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Recent observations of (6)Li in metal poor stars suggest a large production of this isotope during big bang nucleosynthesis (BBN). In standard BBN calculations, the (2)H(α,γ)(6)Li reaction dominates (6)Li 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 (2)H and α at energies below the Coulomb barrier. Indirect measurements using the Coulomb dissociation of (6)Li only give upper limits owing to the dominance of nuclear breakup processes. Here, we report on the results of the first measurement of the (2)H(α,γ)(6)Li cross section at big bang energies. The experiment was performed deep underground at the LUNA 400 kV accelerator in Gran Sasso, Italy. The primordial (6)Li/(7)Li 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 (6)Li/(7)Li values reported for halo stars will likely require a nonstandard physics explanation, as discussed in the literature.

The 3 He(α, γ) 7 Be reaction affects not only the production of 7 Li in Big Bang nucleosynthesis, but also the fluxes of 7 Be and 8 B neutrinos from the Sun. This double role is exploited here to constrain the former by the latter. A number of recent experiments on 3 He(α, γ) 7 Be provide precise cross section data at E = 0.5-1.0 MeV center-of-mass energy. However, there is a scarcity of precise data at Big Bang energies, 0.1-0.5 MeV, and below. This problem can be alleviated, based on precisely calibrated 7 Be and 8 B neutrino fluxes from the Sun that are now available, assuming the neutrino flavour oscillation framework to be correct. These fluxes and the standard solar model are used here to determine the 3 He(α, γ) 7 Be astrophysical S-factor at the solar Gamow peak, S ν 34 (23 +6 −5 keV) = 0.548±0.054 keV b. This new data point is then included in a re-evaluation of the 3 He(α, γ) 7 Be S-factor at Big Bang energies, following an approach recently developed for this reaction in the context of solar fusion studies. The re-evaluated S-factor curve is then used to re-determine the 3 He(α, γ) 7 Be thermonuclear reaction rate at Big Bang energies. The predicted primordial lithium abundance is 7 Li/H = 5.0 ×10 −10 , far higher than the Spite plateau. PACS numbers: 26.35.+c, 26.65.+t, 98.80.Ft

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