Browsing by Author "Bass, Steffen A"

I develop and apply a Bayesian method for quantitatively estimating properties of the quark-gluon plasma (QGP), an extremely hot and dense state of fluid-like matter created in relativistic heavy-ion collisions.

The QGP cannot be directly observed---it is extraordinarily tiny and ephemeral, about 10^(-14) meters in size and living 10^(-23) seconds before freezing into discrete particles---but it can be indirectly characterized by matching the output of a computational collision model to experimental observations.

The model, which takes the QGP properties of interest as input parameters, is calibrated to fit the experimental data, thereby extracting a posterior probability distribution for the parameters.

In this dissertation, I construct a specific computational model of heavy-ion collisions and formulate the Bayesian parameter estimation method, which is based on general statistical techniques.

I then apply these tools to estimate fundamental QGP properties, including its key transport coefficients and characteristics of the initial state of heavy-ion collisions.

Perhaps most notably, I report the most precise estimate to date of the temperature-dependent specific shear viscosity eta/s, the measurement of which is a primary goal of heavy-ion physics.

The estimated minimum value is eta/s = 0.085(-0.025)(+0.026) (posterior median and 90% uncertainty), remarkably close to the conjectured lower bound of 1/4pi =~ 0.08.

The analysis also shows that eta/s likely increases slowly as a function of temperature.

Other estimated quantities include the temperature-dependent bulk viscosity zeta/s, the scaling of initial state entropy deposition, and the duration of the pre-equilibrium stage that precedes QGP formation.

Heavy flavor observables provide valuable information on the properties of the hot and dense Quark-Gluon Plasma (QGP) created in ultra-relativistic heavy-ion collisions.

Previous study has made significant progress regarding the heavy quark in-medium interaction, energy loss and collective behaviors. Various theoretical models are developed to describe the evolution of heavy quarks in heavy-ion collisions, but also show limited performance as they experience challenges to simultaneously describe all the experimental data.

In this thesis, I present a state-of-the-art Bayesian model-to-data analysis to calibrate a heavy quark evolution model on the experimental data at different collision systems and different energies: the heavy quark evolution model incorporates an improved Langevin dynamics for heavy quarks with an event-by-event viscous hydrodynamical model for the expanding QGP medium, and considers both heavy quark collisional and radiative energy loss. By applying the Bayesian analysis to such a modularized framework, the heavy quark evolution model is able to describe the heavy flavor observables in multiple collision system and make prediction of unseen observables. In addition, the estimated heavy quark diffusion coefficient shows a strong positive temperature dependence and strong interaction around the critical temperature.

Finally, by comparing the transport coefficients estimated by various theoretical approaches, I have quantitatively evaluated the contribution from different sources of deviation, which can provide a reference for the theoretical uncertainties regarding the heavy quark transport coefficients.

Ultrarelativistic heavy-ion collisions at the Relativistic Heavy-Ion Collider (RHIC) are thought to have produced a state of matter called the Quark-Gluon-Plasma (QGP). The QGP forms when nuclear matter governed by Quantum Chromodynamics (QCD) reaches a temperature and baryochemical potential necessary to achieve the transition of hadrons (bound states of quarks and gluons) to {it deconfined} quarks and gluons. Such conditions have been achieved at RHIC, and the resulting QGP created exhibits properties of a near perfect fluid. In particular, strong evidence shows that the QGP exhibits a very small shear viscosity to entropy density ratio &eta/s, near the lower bound predicted for that quantity by Anti-deSitter space/Conformal Field Theory (AdS/CFT) methods of &eta/s = $hbar$/ 4 &pi $k_B$ where $hbar$ is Planck's constant and $k_B$ is Boltzmann's constant. As the produced matter expands and cools, it evolves through a phase described by a hadron gas with rapidly increasing $eta/s$.

This thesis presents robust calculations of $eta/s$ for hadronic and partonic media as a function of temperature using the Green-Kubo formalism. An analysis is performed for the behavior of $eta/s$ to mimic situations of the hadronic media at RHIC evolving out of chemical equilibrium, and systematic uncertainties are assessed for our method. In addition, preliminary results are presented for the bulk viscosity to entropy density ratio $zeta/s$, whose behavior is not well-known in a relativistic heavy ion collisions. The diffusion coefficient for baryon number is investigated, and an algorithm is presented to improve upon the previous work of investigation of heavy quark diffusion in a thermal QGP.

By combining the results of my investigations for $eta/s$ from our microscopic transport models with what is currently known from the experimental results on elliptic flow from RHIC, I find that the trajectory of $eta/s$ in a heavy ion collision has a rich structure, especially near the deconfinement transition temperature $T_c$. I have helped quantify the viscous hadronic effects to enable investigators to constrain the value of $eta/s$ for the QGP created at RHIC.

Heavy flavor hadrons serve as valuable probes of the transport properties of the quark-gluon plasma (QGP) created in relativistic heavy-ion collisions. In this dissertation, we introduce a comprehensive framework that describes the full-time evolution of heavy flavor in heavy-ion collisions, including its initial production, in-medium evolution inside the QGP matter, hadronization process from heavy quarks to their respective mesonic bound states and the subsequent interactions between heavy mesons and the hadron gas.

The in-medium energy loss of heavy quarks is studied within the framework of a Langevin equation coupled to hydrodynamic models that simulate the space-time evolution of the hot and dense QGP matter. We improve the classical Langevin approach such that, apart from quasi-elastic scatterings between heavy quarks and the medium background, radiative energy loss is incorporated as well by treating gluon radiation as a recoil force term. The subsequent hadronization of emitted heavy quarks is simulated via a hybrid fragmentation plus recombination model. The propagation of produced heavy mesons in the hadronic phase is described using the ultra-relativistic quantum molecular dynamics (UrQMD) model. Our calculation shows that while collisional energy loss dominates the heavy quark motion inside the QGP in the low transverse momentum (pT) regime, contributions from gluon radiation are found to be significant at high pT. The recombination mechanism is important for the heavy flavor meson production at intermediate energies. The hadronic final state interactions further enhance the suppression and the collective flow of heavy mesons we observe. Within our newly developed framework, we present numerical results for the nuclear modification and the elliptic flow of D mesons, which are consistent with measurements at both the CERN Large Hadron Collider (LHC) and the BNL Relativistic Heavy-Ion Collider (RHIC); predictions for B mesons are also provided.

In addition, various transport properties of heavy quarks are investigated within our numerical framework, such as the thermalization process of heavy quarks inside the QGP, and how the initial configuration of the QGP as well as its properties affect the final state spectra and the elliptic flow of heavy mesons and their decay electrons. The effects of initial state fluctuations in heavy-ion collisions are also studied and found to enhance the heavy quark energy loss in a (2+1)-dimensional boost invariant scenario. Furthermore, a new set of observables -- heavy-flavor-tagged angular correlation functions -- are explored and found to be potential candidates for distinguishing different energy loss mechanisms of heavy quarks inside the QGP.

Dynamical models based on relativistic fluid dynamics provide a powerful tool to extract the properties of the strongly-coupled quark-gluon plasma (QGP) produced in the first ${\sim}10^{-23}$ seconds of an ultrarelativistic nuclear collision. The largest source of uncertainty in these model-to-data extractions is the choice of theoretical initial conditions used to model the distribution of energy or entropy at the hydrodynamic starting time.

Descriptions of the QGP initial conditions are generally improved through iterative cycles of testing and refinement. Individual models are compared to experimental data; the worst models are discarded and best models retained. Consequently, successful traits (assumptions) are passed on to subsequent generations of the theoretical landscape. This so-called bottom-up approach correspondingly describes a form of theoretical trial and error, where each trial proposes a first principles solution to the problem at hand.

A natural complement to this strategy is to employ a top-down or data driven approach which is able to reverse engineer properties of the initial conditions from the constraints imposed by the experimental data. In this dissertation, I motivate and develop a parametric model for initial energy and entropy deposition in ultrarelativistic nuclear collisions which is based on a family of functions known as the generalized means. The ansatz closely mimics the variability of first-principle calculations and hence serves as a reasonable parametric form for exploring QGP energy and entropy deposition assuming imperfect knowledge of the complex physical processes which lead to its creation.

With the parametric model in hand, I explore broad implications of the proposed ansatz using recently adapted Bayesian methods to simultaneously constrain properties of the initial conditions and QGP medium using experimental data from the Large Hadron Collider. These analyses show that the QGP initial conditions are highly constrained by available measurements and provide evidence of a unified hydrodynamic description of small and large nuclear collision systems.

Quark-gluon plasma is a deconfined state of quarks and gluons, which can be produced in heavy ion collisions. Highly energetic partons, which are generated at the early time of a heavy ion collision, propagate through the plasma and lose energy by interacting with the plasma. The energy loss of even very energetic partons can be affected by non-perturbative effects from the strongly-coupled plasma. Hard interactions --- those with large momentum transfer between the energetic parton and the plasma --- are expected to have smaller non-perturbative effects, or even be accessible perturbatively, as a consequence of the running of the QCD coupling. On the other hand, ``soft'' parton-plasma interactions with small momentum transfer are expected to suffer the largest non-perturbative effects.

A stochastic treatment of these soft interactions of energetic partons provides an alternative approach to account for non-perturbative effects --- an approach that is agnostic to the strongly- or weakly-coupled nature of the underlying deconfined plasma. The dynamical details of the large number of soft interactions are encoded in a small number of transport coefficients. From a practical point of view, a stochastic description of a large number of soft interactions can also be more efficient numerically than a rate-based approach.

We present the first numerical implementation of a hard-soft factorized parton energy loss model. We first test and validate this factorization of parton energy loss in the weak coupling regime for a static medium. We introduce a dimensionless scale to quantify the kinematic range for which soft interactions can be described accurately with a stochastic approach. We use this scale to discuss a hard-soft factorization model for a strongly-coupled quark-gluon plasma, relevant for phenomenological applications in heavy ion collisions.

We perform a systematic data-driven extraction of the light parton transport properties in a quark-gluon plasma based on the hard-soft factorized model. As in this model, the larger number of soft interactions are described stochastically in terms of a small number of transport coefficients, these soft transport coefficients can capture non-perturbative effects, agnostic to the strongly- or weakly-coupled nature of the underlying deconfined plasma.

We constrain the temperature dependence of these soft transport coefficients by performing a Bayesian model-to-data comparison with jet measurements from RHIC and LHC, allowing us to better understand the non-perturbative effects suffered by soft interactions in heavy ion collisions.

The quark-gluon plasma (QGP) is one of the most interesting forms of matter providing us with insight on quantum chromodynamics (QCD) and the early universe. It is believed that the heavy-ion collision experiments at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) have created the QGP medium by colliding two heavy nuclei at nearly the speed of light. Since the collision happens really fast, we can not observe the QGP directly. Instead, we look at the hundreds or even thousands of final hadrons coming out of the collision. In particular, jet and heavy flavor observables are excellent probes of the transport properties of such a medium. On the theoretical side, computational models are essential to make the connections between the final observables and the plasma. Previously studies have em-ployed a comprehensive multistage modeling approach of both the probes and the medium. In this dissertation, heavy quarks are investigated as probes of the QGP. First, the framework that describes the evolution of both soft and hard particles during the collision are discussed, which include initial condition, hydrodynamical expansion, parton transport, hadronization, and hadronic rescattering. It has recently been organized into the Jet Energy-loss Tomography with a Statistically and Computationally Advanced Program Envelope (JETSCAPE) framework, which allows people to study heavy-ion collision in a more systematic manner. To study the energy loss of hard partons inside the QGP medium, the linear Boltzmann transport model (LBT) and the MATTER formalism are combined and have achieved a simultaneous description of both charged hadron, D meson, and inclusive jet observables. To further extract the transport coefficients, a Bayesian analysis is conducted which constrains the parameters in the transport models.

Heavy-flavor particles are excellent probes of the properties of the hot and dense nuclear medium created in the relativistic heavy-ion collisions. Heavy-flavor transport coefficients in the quark-gluon plasma (QGP) stage of the collisions are particularly interesting, as they contain important information on the strong interaction at finite temperatures. Studying the heavy-flavor evolution in a dynamically evolving medium requires a comprehensive multi-stage modeling approach of both the medium and the probes, with an accurate implementation of the physical ingredients to be tested. For this purpose, I have developed a new partonic transport model (Linear-Boltzmann-plus-Diffusion-Transport-Model) LIDO and applied it to heavy quark propagation inside a QGP. The model has an improved implementation of parton in-medium bremsstrahlung and a flexible treatment of the probe-medium interactions, combining both large angle scatterings and diffusion processes. The model is then coupled to a high-energy event-generator, a hydrodynamic medium evolution and a hadronic transport model. Finally, applying a Bayesian analysis, I extract the heavy quark transport coefficients from a model-to-data comparison. The results, with uncertainty quantification, are found to be consistent with earlier extraction of the light-quark transport coefficients at high momentum and with first-principle calculations of the heavy-flavor diffusion constant at low momentum.