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Higher-Order Fluctuations: Unveiling the Final Frontier of QCD at the LHC with ALICE

Ilya Fokin

TL;DR

This work tests QCD thermodynamics by measuring net-proton cumulants up to sixth order in ALICE pp and Pb–Pb collisions at the LHC and interpreting them with a canonical-ensemble framework that includes volume fluctuations and long-range baryon-number conservation. The results show good agreement with a conservation-driven description and no signs of critical behavior at $\mu_B\approx0$, consistent with LQCD expectations for higher-order ratios such as $\chi_4^B/\chi_2^B$ and $\chi_6^B/\chi_2^B$, while HRG would predict unity for these ratios. By linking measurements to the full QCD partition function, the study demonstrates a path to test fundamental QCD predictions in high-energy collisions, with ALICE 3 poised to extend the reach to sixth and higher-order cumulants and larger acceptances.

Abstract

Lattice QCD (LQCD) calculations predict that chiral symmetry is restored in a smooth crossover transition between a quark-gluon plasma and a hadron resonance gas (HRG) at vanishing net-baryon density, a condition realized in heavy-ion collisions at the LHC. In this regime, the net-baryon number cumulants computed using the HRG and LQCD partition functions are in good agreement up to third order. However, starting with the fourth-order cumulants, the LQCD results are significantly lower than the corresponding HRG results. This offers a unique opportunity to experimentally verify the full QCD partition function by measuring the fourth-order cumulants of the net-proton number distributions. We present net-proton number cumulants up to sixth order in proton-proton collisions at top LHC energy recorded by ALICE to search for effects of a possible chiral phase transition in this small system. An extension of the net-proton measurements in Pb-Pb collisions to fourth order is also presented. In addition to providing experimental access to the full QCD partition function, these measurements will, for the first time, allow to distinguish between different mechanisms of baryon production.

Higher-Order Fluctuations: Unveiling the Final Frontier of QCD at the LHC with ALICE

TL;DR

This work tests QCD thermodynamics by measuring net-proton cumulants up to sixth order in ALICE pp and Pb–Pb collisions at the LHC and interpreting them with a canonical-ensemble framework that includes volume fluctuations and long-range baryon-number conservation. The results show good agreement with a conservation-driven description and no signs of critical behavior at , consistent with LQCD expectations for higher-order ratios such as and , while HRG would predict unity for these ratios. By linking measurements to the full QCD partition function, the study demonstrates a path to test fundamental QCD predictions in high-energy collisions, with ALICE 3 poised to extend the reach to sixth and higher-order cumulants and larger acceptances.

Abstract

Lattice QCD (LQCD) calculations predict that chiral symmetry is restored in a smooth crossover transition between a quark-gluon plasma and a hadron resonance gas (HRG) at vanishing net-baryon density, a condition realized in heavy-ion collisions at the LHC. In this regime, the net-baryon number cumulants computed using the HRG and LQCD partition functions are in good agreement up to third order. However, starting with the fourth-order cumulants, the LQCD results are significantly lower than the corresponding HRG results. This offers a unique opportunity to experimentally verify the full QCD partition function by measuring the fourth-order cumulants of the net-proton number distributions. We present net-proton number cumulants up to sixth order in proton-proton collisions at top LHC energy recorded by ALICE to search for effects of a possible chiral phase transition in this small system. An extension of the net-proton measurements in Pb-Pb collisions to fourth order is also presented. In addition to providing experimental access to the full QCD partition function, these measurements will, for the first time, allow to distinguish between different mechanisms of baryon production.

Paper Structure

This paper contains 5 sections, 3 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Analysis strategy
  3. Net-proton fluctuations in proton-proton collisions
  4. Net-proton fluctuations in Pb--Pb collisions
  5. Conclusions

Figures (2)

  • Figure 1: Net-proton number cumulant ratios $\kappa_2(\mathrm{p} - \overline{\mathrm{p}})/\langle \mathrm{p} + \overline{\mathrm{p}} \rangle$ (left), $\kappa_4(\mathrm{p} - \overline{\mathrm{p}})/\kappa_2(\mathrm{p} - \overline{\mathrm{p}})$ (center) and $\kappa_6(\mathrm{p} - \overline{\mathrm{p}})/\kappa_2(\mathrm{p} - \overline{\mathrm{p}})$ (right) in pp collisions at $13\tera \electronvolt$ as a function of the charged particle multiplicity density at midrapidity $\langle \mathrm{d}N_\mathrm{ch}/\mathrm{d}\eta \rangle_{\mathrm{|\eta| < 0.5}}$.
  • Figure 2: Net-proton number cumulant ratios $\mathrm{\kappa}_2/\langle~\mathrm{p}~+~\overline{\mathrm{p}}~\rangle$ (left), $\mathrm{\kappa}_3/\mathrm{\kappa}_2$ (center) and $\mathrm{\kappa}_4/\mathrm{\kappa}_2$ (right) in Pb--Pb collisions at $5.02\tera \electronvolt$ as a function of the pseudorapidity acceptance $\Delta \eta$ (top) and centrality (bottom).