The cosmological Li-7 problem from a nuclear physics perspective

The primordial abundance of Li-7 as predicted by Big Bang Nucleosynthesis (BBN) is more than a factor 2 larger than what has been observed in metal-poor halo stars. Herein, we analyze the possibility that this discrepancy originates from incorrect assumptions about the nuclear reaction cross sections relevant for BBN. To do this, we introduce an efficient method to calculate the changes in the Li-7 abundance produced by arbitrary (temperature dependent) modifications of the nuclear reaction rates. Then, considering that Li-7 is mainly produced from Be-7 via the electron capture process Be-7+e(-) -> Li-7+nu(e), we assess the impact of the various channels of Be-7 destruction. Differently from previous analysis, we consider the role of unknown resonances by using a complete formalism which takes into account the effect of Coulomb and centrifugal barrier penetration and that does not rely on the use of the narrow-resonance approximation. As a result of this, the possibility of a nuclear physics solution to the Li-7 problem is significantly suppressed. Given the present experimental and theoretical constraints, it is unlikely that the Be-7 + n destruction rate is underestimated by the 2.5 factor required to solve the problem. We exclude, moreover, that resonant destruction in the channels Be-7+t and Be-7+He-3 can explain the Li-7 puzzle. New unknown resonances in Be-7+d and Be-7+alpha could potentially produce significant effects. Recent experimental results have ruled out such a possibility for Be-7 + d. On the other hand, for the Be-7 + alpha channel very favorable conditions are required. The possible existence of a partially suitable resonant level in C-11 is studied in the framework of a coupled-channel model and the possibility of a direct measurement is considered.