2,097 research outputs found
Improving the Excited Nucleon Spectrum in Hard-Wall AdS/QCD
We show that the nucleon spectrum in a hard-wall AdS/QCD model can be
improved by use of a relatively large IR cutoff. All of the spin-1/2 nucleon
masses listed in PDG can be fit quite well within 11%. The average error is
remarkably only 4.66%.Comment: 11 pages, 2 figures. v2: references added. v3: add a section about
the pion-nucleon coupling, published versio
Consequences of the partial restoration of chiral symmetry in AdS/QCD
Chiral symmetry is an essential concept in understanding QCD at low energy.
We treat the chiral condensate, which measures the spontaneous breaking of
chiral symmetry, as a free parameter to investigate the effect of partially
restored chiral symmetry on the physical quantities in the frame work of an
AdS/QCD model. We observe an interesting scaling behavior among the nucleon
mass, pion decay constant and chiral condensate. We propose a phenomenological
way to introduce the temperature dependence of a physical quantity in the
AdS/QCD model with the thermal AdS metric.Comment: 11 pages, 6 figure
Heavy quarkonium in a holographic QCD model
Encouraged by recent developments in AdS/QCD models for light quark system,
we study heavy quarkonium in the framework of the AdS/QCD models. We calculate
the masses of vector meson states using the AdS/QCD models at zero
and at finite temperature. Among the models adopted in this work, we find that
the soft wall model describes the low-lying heavy quark meson states at zero
temperature relatively well. At finite temperature, we observe that once the
bound state is above , its mass will increase with temperature until it
dissociates at a temperature of around . It is shown that the
dissociation temperature is fixed by the infrared cutoff of the models. The
present model serves as a unified non perturbative model to investigate the
properties of bound quarkonium states above .Comment: 9 pages, 1 figure, minor revision, to appear in phys. Rev.
Holographic Electroweak Symmetry Breaking from D-branes
We observe several interesting phenomena in a technicolor-like model of
electroweak symmetry breaking based on the D4-D8-D8bar system of Sakai and
Sugimoto. The benefit of holographic models based on D-brane configurations is
that both sides of the holographic duality are well understood. We find that
the lightest technicolor resonances contribute negatively to the
Peskin-Takeuchi S-parameter, but heavy resonances do not decouple and lead
generically to large, positive values of S, consistent with standard estimates
in QCD-like theories. We study how the S parameter and the masses and decay
constants of the vector and axial-vector techni-resonances vary over a
one-parameter family of D8-brane configurations. We discuss possibilities for
the consistent truncation of the theory to the first few resonances and suggest
some generic predictions of stringy holographic technicolor models.Comment: REVTeX, 25 pages, 8 eps figures, version published in PR
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
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
Geometric approach to condensates in holographic QCD
An SU(Nf)xSU(Nf) Yang-Mills theory on an extra-dimensional interval is
considered, with appropriate symmetry-breaking boundary conditions on the IR
brane. UV-brane to UV-brane correlators at high energies are compared with the
OPE of two-point functions of QCD quark currents. Condensates correspond to
departure from AdS of the (different) metrics felt by vector and axial
combinations, away from the UV brane. Their effect on hadronic observables is
studied: the extracted condensates agree with the signs and orders of magnitude
expected from QCD.Comment: References added: published version plus misprints correction
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
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
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