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

The correct prediction of the abundances of the light nuclides produced during of Big Bang Nucleosynthesis (BBN) is one of the main topics of modern cosmology. In order to precisely determine BBN 6 Li production the cross-section of the nuclear reaction 2 H(α, γ) 6 Li must be directly measured within the astrophysical energy range of 30-400 keV. This measure- ment requires ultra low gamma-ray background, as obtained at LUNA, the deep underground accelerator laboratory installed in the INFN Gran Sasso National Laboratory (LNGS), Italy. On the basis of the new experimental data, the 2 H(α, γ) 6 Li thermonuclear reaction rate has been derived. Our rate is even lower than previously reported, thus increasing the discrepancy between predicted Big Bang 6 Li abundance and the amount of primordial 6 Li inferred from observations. The primordial 6 Li/ 7 Li isotopic abundance ratio has been consequently deter- mined to be (1.5 ± 0.3) × 10 −5 within standard BBN theory. The much higher 6 Li/ 7 Li values reported for halo stars will likely require a non-standard physics explanation, as discussed in the literature.