First direct measurement of the H2(α,γ)Li6 cross section at Big Bang energies and the primordial lithium problem

Recent observations of Li6 in metal poor stars suggest a large production of this isotope during big bang nucleosynthesis (BBN). In standard BBN calculations, the H2(α,γ)Li6 reaction dominates Li6 production. This reaction has never been measured inside the BBN energy region because its cross section drops exponentially at low energy and because the electric dipole transition is strongly suppressed for the isoscalar particles H2 and α at energies below the Coulomb barrier. Indirect measurements using the Coulomb dissociation of Li6 only give upper limits owing to the dominance of nuclear breakup processes. Here, we report on the results of the first measurement of the H2(α,γ)Li6 cross section at big bang energies. The experiment was performed deep underground at the LUNA 400 kV accelerator in Gran Sasso, Italy. The primordial Li6/Li7 isotopic abundance ratio has been determined to be (1.5±0.3)×10−5, from our experimental data and standard BBN theory. The much higher Li6/Li7 values reported for halo stars will likely require a nonstandard physics explanation, as discussed in the literature.