Acceptance effect on the $N_{\rm t} N_{\rm p}/N_{\rm d}^2$ ratio of light nuclei coalescence yields as a probe of nucleon density fluctuations

Abstract

We employ a coalescence model to form deuterons ($\rm d$), tritons (${\rm t}$) and helium-3 ($^3{\rm He}$) nuclei from a uniformly distributed volume of protons ({\rm p}) and neutrons ({\rm n}). We study the ratio $N_{\rm t} N_{\rm p}/N_{\rm d}^2$ of light nuclei yields as a function of the neutron density fluctuations. We investigate the effect of finite transverse momentum ($p_{\rm T}$) acceptance on the ratio, in particular, the "extrapolation factor" ($f$) for the ratio as functions of the $p_{\rm T}$ spectral shape and the magnitude of neutron density fluctuations. It is found that $f$ is monotonic in $p_{\rm T}$ spectra "temperature" parameter and neutron density fluctuation magnitude; variations in the latter are relatively small. We also examine $f$ in realistic simulations using kinematic distributions of protons measured in heavy ion collision data. It is found that $f$ is smooth and monotonic as a function of beam energy. We conclude that extrapolation from limited $p_{\rm T}$ ranges would not create, enhance, or reduce a local peak of the $N_{\rm t} N_{\rm p}/N_{\rm d}^2$ ratio in beam energy. Our study provides a necessary benchmark for light nuclei ratios as a probe of nucleon density fluctuations, an important observable in the search for the critical point of nuclear matter.