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A reminder of our previous brief look at understanding the equation of state of nuclear matter. We will briefly review, then finally get to our only existing theory of nuclear matter, based on the lattice gauge theory of the strong interaction.
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 The BES (Beam Energy Scan Project) supposedly allowed direct study of the critical point. Not so fast! |
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When nuclear matter is compressed so that nucleon
probability distributions overlap, confinement vanishes, color
is "freed," and quarks are essentially free particles in a Fermi
gas, with the result that they can form Cooper pairs
(like electrons in some metals at very low temperatures) giving
rise to "color
superconductivity" in analogy to Type 1 superconductivity
in some metals at very low temperatures.
From Wikipedia: "Theoretical and experimental works [suggest] that the formation of a quark–gluon plasma occurs at the temperature of T ≈ 150–160 MeV (the Hagedorn temperature) and an energy density of ≈ 0.4–1 GeV/cubic fm. While at first a phase transition was expected, present day theoretical interpretations propose [instead] a phase transformation similar to the process of ionisation of normal matter into ionic and electron plasma." QGP has been supposedly observed in multiple experiments, and its existence is widely accepted in the physics community, but the absence of a single, universally agreed “smoking gun” is due to: (1) the complexity and mimicry of hadronic effects, (2) the energy‑dependent nature of QGP formation, and (3) recent theoretical findings that challenge the reliability of some traditional probes. Currently in Germany, a new accelerator is under construction, which can produce beams of antiprotons, and of all nuclei, at roughly 1 GeV/u. This accelerator will be able to explore regions of the nuclear equation of state that are currently not possible to access.
In lattice gauge QCD, quarks are placed at specific sites on a lattice, and gluons are placed on the links between lattice sites. This approach was first suggested in the early 1970s by Kenneth Wilson (1936 - 2013), and has ever since been the most often-used approach to solving strong-interaction problems. At any given time, such calculations consume all the power of the largest and fastest available computers. Eventually supercomputers were specifically designed with lattice gauge QCD calculations in mind.
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The largest cube currently possible in lattice gauge calculations of nuclear matter is around 160³ lattice points (≈ 16 fm per side) for classical simulations, with lattice spacings of ~ 0.1 fm. Quantum simulation methods may eventually allow much larger cubes, but this is still in the research stage.
In recent years it has been argued that no genuinely advanced LGT calculations can be done except on future quantum computers.
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