The LBT Y p Project. I. An Improved Determination of the Primordial Helium Abundance—Project Description, Sample Selection, Observations, and Methodology

Abstract

Abstract Extremely low-metallicity H ii regions have been observed with the goal of determining the primordial helium abundance ( Y p ). Y 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 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 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 H ii regions with uncertainties of approximately 2% or less. Archival LBT/MODS spectra of standard stars over a 4 yr 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 p with a precision of ∼0.5%, sufficient to provide an independent constraint on the effective number of neutrino families of ∼3%.

The authors' abstract, as published at the source. The Astrophysical Journal, 2026 · DOI ↗