Robustness of the scaled transverse-momentum correlator and the contribution of meson--baryon fluctuations at RHIC energies

Abstract

The search for the QCD critical point relies on identifying non-monotonic signatures in event-by-event fluctuation observables, such as the scaled two-particle transverse-momentum correlator $C_{p_T}$. Using a microscopic transport model (UrQMD), we establish the robustness of $C_{p_T}$ against the variations that distinguish the datasets in the relevant collision energy regime, namely the choice of centrality estimator, finite tracking efficiency, and acceptance, including the difference between collider and fixed-target running. In addition, it has recently been proposed, based on UrQMD yields, that the measured non-monotonicity requires no critical dynamics and instead follows from the event-by-event composition of the final state. We show that this contribution is governed by event-by-event fluctuations in the meson-baryon composition, rather than by the mean yields alone. Using the composition fluctuations obtained from UrQMD, we find that the minimum in the two-component calculation disappears.