QCD phase diagram at high temperature and density

4 problems

• Is the STAR net-proton fluctuation dip a critical-point signal?

  Contested

  The STAR experiment counted how the number of protons minus antiprotons varies from collision to collision as it lowered the collision energy. In 2025 it reported a dip near one energy, and whether a critical point causes it is unclear.

• Order of the chiral transition for massless light quarks

  Partially resolved

  If the up and down quarks had zero mass, QCD would have an exact symmetry that breaks as matter cools, and the transition would be either sudden or continuous. The answer constrains where a critical point can lie in the real phase diagram.

• Existence and location of the QCD critical point

  Open

  At zero net baryon density, ordinary matter turns into quark-gluon plasma smoothly as it heats. At higher density the change may become abrupt, and the critical point is where smooth turns abrupt, if it exists.

• QCD equation of state at large baryon chemical potential

  Open

  Supercomputer calculations of QCD work well at zero net density but fail at high density because of the sign problem (contributions of opposite sign that cancel almost exactly). Without a fix, the dense part of the phase diagram cannot be computed directly.