Backgrounds and Detector Effects in the Search for the QCD Critical Point

Fluctuations of conserved charges in heavy-ion collisions are expected to be sensitive to a critical point in the phase diagram of QCD matter. These fluctuations can be quantified by measuring various orders of cumulants, $C_n$, of the distributions of baryon number, charge, strangeness, and $p_T$. This talk addresses experimental backgrounds in the critical-point search. We present a discussion of the difficulties with using multiple detectors in measurements of high-order cumulants. The effect of the correlation between particle identification detectors and multiplicity detectors will be explored. We motivate this by considering how the two-dimensional space of proton-number versus multiplicity is shaped by various analysis decisions. We then demonstrate these backgrounds 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. Implications for STAR's Beam Energy Scan II and Fixed-Target Program measurements will be discussed.