Approaching Stable Quark Matter

The determination of whether the ground state of baryon matter in Quantum Chromodynamics (QCD) is the ordinary nucleus or a quark matter state remains a long-standing question in physics. A critical parameter in this investigation is the bag parameter $B$, which quantifies the QCD vacuum energy and can be computed using nonperturbative methods such as Lattice QCD (LQCD). By combining the equation of state derived from perturbative QCD (pQCD) with the bag parameter to fit the LQCD-simulated data for isospin-dense matter, we address the stability of quark matter within the LQCD+pQCD framework. Our findings suggest that the current data imposes an upper bound on $B^{1/4} \lesssim 160$ MeV, approaching a conclusive statement on quark matter stability. Given the lower bound on $B$ from the quark condensate contribution to the vacuum energy, the stable 2-flavor quark matter remains possible, whereas the stable 2+1-flavor quark matter is excluded, assuming complete deconfinement and chiral-symmetry restoration and the reliability of pQCD at baryon chemical potentials around the proton mass. Additionally, we derive more general thermodynamic bounds on the quark matter energy-per-baryon and $B$, which, while weaker, provide complementary insights.

💡 Research Summary

The paper tackles the long‑standing question of whether bulk baryonic matter in Quantum Chromodynamics (QCD) prefers the ordinary nuclear phase or a deconfined quark‑matter phase as its true ground state. The authors focus on the bag parameter (B), which quantifies the difference in vacuum energy between the confined (chiral‑symmetry‑broken) and deconfined (chiral‑symmetry‑restored) phases. While (B) is a non‑perturbative quantity, it can be accessed through lattice QCD (LQCD) calculations, whereas the high‑density equation of state (EOS) of quark matter can be computed perturbatively (pQCD).

The methodology consists of two main steps. First, the authors compute the zero‑temperature quark‑matter EOS up to order (\alpha_s^2) in perturbation theory, including the contribution from a possible color‑superconducting (CS) condensate. The pressure of the system is written as
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