Extracting the speed of sound of QCD from transverse momentum fluctuations

TL;DR

This paper determines the speed of sound in quark-gluon plasma using transverse momentum fluctuations from ATLAS data, correcting for detection biases and noise, and finds results consistent with lattice QCD predictions.

Contribution

The study introduces a novel method to extract the speed of sound from experimental data, accounting for detection biases and noise, and confirms theoretical predictions.

Findings

Measured $c_s/c=0.496\, ext{at}\,T=221 ext{MeV}$

Corrected for low-$p_T$ detection bias using variance and $v_0(p_T)$ data

Achieved results in agreement with lattice QCD calculations

Abstract

We extract the speed of sound ($c_s$) in the quark-gluon plasma from ATLAS data on the probability distribution of the transverse momentum per particle, $[p_T]$, in ultra-central Pb+Pb collisions. With an ideal detector, $c_s$ can be inferred from the rise of the mean $[p_T]$ with the collision multiplicity. In practice, however, low-$p_T$ particles escape detection, which biases the analysis. We show how to correct for this bias by using data on the variance of $[p_T]$, as well as information from the recently-measured $v_0(p_T)$. We also introduce a systematic method for deblurring the noise from the hadronization process. Assuming that the size of the quark-gluon plasma is independent of the hadron multiplicity in collisions at zero impact parameter, which is the scenario preferred both by high-energy QCD and heavy-ion data, we obtain $c_s/c=0.496\pm 0.008$ at temperature $T=221\pm 13$ MeV, in perfect agreement with first-principles calculations from lattice QCD.