Master Thesis defense by Yifan Zhang

Title: Studying the speed of sound in quark-gluon plasma with heavy ion collisions

Abstract: Quark-Gluon Plasma (QGP) is a unique state of strongly interacting matter that can only exist at extremely high temperatures and densities. To experimentally study the Quark-Gluon Plasma (QGP), it can now be produced through relativistic heavy-ion collisions. In the Large Hadron Collider (LHC), QGP have been recreated using Pb-Pb collisions at the ultra-relativistic energies. Previous studies have shown that QGP behaves like a fluid. To understand its properties, specifically the equation of state of a relativistic fluid, investigating the speed of sound is crucial.

In this thesis, the analysis of the mean transverse momentum distribution with respect to the number of outgoing charged particles in ultra-central Pb-Pb collisions at $\sqrt{s_\text{NN}}$ = 5.02 TeV is used to determine the speed of sound in QGP. Additionally, the cumulant method is employed to analyze the transverse momentum fluctuation from higher orders, providing important insights into the study of the speed of sound. The study concludes that the transverse momentum fluctuation aligns well with hydrodynamic model calculations and determines a range for the speed of sound that is similar to the hydrodynamic model predictions. Finally, it is found that sound has a very large speed in the QGP, which is close to half the speed of light.