Did Something Decay, Evaporate, or Annihilate during Big Bang Nucleosynthesis?

Abstract

Results of a detailed examination of the cascade nucleosynthesis resulting from the putative hadronic decay, evaporation, or annihilation of a primordial relic during the Big Bang nucleosynthesis (BBN) era are presented. It is found that injection of energetic nucleons around cosmic time 1000 sec may lead to an observationally favored reduction of the primordial Li7/H yield by a factor 2 - 3. Moreover, such sources also generically predict the production of the Li6 isotope with magnitude close to the as yet unexplained high Li6 abundances in low-metallicity stars. The simplest of these models operate at fractional contribution to the baryon density Omega_b h^2> 0.025, slightly larger than that inferred from standard BBN. Though further study is required, such sources, as for example due to the decay of the next-to-lightest supersymmetric particle into GeV gravitinos or the decay of an unstable gravitino in the TeV range of abundance Ω_G h^2\sim 5\times 10^{-4} show promise to explain both the Li6 and Li7 abundances in low metallicity stars.

Figures (3)

• Figure 1: Light element evolution as function of temperature with (solid) and without (dashed) a thermal neutron source. Shown are, from top to bottom (at lower temperatures), the number ratios $n_{\rm D}/n_p$, $n_{\rm ^3He}/n_p$, $n_n/n_p$, $n_{\rm ^7Be}/n_p$, and $n_{\rm ^7Li}/n_p$, respectively. See text for further detail.
• Figure 2: Abundance yields of D/H, $^{7}$Li/H, and $^{7}$Li/$^{6}$Li in an $\Omega_bh^2 = 0.026$ Universe as function of the hadronic decay time $\tau$ of a putative primordial relic. The models are decay of a $m_{\chi} = 10\,$GeV particle (long-dashed), decay of a $m_{\chi} = 200\,$GeV particle (solid), decay of a $m_{\chi} = 4\,$TeV particle (dashed-dotted), injection of monoenergetic nucleons of $E_{kin} = 250\,$MeV (short-dashed), and extended power-law injection due to a $m_{\chi} = 200\,$GeV particle (dotted). Also shown are the two-sigma ranges of the inferred primordial D/H and $^{7}$Li/H abundances Kirk:03Ryan:99 as well as the $^{6}$Li/$^{7}$Li ratio as inferred in the low-metallicity star HD84937 li6:highZ. See text for further details.
• Figure 3: Abundance yields of D/H, $^{7}$Li/H, and $^{7}$Li/$^{6}$Li in the presence of residual neutralino annihilation for a neutralino fractional contribution to the critical density of $\Omega{\chi}h^2 = 0.1126$. Three models are shown: $m_{\chi} = 5\,$GeV and $\langle\sigma v\rangle = 3\times 10^{-26}{\rm cm^3/s}$ (light solid), $m_{\chi} = 5\,$GeV and $\langle\sigma v\rangle = 5\times 10^{-26}{\rm cm^3/s}$ (dashed), and $m_{\chi} = 10\,$GeV and $\langle\sigma v\rangle = 3\times 10^{-26}{\rm cm^3/s}$ (dotted). For comparison, the SBBN yields are shown by the heavy solid lines. Note the absence of appreciable $^{6}$Li production in SBBN. Observational data on the D/H, $^{7}$Li/H, and $^{6}$Li/$^{7}$Li ratios indicated in the figure are as in Fig.2.