The LBT $Y_{\rm p}$ Project I: An Improved Determination of the Primordial Helium Abundance -- Project Description, Sample Selection, Observations, and Methodology

TL;DR

This paper describes the LBT $Y_{ m p}$ Project aimed at a high-precision determination of the primordial helium abundance, $Y_{ m p}$, using extremely metal-poor H II regions and a new analysis framework. It details a coordinated observational program with the Large Binocular Telescope (MODS and LUCI) and a comprehensive methodology that includes updated H and He emissivities and radiative‑transfer corrections to reduce systematics. The authors outline an optimized, homogeneous low‑metallicity sample and a plan to publish a public 1D spectral database, forming the first installment in a six‑paper series that will present data reductions, improved $Y_{ m p}$ derivation, and implications for the effective number of neutrino species, $N_ u$. By targeting a precision of about $0.5 ext{\%}$ on $Y_{ m p}$, the project seeks to tighten constraints on early‑Universe physics and the Standard Model, with broad implications for cosmology and particle physics.

Abstract

Extremely low metallicity HII regions have been observed with the goal of determining the primordial helium abundance ($Y_{\rm p}$). $Y_{\rm p}$, combined with standard big bang nucleosynthesis and the half-life of the neutron, provides a direct measurement of the number of neutrino families, but $Y_{\rm p}$ must be measured very precisely to provide meaningful constraints on physics beyond the Standard Model. Here we describe a program to combine new Large Binocular Telescope (LBT) observations with a new analysis methodology to significantly improve the determination of $Y_{\rm p}$. The LBT, with its MODS and LUCI instruments, produces spectra, which, when combined with our new analysis methodology, are capable of delivering He abundances in individual HII regions with uncertainties of approximately 2% or less. Archival LBT/MODS spectra of standard stars over a four-year period enable the determination of a wavelength-dependent uncertainty in the MODS spectral response, resulting in improved relative emission line uncertainties. An optimized sample of low-metallicity galaxies has been selected with the goal of producing a determination of $Y_{\rm p}$ with a precision of $\sim$ 0.5%, sufficient to provide an independent constraint on the effective number of neutrino families of $\sim$ 3%.

Figures (2)

• Figure 1: A comparison of recent $Y_{\rm p}$ results. The SBBN result based on the Planck determined baryon density aghanim2020 is plotted as a vertical band in light blue Yeh:2022heqYeh:2023nve.
• Figure 2: Spectral flux calibration error functions for the MODS blue and red grating spectrograph channels derived from archival and new observations of HST primary flux calibration stars. Curves show the fractional error in relative flux as a function of wavelength.