Decaying Dark Matter and the PAMELA Anomaly

TL;DR

The paper investigates whether decaying dark matter can account for the PAMELA rise in the cosmic-ray positron fraction, relaxing the requirement of absolute DM stability and allowing lifetimes up to the age of the Universe. It adopts a model-independent framework, contrasting fermionic and scalar DM decays across multiple channels and computing the Earth positron flux using a two-zone diffusion model and Green's-function propagation. The analysis finds that heavy DM with $m_{\rm DM}\gtrsim 300\,\mathrm{GeV}$ and lifetimes around $\tau_{\rm DM}\sim 10^{26}\,\mathrm{s}$ can reproduce the observed spectrum when decays inject hard leptons (e.g., $\psi\rightarrow W^\pm e^\mp$, $\psi\rightarrow W^\pm \mu^\mp$, or $\phi\rightarrow \ell^+ \ell^-$); tau-rich or gauge-boson decays tend to produce too-flat spectra or conflict with antiproton/gamma-ray constraints. The results are fairly robust to propagation-model choices, and they highlight the potential role of a subdominant decaying component or future high-energy data (100–300 GeV) in further testing these scenarios. Cross-consistency with gamma-ray and antiproton measurements will provide complementary tests of decaying dark matter as the PAMELA anomaly’s origin.

Abstract

Astrophysical and cosmological observations do not require the dark matter particles to be absolutely stable. If they are indeed unstable, their decay into positrons might occur at a sufficiently large rate to allow the indirect detection of dark matter through an anomalous contribution to the cosmic positron flux. In this paper we discuss the implications of the excess in the positron fraction recently reported by the PAMELA collaboration for the scenario of decaying dark matter. To this end, we have performed a model-independent analysis of possible signatures by studying various decay channels in the case of both a fermionic and a scalar dark matter particle. We find that the steep rise in the positron fraction measured by PAMELA at energies larger than 10 GeV can naturally be accommodated in several realizations of the decaying dark matter scenario.

Figures (6)

• Figure 1: Positron fraction from the decay of the fermionic dark matter particle in the channel $\psi\rightarrow Z^0 \nu$ when the dark matter mass is, from left to right, $m_{\rm DM}= 150,\;300,\; 600,\; 1000\;{\rm GeV}$. The lifetime has been chosen to provide a qualitatively good fit to the data and is $\sim 5\times 10^{25}\;{\rm s}$ in all the cases.
• Figure 2: Positron fraction from the decay of the fermionic dark matter particle in the channels $\psi\rightarrow W^\pm e^\mp$ (top-left panel), $\psi\rightarrow W^\pm \mu^\mp$ (top-right panel) and $\psi\rightarrow W^\pm \tau^\mp$ (bottom panel), when the dark matter mass is, from left to right, $m_{\rm DM}= 150,\;300,\; 600,\; 1000\;{\rm GeV}$. The lifetime, which ranges between $10^{26}\;{\rm s}$ and $5\times 10^{26}\;{\rm s}$, is different in each case and has been chosen to provide a qualitatively good fit to the data.
• Figure 3: Positron fraction from the decay of the fermionic dark matter particle in the channels $\psi\rightarrow e^+ e^- \nu$ (top-left panel), $\psi\rightarrow \mu^+ \mu^- \nu$ (top-right panel) and $\psi\rightarrow \tau^+ \tau^- \nu$ (bottom panel), when the dark matter mass is, from left to right, $m_{\rm DM}= 150,\;300,\; 600,\; 1000\;{\rm GeV}$. The lifetime, which ranges between $5\times 10^{25}\;{\rm s}$ and $8\times 10^{26}\;{\rm s}$, is different in each case and has been chosen to provide a qualitatively good fit to the data.
• Figure 4: Positron fraction from the decay of a scalar dark matter particle in the channels $\phi\rightarrow Z^0 Z^0$ (top-left panel) and $\phi\rightarrow W^+ W^-$ (top-right panel) when the dark matter mass is, from left to right, $m_{\rm DM}= 300,\; 600,\; 1000\;{\rm GeV}$. The lifetime has been chosen to provide a qualitatively good fit to the data and is $\sim 2\times 10^{26}\;{\rm s}$ ($\sim 10^{26}\;{\rm s}$) for the decay into $Z$ ($W$) bosons.
• Figure 5: Positron fraction from the decay of a scalar dark matter particle in the channels $\phi\rightarrow e^+ e^-$ (top-left panel), $\phi\rightarrow \mu^+ \mu^-$ (top-right panel) and $\phi\rightarrow \tau^+ \tau^-$ (bottom panel), when the dark matter mass is, from left to right, $m_{\rm DM}= 150,\;300,\; 600,\; 1000\;{\rm GeV}$. The lifetime, which ranges between $10^{26}\;{\rm s}$ and $10^{27}\;{\rm s}$, is different in each case and has been chosen to provide a qualitatively good fit to the data.