Why Quarks Look Free Above the Chiral Crossover Even When Confinement Persists

A longstanding puzzle in hot quantum chromodynamics has just received a crisp theoretical answer. On the lattice, certain fluctuation observables of conserved charges — baryon number, electric charge and strangeness — start behaving just above the chiral crossover temperature of roughly 155 MeV as if the quarks in the fireball were non-interacting particles. For two decades this was widely read as direct evidence that quarks had been liberated from confinement, a hallmark of the quark-gluon plasma. A new theoretical analysis published in The European Physical Journal C by L. Ya. Glozman argues that this interpretation is wrong, and explains precisely why the apparent paradox arises.

The resolution hinges on a subtle but crucial distinction between two kinds of correlation functions computed in lattice QCD. Mesonic propagators, which track how quark-antiquark pairs propagate in Euclidean time, show a dramatic rearrangement above the chiral crossover: they fall into multiplets organized by newly emergent chiral spin and SU(4) symmetries. These symmetries indicate that the propagating degrees of freedom are chirally symmetric, essentially massless quarks tied together into color singlets by the chromoelectric confining string. When these full QCD mesonic correlators are compared with correlators computed on the same lattice with free, non-interacting quarks — the so-called free quark gas that represents an idealized quark-gluon plasma — the two are qualitatively completely different, especially for the scalar and pseudoscalar channels.

The conserved charge densities tell a strikingly different story. Quantities such as the conserved quark number density cannot propagate in time, but they do propagate in spatial directions, and it is their spatial propagators that determine the fluctuation cumulatives measured on the lattice and accessible to heavy-ion experiments. When Glozman compared the full QCD spatial correlators of the conserved charge densities at a temperature of 380 MeV with the corresponding free quark loop correlators, the two essentially coincided. Unlike the mesonic correlators, the conserved charges simply do not see the confining force. The effect sets in surprisingly early: already at about 193 MeV, soon above the chiral crossover, the difference between the full QCD and free quark gas charge density correlators is insignificant, which explains why the cumulants approach their free quark gas values around 220 MeV.

This dichotomy is the core of the new interpretation. The mesonic propagators, sensitive to the confining infrared physics, prove that the medium above the crossover is still in a confining regime — there are no deconfined gluons, and only color singlets propagate in time. Meanwhile, the conserved charge fluctuations are blind to that same confining force and therefore mimic free quarks. Fluctuation cumulants, in other words, are simply not a sensitive diagnostic of deconfinement, and their free-quark-like behavior should not be taken as proof that the quark-gluon plasma has formed.

Far from being an exotic novelty, this situation has a well-understood analogue at zero temperature: quark-hadron duality in electron-positron annihilation. In the process e+e- going to hadrons, the vacuum is fully confining and only color-singlet hadrons are registered in detectors, yet the inclusive cross-section at invariant masses above about 2 GeV is accurately reproduced by a free quark loop plus small perturbative corrections. Nobody concludes from that agreement that quarks are deconfined in the vacuum; it merely means that this particular observable is insensitive to the confining infrared contributions. Glozman argues that the analogy is direct: mesonic propagators above the crossover are sensitive to confinement, while fluctuations of conserved charges are not, in exactly the same way that inclusive annihilation rates are dual to free quarks while exclusive observables are not.

The analysis also situates the result within a broader three-regime picture of the QCD phase diagram at small baryon chemical potential. Below the chiral crossover temperature of roughly 155 MeV lies the hadron gas, in which pressure, energy density and entropy density scale with the number of colors Nc to the zeroth power. Between the crossover and roughly three times that temperature lies the stringy fluid, whose thermodynamic quantities scale as Nc — the signature of a medium built from color singlets rather than deconfined gluons. Only at higher temperatures does the true quark-gluon plasma emerge, with its Stefan-Boltzmann-like Nc-squared scaling and T to the fourth power behavior, which lattice studies show sets in only above roughly 400 to 500 MeV. The smooth transition out of the stringy fluid is most likely centered near 300 MeV, the deconfinement temperature of the pure glue theory, a value also supported by center vortex percolation calculations and the Hagedorn temperature of the string description.

To make sense of what the stringy fluid actually is microscopically, the paper draws on large-Nc model calculations that are manifestly confining yet chirally symmetric. In such models, chiral symmetry restoration above the crossover happens because thermal excitations of quarks and antiquarks Pauli-block the quark levels that are required for the existence of the chiral condensate. The same Pauli blocking causes mesons to swell to several times their vacuum size — becoming infinitely large in the chiral limit. The resulting medium is a densely packed, highly collective system of overlapping color-singlet meson-like objects with a very small mean free path for their constituents, and with no deconfined gluons anywhere in it.

Within this picture, the free-quark-like fluctuations of conserved charges acquire a natural explanation. Because the color-singlet clusters overlap so strongly, the Pauli principle forces quarks to be exchanged between neighboring clusters. These interchanges make the quarks quasi-free for the purposes of conserved charge transport across space, even though no colored quark ever propagates freely through time. Only the color singlets, in which massless quarks and antiquarks remain connected by the confining chromoelectric string, propagate in time. The free-quark-loop behavior of the spatial charge density propagator is, in this view, the dual manifestation of quark interchange dynamics in the dense stringy fluid.

The interpretation resolves several otherwise conflicting lattice observations into a single coherent framework. The apparent contradiction between free-quark-like cumulants and the confining evidence from mesonic correlators disappears once the different infrared sensitivities of the two observables are recognized. It is likewise consistent with the pion spectral function, which shows clean pion and pion-prime peaks above the crossover, and with the bottomonium spectrum, which remains essentially unchanged from its vacuum form below about 300 MeV even as the states broaden with temperature.

The proposed picture of the stringy fluid remains a hypothesis to be tested and potentially falsified in lattice simulations, and the paper outlines concrete checks. The predicted several-fold swelling of pion-like excitations above the crossover can be probed with standard lattice techniques. More decisively, simulations of pressure, energy density and entropy density at Nc equal to 5 and 7, performed exactly as they were for three colors, can settle whether the regime above the crossover scales as Nc — as expected for a fluid of overlapping color singlets — or as Nc-squared, which would signal the presence of deconfined gluons. Either outcome would sharpen our map of the phases of hot strongly interacting matter.

Subject of Research: Why conserved charge fluctuations in hot QCD appear quasifree in the confining regime above the chiral crossover temperature

Article Title: Why fluctuations of conserved charges in the confining regime above \(T_{ch}\) behave as if the quarks were free?

Article References: Glozman, L. Y. (2026). Why fluctuations of conserved charges in the confining regime above $$T_{ch}$$ behave as if the quarks were free?. The European Physical Journal C, 86(10), Article 1223. https://doi.org/10.1140/epjc/s10052-026-16479-1

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16479-1

Keywords: lattice QCD, confinement, chiral crossover, conserved charge fluctuations, stringy fluid, quark-hadron duality, quark-gluon plasma, chiral symmetry restoration, mesonic correlators, quark-gluon plasma threshold, Pauli blocking, heavy-ion physics