Wilke van der Schee

Fully Dynamical Simulation of Central Nuclear Collisions

We present a fully dynamical simulation of central nuclear collisions around midrapidity 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 anti-de Sitter space/conformal field theory for the preequilibrium stage, viscous hydrodynamics for the plasma equilibrium stage, and kinetic theory for the low-density hadronic stage. Our preequilibrium 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.

Strong Coupling Isotropization of Non-Abelian Plasmas Simplified

We study the isotropization of a homogeneous, strongly coupled, non-abelian plasma by means of its gravity dual. We compare the time evolution of a large number of initially anisotropic states as determined, on the one hand, by the full nonlinear Einsteins equations and, on the other, by the Einsteins equations linearized around the final equilibrium state. The linear approximation works remarkably well even for states that exhibit large anisotropies. For example, it predicts with a 20% accuracy the isotropization time, which is of the order of t(iso)≲1/T, with T the final equilibrium temperature. We comment on possible extensions to less symmetric situations.

Holographic isotropization linearized

A bstractThe holographic isotropization of a highly anisotropic, homogeneous, strongly coupled, non-Abelian plasma was simplified in ref. [1] by linearizing Einstein’s equations around the final, equilibrium state. This approximation reproduces the expectation value of the boundary stress tensor with a 20% accuracy. Here we elaborate on these results and extend them to observables that are directly sensitive to the bulk interior, focusing for simplicity on the entropy production on the event horizon. We also consider next-to-leading-order corrections and show that the leading terms alone provide a better description of the isotropization process for the states that are furthest from equilibrium.

Holographic thermalization with radial flow

Recently, a lot of effort has been put into describing the thermalization of the quark-gluon plasma using the gauge/gravity duality. In this context we here present a full numerical solution of the early far-from-equilibrium formation of the plasma, which is expanding radially in the transverse plane and is boost invariant along the collision axis. This can model the early stage of a head-on relativistic heavy ion collision. The resulting momentum distribution quickly reaches local equilibrium, after which they can be evolved using ordinary hydrodynamics. We comment on general implications for these hydrodynamic simulations, both for central and non-central collisions, and including fluctuations in the initial state.

Longitudinal coherence in a holographic model of asymmetric collisions.

As a model of the longitudinal structure in heavy ion collisions, we simulate gravitational shock wave collisions in anti-de Sitter space in which each shock is composed of multiple constituents. We find that all constituents act coherently, and their separation leaves no imprint on the resulting plasma, when this separation is ≲0.26/T_{hyd}, with T_{hyd} the temperature of the plasma at the time when hydrodynamics first becomes applicable. In particular, the center-of-mass of the plasma coincides with the center-of-mass of all the constituents participating in the collision, as opposed to the center-of-mass of the individual collisions. We discuss the implications for nucleus-nucleus and proton-nucleus collisions.

Early thermalization, hydrodynamics and energy loss in AdS/CFT

Gauge/gravity duality has provided unprecedented opportunities to study dynamics in certain strongly coupled gauge theories. This review aims to highlight several applications to heavy ion collisions including far-from-equilibrium dynamics, hydrodynamics and jet energy loss at strong coupling.

Weak and strong coupling equilibration in nonabelian gauge theories

A bstractWe present a direct comparison studying equilibration through kinetic theory at weak coupling and through holography at strong coupling in the same set-up. The set-up starts with a homogeneous thermal state, which then smoothly transitions through an out-of-equilibrium phase to an expanding system undergoing boost-invariant flow. This first apples-to-apples comparison of equilibration provides a benchmark for similar equilibration processes in heavy-ion collisions, where the equilibration mechanism is still under debate. We find that results at weak and strong coupling can be smoothly connected by simple, empirical power-laws for the viscosity, equilibration time and entropy production of the system.

New dynamical instability in asymptotically anti–de Sitter spacetime

We present fully dynamical solutions to Einstein-scalar theory in asymptotically anti–de Sitter spacetime with a scalar potential containing particularly rich physics. Depending on one parameter in the potential, we find an especially interesting regime, which exhibits a dynamically unstable black brane, already at zero momentum, while nevertheless having positive specific heat. We show this using the nonlinear dynamics and give a clear interpretation in terms of the spectrum of linearized perturbations. Our results translate directly to their dual strongly coupled nonconformal field theories, whereby we in particular provide two mechanisms to obtain equilibration times much larger than the inverse temperature.

Universal hydrodynamic flow in holographic planar shock collisions

A bstractWe study the collision of planar shock waves in AdS5 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.

Absence of a local rest frame in far from equilibrium quantum matter

A bstractIn 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.