Big Bang nucleosynthesis and the neutrino-extended Standard Model Effective Field Theory

A bstract We study the impact of light GeV-scale heavy neutral leptons (HNLs) on Big Bang nucleosynthesis (BBN) in the neutrino-extended Standard Model Effective Field Theory ( ν SMEFT). We show that, based on very general considerations, BBN constraints complement laboratory searches at colliders, beam dumps, and neutrinoless double beta decay, by providing an upper bound on the cut-off scale of the effective field theory for HNL masses above ~ 100 MeV. We identify target regions for future laboratory probes of the ν SMEFT parameter space, bounded from above and below, and illustrate them in representative scenarios, including EFT benchmarks based on operator subsets with universal Wilson coefficients.