Bayesian parameter estimation for relativistic heavy-ion collisions
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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 η/s, the measurement of which is a primary goal of heavy-ion physics. The estimated minimum value is η/s = 0.085_-0.025^+0.026 (posterior median and 90 remarkably close to the conjectured lower bound of 1/4π≃ 0.08. The analysis also shows that η/s likely increases slowly as a function of temperature. Other estimated quantities include the temperature-dependent bulk viscosity ζ/s, the scaling of initial state entropy deposition, and the duration of the pre-equilibrium stage that precedes QGP formation.
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