151 research outputs found
Holographic thermalization in N=4 Super Yang-Mills theory at finite coupling
We investigate the behavior of energy momentum tensor correlators in
holographic super Yang-Mills plasma, taking finite coupling
corrections into account. In the thermal limit we determine the flow of
quasinormal modes as a function of the 't Hooft coupling. Then we use a
specific model of holographic thermalization to study the deviation of the
spectral densities from their thermal limit in an out-of-equilibrium situation.
The main focus lies on the thermalization pattern with which the plasma
constituents approach their thermal distribution as the coupling constant
decreases from the infinite coupling limit. All obtained results point towards
the weakening of the usual top-down thermalization pattern.Comment: 18 pages, 7 figures, v3: major revisio
Hyperfine Splitting and the Zeeman Effect in Holographic Heavy-Light Mesons
We inspect the mass spectrum of heavy-light mesons in deformed N=2 super
Yang-Mills theory using the AdS/CFT correspondence. We demonstrate how some of
the degeneracies of the supersymmetric meson spectrum can be removed upon
breaking the supersymmetry, thus leading to the emergence of hyperfine
structure. The explicit SUSY breaking scenarios we consider involve on one hand
tilting one of the two fundamental D7 branes inside the internal R^6 space, and
on the other hand applying an external magnetic field on the (untilted) branes.
The latter scenario leads to the well-known Zeeman effect, which we inspect for
both weak and strong magnetic fields.Comment: 5 pages, 1 figur
Thermalization at intermediate coupling
We use the AdS/CFT conjecture to investigate the thermalization of large-N_c
N=4 Super Yang-Mills plasma in the limit of large but finite 't Hooft coupling.
On the gravity side, we supplement the type IIB supergravity action by the full
set of O(\alpha'^3) operators, which enables us to derive O(\lambda^{-3/2})
corrections to the emission spectrum of prompt photons in one model of
holographic thermalization. Decreasing the coupling strength from the
\lambda=\infty limit, we observe a qualitative change in the way the photon
spectral density approaches its thermal limit as a function of the photon
energy. We interpret this behavior as a sign of the thermalization pattern of
the plasma shifting from top/down towards bottom/up.Comment: 5 pages, 3 figures; v2: minor corrections, added reference
Exploring nonlocal observables in shock wave collisions
We study the time evolution of 2-point functions and entanglement entropy in
strongly anisotropic, inhomogeneous and time-dependent N=4 super Yang-Mills
theory in the large N and large 't Hooft coupling limit using AdS/CFT. On the
gravity side this amounts to calculating the length of geodesics and area of
extremal surfaces in the dynamical background of two colliding gravitational
shockwaves, which we do numerically. We discriminate between three classes of
initial conditions corresponding to wide, intermediate and narrow shocks, and
show that they exhibit different phenomenology with respect to the nonlocal
observables that we determine. Our results permit to use (holographic)
entanglement entropy as an order parameter to distinguish between the two
phases of the cross-over from the transparency to the full-stopping scenario in
dynamical Yang-Mills plasma formation, which is frequently used as a toy model
for heavy ion collisions. The time evolution of entanglement entropy allows to
discern four regimes: highly efficient initial growth of entanglement, linear
growth, (post) collisional drama and late time (polynomial) fall off.
Surprisingly, we found that 2-point functions can be sensitive to the geometry
inside the black hole apparent horizon, while we did not find such cases for
the entanglement entropy.Comment: 28 pp, 9 figs; v2: updated references, changed color bars in Figure 2
and Figure
Gauge Independence of IR singularities in Non-Commutative QFT - and Interpolating Gauges
IR divergences of a non-commutative U(1) Maxwell theory are discussed at the
one-loop level using an interpolating gauge to show that quadratic IR
divergences are independent not only from a covariant gauge fixing but also
independent from an axial gauge fixing.Comment: 11 pages, 2 figures, v1 minor correction
Holographic dilepton production in a thermalizing plasma
We determine the out-of-equilibrium production rate of dileptons at rest in
strongly coupled N=4 Super Yang-Mills plasma using the AdS/CFT correspondence.
Thermalization is achieved via the gravitational collapse of a thin shell of
matter in AdS_5 space and the subsequent formation of a black hole, which we
describe in a quasistatic approximation. Prior to thermalization, the dilepton
spectral function is observed to oscillate as a function of frequency, but the
amplitude of the oscillations decreases when thermal equilibrium is approached.
At the same time, we follow the flow of the quasinormal spectrum of the
corresponding U(1) vector field towards its equilibrium limit.Comment: 21 pages, 7 figures. v2: Version accepted for publication in JHEP;
minor modifications, added reference
Remarks on Heavy-Light Mesons from AdS/CFT
We use the AdS/CFT correspondence to compute the energy spectrum of
heavy-light mesons in a N=2 SU(N) super Yang-Mills theory with two massive
hypermultiplets. In the heavy quark limit, similar to QCD, we find that the
excitation energies are independent of the heavy quark mass. We also make some
remarks about related AdS/CFT models of flavor with less supersymmetry.Comment: 29 pages, 9 figures; v2 section 5 rewritten to include discussion of
long strings, other minor improvements, ref adde
Anomalies and the chiral magnetic effect in the Sakai-Sugimoto model
In the chiral magnetic effect an imbalance in the number of left- and
right-handed quarks gives rise to an electromagnetic current parallel to the
magnetic field produced in noncentral heavy-ion collisions. The chiral
imbalance may be induced by topologically nontrivial gluon configurations via
the QCD axial anomaly, while the resulting electromagnetic current itself is a
consequence of the QED anomaly. In the Sakai-Sugimoto model, which in a certain
limit is dual to large-N_c QCD, we discuss the proper implementation of the QED
axial anomaly, the (ambiguous) definition of chiral currents, and the
calculation of the chiral magnetic effect. We show that this model correctly
contains the so-called consistent anomaly, but requires the introduction of a
(holographic) finite counterterm to yield the correct covariant anomaly.
Introducing net chirality through an axial chemical potential, we find a
nonvanishing vector current only before including this counterterm. This seems
to imply the absence of the chiral magnetic effect in this model. On the other
hand, for a conventional quark chemical potential and large magnetic field,
which is of interest in the physics of compact stars, we obtain a nontrivial
result for the axial current that is in agreement with previous calculations
and known exact results for QCD.Comment: 35 pages, 4 figures, v2: added comments about frequency-dependent
conductivity at the end of section 4; references added; version to appear in
JHE
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