30 research outputs found
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
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
Axions and Superfluidity in Weyl Semimetals
An effective field theory (EFT) for dynamical axions in Weyl semimetals
(WSMs) is presented. A pseudoscalar axion excitation is predicted in WSMs at
sufficiently low temperatures, independently of the strength of the Weyl
fermion self-coupling. For strong fermion self-coupling the axion is the
gapless Goldstone boson of chiral spontaneous symmetry
breaking. For weak fermion self-coupling an axion is also generated at non-zero
chiral density for Weyl nodes displaced in energy, as a gapless collective mode
of correlated fermion pair excitations of the Fermi surface. This is an
explicit example of the extension of Goldstone's theorem to symmetry breaking
by the axial anomaly itself. In both cases the axion is a chiral density wave
or phason mode of the superfluid state of the WSM, and the Weyl fermions form a
chiral condensate at low temperatures. In the
presence of an applied magnetic field the axion mode becomes gapped, in analogy
to the Anderson-Higgs mechanism in a superconductor. 't Hooft anomaly matching
from ultraviolet to infrared scales is directly verified in the EFT approach.
WSMs thus provide an interesting quantum system in which superfluid, non-Fermi
liquid behavior, and a dynamical axion are predicted to follow directly from
the axial anomaly in a consistent EFT that may be tested experimentally.Comment: 50 pages, 7 figure
Jets in evolving matter within the opacity expansion approach
In a recent study [1] we have extended the opacity expansion approach to
describe jet-medium interactions including medium motion effects in the context
of heavy-ion collisions. We have computed color field of the in-medium sources,
including the effects of the transverse field components and the energy
transfer between the medium and jet. The corresponding contributions are
sub-eikonal in nature, and were previously ignored in the literature. Here we
discuss how our approach can be applied to describe the medium motion effects
in the context of Deep Inelastic Scattering.Comment: 6 pages, 1 figure, DIS2021 proceeding
Picturing QCD jets in anisotropic matter: from jet shapes to Energy Energy Correlators
Recent theoretical developments in the description of jet evolution in the
quark gluon plasma have allowed to account for the effects of hydrodynamic
gradients in the medium modified jet spectra. These constitute a crucial step
towards using jets as tomographic probes of the nuclear matter they traverse.
In this work, we complement these studies by providing leading order
calculations of widely studied jet observables, taking into account matter
anisotropies. We show that the energy distribution inside a jet is pushed
towards the direction of the largest matter anisotropy, while the away region
is depleted. As a consequence, the jet mass and girth gain a non-trivial
azimuthal dependence, with the average value of the distribution increasing
along the direction of largest gradients. However, we find that, for these jet
shapes, matter anisotropic effects can be potentially suppressed by vacuum
Sudakov factors. We argue that the recently proposed measurements of energy
correlations within jets do not suffer from such effects, with the azimuthal
dependence being visible in a large angular window, regardless of the shape of
the distribution.Comment: 9 pages, 6 figure