The antiproton puzzle, QCD critical point, and fireball properties in heavy-ion collisions

¹ Department of Physics, University of Houston, Houston, TX 77204, USA
² AGH University of Krakow, Faculty of Physics and Applied Computer Science, 30-059 Kraków, Poland
³ Nuclear Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA

Published online: 17 April 2026

Abstract

We discuss factorial cumulants of protons and antiprotons as a tool to probe the properties of the fireball created in heavy-ion collisions. First, we discuss the new RHIC BES-II data on factorial cumulants of protons and their deviation from the non-critical baseline at √s_NN≲10−20 GeV in the context of the critical point search. We then show that the acceptance dependence of the appropriately scaled factorial cumulants is flat if correlations are driven solely by long-range effects, such as global baryon conservation and volume fluctuations. The analysis of RHIC BES-I data reveals an antiproton puzzle: sizable differences between p and p̅ second-order fluctuations at RHIC energies that are not reproduced by a single-fluid picture. We argue that such a splitting indicates incomplete equilibration between stopped and produced matter, which can be further probed with upcoming high-statistics data from RHIC BES-II.