Extracting the QCD speed of sound in the presence of quantum fluctuations

The equation of state of strongly interacting matter plays a %is one of the central quantities characterizing central role in characterizing quark--gluon plasma created in relativistic heavy-ion collisions. It has recently been recognized that the QCD speed of sound is responsible for the linear correlations between the mean transverse momentum and charged-particle multiplicity in ultra-central collisions. In realistic collisions, however, this thermodynamic correlation is inevitably biased by quantum-initiated event-by-event fluctuations. In this work, we present a novel statistical framework to correct this bias by leveraging the Gaussianity condition of the quantum fluctuations. Furthermore, we demonstrate that this approach can also serve as a probe of thermalization. Validated by the event-by-event hydrodynamic simulations, the extracted speed of sound in our framework is successfully consistent with the predictions from lattice QCD, from large to small collision systems.