485 research outputs found
AdS/QCD Phenomenological Models from a Back-Reacted Geometry
We construct a fully back-reacted holographic dual of a four-dimensional
field theory which exhibits chiral symmetry breaking. Two possible models are
considered by studying the effects of a five-dimensional field, dual to the
operator. One model has smooth geometry at all radii and the other
dynamically generates a cutoff at finite radius. Both of these models satisfy
Einstein's field equations. The second model has only three free parameters, as
in QCD, and we show that this gives phenomenologically consistent results. We
also discuss the possibility that in order to obtain linear confinement from a
back-reacted model it may be necessary to consider the condensate of a
dimension two operator.Comment: 13 pages, 4 figures, Replaced with minor correction
A Note on Chiral Symmetry Breaking from Intersecting Branes
In this paper, we will consider the chiral symmetry breaking in the
holographic model constructed from the intersecting brane configuration, and
investigate the Nambu-Goldstone bosons associated with this symmetry breaking.Comment: 16 pp, minor changes, to appear PR
Canonical Coordinates and Meson Spectra for Scalar Deformed N=4 SYM from the AdS/CFT Correspondence
Five supersymmetric scalar deformations of the AdS_5xS^5 geometry are
investigated. By switching on condensates for the scalars in the N=4 multiplet
with a form which preserves a subgroup of the original R-symmetry, disk and
sphere configurations of D3-branes are formed in the dual supergravity
background. The analytic, canonical metric for each geometry is formulated and
the singularity structure is studied. Quarks are introduced into two of the
corresponding field theories using D7-brane probes and the pseudoscalar meson
spectrum is calculated. For one of the condensate configurations, a mass gap is
found and shown analytically to be present in the massless limit. It is also
found that there is a stepped spectrum with eigenstate degeneracy in the limit
of small quark masses. In the case of a second, similar deformation, it is
necessary to understand the full D3-D7 brane interaction to study the limit of
small quark masses. It is seen that simple solutions to the equations of motion
for the other three geometries are unlikely to exist.Comment: 16 pages, 7 figures, references added, typos correcte
A note on conductivity and charge diffusion in holographic flavour systems
We analyze the charge diffusion and conductivity in a Dp/Dq holographic setup
that is dual to a supersymmetric Yang-Mills theory in p+1 dimensions with N_f<<
N_c flavour degrees of freedom at finite temperature and nonvanishing U(1)
baryon number chemical potential. We provide a new derivation of the results
that generalize the membrane paradigm to the present context. We perform a
numerical analysis in the particular case of the D3/D7 flavor system. The
results obtained support the validity of the Einstein relation at finite
chemical potential.Comment: 15 pages, 3 figures, v2 with minor correction
Sum rules, plasma frequencies and Hall phenomenology in holographic plasmas
We study the AC optical and hall conductivities of Dp/Dq-branes intersections
in the probe approximation and use sum-rules to study various associated
transport coefficients. We determine that the presence of massive fundamental
matter, as compared to massless fundamental matter described holographically by
a theory with no dimensional defects, reduces the plasma frequency. We further
show that this is not the case when the brane intersections include defects. We
discuss in detail how to implement correctly the regularization of retarded
Green's functions so that the dispersion relations are satisfied and the low
energy behaviour of the system is physically realistic.Comment: 25 pages, 5 figures. v2.minor changes, published versio
Linear square-mass trajectories of radially and orbitally excited hadrons in holographic QCD
We consider a new approach towards constructing approximate holographic duals
of QCD from experimental hadron properties. This framework allows us to derive
a gravity dual which reproduces the empirically found linear square-mass
trajectories of universal slope for radially and orbitally excited hadrons.
Conformal symmetry breaking in the bulk is exclusively due to infrared
deformations of the anti-de Sitter metric and governed by one free mass scale
proportional to Lambda_QCD. The resulting background geometry exhibits dual
signatures of confinement and provides the first examples of holographically
generated linear trajectories in the baryon sector. The predictions for the
light hadron spectrum include new relations between trajectory slopes and
ground state masses and are in good overall agreement with experiment.Comment: 33 pages, 5 figures, updated to the extended version published in
JHEP, vector meson bulk potential and metric corrected, comments and
references added, phenomenology and conclusions unchange
Challenges facing holographic models of QCD
This paper, written in memory of Manoj Banerjee, takes a critical look at
holographic models of QCD focusing on ``practical'' models in which the five
dimensional theory is treated classically. A number of theoretical and
phenomenological challenges to the approach are discussed.Comment: This paper was written for an issue in memory of Manoj Banerjee in
the Indian Journal of Physic
Chiral phase transitions and quantum critical points of the D3/D7(D5) system with mutually perpendicular E and B fields at finite temperature and density
We study chiral symmetry restoration with increasing temperature and density
in gauge theories subject to mutually perpendicular electric and magnetic
fields using holography. We determine the chiral symmetry breaking phase
structure of the D3/D7 and D3/D5 systems in the temperature-density-electric
field directions. A magnetic field may break the chiral symmetry and an
additional electric field induces Ohm and Hall currents as well as restoring
the chiral symmetry. At zero temperature the D3/D5 system displays a line of
holographic BKT phase transitions in the density-electric field plane, while
the D3/D7 system shows a mean-field phase transition. At intermediate
temperatures, the transitions in the density-electric field plane are of first
order at low density, transforming to second order at critical points as
density rises. At high temperature the transition is only ever first order.Comment: 15 pages, 7 figures, v2: Added a referenc
NGN, QCD_2 and chiral phase transition from string theory
We construct a D2-D8- configuration in string theory, it can be
described at low energy by two dimensional field theory. In the weak coupling
region, the low energy theory is a nonlocal generalization of Gross-Neveu(GN)
model which dynamically breaks the chiral flavor symmetry at large and finite . However, in the strong coupling
region, we can use the SUGRA/Born-Infeld approximation to describe the low
energy dynamics of the system. Also, we analyze the low energy dynamics about
the configuration of wrapping the one direction of D2 brane on a circle with
anti-periodic boundary condition of fermions. The fermions and scalars on D2
branes get mass and decouple from the low energy theory. The IR dynamics is
described by the at weak coupling. In the opposite region, the dynamics
has a holographic dual description. And we have discussed the phase transition
of chiral symmetry breaking at finite temperature. Finally, after performing
T-duality, this configuration is related to some other brane configurations.Comment: 30 pages, 3 figures, minor change
Universal Holographic Chiral Dynamics in an External Magnetic Field
In this work we further extend the investigation of holographic gauge
theories in external magnetic fields, continuing earlier work. We study the
phenomenon of magnetic catalysis of mass generation in 1+3 and 1+2 dimensions,
using D3/D7- and D3/D5-brane systems, respectively. We obtain the low energy
effective actions of the corresponding pseudo Goldstone bosons and study their
dispersion relations. The D3/D7 system exhibits the usual
Gell-Mann--Oakes--Renner (GMOR) relation and a relativistic dispersion
relation, while the D3/D5 system exhibits a quadratic non-relativistic
dispersion relation and a modified linear GMOR relation. The low energy
effective action of the D3/D5 system is related to that describing magnon
excitations in a ferromagnet. We also study properties of general Dp/Dq systems
in an external magnetic field and verify the universality of the magnetic
catalysis of dynamical symmetry breaking.Comment: 41 pages, 11 figures, references adde
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