The Case for a 700+ GeV WIMP: Cosmic Ray Spectra from PAMELA, Fermi and ATIC

TL;DR

The paper argues that a TeV-scale WIMP annihilating predominantly to leptons, potentially via a light mediator with Sommerfeld enhancement, can jointly explain the PAMELA $e^+$ rise, Fermi/HESS $e^+e^-$ hardening, ATIC/PPB-BETS features, and the WMAP Haze. It employs GALPROP-based propagation with an Einasto DM profile to relate local high-energy signals to a robust inverse-Compton gamma-ray signal toward the Galactic Center, a key smoking-gun prediction testable by Fermi/GLAST. Leptonic channels, especially direct $e^+e^-$ and φ→$e^+e^-$, fit the high-energy data with modest boost factors, while τ and hadronic channels tend to be softer or require larger boosts; gamma-ray limits from EGRET constrain some modes but do not rule out the leptonic scenarios within uncertainties. Overall, the study presents a coherent, testable DM interpretation of multiple astrophysical anomalies and highlights GC ICS gamma rays as a critical observable for future confirmation.

Abstract

Multiple lines of evidence indicate an anomalous injection of high-energy e+- in the Galactic halo. The recent $e^+$ fraction spectrum from the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) shows a sharp rise up to 100 GeV. The Fermi Gamma-ray Space Telescope has found a significant hardening of the e+e- cosmic ray spectrum above 100 GeV, with a break, confirmed by HESS at around 1 TeV. The Advanced Thin Ionization Calorimeter (ATIC) has also detected detected a similar excess, falling back to the expected spectrum at 1 TeV and above. Excess microwaves towards the galactic center in the WMAP data are consistent with hard synchrotron radiation from a population of 10-100 GeV e+- (the WMAP ``Haze''). We argue that dark matter annihilations can provide a consistent explanation of all of these data, focusing on dominantly leptonic modes, either directly or through a new light boson. Normalizing the signal to the highest energy evidence (Fermi and HESS), we find that similar cross sections provide good fits to PAMELA and the Haze, and that both the required cross section and annihilation modes are achievable in models with Sommerfeld-enhanced annihilation. These models naturally predict significant production of gamma rays in the galactic center via a variety of mechanisms. Most notably, there is a robust inverse-Compton scattered (ICS) gamma-ray signal arising from the energetic electrons and positrons, detectable at Fermi/GLAST energies, which should provide smoking gun evidence for this production.

Figures (16)

• Figure 1: (a) A new power-law component of $e^+e^-$ fit to the PAMELA excess (long-dashed) with expected background $e^+$ model (solid) and total positron fraction (short-dashed). (b) Extrapolation of this new component to higher energies, assuming equal parts $e^+$ and $e^-$. For reference, the new component positrons are about 12% of the PAMELA fraction at 100 GeV, and therefore constitute about 1/4 of the Fermi flux $\phi(e^+)+\phi(e^-)$ at 100 GeV. The fact that the high energy electron+positron and PAMELA excesses are connected by such a generic argument suggests that a single mechanism, producing equal numbers of $e^+$ and $e^-$, explains both.
• Figure 2: The dominant contributions to gamma rays in the galactic center (inner $5^\circ$) from conventional astrophysical CR interactions. Shown are the photons from $\pi^0$'s, electron ICS signals and bremsstrahlung. In this and following figures, "ISM" denotes gammas produced by CR interactions with the interstellar radiation field, as well as the interstellar medium (gas and dust). The $\pi^0$ line refers to $\pi^0$$\gamma$-rays produced by the hadronic CRs interacting with the interstellar medium. The slope of the $\pi^0$ curve at high energies is tied to the primary $p$ CR spectrum, and is better constrained than the amplitude.
• Figure 3: The cosmic ray signals from dark matter annihilations $\chi \chi \rightarrow e^+ e^-$. Upper left: Predicted positron fraction vs. energy (solid and dashed lines), expected positron fraction vs. energy due to secondary production only (dotted), and PAMELA Adriani:2008zr data points. BF is the boost factor required relative to $\langle\sigma_Av\rangle = 3\times10^{-26}{\rm ~cm^3/s}$ and the reference local DM density of $\rho_0 = 0.3 {\rm ~GeV} {\rm ~cm}^{-3}$. Upper right: Spectrum of DM $e^+ e^-$ (solid and dashed), background $e^+ e^-$ (dot-dashed), and total (solid and dashed) with data from Fermi Abdo:2009zk, ATIC ATIClatest and PPB-BETS Torii:2008xu. Lower left: Predicted WMAP Haze signal vs. galactic latitude at 23 GHz (solid and dashed) and data points from WMAP Dobler:2007wv. Error bars are statistical only. Lower right: Total diffuse gamma ray spectrum (solid and dashed) and background diffuse gamma ray spectrum (dotted) for the inner $5^\circ$ of the Milky Way, computed with GALPROP Strong:1999sv. Data points are from the Strong et al. re-analysis of the EGRET data Strong:2005zx, which found a harder spectrum at $10-100$ GeV within a few degrees of the GC, using improved sensitivity estimates from Thompson:2005.
• Figure 4: The cosmic ray signals of dark matter annihilations as in Figure \ref{['fig:electrons']}, but with $\chi \chi \rightarrow \mu^+ \mu^-$.
• Figure 5: The cosmic ray signals of dark matter annihilations $\chi \chi \rightarrow \tau^+ \tau^-$.