First direct measurement of the 2H(α,γ)6Li cross section at Big Bang energies and the primordial lithium problem

Recent observations of 6Li in metal poor stars suggest a large production of this isotope during Big Bang Nucleosynthesis (BBN). In standard BBN calculations, the 2H(α,γ)6Li reaction dominates 6Li production. Unfortunately, this reaction has never been directly measured inside the BBN energy region because its cross section drops exponentially at low energy. Indirect measurements using the Coulomb dissociation of 6Li only give upper limits owing to the dominance of nuclear breakup processes. Here, we report on the results of the first measurement of the 2H(α,γ)6Li cross section directly at Big Bang energies. The experiment was performed deep underground at the LUNA 400kV accelerator in Gran Sasso, Italy. The 6Li/7Li isotopic abundance ratio from standard BBN has been determined to be (1.5±0.3)×10−5, entirely from experimental data. The much higher 6Li/7Li values reported for halo stars will likely require a non-standard physics explanation.