Extracting QCD baryon susceptibilities from proton fluctuations at BES energies

Abstract

We present a new framework for connecting proton fluctuation measurements in relativistic heavy-ion collisions to QCD baryon-number susceptibilities. The pipeline combines viscous hydrodynamic simulations with a maximum-entropy construction of proton and antiproton fluctuations from baryon-number fluctuations on the particlization hypersurface. Finite experimental acceptance and exact global charge conservation are incorporated using the newly developed Subensemble Acceptance Method, SAM 3.0. Applying this framework to proton cumulants and factorial cumulants in Au+Au collisions from the RHIC Beam Energy Scan, we perform a Bayesian extraction of the second-order baryon-number susceptibility. We find a growing tension with lattice-QCD-based 4D Taylor-expansion predictions at larger baryon chemical potential, suggesting that proton fluctuation data may provide direct sensitivity to changes in the underlying QCD thermodynamics.