Li6 production by the radiative decay of long-lived particles

Recent spectroscopic observations of metal-poor stars have indicated that both Li7 and Li6 have abundance plateaus with respect to the metallicity. Abundances of Li7 are about a factor three lower than the primordial abundance predicted by standard big bang nucleosynthesis (SBBN), and Li6 abundances are ∼1/20 of Li7, whereas SBBN predicts negligible amounts of Li6 compared to the detected level. These discrepancies suggest that Li6 has another cosmological or Galactic origin than the SBBN. Furthermore, it would appear that Li7 (and also Li6) 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 D, T, He3, He4, Li6, and Li7. 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 Li6 at the level detected in metal-poor halo stars (MPHSs), when the lifetime of the unstable particles is of the order ∼108-1012s and their initial abundance with respect to that of the photons is ∼(10-13-10-12GeV)/Eγ0, where Eγ0 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.