Hydrodynamics of Unitary Fermi Gases

In this thesis we present fully nonlinear dissipative hydrodynamics simulations using the lattice Boltzmann method which we have developed to study hydrodynamics in cold atomic gases. We motivate and derive the specifics of the lattice Boltzmann implementation for this system, and then use our simulations to study collective oscillations in two-dimensional Fermi gases. We show that all dominant frequencies and damping rates of the breathing and quadrupole oscillatory modes agree quantitatively with analytic solutions in the hydrodynamic limit for a gas with ideal equation of state in a harmonic trap. We suggest improvements to our numerical methods which we expect to improve the quantitative agreement of the quadrupole mode frequency with the expected value outside of the hydrodynamic limit. We also suggest techniques for extracting the non-dominant (non-hydrodynamic) damping rate of the quadrupole mode in the hydrodynamic regime with higher precision. Additionally, we propose that the observed non-hydrodynamic damping of the quadrupole mode would be interesting to study experimentally for applications to collective oscillations in the quark-gluon plasma

Jets as a probe of the quark-gluon plasma

The suppression and modification of high-energy objects, like jets, in heavy-ion collisions provide an important window to access the degrees of freedom of the quark-gluon plasma on different length scales. Despite increasingly precise and differential measurements of the properties of jets in heavy-ion collisions, however, it has remained challenging to use jets to make unambiguous and model-independent statements about the quark-gluon plasma. Here I will give a personal take on some origins of these challenges, including the difficulty of modelling and biases from jet selection that obfuscate the direct interpretation of jet modification measurements. I will discuss a few model studies that have helped to disentangle the source of non-intuitive effects in measurements, and finally highlight data-driven approaches as an interesting opportunity toward studying the quark-gluon plasma in a model-independent way using jets.Comment: 9 pages, 0 figures; proceedings of Hard Probes 202

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

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

Thermalization of a jet wake in QCD kinetic theory

We study the energy deposition of a high-momentum parton traveling through a Quark-Gluon Plasma using QCD kinetic theory. We show that the energy is first transported to the soft sector by collinear cascade and then isotropised by elastic scatterings. Remarkably, we find that the jet wake can be well described by a thermal distribution function with angle-dependent temperature. This could be used for effective phenomenological descriptions of jet thermalization in realistic heavy-ion collision simulations.Comment: 6 pages, 7 figures, proceedings of the 11th International Conference on Hard and Electromagnetic Probes of High Energy Nuclear Collisions (Hard Probes 2023

Sorting out quenched jets

We introduce a new 'quantile' analysis strategy to study the modification of jets as they traverse through a droplet of quark-gluon plasma. To date, most jet modification studies have been based on comparing the jet properties measured in heavy-ion collisions to a proton-proton baseline at the same reconstructed jet transverse momentum ($p_T$). It is well known, however, that the quenching of jets from their interaction with the medium leads to a migration of jets from higher to lower $p_T$, making it challenging to directly infer the degree and mechanism of jet energy loss. Our proposed quantile matching procedure is inspired by (but not reliant on) the approximate monotonicity of energy loss in the jet $p_T$. In this strategy, jets in heavy-ion collisions ordered by $p_T$ are viewed as modified versions of the same number of highest-energy jets in proton-proton collisions, and the fractional energy loss as a function of jet $p_T$ is a natural observable ($Q_{\rm AA}$). Furthermore, despite non-monotonic fluctuations in the energy loss, we use an event generator to validate the strong correlation between the $p_T$ of the parton that initiates a heavy-ion jet and the $p_T$ of the vacuum jet which corresponds to it via the quantile procedure ($p_T^{\rm quant}$). We demonstrate that this strategy both provides a complementary way to study jet modification and mitigates the effect of $p_T$ migration in heavy-ion collisions.info:eu-repo/semantics/publishedVersio