Neutralinos, Big Bang Nucleosynthesis and 6Li in Low-Metallicity Stars

Abstract

The synthesis of Li6 during the epoch of Big Bang nucleosynthesis (BBN) due to residual annihilation of dark matter particles is considered. By comparing the predicted Li6 to observations of this isotope in low-metallicity stars, generic constraints on s-wave dark matter annihilation rates into quarks, gauge bosons, and Higgs bosons are derived. It may be shown that, for example, wino dark matter in anomaly-mediated SUSY breaking scenarios with masses Mx < 250 GeV or light neutralinos with Mx < 20 GeV annihilating into light quarks are, taken face value, ruled out. These constraints may only be circumvented if significant Li6 depletion has occurred in all three low-metallicity stars in which this isotope has been observed to date. In general, scenarios invoking non-thermally generated neutralinos with enhanced annihilation rates for a putative explanation of cosmic ray positron or galactic center as well as diffuse background gamma-ray signals by present-day neutralino annihilation will have to face a stringent Li6 overproduction problem. On the other hand, it is possible that Li6 as observed in low-metallicity stars is entirely due to residual dark matter annihilation during BBN, even for neutralinos undergoing a standard thermal freeze-out.

Figures (3)

• Figure 1: Final $^{6}$Li yield functions defined via Eq. (\ref{['yield']}) as a function of neutralino mass for various annihilation channels as labeled in the key. The 1-$\sigma$ range of the $^{6}$Li abundance in HD84937 is also shown.
• Figure 2: Annihilation-channel dependent constraints on the s-wave annihilation rate due to possible $^{6}$Li overproduction as a function of neutralino mass. For simplicity, only the $u\bar{u}$, $b\bar{b}$ and $W^-W^+$ channels are shown with heavy lines and line styles as indicated in Fig.1. Constraints on other quark- and gauge boson- annihilation channels are virtually identical to those shown as may be verified by inspection of Fig.1. Annihilation rates above the lines are ruled out. Also shown are the annihilation rates required remark4a to produce $\Omega_{\chi}h^2 = 0.1126$ during standard thermal freeze-out (solid line) or during thermal freeze-out when extra degrees of freedom (contributing to the Hubble expansion during freeze-out) are present (the upper two solid lines with $\delta g_H^f/g_H^f$ as labeled). The dotted diagonal lines correspond to the required $\langle\sigma v\rangle$ for post thermal freeze-out non-thermal generation of $\Omega_{\chi}h^2 = 0.1126$ at temperatures, from top to bottom, $0.1,0.2,0.5,1,2,$ and $5\,$GeV, respectively. Here the QCD phase transition has been assumed to occur at $200\,$MeV. The curved dotted line shows the annihilation rate in case of AMSB winos Moroi:1999zb.
• Figure 3: S-wave annihilation rate required to produce within the $2-\sigma$ limits the $^{6}$Li abundance of HD84937. The heavy lines indicate the central value of HD84937, whereas lighter lines the $2-\sigma$ ranges. For simplicity only the $u\bar{u}$ (solid) and $W^-W^+$ (dotted) channels are shown with results for other channels similar (cf. Fig. 1).