6 research outputs found
Bulk Filling Branes and the Baryon Density in AdS/QCD with gravity back-reaction
We consider the gravity back reaction on the metric due to the baryon density
in effective ads/qcd model by reconsidering the role of the charged AdS black
hole. Previously it has been known that the U(1) charge is dual to the
R-charge. Here we point out that if we consider the case where is
completely filled with flavor branes, the gravity back reaction produces
charged AdS black hole where the effect of charge on the metric is proportional
to . As a consequence, phase diagram changes qualitatively if we allow
finite: it closes at the finite density unlike the probe brane
embedding approach.
Another issue we discuss here is the question whether there is any chemical
potential dependence in the confining phase. We consider this problem in the
hard wall model with baryon charge. We conclude that there is a non-trivial
dependence on the chemical potential in this case also.Comment: 17 pages 3x2 figures, v2: references added;v3 published version,
title change and reference adde
Dynamics of Baryons from String Theory and Vector Dominance
We consider a holographic model of QCD from string theory, a la Sakai and
Sugimoto, and study baryons. In this model, mesons are collectively realized as
a five-dimensional \ Yang-Mills field and baryons
are classically identified as solitons with a unit Pontryagin number
and electric charges. The soliton is shown to be very small in the large
't Hooft coupling limit, allowing us to introduce an effective field . Its coupling to the mesons are dictated by the soliton structure, and
consists of a direct magnetic coupling to the field strength as well
as a minimal coupling to the gauge field. Upon the dimensional
reduction, this effective action reproduces all interaction terms between
nucleons and an infinite tower of mesons in a manner consistent with the large
expansion. We further find that all electromagnetic interactions, as
inferred from the same effective action via a holographic prescription, are
mediated by an infinite tower of vector mesons, rendering the baryon
electromagnetic form factors completely vector-dominated as well. We estimate
nucleon-meson couplings and also the anomalous magnetic moments, which compare
well with nature.Comment: 65pages, 3 figures, vector mesons and axial-vector mesons are now
canonically normalized (comparisons with data and conclusions unaffected
Holographic Nuclear Physics
We analyze the phases of the Sakai-Sugimoto model at finite temperature and
baryon chemical potential. Baryonic matter is represented either by 4-branes in
the 8-branes or by strings stretched from the 8-branes to the horizon. We find
the explicit configurations and use them to determine the phase diagram and
equation of state of the model. The 4-brane configuration (nuclear matter) is
always preferred to the string configuration (quark matter), and the latter is
also unstable to density fluctuations. In the deconfined phase the phase
diagram has three regions corresponding to the vacuum, quark-gluon plasma, and
nuclear matter, with a first-order and a second-order phase transition
separating the phases. We find that for a large baryon number density, and at
low temperatures, the dominant phase has broken chiral symmetry. This is in
qualitative agreement with studies of QCD at high density.Comment: 27 pages, 26 figures. v2: Added a comment about higher derivative
corrections to the DBI action in the smeared instanton in section 2.1. v3:
References added, version published in JHEP. v4: misprints correcte
Quantum Criticality via Magnetic Branes
Holographic methods are used to investigate the low temperature limit,
including quantum critical behavior, of strongly coupled 4-dimensional gauge
theories in the presence of an external magnetic field, and finite charge
density. In addition to the metric, the dual gravity theory contains a Maxwell
field with Chern-Simons coupling. In the absence of charge, the magnetic field
induces an RG flow to an infrared AdS geometry, which is
dual to a 2-dimensional CFT representing strongly interacting fermions in the
lowest Landau level. Two asymptotic Virasoro algebras and one chiral Kac-Moody
algebra arise as {\sl emergent symmetries} in the IR. Including a nonzero
charge density reveals a quantum critical point when the magnetic field reaches
a critical value whose scale is set by the charge density. The critical theory
is probed by the study of long-distance correlation functions of the boundary
stress tensor and current. All quantities of major physical interest in this
system, such as critical exponents and scaling functions, can be computed
analytically. We also study an asymptotically AdS system whose magnetic
field induced quantum critical point is governed by a IR Lifshitz geometry,
holographically dual to a D=2+1 field theory. The behavior of these holographic
theories shares important similarities with that of real world quantum critical
systems obtained by tuning a magnetic field, and may be relevant to materials
such as Strontium Ruthenates.Comment: To appear in Lect. Notes Phys. "Strongly interacting matter in
magnetic fields" (Springer), edited by D. Kharzeev, K. Landsteiner, A.
Schmitt, H.-U. Ye