Be7charge exchange betweenBe3+7ion and an exotic long-lived negatively charged massive particle in big bang nucleosynthesis

Abstract

The existence of an exotic long-lived negatively charged massive particle, i.e., ${X}^{\ensuremath{-}}$, during big bang nucleosynthesis can affect primordial light element abundances. In particular, the final abundance of $^{7}\mathrm{Li}$, mainly originating from the electron capture of $^{7}\mathrm{Be}$, has been suggested to be made smaller by the $^{7}\mathrm{Be}$ destruction via the radiative ${X}^{\ensuremath{-}}$ capture of $^{7}\mathrm{Be}$ followed by the radiative proton capture of the bound state of $^{7}\mathrm{Be}$ and ${X}^{\ensuremath{-}}$ ($^{7}\mathrm{Be}_{X}$). We suggest a new route of $^{7}\mathrm{Be}_{X}$ formation, that is the $^{7}\mathrm{Be}$ charge exchange at the reaction of $^{7}\mathrm{Be}^{3+}$ ion and ${X}^{\ensuremath{-}}$. The formation rate depends on the number fraction of the $^{7}\mathrm{Be}^{3+}$ ion, the charge exchange cross section of $^{7}\mathrm{Be}^{3+}$, and the probability that produced excited states $^{7}\mathrm{Be}_{X}^{*}$ are converted to the ground state. We estimate respective quantities affecting the $^{7}\mathrm{Be}_{X}$ formation rate and find that this reaction pathway can be more important than the ordinary radiative recombination of $^{7}\mathrm{Be}$ and ${X}^{\ensuremath{-}}$. The effect of the charge exchange reaction is then shown in a latest nuclear reaction network calculation. Quantum physical model calculations for related reactions are needed to precisely estimate the efficiency of this pathway in the future.