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arXiv · 2603.09647

Extracting the speed of sound of QCD from transverse momentum fluctuations

Mubarak Alqahtani·Tribhuban Parida·Jean-Yves Ollitrault
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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.

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Mubarak Alqahtani, Tribhuban Parida, Jean-Yves Ollitrault. 2026-03-10. Extracting the speed of sound of QCD from transverse momentum fluctuations. https://doi.org/10.1016/j.physletb.2026.140730

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