Recent baryon acoustic oscillation (BAO) distance measurements, when combined with Cosmic Microwave Background (CMB) observations in the ΛCDM framework, lead to a preference for negative neutrino masses. We investigate whether this neutrino mass anomaly can be alleviated by a class of astrophysically motivated reionization histories. Using a frequentist analysis, we find that some reionization histories can move the best-fit value of ∑mν to a positive value and bring ∑mν≃0.06eV into the 95\% confidence interval. To separate the effect of the total optical depth from that of the details of the reionization history, we compare a high-τ history with a two-step tanh-like reionization history of the same τ. The resulting Δχ2(∑mν) profiles are nearly identical. This indicates that the effect is mainly driven by the total optical depth, while the details of the reionization history play only a minor role.
@article{arxiv.2605.10116,
title = {Reionization History and Neutrino Mass},
author = {YiCheng Dai and Wei Liao},
journal= {arXiv preprint arXiv:2605.10116},
year = {2026}
}