The volume of the quark-gluon plasma (QGP) in ultra-central heavy-ion collisions is mostly fixed, while the total entropy of the system can vary significantly. The entropy density, which is proportional to the charged-particle multiplicity, can also vary within a few percent, leading to an increase in the temperature of the system and resulting in a rise of the average transverse momentum $(\langle p_{\mathrm{T}} \rangle)$ of the charged hadrons in the final state.

This study measures the $p_{\mathrm{T}}$ distributions of charged particles in ultra-central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02~\mathrm{TeV}$. Different centrality estimators are used to select ultra-central collisions based on the number of produced charged particles and the event transverse energy . The influence of charged particles produced in jets and its potential effects in the rise of the mean transverse momentum is tested by varying the pseudorapidity gap between the region used to define centrality and the region to perform the measurement.

The pronounced rise in the measured $\langle p_{\mathrm{T}} \rangle$ in ultra-central collisions is used to extract the speed of sound in the QGP. The above results are compared with predictions from Trajectum, a hydrodynamic-based model, and QCD-inspired Monte Carlo event generators such as PYTHIA and EPOS-LHC.