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

TL;DR

This paper explores whether resonant enhancement of Be7 burning reactions, due to a specific nuclear level in B9, can resolve the discrepancy between predicted and observed cosmological lithium abundance, suggesting a potential nuclear physics solution.

Contribution

It identifies a specific nuclear resonance in B9 that could significantly enhance Be7 burning, potentially solving the lithium problem.

Findings

Resonance near 170-220 keV can deplete lithium abundance.

Large interaction radius (≥9 fm) is required for significant width.

Current lithium predictions should have larger error margins until further study.

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}.