New state of matter between the hadronic phase and the quark-gluon plasma?

Journal Article · Mon Oct 06 00:00:00 EDT 2025 · Physical Review. D.

DOI:https://doi.org/10.1103/h71y-km92· OSTI ID:3019303

Fujimoto, Yuki ^[1]; Fukushima, Kenji ^[2]; Hidaka, Yoshimasa ^[3]; McLerran, Larry ^[4]

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1. University of California, Berkeley, CA (United States); RIKEN, Wako (Japan)
2. The University of Tokyo (Japan)
3. Kyoto University (Japan); RIKEN, Wako (Japan)
4. University of Washington, Seattle, WA (United States)

Lattice-quantum chromodynamics (QCD) simulations and theoretical arguments hint at the existence of an intermediate phase of strongly interacting matter between a confined hadron gas and a deconfined quark-gluon plasma (QGP). We qualitatively and semiquantitatively explore and differentiate the phase structures in the temperature window from the QCD pseudocritical temperature 𝑇_c ≃ 160 MeV to the pure gluonic deconfinement temperature 𝑇_d ≃ 285 MeV. We propose a three-regime picture using a hadron resonance gas description augmented with the exponential spectrum of strings, corresponding to highly excited mesons and glueballs, based on the analysis of a large number 𝑁_c of colors. We estimate the entropy density from our model to confirm that the lattice-QCD data are bracketed with three regimes, i.e., a hadron gas, a QGP, and a new phase for 𝑇_c ≲ 𝑇 ≲ 𝑇_d. In this new phase, which we name a spaghetti of quarks with glueballs (SQGBs), thermal degrees of freedom of quarks are liberated, yet gluons remain confined in glueballs. Since the Hagedorn temperature 𝑇_H ∼ 285 MeV is universal in the meson and the glueball sectors, in the infinite-𝑁_c limit, the phase diagram in the plane of the baryon chemical potential and the temperature is reduced to one with the confined and deconfined phases and quarkyonic matter at high density. At large but finite 𝑁_c, an SQGB window may open between these phases. We point out that the SQGB has interesting similarities with quarkyonic matter and that this matter in the large-𝑁_c limit is confined as measured by the interaction between heavy quarks, but behaves in other respects like a quasifree gas of quarks. As a result of the extrapolation to 𝑁_c = 3, we present a revised phase diagram with the SQGB phase bounded by thermal crossovers. Finally, we give a quantitative analysis of chiral-symmetry restoration in the SQGB phase.

• Research Organization: University of Washington, Seattle, WA (United States)
• Sponsoring Organization: Adopting Sustainable Partnerships for Innovative Research Ecosystem (ASPIRE); Heising-Simons Foundation; JSPS KAKENHI; Japan Science and Technology Agency (JST); National Science Foundation (NSF); USDOE
• Grant/Contract Number: FG02-00ER41132
• OSTI ID: 3019303
• Journal Information: Physical Review. D., Journal Name: Physical Review. D. Journal Issue: 7 Vol. 112; ISSN 2470-0010; ISSN 2470-0029
• Publisher: American Physical Society (APS)
• Country of Publication: United States
• Language: English

Related Subjects

Color confinement
Large-N expansion in field theory
QCD phase transitions
Quark matter
Quark-gluon plasma
Relativistic heavy-ion collisions
Strings & branes