Resonant enhancement of nuclear reactions as a possible solution to the cosmological lithium problem

Abstract

There is a significant discrepancy between the current theoretical prediction of the cosmological lithium abundance, mostly produced as Be7 during the Big Bang, and its observationally inferred value. We investigate whether the resonant enhancement of Be7 burning reactions may alleviate this discrepancy. We identify one narrow nuclear level in B9, E_{5/2^+} \simeq 16.7 MeV that is not sufficiently studied experimentally, and being just \sim 200 keV above the Be7+d threshold, may lead to the resonant enhancement of Be7(d,\gamma)B9 and Be7(d,p)\alpha\alpha reactions. We determine the relationship between the domain of resonant energies E_r and the deuterium separation width \Gamma_d that results in the significant depletion of the cosmological lithium abundance and find that (E_r, ~\Gamma_{d}) \simeq (170-220,~10-40) keV can eliminate current discrepancy. Such a large width at this resonant energy can be only achieved if the interaction radius for the deterium entrance channel is very large, a_{27} \ge 9 fm. Our results also imply that before dedicated nuclear experimental and theoretical work is done to clarify the role played by this resonance, the current conservative BBN prediction of lithium abundance should carry significantly larger error bars, [Li7/H]_{\rm BBN} = (2.5-6)\times 10^{-10}.