Dark Matter Signals from Cascade Annihilations

TL;DR

This paper investigates whether dark matter annihilation, especially through cascade decays into light resonances that subsequently decay to leptons, can explain the PAMELA/ATIC electron/positron excess while remaining consistent with gamma-ray and neutrino bounds. The authors compute cascade spectra via an iterative convolution and fit them to PAMELA/ATIC data, finding that DM masses on the TeV scale with boost factors around 10^3 provide good fits, with the required parameters increasing roughly as $2^n$ for an $n$-step cascade. They demonstrate that cascade annihilations soften the primary gamma-ray flux from final-state radiation, weakening FSR bounds, but neutrino bounds from muon cascades remain robust and can be competitive, particularly for NFW/Einasto halos. The analysis also extends to axion-portal scenarios, showing spectra and constraints are typically between 1- and 2-step cascades, and provides branching-ratio limits for rare decays. Overall, cascade annihilations can reconcile the PAMELA/ATIC signals with existing gamma-ray and neutrino limits within halo-profile uncertainties, with future gamma-ray and neutrino observations (and Fermi ICS considerations) poised to further test these models.

Abstract

A leading interpretation of the electron/positron excesses seen by PAMELA and ATIC is dark matter annihilation in the galactic halo. Depending on the annihilation channel, the electron/positron signal could be accompanied by a galactic gamma ray or neutrino flux, and the non-detection of such fluxes constrains the couplings and halo properties of dark matter. In this paper, we study the interplay of electron data with gamma ray and neutrino constraints in the context of cascade annihilation models, where dark matter annihilates into light degrees of freedom which in turn decay into leptons in one or more steps. Electron and muon cascades give a reasonable fit to the PAMELA and ATIC data. Compared to direct annihilation, cascade annihilations can soften gamma ray constraints from final state radiation by an order of magnitude. However, if dark matter annihilates primarily into muons, the neutrino constraints are robust regardless of the number of cascade decay steps. We also examine the electron data and gamma ray/neutrino constraints on the recently proposed "axion portal" scenario.

Figures (11)

• Figure 1: For an $n$-step cascade annihilation, dark matter $\chi$ annihilates into $\phi_n \phi_n$. The cascade annihilation then occurs through $\phi_{i+1} \rightarrow \phi_i \phi_i$ ($i=1,\cdots,n-1$), and in the last stage, $\phi_1$ decays into standard model particles. The figure represents the cases where $\phi_1 \rightarrow \ell^+ \ell^-$.
• Figure 2: The best fit regions for the dark matter mass $m_{\rm DM}$ and boost factor $B$ in the cases of direct, $1$-step, and $2$-step annihilations into $e^+ e^-$ for different halo profiles and propagation models. The best fit values are indicated by the crosses, and the contours are for $1\sigma$ and $2\sigma$.
• Figure 3: The same as Figure \ref{['fig:positron_e']} but for annihilations into $\mu^+ \mu^-$.
• Figure 4: The predicted $e^\pm$ intensities compared to the PAMELA (left) and ATIC (right) data for direct (solid), $1$-step (dashed), and $2$-step (dotted) annihilations into electron final states. The NFW halo profile and the MED propagation model are chosen, and the $e^\pm$ backgrounds are marginalized as described in Eq. (\ref{['eq:marginalize']}). Note that we fit the PAMELA data only for $E \mathrel{\hbox{$>$$\sim$}} 10~{\rm GeV}$ because solar modulation effects are important at lower energies.
• Figure 5: The same as Figure \ref{['fig:spectra_e']} but for annihilations into muon final states.