the bulk, that is, treated as a local symmetry. Imposing such a local symmetry
admits fewer terms coupling the (axial) vectors and (pseudo)scalars than if a
global symmetry is imposed. The latter is the case in standard low-energy
effective Lagrangians. Here we incorporate these additional, a priori only
globally invariant terms into a holographic treatment by means of a
Stueckelberg completion and alternatively by means of a Legendre
transformation. This work was motivated by our investigations concerning
dynamical electroweak symmetry breaking by walking technicolour and we apply
our findings to these theories.Comment: 12 pages, 5 figure
Towards understanding Regge trajectories in holographic QCD
We reassess a work done by Migdal on the spectrum of low-energy vector mesons
in QCD in the light of the AdS-QCD correspondence. Recently, a tantalizing
parallelism was suggested between Migdal's work and a family of holographic
duals of QCD. Despite the intriguing similarities, both approaches face a major
drawback: the spectrum is in conflict with well-tested Regge scaling. However,
it has recently been shown that holographic duals can be modified to accomodate
Regge behavior. Therefore, it is interesting to understand whether Regge
behavior can also be achieved in Migdal's approach. In this paper we
investigate this issue. We find that Migdal's approach, which is based on a
modified Pade approximant, is closely related to the issue of quark-hadron
duality breakdown in QCD.Comment: 17 pages, 1 figure. Typos fixed, references added, improved
discussion. Minor changes to match the journal versio
Baryon Number-Induced Chern-Simons Couplings of Vector and Axial-Vector Mesons in Holographic QCD
We show that holographic models of QCD predict the presence of a Chern-Simons
coupling between vector and axial-vector mesons at finite baryon density. In
the AdS/CFT dictionary, the coefficient of this coupling is proportional to the
baryon number density, and is fixed uniquely in the five-dimensional
holographic dual by anomalies in the flavor currents. For the lightest mesons,
the coupling mixes transverse and polarization states. At
sufficiently large baryon number densities, it produces an instability, which
causes the and mesons to condense in a state breaking both
rotational and translational invariance.Comment: 4 page
Massive Pions, Anomalies and Baryons in Holographic QCD
We consider a holographic model of QCD, obtained by a very simple
modification of the original construction, which describes at the same time the
pion mass, the QCD anomalies and the baryons as topological solitons. We study
in detail its phenomenological implications in both the mesonic and baryonic
sectors and compare with the observations.Comment: 31 pages, 2 figures; v2: Version published in Nucl. Phys.
Light custodians in natural composite Higgs models
We present a class of composite Higgs models arising from a warped extra
dimension that can satisfy all the electroweak precision tests in a significant
portion of their parameter space. A custodial symmetry plays a crucial role in
keeping the largest corrections to the electroweak observables below their
experimental limits. In these models the heaviness of the top quark is not only
essential to trigger the electroweak symmetry breaking, but it also implies
that the lowest top resonance and its custodial partners, the custodians, are
significantly lighter than the other resonances. These custodians are the
trademark of these scenarios. They are exotic colored fermions of
electromagnetic charges 5/3, 2/3 and -1/3, with masses predicted roughly in the
range 500-1500 GeV. We discuss their production and detection at the LHC.Comment: 23 pages, 2 figure
Structure of Vector Mesons in Holographic Model with Linear Confinement
Wave functions and form factors of vector mesons are investigated in the
holographic dual model of QCD with a smooth oscillator-like wall. We introduce
wave functions conjugate to solutions of the 5D equation of motion and develop
a formalism based on these wave functions, which are very similar to those of a
quantum-mechanical oscillator. For the lowest bound state (rho-meson), we show
that, in this model, the basic elastic form factor exhibits the perfect vector
meson dominance, i.e., it is given by the rho-pole contribution alone. The
electric radius of the rho-meson is calculated, _C = 0.655 fm^2, which
is larger than in case of the hard-wall cutoff. The squared radii of higher
excited states are found to increase logarithmically rather than linearly with
the radial excitation number. We calculate the coupling constant f_rho and find
that the experimental value is closer to that calculated in the hard-wall
model.Comment: 8 pages, RevTex4, references, comments and a figure added. Some
terminoloy change
Interacting fermions and domain wall defects in 2+1 dimensions
We consider a Dirac field in 2+1 dimensions with a domain wall like defect in
its mass, minimally coupled to a dynamical Abelian vector field. The mass of
the fermionic field is assumed to have just one linear domain wall, which is
externally fixed and unaffected by the dynamics. We show that, under some
general conditions on the parameters, the localized zero modes predicted by the
Callan and Harvey mechanism are stable under the electromagnetic interaction of
the fermions
Pion distribution amplitude from holographic QCD and the electromagnetic form factor F_pi(Q2)
The holographic QCD prediction for the pion distribution amplitude (DA)
is used to compute the pion spacelike electromagnetic form
factor within the QCD light-cone sum rule method. In
calculations the pion's renormalon-based model twist-4 DA, as well as the
asymptotic twist-4 DA are employed. Obtained theoretical predictions are
compared with experimental data and with results of the holographic QCD
Effective Action and Holography in 5D Gauge Theories
We apply the holographic method to 5D gauge theories on the warped interval.
Our treatment includes the scalars associated with the fifth gauge field
component, which appear as 4D Goldstone bosons in the holographic effective
action. Applications are considered to two classes of models in which these
scalars play an important role. In the Composite-Higgs (and/or Gauge-Higgs
Unification) scenario, the scalars are interpreted as the Higgs field and we
use the holographic recipe to compute its one-loop potential. In AdS/QCD
models, the scalars are identified with the mesons and we compute
holographically the Chiral Perturbation Theory Lagrangian up to p^4 order. We
also discuss, using the holographic perspective, the effect of including a
Chern-Simons term in the 5D gauge Lagrangian. We show that it makes a
Wess-Zumino-Witten term to appear in the holographic effective action. This is
immediately applied to AdS/QCD, where a Chern-Simons term is needed in order to
mimic the Adler-Bardeen chiral anomaly.Comment: 37 pages; v2, minor changes, one reference added; v3, minor
corrections, version published in JHE
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