Estimating initial state and quark-gluon plasma medium properties using a hybrid model with nucleon substructure calibrated to $p$-Pb and Pb-Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV

TL;DR

This paper develops a hybrid hydrodynamic and microscopic model to analyze quark-gluon plasma properties in small and large collision systems at high energies, calibrated with Bayesian inference to experimental data.

Contribution

It introduces a unified model with nucleon substructure and Bayesian calibration to extract initial state and medium properties of QGP in p-Pb and Pb-Pb collisions.

Findings

Supports hydrodynamic flow in small systems at high energies

Flow develops at sub-proton length scales

Provides posterior estimates of QGP transport coefficients

Abstract

We posit a unified hydrodynamic and microscopic description of the quark-gluon plasma (QGP) produced in ultrarelativistic $p$-Pb and Pb-Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV and evaluate our assertion using Bayesian inference. Specifically, we model the dynamics of both collision systems using initial conditions with parametric nucleon substructure, a pre-equilibrium free-streaming stage, event-by-event viscous hydrodynamics, and a microscopic hadronic afterburner. Free parameters of the model which describe the initial state and QGP medium are then simultaneously calibrated to fit charged particle yields, mean $p_T$, and flow cumulants. We argue that the global agreement of the calibrated model with the experimental data strongly supports the existence of hydrodynamic flow in small collision systems at ultrarelativistic energies, and that the flow produced develops at length scales smaller than a single proton. Posterior estimates for the model's input parameters are obtained, and new insights into the temperature dependence of the QGP transport coefficients and event-by-event structure of the proton are discussed.