Cosmic Rays from Dark Matter Annihilation and Big-Bang Nucleosynthesis

TL;DR

This work quantifies how dark matter annihilation during the Big-Bang Nucleosynthesis era injects high-energy particles that alter light-element abundances, providing robust constraints on annihilation cross sections. By modeling both electromagnetic cascades (photo-dissociation) and hadronic processes (hadro-dissociation and $p\leftrightarrow n$ inter-conversions) and anchoring to current primordial abundance observations, the authors derive upper bounds on $\langle\sigma v\rangle$ for various final states. They find that leptonic annihilation channels are compatible with PAMELA/ATIC signals within BBN limits, while hadronic channels are more tightly constrained and may require a boost factor to maintain compatibility. The results emphasize that BBN constraints are a powerful, relatively astrophysics-insensitive cross-check on DM explanations of cosmic-ray anomalies and guide the viable parameter space for DM models. Overall, the paper demonstrates that early-universe energy injection from DM annihilation can be as constraining as late-time cosmic-ray observations for discriminating DM scenarios.

Abstract

Recent measurements of cosmic-ray electron and positron fluxes by PAMELA and ATIC experiments may indicate the existence of annihilating dark matter with large annihilation cross section. We show that the dark matter annihilation in the big-bang nucleosynthesis epoch affects the light element abundances, and it gives stringent constraints on such annihilating dark matter scenarios for the case of hadronic annihilation. Constraints on leptonically annihilating dark matter models are less severer.

Figures (5)

• Figure 1: BBN constraints on the annihilation cross section of DM particles when we take $D_{7}$ = 0.35. The red, cyan, blue, green and brown solid curves represent the upper bounds from observational constraints on $^3$He/D, D, $^6$Li, $^7$Li and $^4$He, respectively. The name of the element is also written by each line. As for D, we plot both upper limits from High and Low values. Here we assume that the dark matter annihilates only into $e^+ e^-$, which means the fraction of the visible energy is $E_{\rm vis}/m=2$. The lines of $^4$He, $^6$Li and $^7$Li do not appear in this figure. For reference, the region sandwiched by two dashed lines is allowed by $^{6}$Li when we take $D_{7}$ = 0.
• Figure 2: Same as Fig. \ref{['fig:BBNrad']} but for the annihilation into $W^{+}W^{-}$ pair. Then the fraction of the visible energy is $E_{\rm vis} / m = 0.94$.
• Figure 3: Same as Fig. \ref{['fig:BBNrad']} but for the annihilation into $b\bar{b}$ pair (top panel). Then the fraction of the visible energy is $E_{\rm vis} / m = 1.04$. For comparison, we also plot the case of $b+\bar{b}$ emission at $1\%$ in the bottom panel virtually by changing the hadronic branching ratio from 1 to 0.01 by hand with keeping the same value of $E_{\rm vis}$.
• Figure 4: Positron fraction $R$ as a function of positron energy $E$. (Top) We assume the DM annihilating into $e^+e^-$ with annihilation cross section $\langle \sigma v\rangle=5\times 10^{-24}~{\rm cm^{3}s^{-1}}$ for $m_{\rm DM}=650~{\rm GeV}$, and into $\mu^+ \mu^-$ with $\langle \sigma v\rangle=15\times 10^{-24}~{\rm cm^{3}s^{-1}}$ for $m_{\rm DM}=900~{\rm GeV}$ for the MED propagation model. (Bottom) We assume DM annihilating into ${\tau}^+{\tau}^-$ with annihilation cross section $\langle \sigma v\rangle=4\times 10^{-23}~{\rm cm^{3}s^{-1}}$ for $m_{\rm DM}=1~{\rm TeV}$, and into $W^+W^-$ with $\langle \sigma v\rangle=3\times 10^{-23}~{\rm cm^{3}s^{-1}}$ for $m_{\rm DM}=800~{\rm GeV}$ for the M2 propagation model. Results of PAMELA and HEAT experiments are also shown.
• Figure 5: Total electron and positron flux (times $E^3$) as a function of their energy. Model parameters are same as Fig. \ref{['fig:R']}. Results of BETS, PPB-BETS and ATIC are plotted.