Constraining the Low-$p_T$ $η/π^0$ Ratio for Direct-Photon Analyses with Blast-Wave Fits to $π$, $K$, and $p$ Spectra

Abstract

We predict the $η/π^0$ ratio at low $p_T$ ($p_T \lesssim 3~\mathrm{GeV}/c$) using the measured charged $K/π$ ratio and model input from a blast-wave framework with feeddown contributions. This approach can provide improved, data-constrained background estimates for direct-photon and dilepton measurements in heavy-ion collisions. In this approach, the explicit modeling of radial flow and hadronic feeddown enables an uncertainty estimate for the low-$p_T$ extrapolation of the $η/π^0$ ratio. Using central Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV as an example, we find that the $η$-related decay-photon uncertainty at $p_T \approx 1~\mathrm{GeV}/c$ is of order 10\% of the expected direct-photon signal.