${}^6$Li Production by the Radiative Decay of Long-Lived Particles

Abstract

Recent spectroscopic observations of metal poor stars have indicated that both ${}^7$Li and ${}^6$Li have abundance plateaus with respect to the metallicity. Abundances of ${}^7$Li are about a factor three lower than the primordial abundance predicted by standard big-bang nucleosynthesis (SBBN), and ${}^6$Li abundances are $\sim 1/20$ of ${}^7$Li, whereas SBBN predicts negligible amounts of ${}^6$Li compared to the detected level. These discrepancies suggest that ${}^6$Li has another cosmological or Galactic origin than the SBBN. Furthermore, it would appear that ${}^7$Li (and also ${}^6$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 non-thermal nucleosynthesis associated with the radiative decay and 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 non-thermal nucleosynthesis produces ${}^6$Li at the level detected in metal poor halo stars (MPHSs), when the lifetime of the unstable particles is of the order $\sim 10^8-10^{12}$~s and their initial abundance with respect to that of the photons is $\sim (10^{-13}-10^{-12}\mathrm{GeV})/E_{\gamma 0}$, where $E_{\gamma 0}$ is the emitted photon energy in the radiative decay. We conclude that the most likely nucleosynthetic scenario involves two different processes. First, a non-thermal cosmological nucleosynthesis associated with the radiative decay of unstable particles; and second, the stellar depletion of both of the primordial lithium isotopic abundances.

12 pages, 4 Postscript figures; references added, typos corrected

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