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Extracting the speed of sound in quark-gluon plasma with ultrarelativistic lead-lead collisions at the LHC (2401.06896v3)

Published 12 Jan 2024 in nucl-ex and hep-ex

Abstract: Ultrarelativistic nuclear collisions create a strongly interacting state of hot and dense quark-gluon matter that exhibits a remarkable collective flow behavior with minimal viscous dissipation. To gain deeper insights into its intrinsic nature and fundamental degrees of freedom, we determine the speed of sound in an extended volume of quark-gluon plasma using lead-lead (PbPb) collisions at a center-of-mass energy per nucleon pair of 5.02 TeV. The data were recorded by the CMS experiment at the CERN LHC and correspond to an integrated luminosity of 0.607 nb${-1}$. The measurement is performed by studying the multiplicity dependence of the average transverse momentum of charged particles emitted in head-on PbPb collisions. Our findings reveal that the speed of sound in this matter is nearly half the speed of light, with a squared value of 0.241 $\pm$ 0.002 (stat) $\pm$ 0.016 (syst) in natural units. The effective medium temperature, estimated using the mean transverse momentum, is 219 $\pm$ 8 (syst) MeV. The measured squared speed of sound at this temperature aligns precisely with predictions from lattice quantum chromodynamic (QCD) calculations. This result provides a stringent constraint on the equation of state of the created medium and direct evidence for a deconfined QCD phase being attained in relativistic nuclear collisions.

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Citations (8)
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Summary

  • The paper determines a squared speed of sound value of 0.241, revealing nearly half the speed of light propagation in QGP.
  • It employs innovative hydrodynamic modeling and transverse momentum analysis from billions of Pb–Pb collision events.
  • The results closely match lattice QCD predictions, providing stringent constraints on theoretical models of deconfined matter.

Extraction of the Speed of Sound in Quark-Gluon Plasma from Ultrarrelativistic Lead-Lead Collisions at the LHC

The paper examines the propagation of sound in the quark-gluon plasma (QGP) created in ultrarelativistic lead-lead (Pb-Pb) collisions at the Large Hadron Collider (LHC). This exotic state of matter is characterized by its creation at extreme temperatures, where quarks and gluons are no longer confined within hadrons but exist as a deconfined, strongly interacting medium. The central aim of the paper is to measure the speed of sound in this medium, which serves as an essential parameter for understanding its equation of state (EoS).

Background

Quark-gluon plasma is a fundamental phase predicted by quantum chromodynamics (QCD) to occur at high energy densities, a condition achieved in heavy-ion collisions. The EoS of a system, providing a relation between pressure and energy density, serves as a critical tool in defining the properties of the QGP, particularly how it responds to perturbations such as sound waves. The speed of sound, csc_s, is intrinsically linked to the EoS as it describes how pressure changes with respect to energy density (cs2=∂P∂εc_s^2 = \frac{\partial P}{\partial \varepsilon}). This paper leverages experimental data to extract the speed of sound, aiming to verify and potentially constrain theoretical models, such as lattice QCD predictions.

Methodology

The paper utilizes collision data at a center-of-mass energy of 5.02 TeV per nucleon pair, recorded by the CMS experiment at the CERN LHC. By analyzing the dependence of the average transverse momentum (⟨pT⟩\langle p_T \rangle) of charged particles on the charged-particle multiplicity in ultra-central collisions, the research provides a unique approach to derive the speed of sound in QGP.

The analysis involves:

  • Data Collection: 4.27 billion minimum bias events corresponding to an integrated luminosity of 0.607 nb−1^{-1} were analyzed.
  • Estimation Techniques: Using developments in hydrodynamic modeling, the research introduces a robust, innovative methodology by examining ultra-central collisions—where nuclei collide almost frontally—resulting in high precision in determining the speed of sound.
  • Extrapolation and Fitting: The paper fits the transverse momentum spectra using a Hagedorn function, extrapolating for full coverage, and employs a functional form derived from hydrodynamic simulations to fit the ⟨pT⟩\langle p_T \rangle as a function of normalized multiplicity, NchN_{\text{ch}}.

Results

The extracted squared speed of sound, cs2=0.241±0.002stat±0.016systc_s^2 = 0.241 \pm 0.002_{\text{stat}} \pm 0.016_{\text{syst}}, implies that the sound waves propagate at nearly half the speed of light in natural units. The medium temperature is estimated at 219±8syst MeV219 \pm 8_{\text{syst}} \text{ MeV}, aligning closely with lattice QCD predictions. This alignment provides significant confirmation of the deconfined phase of QCD matter at these temperature levels.

Implications

The precision of this measurement provides stringent constraints on theoretical models describing the QGP. The comparisons with lattice QCD reinforce the predicted behaviors of QGP and confirm that these experiments achieve the necessary conditions for deconfinement and collectivity characteristic of QCD matter.

The success of these measurements holds potential for future experiments at the LHC and similar facilities, indicating possibilities for even tighter constraints on QGP properties. These findings contribute significantly to our understanding of early universe conditions, where a similar QGP phase would have existed just microseconds after the Big Bang.

Conclusion

This paper exemplifies a significant advance in the field of high-energy nuclear physics by providing a precise determination of the speed of sound in QGP. The close agreement of the experimental results with theoretical predictions underscores the robustness of both experimental techniques and theoretical models, enhancing our comprehension of matter under extreme conditions.

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  1. Origin of the local minimum in normalized mean transverse momentum versus multiplicity
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Authors (1)

  1. CMS Collaboration
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