5,730 research outputs found
Electromagnetic Baryon Form Factors from Holographic QCD
In the holographic model of QCD suggested by Sakai and Sugimoto, baryons are
chiral solitons sourced by D4 instantons in bulk of size 1/\sqrt{\lambda} with
\lambda=g^2N_c. We quantize the D4 instanton semiclassically using
\hbar=1/(N_c\lambda) and non-rigid constraints on the vector mesons. The
holographic baryon is a small chiral bag in the holographic direction with a
Cheshire cat smile. The vector-baryon interactions occur at the core boundary
of the instanton in D4. They are strong and of order 1/\sqrt{\hbar}. To order
\hbar^0 the electromagnetic current is entirely encoded on the core boundary
and vector-meson dominated. To this order, the electromagnetic charge radius is
of order \lambda^0. The meson contribution to the baryon magnetic moments sums
identically to the core contribution. The proton and neutron magnetic moment
are tied by a model independent relation similar to the one observed in the
Skyrme model.Comment: 26 pages, 2 figure
Holographic d-wave superconductors
We construct top down models for holographic d-wave superfluids in which the
order parameter is a charged spin two field in the bulk. Close to the
transition temperature the condensed phase can be captured by a charged spin
two field in an R-charged black hole background (downstairs picture) or
equivalently by specific graviton perturbations of a spinning black brane
(upstairs picture). We analyse the necessary conditions on the mass and the
charge of the spin two field for a condensed phase to exist and we discuss the
competition of the d-wave phase with other phases such as s-wave superfluids.Comment: 58 pages, 7 figure
Nucleon-Nucleon Potential from Holography
In the holographic model of QCD, baryons are chiral solitons sourced by D4
flavor instantons in bulk of size 1/\sqrt{\lambda} with \lambda=g^2*N_c. Using
the ADHM construction we explicit the exact two-instanton solution in bulk. We
use it to construct the core NN potential to order N_c/\lambda. The core
sources meson fields to order \sqrt{N_c/\lambda} which are shown to contribute
to the NN interaction to order N_c/\lambda. In holographic QCD, the NN
interaction splits into a small core and a large cloud contribution in line
with meson exchange models. The core part of the interaction is repulsive in
the central, spin and tensor channels for instantons in the regular gauge. The
cloud part of the interaction is dominated by omega exchange in the central
channel, by pion exchange in the tensor channel and by axial-vector exchange in
the spin and tensor channels. Vector meson exchanges are subdominant in all
channels.Comment: 44 pages, 9 figure
Holographic Nambu Jona-Lasinio Interactions
NJL interactions are introduced into the D3/ probe D7 system using Witten's
double trace operator prescription which includes the operator as a classical
term in the effective potential. In the supersymmetric system they do not
induce chiral symmetry breaking which we attribute to the flat effective
potential with quark mass in the supersymmetric theory. If additional
supersymmetry breaking is introduced then standard NJL behaviour is realized.
In examples where chiral symmetry breaking is not preferred such as with a B
field plus an IR cut off chiral condensation is triggered by the NJL
interaction at a second order transition after a finite critical coupling. If
the model already contains chiral symmetry breaking, for example in the B field
case with no IR cut off, then the NJL interaction enhances the quark mass at
all values of the NJL coupling. We also consider the system at finite
temperature: the temperature discourages condensation but when combined with a
magnetic field we find regions of parameter space where the NJL interaction
triggers a first order chiral transition above a critical coupling.Comment: 7 pages, 6 figure
Diffusion and Butterfly Velocity at Finite Density
We study diffusion and butterfly velocity () in two holographic models,
linear axion and axion-dilaton model, with a momentum relaxation parameter
() at finite density or chemical potential (). Axion-dilaton model
is particularly interesting since it shows linear--resistivity, which may
have something to do with the universal bound of diffusion. At finite density,
there are two diffusion constants describing the coupled diffusion of
charge and energy. By computing exactly, we find that in the incoherent
regime () is identified with the charge
diffusion constant () and is identified with the energy diffusion
constant (). In the coherent regime, at very small density, are
`maximally' mixed in the sense that is identified with ,
which is opposite to the case in the incoherent regime. In the incoherent
regime where or 1 so it is
universal independently of and . However, where or so, in general,
may not saturate to the lower bound in the incoherent regime, which suggests
that the characteristic velocity for charge diffusion may not be the butterfly
velocity. We find that the finite density does not affect the diffusion
property at zero density in the incoherent regime.Comment: 24 pages, 6 figures, v2 minor edits and references adde
On the Chaos Bound in Rotating Black Holes
We study out-of-time-order correlators (OTOCs) of rotating BTZ black holes
using two different approaches: the elastic eikonal gravity approximation, and
the Chern-Simons formulations of 3-dimensional gravity. Within both methods the
OTOC is given as a sum of two contributions, corresponding to left and right
moving modes. The contributions have different Lyapunov exponents,
, where
is the angular velocity and is the AdS radius. Since , there is an apparent contradiction with
the chaos bound. We discuss how the result can be made consistent with the
chaos bound if one views as the effective
inverse temperatures of the left and right moving modes.Comment: 35 pages, 2 figures. v2: references added, typos corrected, and
clarifications added to the discussion sectio
E, B, \mu, T Phase Structure of the D3/D7 Holographic Dual
The large N_c N=4 gauge theory with quenched N=2 quark matter displays chiral
symmetry breaking in the presence of a magnetic field. We previously studied
the temperature and chemical potential phase structure of this theory in the
grand canonical ensemble - here we, in addition, include the effect of an
electric field which acts to counter chiral symmetry breaking by disassociating
mesons. We compute using the gravity dual based on the D3/probe-D7 brane
system. The theory displays two transition at one of which chiral symmetry is
restored. At the other transition density switches on, the mesons of the theory
become unstable and a current forms, making it a conductor-insulator
transition. Through the temperature, electric field, chemical potential volume
(at fixed magnetic field parallel to the electric field) these transitions can
coincide or separate at critical points, and be first order or second order. We
map out this full phase structure which provides varied computable examples
relevant to strongly coupled gauge theories and potentially condensed matter
systems.Comment: 20 pages, 7 figure
The open string membrane paradigm with external electromagnetic fields
We study the effective geometry felt by the fluctuations of open strings
living on the worldvolume of probe D-branes in the presence of background
electromagnetic fields. This is captured by an effective action consisting of a
Maxwell term and a topological term, with the role of the metric played by the
open string metric. Studying generalized Eddington-Finkelstein coordinates for
stationary but non-static manifolds, we consider an open string membrane
paradigm to obtain a generic formula for the DC transport coefficients,
including the effect of external electromagnetic fields present on the
worldvolume of the probe branes. We show that the previously studied singular
shell, present when a critical electric field strength is turned on, behaves as
a horizon for the open string degrees of freedom. The results of this analysis
can be used to define a membrane paradigm for a very general class of
spacetimes with non-diagonal metrics.Comment: 31 pages, 3 figures, v2: Appendix added, minor correction
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