47 research outputs found
(Non)renormalization of Anomalous Conductivities and Holography
The chiral magnetic and the chiral vortical effects are recently discovered
phenomena arising from chiral gauge and gravitational anomalies that lead to
generation of electric currents in presence of magnetic field or vorticity. The
magnitude of these effects is determined by the anomalous conductivities. These
conductivities can be calculated by the linear response theory, and in the
strong coupling limit this calculation can be carried out by the holographic
techniques. Earlier calculations in case of conformal field theories indicate
non-renormalization of these conductivities where the holographic calculation
agrees with the free field limit. We extend this holographic study to
non-conformal theories exhibiting mass-gap and confinement-deconfinement type
transitions in a holographic model based on the analytic black hole solution of
Gao and Zhang. We show that radiative corrections are also absent in these
non-conformal theories confirming indirect arguments of Jensen et al in a
direct and non-trivial fashion. There are various indications in field theory
that such radiative corrections should arise when contribution of dynamical
gluon fields to the chiral anomaly is present. Motivated by this, we seek for
such corrections in the holographic picture and argue that such corrections
indeed arise through mixing of the background and its fluctuations with the
axion and the one-form fields that couple to the flavor and probe gauge branes
through the Wess-Zumino terms. These corrections are non-vanishing when the
flavor to color ratio is finite, therefore they are only visible in
the Veneziano limit at large .Comment: 35 pages and appendices; v2:references adde
Holographic equilibration in confining gauge theories under external magnetic fields
We investigate the effect of external magnetic fields on equilibration in the
improved holographic QCD theory in the deconfined phase using the AdS/CFT
correspondence. In particular we calculate the quasinormal mode spectra in the
corresponding black brane solutions and study their dependence on temperature,
momentum and magnetic field, both in the scalar and the shear channels. We find
complex patterns in the motion of quasinormal modes on the complex plane,
including certain cross overs between the lowest lying modes under varying
magnetic field, momentum and temperature. We also discover a critical value of
the magnetic field above which the hydrodynamic approximation breaks
down, as the imaginary part of the first excited quasi-normal mode in the shear
channel becomes smaller than that of the hydro mode.Comment: 20 pages, 13 figures. v2: discussion on the instability we reported
in the first version is corrected. v3: range of B in figures increased,
discussion on the breakdown of hydrodynamics added, abstract and references
update
Horizon universality and anomalous conductivities
We show that the value of chiral conductivities associated with anomalous
transport is universal in a general class of strongly coupled quantum field
theories. Our result applies to theories with no dynamical gluon fields and
admitting a gravitational holographic dual in the large N limit. On the gravity
side the result follows from near horizon universality of the fluctuation
equations, similar to the holographic calculation of the shear viscosity.Comment: 13 page
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
Late time behavior of non-conformal plasmas
We determine analytically the dependence of the approach to thermal
equilibrium of strongly coupled plasmas on the breaking of scale invariance.
The theories we consider are the holographic duals to Einstein gravity coupled
to a scalar with an exponential potential. The coefficient in the exponent,
, is the parameter that controls the deviation from the conformally
invariant case. For these models we obtain analytic solutions for the plasma
expansion in the late-time limit, under the assumption of boost-invariance, and
we determine the scaling behaviour of the energy density, pressure, and
temperature as a function of time. We find that the temperature decays as a
function of proper time as with determined in terms of
the non-conformality parameter as . This agrees with the
result of Janik and Peschanski, , for the conformal plasmas and
generalizes it to non-conformal plasmas with . We also consider more
realistic potentials where the exponential is supplemented by power-law terms.
Even though in this case we cannot have exact solutions, we are able under
certain assumptions to determine the scaling of the energy, that receives
logarithmic corrections.Comment: 31 page
Instabilities near the QCD phase transition in the holographic models
The paper discusses phenomena close to the critical QCD temperature, using
the holographic model. One issue studied is the overcooled high-T phase, in
which we calculate quasi normal sound modes. We do not find instabilities
associated with other first order phase transitions, but nevertheless observe
drastic changes in sound propagation/dissipation. The rest of the paper
considers a cluster of the high-T phase in the UV in coexistence with the low-T
phase, in a simplified ansatz in which the wall separating them is positioned
only in the holographic coordinate. This allows to find the force on the wall
and classical motion of the cluster. When classical motion is forbidden, we
evaluate tunneling probability through the remaining barrier.Comment: 11 pages, 7 figure
Magnetohydrodynamics and charged currents in heavy ion collisions
The hot QCD matter produced in any heavy ion collision with a nonzero impact
parameter is produced within a strong magnetic field. We study the imprint the
magnetic fields produced in non-central heavy ion collisions leave on the
azimuthal distributions and correlations of the produced charged hadrons. The
magnetic field is time-dependent and the medium is expanding, which leads to
the induction of charged currents due to the combination of Faraday and Hall
effects. We find that these currents result in a charge-dependent directed flow
that is odd in rapidity and odd under charge exchange. It can be detected
by measuring correlations between the directed flow of charged hadrons at
different rapidities, .Comment: Contribution to the Proceedings of the Quark Matter 2014 conference;
talk given by U. Gurso
Holographic QCD in the Veneziano limit at finite Magnetic Field and Chemical Potential
We investigate the phase diagram of QCD-like gauge theories at strong
coupling at finite magnetic field , temperature and baryon chemical
potential using the improved holographic QCD model including the full
backreaction of the quarks in the plasma. In addition to the phase diagram we
study the behavior of the quark condensate as a function of , and
and discuss the fate of (inverse) magnetic catalysis at finite . In
particular we observe that inverse magnetic catalysis exists only for small
values of the baryon chemical potential. The speed of sound in this holographic
quark-gluon plasma exhibits interesting dependence on the thermodynamic
parameters.Comment: 7 pages, 6 figure
Magnetohydrodynamics, charged currents and directed flow in heavy ion collisions
The hot QCD matter produced in any heavy ion collision with a nonzero impact
parameter is produced within a strong magnetic field. We study the imprint that
these fields leave on the azimuthal distributions and correlations of the
produced charged hadrons. The magnetic field is time-dependent and the medium
is expanding, which leads to the induction of charged currents due to the
combination of Faraday and Hall effects. We find that these currents result in
a charge- dependent directed flow v1 that is odd in rapidity and odd under
charge exchange. It can be detected by measuring correlations between the
directed flow of charged hadrons at different rapidities, .Comment: 13 pages, 9 figure