Revision 1

Accepted · first revision · record

Title: Is the STAR net-proton fluctuation dip a critical-point signal?
Plain: 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: 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.
Settled by: 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.
Kind: phenomenon
Answer type: identification
References: (empty)
Literature status: contested
Status 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.
Posed since: 2025