Resonant destruction as a possible solution to the cosmological lithium problem

We explore a nuclear physics resolution to the discrepancy between the predicted standard big-bang nucleosynthesis (BBN) abundance of ${}^7\mathrm{Li}$ and its observational determination in metal-poor stars. The theoretical ${}^7\mathrm{Li}$ abundance is 3–4 times greater than the observational values, assuming the baryon-to-photon ratio, ${\eta }_{\mathrm{wmap}}$, determined by WMAP. The ${}^7\mathrm{Li}$ problem could be resolved within the standard BBN picture if additional destruction of $A=7$ isotopes occurs due to new nuclear reaction channels or upward corrections to existing channels. This could be achieved via missed resonant nuclear reactions, which is the possibility we consider here. We find some potential candidate resonances which can solve the lithium problem and specify their required resonant energies and widths. For example, a $1^{-}$ or $2^{-}$ excited state of ${}^{10}\mathrm{C}$ sitting at approximately 15.0 MeV above its ground state with an effective width of order 10 keV could resolve the ${}^7\mathrm{Li}$ problem; the existence of this excited state needs experimental verification. Other examples using known states include ${}^7\mathrm{Be}+t\to {}^{10}\mathrm{B}(18.80\mathrm{MeV})$, and ${}^7\mathrm{Be}+d\to {}^9\mathrm{B}(16.71\mathrm{MeV})$. For all of these states, a large channel radius ($a>10\mathrm{fm}$) is needed to give sufficiently large widths. Experimental determination of these reaction strengths is needed to rule out or confirm these nuclear physics solutions to the lithium problem.

Figure 1

The effect of resonances in the ${}^8\mathrm{Be}$ compound nucleus involving initial states ${}^7\mathrm{Be}+n$. It shows the range of values for the product of the resonant state spin degeneracy and resonance strength $(2J+1){\Gamma }_{\mathrm{eff}}$ versus the resonance energy. Contours indicate where the lithium abundance is reduced to ${}^7\mathrm{Li}/H=1.23\times {10}^{-10}$, $2.0\times {10}^{-10}$, $3.0\times {10}^{-10}$, $4.0\times {10}^{-10}$, and $5.0\times {10}^{-10}$. Normal resonances have $E_{\mathrm{res}}>0$, while subthreshold resonances lie in the $E_{\mathrm{res}}<0$. The horizontal dot-dashed line is the experimental value of the strength of the resonance corresponding to the 19.40 MeV energy level. The vertical dashed line shows the position of $E_{\mathrm{res}}$ for the same state.

Figure 2

As in Fig. 1, for the resonances in the ${}^8\mathrm{B}$ compound nucleus involving initial states ${}^7\mathrm{Be}+p$.

Figure 3

As in Fig. 1, for the resonances in the ${}^9\mathrm{Be}$ compound nucleus.

Figure 4

As in Fig. 1, for the resonances in the ${}^9\mathrm{B}$ compound nucleus. The vertical dashed line at 220 keV indicates the experimental central value of the resonance energy of the 16.71 MeV level.

Figure 5

As in Fig. 1, for the resonances in the ${}^{10}\mathrm{Be}$ compound nucleus involving the initial state ${}^7\mathrm{Li}+t$ (left), and in the ${}^{10}\mathrm{B}$ compound nucleus involving the initial state ${}^7\mathrm{Li}+{}^3\mathrm{He}$. (right).

Figure 6

As in Fig. 1, for the resonances in the ${}^{10}\mathrm{B}$ compound nucleus involving initial states ${}^7\mathrm{Be}+t$.

Figure 7

As in Fig. 1, for the resonances in the reaction ${}^7\mathrm{Be}(t,{}^3\mathrm{He}){}^7\mathrm{Li}$.

Figure 8

As in Fig. 1, for the resonances in ${}^{10}\mathrm{C}$ involving initial state ${}^7\mathrm{Be}+{}^3\mathrm{He}$.

Figure 9

As in Fig. 1, for the resonances in ${}^{11}\mathrm{C}$ involving initial states ${}^7\mathrm{Be}+\alpha$.