How Not to Measure a False QCD Critical Point

TL;DR

This paper discusses how measurement distortions in heavy-ion collision experiments can falsely suggest a QCD critical point, emphasizing analysis choices that mitigate such false signals.

Contribution

It identifies how correlations between particle identification and centrality measurements can create misleading high-order cumulant signals, and proposes strategies to avoid false critical point signatures.

Findings

Simulated data shows measurement distortions can mimic critical signals.

Certain analysis strategies reduce vulnerability to false signals.

Guidelines are provided for more reliable fluctuation measurements.

Abstract

Fluctuations of conserved charges are a golden channel for measuring a QCD critical point in relativistic heavy-ion collisions. These fluctuations are quantified by measuring high-order cumulants of baryon-number distributions at a given centrality. Using simulated proton-number cumulants as an example, we discuss how the correlation between particle identification and centrality measurements can distort particle-number distributions. These distortions can easily create large fluctuations in high-order cumulants that might be mistaken for a critical-point signature. We show that certain measurement choices can make the analysis more or less vulnerable to these false signals. We motivate this by considering how the two-dimensional probability space of proton-number versus multiplicity is shaped by analysis choices. We then demonstrate this vulnerability with simulated Au+Au collisions at $\sqrt{s_{NN}}=3.9$ GeV in UrQMD, and two toy models of detector responses to certain classes of events. We explain how an analyzer might observe a false critical signature, and how to avoid doing so, even in a challenging experimental environment.