Li6production by the radiative decay of long-lived particles

Abstract

Recent spectroscopic observations of metal-poor stars have indicated that both $^{7}\mathrm{Li}$ and $^{6}\mathrm{Li}$ have abundance plateaus with respect to the metallicity. Abundances of $^{7}\mathrm{Li}$ are about a factor three lower than the primordial abundance predicted by standard big bang nucleosynthesis (SBBN), and $^{6}\mathrm{Li}$ abundances are $\ensuremath{\sim}1/20$ of $^{7}\mathrm{Li}$, whereas SBBN predicts negligible amounts of $^{6}\mathrm{Li}$ compared to the detected level. These discrepancies suggest that $^{6}\mathrm{Li}$ has another cosmological or Galactic origin than the SBBN. Furthermore, it would appear that $^{7}\mathrm{Li}$ (and also $^{6}\mathrm{Li}$) has been depleted from its primordial abundance by some post-BBN processes. In this paper we study the possibility that the radiative decay of long-lived particles has affected the cosmological lithium abundances. We calculate the nonthermal nucleosynthesis associated with the radiative decay taking account both of the primary nuclear production reactions and the effects of secondary production as well as the destruction processes of energetic nuclides $\mathrm{D}$, $\mathrm{T}$, $^{3}\mathrm{He}$, $^{4}\mathrm{He}$, $^{6}\mathrm{Li}$, and $^{7}\mathrm{Li}$. We explore the allowed region of the parameters specifying the properties of long-lived particles. We also impose constraints from observations of the CMB energy spectrum. We find that nonthermal nucleosynthesis produces $^{6}\mathrm{Li}$ at the level detected in metal-poor halo stars (MPHSs), when the lifetime of the unstable particles is of the order $\ensuremath{\sim}{10}^{8}--{10}^{12}\text{ }\text{ }\mathrm{s}$ and their initial abundance with respect to that of the photons is $\ensuremath{\sim}({10}^{\ensuremath{-}13}--{10}^{\ensuremath{-}12}\text{ }\text{ }\mathrm{GeV})/{E}_{\ensuremath{\gamma}0}$, where ${E}_{\ensuremath{\gamma}0}$ is the emitted photon energy in the radiative decay. We conclude that the most likely nucleosynthetic scenario involves two different processes. First, a nonthermal cosmological nucleosynthesis associated with the radiative decay of unstable particles; and second, the stellar depletion of both of the primordial lithium isotopic abundances.