Nuclear matter is a hypothetic substance consisting of interacting quarks and gluons in large enough numbers that the system can be considered infinite.
Left: tracks in the STAR detector from collisions of a 3.85 GeV gold beam in the Relativistic Heavy Ion Collider hitting a gold target from the left. Upper right: radial projection perpendicular to the beam. Lower right: side projection along the beam. Image credit: STAR Collaboration.
Nuclear matter consists of interacting quarks and gluons, described by the theory of quantum chromodynamics.
Like ordinary matter, it can exist in different phases, in today’s world predominantly as protons and neutrons in atomic nuclei.
Collisions of heavy nuclei provide a unique environment to study its properties.
The well-known hadronic phase of nuclear matter consists of baryons (protons and neutrons) and a large number of elementary particles.
Heavy ion collisions have revealed a new phase of nuclear matter with freely moving quarks and gluons, the quark-gluon plasma (QGP).
The QGP is like the matter in the early Universe and in dense compact stars.
Despite these discoveries, the structure of nuclear matter and the transition between its hadronic and QGP phases are largely unknown.
Physicists from the STAR Collaboration at the Relativistic Heavy Ion Collider (RHIC) are aiming to establish if a critical point exists in the phase diagram of nuclear matter.
At this critical point, the QGP would coexist with a gas of protons, neutrons, and other particles.
The STAR researchers found that gold-gold collisions at the lowest energies accessible at RHIC produce predominantly hadronic matter and do not form a QGP. In contrast, experiments at 20 GeV and above form a QGP.
At collision energies of 3 GeV, the data taken by the STAR experiment in the fixed target mode can be reproduced by theoretical model calculations that only consider hadronic interactions.
“This indicates that the critical point, if it exists, should be between collision energies of 3 GeV and 20 GeV,” the authors said.
In addition, they measured kinematic distributions for the first time for light hyper-nuclei that involve hadrons with strange quarks as well as ordinary nucleons (protons and neutrons).
“This is the beginning of the era for studying hyperon-nucleon interaction, an essential step for understanding the inner structure of compact stars,” they concluded.
The findings were published in the journal Physics Letters B and the journal Physical Review Letters.
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M.S. Abdallah et al. (STAR Collaboration). 2022. Disappearance of partonic collectivity in √SNN=3 GeV Au+Au collisions at RHIC. Physics Letters B 827: 137003; doi: 10.1016/j.physletb.2022.137003
M.S. Abdallah et al. (STAR Collaboration). 2022. Probing strangeness canonical ensemble with K−, ϕ(1020) and Ξ− production in Au+Au collisions at √SNN=3 GeV. Physics Letters B 831: 137152; doi: 10.1016/j.physletb.2022.137152
M.S. Abdallah et al. (STAR Collaboration). 2022. Measurements of Proton High-Order Cumulants in √sNN=3 GeV Au+Au Collisions and Implications for the QCD Critical Point. Phys. Rev. Lett 128 (20): 202303; doi: 10.1103/PhysRevLett.128.202303
M.S. Abdallah et al. (STAR Collaboration). 2022. Measurements of 3ΛH and 4ΛH Lifetimes and Yields in Au+Au Collisions in the High Baryon Density Region. Phys. Rev. Lett 128 (20): 202301; doi: 10.1103/PhysRevLett.128.202303
