Two component dark matter

TL;DR

The paper tackles the PAMELA/ATIC positron excess by proposing a two-component dark matter model in which a stable χ1 and a metastable χ2 co-exist in thermal equilibrium and χ2 decays to χ1 after ~10^-8 s. Through coupled Boltzmann dynamics, the late decay enhances the final χ1 abundance Y1(∞) and generates a large boost factor B without requiring steep local density spikes; in the limit Γ2 ≪ Γ_Ai one finds Y1(∞) ≈ Y1^{Th.rel.} + N_dec Y2^{Th.rel.} and B ∝ (cross-section ratio) × (Y1(∞)/Y1^{Th.rel.}). Constraints on Γ2, m1, m2, and N_dec ensure the enhancement survives without washout and remains consistent with Big Bang Nucleosynthesis, with dimension-5 operators linking the decay width to a high new-physics scale Λ. The work shows that a concrete particle-physics realization, potentially testable at colliders, can reproduce the needed boost factors and offers a framework to connect cosmic-ray hints with a testable dark sector. Overall, the 2DM scenario provides a plausible, testable mechanism to explain the PAMELA/ATIC signals within a thermal relic paradigm while guiding future collider and cosmological probes.

Abstract

We explain the PAMELA positron excess and the PPB-BETS/ATIC e+ + e- data using a simple two component dark matter model (2DM). The two particle species in the dark matter sector are assumed to be in thermal equilibrium in the early universe. While one particle is stable and is the present day dark matter, the second one is metastable and decays after the universe is 10^-8 s old. In this model it is simple to accommodate the large boost factors required to explain the PAMELA positron excess without the need for large spikes in the local dark matter density. We provide the constraints on the parameters of the model and comment on possible signals at future colliders.

Figures (2)

• Figure 1: The solution for $Y_1(z)$ (black solid line) and $Y_2(z)$ (blue solid line) for $m_1=1$ TeV, $m_2=3$ TeV, $\Gamma_2=10^{-24}$ GeV, $N_{\rm dec}=1$ and three different values of $R$ as denoted. Dashed lines denote the thermal relic values of $Y_i(z)$. On the upper figure $\langle \sigma_{A2} v_2\rangle$ is held fixed to $\langle \sigma_A v\rangle_{\rm CDM}=3\times 10^{-26} {\rm cm}^3/{\rm s}$, so that without decay this would give correct DM relic density with $\chi_2$ the DM particle. Through decay this is transfered to $\chi_1$. For illustration we also show the lower figure, where $\langle \sigma_{A1} v_1\rangle$ is held fixed to $3\times 10^{-26} {\rm cm}^3/{\rm s}$.
• Figure 2: The boost factor $B$ as a function of $m_1$ and $R$, in the case where $\Gamma_2$ is small enough that wash-out can be neglected. The framed numbers ($n$) labeling contours give boost factors as $B=10^n$.