Positron/Gamma-Ray Signatures of Dark Matter Annihilation and Big-Bang Nucleosynthesis

TL;DR

This work investigates whether dark matter annihilation sufficient to explain the PAMELA positron excess can simultaneously alleviate the primordial lithium problem via effects on Big-Bang Nucleosynthesis. Using a concrete wino-like neutralino in anomaly-mediated SUSY breaking, the authors connect cosmic positron signatures, BBN constraints, and Galactic-center gamma-ray flux, highlighting testable predictions for Fermi. They show that a light wino with $m_\chi \sim 150$–$200$ GeV can fit PAMELA with modest boost factors and, with Li-depletion, can reduce ${}^{7}$Li while maintaining other light-element abundances, and that a Galactic-center gamma-ray signal offers a complementary probe; they also discuss uncertainties from antiprotons and synchrotron emission and note that heavier winos around $\sim 2$ TeV can be relevant with non-perturbative enhancements or non-thermal production.

Abstract

The positron excess observed by the PAMELA experiment may come from dark matter annihilation, if the annihilation cross section is large enough. We show that the dark matter annihilation scenarios to explain the positron excess may also be compatible with the discrepancy of the cosmic lithium abundances between theory and observations. The wino-like neutralino in the supersymmetric standard model is a good example for it. This scenario may be confirmed by Fermi satellite experiment.

Figures (4)

• Figure 1: Positron fraction for $m_{\chi}=$ 150 and 200 GeV with boost factor 2 and 3, as a function of positron energy. HEAT and PAMELA results are also shown.
• Figure 2: Allowed regions at 95$\%$ C.L. from observational light element abundances in $m_{\chi}$--$\langle \sigma v \rangle$ plane. The name of each element is written in the close vicinity of the line. For $^{6}$Li and $^{7}$Li, regions sandwiched between two lines are allowed, respectively. Except for lithiums, each line means the upper bound. The total cross section of the annihilation and its major four modes are also plotted. The calculation is performed by assuming 100$\%$$WW$ emission for simplicity.
• Figure 3: Same as Fig. \ref{['fig:BBNallowed']} except for including the depletion of lithium ($\Delta \log_{10}({^7{\rm Li}}/{\rm H}) =0.4\Delta \log_{10}({^6{\rm Li}}/{\rm H})=0.25$).
• Figure 4: Gamma-ray flux produced by the wino-like DM annihilation with mass 150 GeV for both NFW and isothermal profile, and 200 GeV for NFW profile from the Galactic center within the region $-5^\circ <l<5^\circ$ and $-2^\circ <b<2^\circ$. The result of EGRET observation is also shown.