Acceptance dependence of fluctuation measures near the QCD critical point

We argue that a crucial determinant of the acceptance dependence of fluctuation measures in heavy-ion collisions is the range of correlations in the momentum space, e.g., in rapidity, $Δy_{\rm corr}$. The value of $Δy_{\rm corr}\sim1$ for critical thermal fluctuations is determined by the thermal rapidity spread of the particles at freezeout, and has little to do with position space correlations, even near the critical point where the spatial correlation length $ξ$ becomes as large as $2-3$ fm (this is in contrast to the magnitudes of the cumulants, which are sensitive to $ξ$). When the acceptance window is large, $Δy\ggΔy_{\rm corr}$, the cumulants of a given particle multiplicity, $κ_k$, scale linearly with $Δy$, or mean multiplicity in acceptance, $\langle N\rangle$, and cumulant ratios are acceptance independent. While in the opposite regime, $Δy\llΔy_{\rm corr}$, the factorial cumulants, $\hatκ_k$, scale as $(Δy)^k$, or $\langle N\rangle^k$. We demonstrate this general behavior quantitatively in a model for critical point fluctuations, which also shows that the dependence on transverse momentum acceptance is very significant. We conclude that extension of rapidity coverage proposed by STAR should significantly increase the magnitude of the critical point fluctuation signatures.

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