Solar neutrino constraints on the BBN production of Li

Using the recent WMAP determination of the baryon-to-photon ratio, 10¹⁰η=6.14 to within a few percent, big bang nucleosynthesis (BBN) calculations can make relatively accurate predictions of the abundances of the light element isotopes which can be tested against observational abundance determinations. At this value of η, the ⁷Li abundance is predicted to be significantly higher than that observed in low metallicity halo dwarf stars. Among the possible resolutions to this discrepancy are (1) ⁷Li depletion in the atmosphere of stars, (2) systematic errors originating from the choice of stellar parameters—most notably the surface temperature, and (3) systematic errors in the nuclear cross sections used in the nucleosynthesis calculations. Here, we explore the last possibility, and focus on possible systematic errors in the ³He(α,γ)⁷Be reaction, which is the only important ⁷Li production channel in BBN. The absolute value of the cross section for this key reaction is known relatively poorly both experimentally and theoretically. The agreement between the standard solar model and solar neutrino data thus provides additional constraints on variations in the cross section (S₃₄). Using the standard solar model of Bahcall, and recent solar neutrino data, we can exclude systematic S₃₄ variations of the magnitude needed to resolve the BBN ⁷Li problem at the ≳95% C.L., or more strongly, depending on the Li observations used. Additional laboratory data on ³He(α,γ)⁷Be will sharpen our understanding of both BBN and solar neutrinos, particularly if care is taken in determining the absolute cross section and its uncertainties. Nevertheless, it is already clear that this “nuclear fix” to the ⁷Li BBN problem is unlikely; other possible solutions are briefly discussed.