42 research outputs found
Chiral Anomalous Dispersion
The linearized Einstein equation describing graviton propagation through a
chiral medium appears to be helicity dependent. We analyze features of the
corresponding spectrum in a collision-less regime above a flat background. In
the long wave-length limit, circularly polarized metric perturbations travel
with a helicity dependent group velocity that can turn negative giving rise to
a new type of an anomalous dispersion. We further show that this chiral
anomalous dispersion is a general feature of polarized modes propagating
through chiral plasmas extending our result to the electromagnetic sector.Comment: 13 pages
Drag suppression in anomalous chiral media
We study a heavy impurity moving longitudinal with the direction of an
external magnetic field in an anomalous chiral medium. Such system would carry
a non-dissipative current of chiral magnetic effect associated with the
anomaly. We show, by generalizing Landau's criterion for superfluidity, that
the "anomalous component" which gives rise to the anomalous transport will {\it
not} contribute to the drag experienced by an impurity. We argue on a very
general basis that those systems with a strong magnetic field would exhibit an
interesting transport phenomenon -- the motion of the heavy impurity is
frictionless, in analogy to the case of a superfluid. We demonstrate and
confirm our general results with two complementary examples: weakly coupled
chiral fermion gases and strongly interacting chiral liquids.Comment: 6 pages, 1 figure, version accepted in PR
Chiral Vortical Effect for Bosons
The thermal contribution to the chiral vortical effect is believed to be
related to the axial anomaly in external gravitational fields. We use the
universality of the spin-gravity interaction to extend this idea to a wider set
of phenomena. We consider the Kubo formula at weak coupling for the spin
current of a vector field and derive a novel anomalous effect caused by the
medium rotation: the chiral vortical effect for bosons. The effect consists in
a spin current of vector bosons along the angular velocity of the medium. We
argue that it has the same anomalous nature as in the fermionic case and show
that this effect provides a mechanism for helicity transfer, from flow helicity
to magnetic helicity.Comment: 6 pages, 1 figure, version accepted in PR
Anomalous Transport and Generalized Axial Charge
In this paper we continue studying the modification of the axial charge in
chiral media by macroscopic helicities. Recently it was shown that magnetic
reconnections result in a persistent current of zero mode along flux tubes.
Here we argue that in general a change in the helical part of the generalized
axial charge results in the same phenomenon. Thus one may say that there is a
novel realization of chiral effects requiring no initial chiral asymmetry. The
transfer of flow helicity to zero modes is analyzed in a toy model based on a
vortex reconnection in a chiral superfluid. Then, we discuss the balance
between the two competing processes effect of reconnections and the chiral
instability on the example of magnetic helicity. We argue that in the general
case there is a possibility for the distribution of the axial charge between
the magnetic and fermionic forms at the end of the instability.Comment: 19 pages, version accepted in PR
Jet shape modifications in holographic dijet systems
We present a coherent model that combines jet production from perturbative
QCD with strongly-coupled jet-medium interactions described in holography. We
use this model to study the modification of an ensemble of jets upon
propagation through a quark-gluon plasma either resembling central heavy ion
collisions or proton-ion collisions. Here the modification of the dijet
asymmetry depends strongly on the subleading jet width, which can therefore be
an important observable for studying jet-medium interactions. We furthermore
show that the modification of the shape of the leading jet is relatively
insensitive to the dijet asymmetry, whereas the subleading jet shape
modification is much larger for more imbalanced dijets.Comment: 6 pages, 4 figure
Drag force to all orders in gradients
We study the energy loss of a heavy quark slowly moving through an evolving
strongly coupled plasma. We use the linearized fluid/gravity correspondence to
describe small perturbations of the medium flow with general spacetime
dependence. This all order linearized hydrodynamics results in a drag force
exerted on a heavy quark even when it is at rest with the fluid element. We
show how the general contribution to the drag force can be derived order by
order in the medium velocity gradients and provide explicit results valid up to
the third order. We then obtain an approximate semi-analytic result for the
drag force to all orders in the gradient expansion but linearized in the medium
velocity. Thus, the effects of a class of hydrodynamic gradients on the drag
force are re-summed, giving further insight into the dissipative properties of
strongly coupled plasmas. The all order result allows us to study the drag
force in the non-hydrodynamic regime of linear medium perturbations that vary
rapidly in space and time.Comment: 25 pages, 4 figures. v2: Journal version, references and figure adde
Evolution of the Mean Jet Shape and Dijet Asymmetry Distribution of an Ensemble of Holographic Jets in Strongly Coupled Plasma
Some of the most important probes of the quark-gluon plasma (QGP) produced in
heavy ion collisions come from the analysis of how the shape and energy of jets
are modified by passage through QGP. We model an ensemble of back-to-back
dijets to gain a qualitative understanding of how the shapes of the individual
jets and the asymmetry in the energy of the pairs of jets are modified by
passage through an expanding droplet of strongly coupled plasma, as modeled in
a holographic gauge theory. We do so by constructing an ensemble of strings in
the gravitational description of the gauge theory. We model QCD jets in vacuum
using strings whose endpoints move "downward" into the gravitational bulk
spacetime with some fixed small angle that represents the opening angle (ratio
of jet mass to jet energy) that the QCD jet would have in vacuum. Such strings
must be moving through the gravitational bulk at (close to) the speed of light;
they must be (close to) null. This condition does not specify the energy
distribution along the string, meaning that it does not specify the shape of
the jet being modeled. We study the dynamics of strings that are initially not
null and show that strings with a wide range of initial conditions rapidly
accelerate and become null and, as they do, develop a similar distribution of
their energy density. We use this distribution of the energy density along the
string, choose an ensemble of strings whose opening angles and energies are
distributed as in perturbative QCD, and show that we can then fix one model
parameter such that the mean jet shape in our ensemble matches that measured in
p-p collisions reasonably well. We send our strings through the plasma,
choosing the second model parameter to get a reasonable suppression in the
number of jets, and study how the mean jet shape and the dijet asymmetry are
modified, comparing both to data from LHC heavy ion collisions.Comment: References added; 34 pages, 11 figure