189 research outputs found
Thermodynamics and CP-odd transport in Holographic QCD with Finite Magnetic Field
We consider a bottom-up holographic model of QCD at finite temperature T and
magnetic field B, and study dependence of thermodynamics and CP-odd transport
on these variables. As the magnetic field couples to the flavor sector only,
one should take the Veneziano limit where the number of flavors and colors are
large while their ratio is kept fixed. We investigate the corresponding
holographic background in the approximation where the ratio of flavors to
colors is finite but small. We demonstrate that B-dependence of the entropy of
QCD is in qualitative agreement with the recent lattice studies. Finally we
study the CP-odd transport properties of this system. In particular, we
determine the Chern-Simons decay rate at finite B and T, that is an important
ingredient in the Chiral Magnetic Effect.Comment: 24 pages, 3 Figure
The anomalous transport of axial charge: topological vs non-topological fluctuations
Axial charge imbalance is an essential ingredient in novel effects associated
with chiral anomaly such as chiral magnetic effects (CME). In a non-Abelian
plasma with chiral fermions, local axial charge can be generated a) by
topological fluctuations which would create domains with non-zero winding
number b) by conventional non-topological thermal fluctuations. We provide a
holographic evaluations of medium's response to dynamically generated axial
charge density in hydrodynamic limit and examine if medium's response depends
on the microscopic origins of axial charge imbalance. We show a local domain
with non-zero winding number would induce a non-dissipative axial current due
to chiral anomaly. We illustrate holographically that a local axial charge
imbalance would be damped out with the damping rate related to Chern-Simon
diffusive constant. By computing chiral magnetic current in the presence of
dynamically generated axial charge density, we found that the ratio of CME
current over the axial charge density is independent of the origin of axial
charge imbalance in low frequency and momentum limit. Finally, a stochastic
hydrodynamic equation of the axial charge is formulated by including both types
of fluctuations.Comment: 30 pages, version accepted in JHE
The CP-odd sector and dynamics in holographic QCD
The holographic model of V-QCD is used to analyze the physics of QCD in the
Veneziano large-N limit. An unprecedented analysis of the CP-odd physics is
performed going beyond the level of effective field theories. The structure of
holographic saddle-points at finite is determined, as well as its
interplay with chiral symmetry breaking. Many observables (vacuum energy and
higher-order susceptibilities, singlet and non-singlet masses and mixings) are
computed as functions of and the quark mass . Wherever applicable
the results are compared to those of chiral Lagrangians, finding agreement. In
particular, we recover the Witten-Veneziano formula in the small
limit, we compute the -dependence of the pion mass and we derive the
hyperscaling relation for the topological susceptibility in the conformal
window in terms of the quark mass.Comment: 58 pages plus appendices, 19 figures. V2: section 3.1 improved, typos
corrected, published versio
V-QCD: Spectra, the dilaton and the S-parameter
Zero temperature spectra of mesons and glueballs are analyzed in a class of
holographic bottom-up models for QCD (named V-QCD), as a function of x =
N_f/N_c with the full back-reaction included. It is found that spectra are
discrete and gapped (modulo the pions) in the QCD regime, for x below the
critical value x_c where the conformal transition takes place. The masses
uniformly converge to zero in the walking region x -> x_c due to Miransky
scaling. The ratio of masses all asymptote to non-zero constants as x -> x_c
and therefore there is no "dilaton" in the spectrum. The S-parameter is
computed and found to be of O(1) in the walking regime.Comment: 11 pages, 6 figure
The discontinuities of conformal transitions and mass spectra of V-QCD
Zero temperature spectra of mesons and glueballs are analyzed in a class of
holographic bottom-up models for QCD in the Veneziano limit, N_c -> infinity,
N_f -> infinity, with x = N_f/N_c fixed (V-QCD). The backreaction of flavor on
color is fully included. It is found that spectra are discrete and gapped
(modulo the pions) in the QCD regime, for x below the critical value x_c where
the conformal transition takes place. The masses uniformly converge to zero in
the walking region x -> x_c^- due to Miransky scaling. All the ratios of masses
asymptote to non-zero constants as x -> x_c^- and therefore there is no
"dilaton" in the spectrum. The S-parameter is computed and found to be of O(1)
in units of N_f N_c in the walking regime, while it is always an increasing
function of x. This indicates the presence of a subtle discontinuity of
correlation functions across the conformal transition at x = x_c.Comment: 45 pages plus appendices, 13 figure
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