# Browsing by Author "Bass, Steffen A"

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Item Open Access Bayesian Parameter Estimation for Relativistic Heavy-ion Collisions(2018) Bernhard, JonahI 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.

Item Open Access Coupled Transport Equations for Quarkonium Production in Heavy Ion Collisions(Proceedings of Science, 2020-09-11) Yao, Xiaojun; Ke, Weiyao; Xu, Yingru; Bass, Steffen A; Müller, BerndtMotivated by recent applications of the open quantum system formalism to understand quarkonium transport in the quark-gluon plasma, we develop a set of coupled Boltzmann equations for open heavy quark-antiquark pairs and quarkonia. Our approach keeps track of the correlation between the heavy quark-antiquark pair from quarkonium dissociation and thus is able to account for both uncorrelated and correlated recombination. By solving the coupled Boltzmann equations for current heavy ion collision experiments, we find correlated recombination is crucial to describe the data of bottomonia nuclear modification factors. To further test the importance of correlated recombination in experiments, we propose a new observable: $\frac{R_{AA}[\chi_b(1P)]}{R_{AA}[\Upsilon(2S)]}$. Future measurements of this ratio will help distinguish calculations with and without correlated recombination.Item Open Access Initial Conditions of Bulk Matter in Ultrarelativistic Nuclear Collisions(2019) Moreland, John ScottDynamical 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.

Item Open Access MULTI STAGE HEAVY QUARK TRANSPORT IN ULTRA-RELATIVISTIC HEAVY-ION COLLISIONS(2022) Fan, WenkaiThe 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.

Item Open Access Partonic Transport Model Application to Heavy Flavor(2019) Ke, WeiyaoHeavy-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.