NUC In the literature: contested

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

In plain words

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.

Precise statement

STAR Beam Energy Scan II (PRL 2025) measured the net-proton cumulant ratio $C4/C2$ in 0-5 percent central $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}} = 7.7 \text{ to } 27 \mathrm{GeV}$ and found a minimum near 19.6 GeV deviating from non-critical baselines by 2 to 5 standard deviations. Determine whether this follows from a critical point, using dynamical models with baryon-number conservation, finite size and lifetime, and non-equilibrium evolution, or from non-critical effects. Answer: yes or no with quantified model uncertainty.

What would settle it

Fixed-target data below $7.7\,\mathrm{GeV}$ (STAR FXT, HADES, CBM at FAIR) testing the predicted energy dependence of $C4/C2$, together with dynamical simulations reproducing the full excitation function.

Status in the literature

Unverified note

The significance depends on the choice of non-critical baseline, and no dynamical critical-point model had reproduced the full energy dependence as of 2026.