103 research outputs found
Equilibration and hydrodynamics at strong and weak coupling
We give an updated overview of both weak and strong coupling methods to
describe the approach to a plasma described by viscous hydrodynamics, a process
now called hydrodynamisation. At weak coupling the very first moments after a
heavy ion collision is described by the colour-glass condensate framework, but
quickly thereafter the mean free path is long enough for kinetic theory to
become applicable. Recent simulations indicate thermalization in a time
[1], with the temperature at that time and
the shear viscosity divided by the entropy density. At (infinitely)
strong coupling it is possible to mimic heavy ion collisions by using
holography, which leads to a dual description of colliding gravitational shock
waves. The plasma formed hydrodynamises within a time of . A recent
extension found corrections to this result for finite values of the coupling,
when is bigger than the canonical value of , which leads to
[2]. Future improvements include the
inclusion of the effects of the running coupling constant in QCD.Comment: 7 pages, 4 figures, talk presented at Quark Matter 2017 (Chicago
Coupling constant corrections in a holographic model of heavy ion collisions
We initiate a holographic study of coupling-dependent heavy ion collisions by
analysing for the first time the effects of leading-order, inverse coupling
constant corrections. In the dual description, this amounts to colliding
gravitational shock waves in a theory with curvature-squared terms. We find
that at intermediate coupling, nuclei experience less stopping and have more
energy deposited near the lightcone. When the decreased coupling results in an
80% larger shear viscosity, the time at which hydrodynamics becomes a good
description of the plasma created from high energy collisions increases by 25%.
The hydrodynamic phase of the evolution starts with a wider rapidity profile
and smaller entropy.Comment: V2: 6 pages, 5 figures. Second-order coupling constant corrections
added. Version appeared in PR
Absence of a local rest frame in far from equilibrium quantum matter
In a collision of strongly coupled quantum matter we find that the dynamics
of the collision produces regions where a local rest frame cannot be defined
because the energy-momentum tensor does not have a real time-like eigenvector.
This effect is purely quantum mechanical, since for classical systems, a local
rest frame can always be defined. We study the relation with the null and weak
energy condition, which are violated in even larger regions, and compare with
previously known examples. While no pathologies or instabilities arise, it is
interesting that regions without a rest frame are possibly present in heavy ion
collisions.Comment: 5 pages, 4 figures; v2: fixed typo, v3: added references, matches
published versio
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
A fully dynamical simulation of central nuclear collisions
We present a fully dynamical simulation of central nuclear collisions around
mid-rapidity at LHC energies. Unlike previous treatments, we simulate all
phases of the collision, including the equilibration of the system. For the
simulation, we use numerical relativity solutions to AdS/CFT for the
pre-equilibrium stage, viscous hydrodynamics for the plasma equilibrium stage
and kinetic theory for the low density hadronic stage. Our pre-equilibrium
stage provides initial conditions for hydrodynamics, resulting in sizable
radial flow. The resulting light particle spectra reproduce the measurements
from the ALICE experiment at all transverse momenta.Comment: 4 pages, 6 figures; v2: minor changes; v3: major changes, including a
new section comparing to free streaming, matches published versio
Universal hydrodynamic flow in holographic planar shock collisions
We study the collision of planar shock waves in AdS as a function of
shock profile. In the dual field theory the shock waves describe planar sheets
of energy whose collision results in the formation of a plasma which behaves
hydrodynamically at late times. We find that the post-collision stress tensor
near the light cone exhibits transient non-universal behavior which depends on
both the shock width and the precise functional form of the shock profile.
However, over a large range of shock widths, including those which yield
qualitative different behavior near the future light cone, and for different
shock profiles, we find universal behavior in the subsequent hydrodynamic
evolution. Additionally, we compute the rapidity distribution of produced
particles and find it to be well described by a Gaussian.Comment: 23 pages, 15 figures, published versio
Saturation of the Quantum Null Energy Condition in Far-From-Equilibrium Systems
The Quantum Null Energy Condition (QNEC) is a new local energy condition that
a general Quantum Field Theory (QFT) is believed to satisfy, relating the
classical null energy condition (NEC) to the second functional derivative of
the entanglement entropy in the corresponding null direction. We present the
first series of explicit computations of QNEC in a strongly coupled QFT, using
holography. We consider the vacuum, thermal equilibrium, a homogeneous
far-from-equilibrium quench as well as a colliding system that violates NEC.
For vacuum and the thermal phase QNEC is always weaker than NEC. While for the
homogeneous quench QNEC is satisfied with a finite gap, we find the interesting
result that the colliding system can saturate QNEC, depending on the null
direction.Comment: 5 pages, 5 figure
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