Big Bang 6 Li nucleosynthesis studied deep underground (LUNA collaboration)

The correct prediction of the abundances of the light nuclides produced during the epoch of Big Bang Nucleosynthesis (BBN) is one of the main topics of modern cosmology. For many of the nuclear reactions that are relevant for this epoch, direct experimental cross section data are available, ushering the so-called “age of precision”. The present work addresses an exception to this current status: the ²H(α,γ)⁶Li reaction that controls ⁶Li production in the Big Bang. Recent controversial observations of ⁶Li in metal-poor stars have heightened the interest in understanding primordial ⁶Li production. If confirmed, these observations would lead to a second cosmological lithium problem, in addition to the well-known ⁷Li problem. In the present work, the direct experimental cross section data on ²H(α,γ)⁶Li in the BBN energy range are reported. The measurement has been performed deep underground at the LUNA (Laboratory for Underground Nuclear Astrophysics) 400 kV accelerator in the Laboratori Nazionali del Gran Sasso, Italy. The cross section has been directly measured at the energies of interest for Big Bang Nucleosynthesis for the first time, at E_cm=80, 93, 120, and 133 keV. Based on the new data, the ²H(α,γ)⁶Li thermonuclear reaction rate has been derived. Our rate is even lower than previously reported, thus increasing the discrepancy between predicted Big Bang ⁶Li abundance and the amount of primordial ⁶Li inferred from observations.