Hadron Production in Ultra-relativistic Nuclear Collisions: Quarkyonic Matter and a Triple Point in the Phase Diagram of QCD

Abstract: We argue that features of hadron production in relativistic nuclear collisions, mainly at CERN-SPS energies, may be explained by the existence of three forms of matter: Hadronic Matter, Quarkyonic Matter, and a Quark-Gluon Plasma. We suggest that these meet at a triple point in the QCD phase diagram. Some of the features explained, both qualitatively and semi-quantitatively, include the curve for the decoupling of chemical equilibrium, along with the non-monotonic behavior of strange particle multiplicity ratios at center of mass energies near 10 GeV. If the transition(s) between the three phases are merely crossover(s), the triple point is only approximate.

Hadron Production in Ultra-relativistic Nuclear Collisions: Quarkyonic Matter and a Triple Point in the Phase Diagram of QCD

The study of hadron production in ultra-relativistic nuclear collisions is a subject of significant interest in the field of quantum chromodynamics (QCD). The paper proposes the existence of three distinct phases of matter: Hadronic Matter, Quarkyonic Matter, and the Quark-Gluon Plasma, which intersect at a triple point in the QCD phase diagram. This conceptual framework is supported by experimental observations from heavy-ion collision experiments conducted at CERN-SPS energies, where peculiarities such as non-monotonic behavior in strange particle multiplicity ratios are observed.

The key proposition of the paper is the existence of Quarkyonic Matter, a phase that is essentially confined but has a large baryon number density, distinguishing it from both Hadronic Matter and a fully deconfined Quark-Gluon Plasma. The authors suggest that these phases meet in a triple point in the QCD phase diagram, proposing that the transitions between them might be crossovers rather than true phase transitions.

Experimental Observations

Experimental data from heavy-ion collisions, particularly at CERN's SPS, reveal several intriguing patterns. The temperature at which chemical equilibrium decouples reaches approximately 160 MeV near a center of mass energy of 10 GeV, with a corresponding smooth decrease in baryon chemical potential ($\mu_B$). This temperature and $\mu_B$ combination produces a phase-diagram-like pattern that suggests a distinctive hadronic decoupling surface, which may serve as a boundary between different phases of dense matter.

Further insights are provided by the energy dependence of particle yields where ratios such as the $K^+/\pi^+$ and $\Lambda/\pi$ exhibit maxima, corresponding to collision energies near 10 GeV. This anomaly at SPS energies indicates a transition in the nature of nuclear matter produced in heavy ion collisions, hinting at the possibility of a triple point where hadronic, quarkyonic, and plasma matter intersect.

Theoretical Implications

The paper introduces the theoretical notion of a Hagedorn model with quarkyonic transitions, suggesting that the density of baryonic states may be regulated by a Hagedorn temperature that varies with $\mu_B$ and becomes a key factor in the delineation of phase transition boundaries. This is modeled using a simple dependency based on diminishing temperature as the baryon chemical potential increases.

Moreover, the boundary between Hadronic Matter and the Quark-Gluon Plasma at higher energies is proposed to be relatively independent of $\mu_B$, consistent with lattice QCD simulations that show a rapid but smooth transition. However, the transition from Hadronic to Quarkyonic Matter at large $\mu_B$ is hypothesized as pivotal in dense quark systems where confinement still plays a significant role.

Future Directions in Research

The experimental verification of Quarkyonic Matter and the confirmation of a triple point within the QCD diagram warrant further exploration and data collection in upcoming heavy-ion collision experiments. Efforts should be focused on energies around the proposed triple point as this area holds potential for novel insights into phase transitions in dense nuclear environments.

The paper concludes by reasserting the need for more data from current and future facilities such as RHIC, NA61, CBM at FAIR Darmstadt, and NICA at JINR Dubna to solidify the theoretical predictions. The exploration of the phase boundary between Hadronic and Quarkyonic Matter remains a promising direction for future research, particularly in understanding the nature of chemical freeze-out and the conditions of particle abundances across different energies.

In summary, the paper provides an insightful narrative on the complexity of nuclear matter under extreme conditions and proposes a framework by which the peculiarities observed in experimental data can be understood as manifestations of phase transitions within the QCD phase diagram. These findings have potential implications for our understanding of hadron dynamics, strangeness production, and are fundamental to our grasp of strong force interactions in high-energy physics.